Project Description POLFEL Polish free electron laser

Transcription

1 NATIONAL CENTRE FOR NUCLEAR RESEARCH Świerk Project Description POLFEL Polish free electron laser Oliwia Chołuj-Dziewiecka, Jerzy Lorkiewicz, Robert Nietubyć, Jerzy Pełka, Jacek Sekutowicz, Ryszard Sobierajski, Jarosław Szewiński, Tomasz Wasiewicz, Grzegorz Wrochna Warszawa Otwock Świerk February 2012

2 National Centre for Nuclear Research Otwock, A.Sołtana 7, Poland Publisher Oliwia Chołuj-Dziewiecka M. Eng. National Centre for Nuclear Research Otwock, A.Sołtana 7 Division of Accelerator Physics and Technology Dr. Jerzy Lorkiewicz National Centre for Nuclear Research Otwock, A.Sołtana 7 Division of Accelerator Physics and Technology Dr. Robert Nietubyć National Centre for Nuclear Research Otwock, A.Sołtana 7 Plasma/Ion Beam Technology Division, Plasma Studies Division Tomasz Wasiewicz M. Eng. National Centre for Nuclear Research Otwock, A.Sołtana 7 Plasma/Ion Beam Technology Division Plasma Studies Division Jarosław Szewiński M. Eng. National Centre for Nuclear Research Otwock, A.Sołtana 7 Research Reactor Technology Division, Radiation Detectors Division, Electronics and Detection Systems Division Authors Dr. Hab. Jerzy Pełka Institute of Physics, Polish Academy of Sciences Warszawa, Al. Lotników 32/46 Laboratory of X-Ray and electron microscopy resarch Dr. Ryszard Sobierajski Institute of Physics, Polish Academy of Sciences Warszawa, Al. Lotników 32/46 Laboratory of X-Ray and electron microscopy resarch Dr. Hab. Jacek Sekutowicz Deutsches Elektronen-Synchrotron A Research Centre of the Helmholtz Association Hamburg, Notkestrasse 85 Germany Machine Physics Group Prof. Dr. Hab. Grzegorz Wrochna National Centre for Nuclear Research Otwock, A.Sołtana 7 Director NCNR The authors would like to thank all who contributed to this document _v4 1

3 In brief POLFEL Polish free electron laser will provide tunable coherent electromagnetic radiation in the range from several nanometers (Soft X rays, SX) to several hundred micrometers (Terahertz radiation, THz). The radiation of free electron lasers (FEL) is emitted in impulses, which last from a few femtoseconds for SX to tens of picoseconds for the THz range, with peak power of the order of 1 GW over the whole range of the frequency spectrum. As one of the few free electron lasers, the proposed source will be able to switch to the emission of high average power of the order of several tens of watts over its entire operating range. For the most part of this range it will be one of the most powerful sources of coherent radiation. POLFEL will complement the capabilities of conventional lasers by extending the avaiable spectrum and will provide transverse and longitudinal coherent light, which is impossible for synchrotrons. It will also provide higher intensity by several orders of size and magnitude and pulse durations from ten to ten thousand times faster than existing sources. POLFEL laser radiation will enable the development of fundamental and applied research in physics, chemistry, biology, materials engineering and in other fields. Many of these experiments that POLFEL will enable are impossible to perform at present, due to the limited coherence of the beam power and the excessive duration of the pulse. New experiments will expand our knowledge of: the properties of light and its interaction with matter, including the highly excited states of matter (plasma physics, astrophysics), molecular dynamics and transient states in excitations and chemical reactions the structure of large biological molecules and systems of new semiconductor and magnetic materials (for eg. spintronics). The studies of applications will increase the possibility of: surface modification by irradiation (for e.g. the production of structures of nanometer size), determination of the spectral signatures of materials in different ranges. An important area, which will be revived in Poland by the POLFEL project and in which we can expect the strong development is the physics and technology of particle acceleration. POLFEL will consist of three parts: the electron accelerator, an undulator, where the scattered electrons emit radiation, and the experimental stations. POLFEL s linear accelerator will employ the TESLA technology, which was used in the FLASH laser accelerator and is currently being used in the construction of the European X-FEL, and is planned for the future lepton collider for particle physics, ILC. Undulators which can produce light with variable linear polarization plane and direction of the axis of elliptical polarization will be installed in POLFEL. Achieving the planned wavelength range requires the construction of an accelerator which is able of giving electrons an energy of several hundred megaelectronvolts, the associated undulators and several experimental systems designed to operate in the appropriate ranges of wavelength will be required to exploit fully the experimental possibilities. The construction of POLFEL will be divided into two stages. In the first stage a source will be created capable of emitting radiation in the range from the infrared to terahertz waves. It will be equipped with an accelerator with a final energy of 50 MeV, two undulators and an experimental line. First stage of this project will discover new optimizations which will be used in further phases. A possible development will be to perform experiments exploiting the unique capabilities of the NCBJ center in Swierk by creating a laboratory that uses both FEL photon beams and neutrons produced in the MARIA nuclear reactor. 2

4 The laboratory hosting POLFEL, the free electron laser, will be a center open to users from Poland and abroad. Its task will be to provide a beam of radiation with given parameters to conduct experiments and the development of the physics and techniques for making intense beams of radiation, and the instrumentation and measurement methods which are used in experiments conducted with such beams. The crucial issue for this multidisciplinary center for Polish science is the fact that Polish scientists will have permanent access to a unique facility that will enable them to cooperate with similar centers around the world on a equal basis. Research groups from other countries will bring their research to the Polish scientific community. Such cooperation will improve the value of research carried out in Poland, its significance for Polish science will be much greater than the current participation by Polish scientists in international projects carried out at foreign centers. Also important is the development of advanced technologies, which will be required in the construction of such a complex modern facility. The expansion of POLFEL will provide dozens of jobs in most fields of engineering. 3

5 Table of contents 1. Introduction The history of the development of light sources Polish contribution to the study of the use of FEL lasers and work on their development Meaning of POLFEL for science and the economy Science Economy POLFEL on the ESFRI Roadmap Connections of the POLFEL project with science development programs in the European Union Characteristics of POLFEL laser FEL sources in science and technology Introduction Terahertz radiation range, the basic features and a bit of history The use of FEL-type light sources in the terahertz range (THz-FEL) in science and technology Associated equipment and prospects for development of the POLFEL center Selected applications of a XUV-FEL source (Vis - XUV) Final remarks Technical description Conceptual work Free Electron Laser elements Electron source Acceleration sections Electron bunch compressor Undulator Collector Electron beam diagnostics Cryogenics High-frequency power supply LLRF System Monitoring and Diagnostics The security system Computer control system Phase 0 Test Photoinjector Test Photoinjector cost Thz-POLFEL technical design Accelerating sections RF power sources Cryogenics Undulators Preliminary cost estimation of the THz-POLFEL Preliminary schedule Building, bunker, tunnel Further opportunities for XUV-POLFEL development Investment location Social impact The project on the background of different EU, PL, Mazovian, district and commune documents Summary Literature Appendix 69 A. Glossary of names and abbreviations B. List of publications on FEL lasers with Polish authors C. List of publications on the THz radiation with participation of Polish authors

6 1. Introduction Synchrotron radiation is electromagnetic radiation emitted in a small solid angle by charged particles moving at relativistic speeds. It is produced by, inter alia, synchrotrons and free electron lasers, where systems of magnets affect the path of accelerated particles. These devices are currently used as the main main sources of electromagnetic radiation for scientific purposes. In a synchrotron an accelerated bunch of electrons is dispatched to a storage ring, where it moves along a closed orbit, passing repeatedly various kinds of magnet systems placed around the ring. The simplest of these systems is the dipole curved magnet. Wigglers and undulators, consisting of alternating oriented dipoles are also used. The emitted radiation pulses are transmitted to the experimental systems placed around the synchrotron ring. In this way a radiation beam in the range from IR to hard X rays is produced. The properties of the radiation produced, above all its spectral distribution and brightness, depend on the type and parameters of the magnet systems and on the spatial energy spread and momentum of the electron bunches in the ring. A free electron laser is a device consisting of a source of relativistic electrons and an undulator, producing coherent synchrotron radiation. This radiation is emitted in pulses of femtosecond duration and momentary power reaching the gigawatt level. Figure 1. A free electron laser produces a coherent ultra-short pulse of electromagnetic radiation (3) form a source of relativistic electrons such as a linear accelerator (1) and a periodic magnetic induction field produced for example by an undulator (2). This device may be thought of as a combination of a synchrotron and a conventional laser, although it is composed of very different components an accelerator and an undulator. The active medium of a conventional laser, in which there are discrete electron states, has been replaced by an electron beam in a vacuum, where the distribution of electron states is continuous. The accelerator play the role of the pump, which promotes the electrons from the ground state to the excited state in the active medium of a conventional laser. An accelerator which gives high energy to electrons. In a convenational laser pumping is sufficient to give rise to spontaneous emission; in a free electron laser spontaneous emission occurs as a result of the acceleration which is given to the electrons by the undulator magnetic field. In both cases, coherent radiation is produced by the impact of the center of the radiation produced spontaneously, having for both types of lasers similar characteristics: high intensity, monochromatic, defined phase and radiating in a very small solid angle. However, a Free Electron Laser exceeds the brightness of conventional lasers and has a shorter pulse duration and the possibility of tuning over a wide wavelength range. The tuning range is defined by the ability to change the parameters of the undulator magnetic structure and the electron beam energy range. For a given period, the wavelength of the light depends on the highest attainable energy of the electrons in the accelerator. This depends mainly on the number of accelerator modules and thus on the size and cost of the accelerator. Construction of the POLFEL laser will be divided into two phases. In the first stage a short accelerator will be built, in which the energy of the accelerated electrons will be up to 50 MeV, depending on the operation mode and the number of accelerator modules. This will produce a coherent beam of light with a wavelength between 8 µm and 200 µm, feeding one 5

7 experimental line. In the second stage, the accelerator will be extended to enable the achievement of an electron energy greater than 800 MeV. This will allow laser light with a wavelength below 10 nm to be produced. More measurement and research stations will also be built in the second stage. The primary task of the laboratory thus created will be to deliver the radiation beam to carry out experiments selected by open competition from external users and decided by an independent group of reviewers. A secondary task will be the development of the physics and engineering of light sources. 6

8 2. The history of the development of light sources FELs reached the short-wave limit at nm (for FELs using an optical resonator and amplifier with low gain). In FELs with a small gain the coupling between the radiation and the electron beam is small. To increase the impulse intensity one needs to exchange energy many times between the electrons bunches and the light pulse. This occurs when multiple light pulses pass through the optical resonator mirror, similar to that used in conventional lasers. A FEL with a small gain (sometimes called multipass) operates in the infrared, visible and near ultraviolet range. The use of an optical resonator puts a limit on the smallest wavelength that may be generated, because there are no suitable mirrors to build a proper resonator, nor is there a generator whose signal could be strengthened when the wavelength is very short. Beam intensity is comparable to that which can be obtained from conventional pulsed laser sources. The advantage is the ability to adjust the wavelength. The impossibility of producing radiation in the long ultraviolet or soft X ray range led to the search for another way to generate light. Theoretical work on the emission of light in undulators in the late seventies allowed these difficulties to be avoided. Instead of a resonator and a generator one could use a precise long undulator. During a single electron pass one achieved a sufficiently strong electron interaction with the emitted radiation to lead to modulation of the electron charge density and consequently a bunch in the phase of light emission (SASE). This required the injection into the undulator of a bunch of electrons with a much smaller spatial size and energy and momentum spread compared to the case with a resonator. This was only possible by using a complicated system of acceleration and bunching. Figure 2. Top: diagram of a FEL with a small gain, with a resonance cavity of emission range: FIR-UV. Bottom: schematic of a SASE-FEL with a high gain, working without an optical resonator. Emission range is (FIR)-EUV-HX. G - electron gun, A - electron accelerator modules, U - sections of the undulator, B - bending magnet, M resonance cavity mirrors, S - electron beam trap, X - laser beam, K - electron beam compressor, C - electron beam collimator. In this FEL working mode the whole emission energy accumulates in the pulse interacting with a very dense bunch of relativistic electrons when they travel through the undulator. At the moment the only devices working in this way are SASE-FEL. As the name suggests, they use the phenomenon of Self Amplified Spontanous Emission. In a low gain FEL, in which there is no SASE process, reduction of the radiation intensity from the undulator occurs due to the fact that the electron bunches are much longer than the 7

9 emitted wavelength. As the distribution of electrons in the bunches is fairly homogeneous, the phase of the radiation emitted by them is random - the electrons do not radiate coherently. One way to circumvent this limitation is the phenomenon of microbunching, accompanying the SASE process. The electrons, which lose their energy to the electromagnetic wave must overcome a slightly longer distance in the undulator (because they are more strongly curved in the magnetic field and their trajectories are longer), opposite to electrons, which received the energy from the electromagnetic field (the distance is shorter). As a result, there is a speed modulation of the electrons in the bunch in the undulator. This leads finally to the clustering of electrons into layers (microbunches), in which electrons are divided from each other by distances much smaller than the wavelength, so the electrons concentrated in the same microbunch radiate coherently. As a result of micromodulation all electrons in the undulator emit "the same electromagnetic wave," or in other words, they interfere constructively. So they are a source of powerful electromagnetic radiation whose intensity can be approximately Nc times higher than that obtained without micromodulation. Nc has a value between 106 and Both types of FEL: FELs with a low gain and a resonant cavity and SASE-FELs with a linear electron accelerator based on superconducting accelerator modules are compared in Figure 2. It should be mentioned that the SASE-FEL devices are much more technologically advanced than their older brethren FELs with a small gain. To initiate the process of SASE, one must provide as short as possible a bunch of ultrarelativisitic electrons (with energy from several hundreds of MeV to several GeV) to the undulator entrance and a very high charge density peak. Along with the construction in 1999 of a single pass SASE-FEL, FLASH, in Hamburg, which works without a resonant cavity, the wavelength lower limit shifted step-by-step to nm. In the next decade the lowest wavelength (first harmonic) of the FLASH laser reached 6.3 nm. FEL sources reach wavelength ranges covering from the extreme ultraviolet (XUV) to soft X rays (SX), where laser sources with an optical resonant cavity cannot work. Moreover, the modified principle of operation of SASE-FEL enabled the emission of radiation in very short pulses, from a few to tens of femtoseconds. In this very short time the power of the emitted radiation can have a value up to several GW. In 2009 other free electron lasers begin to operate: FERMI at the ELETTRA synchrotron that uses part of the accelerator and emits radiation in the XUV range, and the LCLS at Stanford University (USA) using a two-kilometre section of the SLC linear accelerator (Stanford Linear Collider), generating for the first time in the world light with a wavelength of 0.1 nm, i.e. hard X rays. The European X-ray laser XFEL in Hamburg is being built with the cooperation of several countries, including Poland. It will have a length of about 3.5 km, with a dedicated linear electron accelerator almost 1.6 km in length. Completion of construction is planned for The use of SASE to produce coherent light, the production of X rays and the beginning of construction of the European X-ray laser is a breakthrough in the development of free electron lasers. Its meaning is expressed by the number of projects for new facilities at different stages of development all over the world. The four properties considered to be the most important for today's FEL research are: short pulse duration, high brightness, an ability to retune and coherence, which is unavailable in other strong short-wave sources. Adding a high average power would allow a better use of these devices by reducing the time needed to conduct experiments. This is particularly important for experiments involving a large number of total photons, such as, for example, investigations of phenomena with a low probability, or performed with dilute samples. On the other hand, it is also desirable for technological applications related to the exposure and modification of the surface, where high average 8

10 power means high efficiency. The average power of existing FEL sources is limited by the HF power pulse duration of the propelling accelerator. This time must be limited due to the high losses of high frequency field energy, which appear as heat. In the intervals between impulses, which together take 99 % of the operating time, the electrons are accelerated and no light is emitted. Extending the duration of the impulse requires a reduction of the power loss through the construction of a fully superconducting accelerator. Many of the existing devices have a superconducting accelerator, yet are forced to work in the impulse mode due to a normal conducting electron gun. The difficulty in the construction of superconducting gun is to develop a superconducting photocathode with sufficient quantum performance and to build a suitable initiation laser that gives microjoule light pulses with a wavelength around 200 nm and a repetition rate of tens of kilohertz. Obtaining high average power at work in continuous mode, i.e. the high frequency field constantly agitated in the resonance cavities of the accelerator, is one of the main scientific themes of the proposed POLFEL laser. 9

11 3. Polish contribution to the study of the use of FEL lasers and work on their development The Polish contribution to the study of the use of FELs and their development involves all three main groups of issues: control and synchronization of LLRF, the development of particle acceleration physics in superconducting accelerators and research into ultrastrong interactions of light impulses with matter. Work has also been carried out in the field of structural research and the use of FEL light in biology and medicine. Topics related to the design and construction of FEL components were actively pursued in Poland long before the breakthroughs of the last decade. The work of groups from the Universities of Technology of Warsaw and Lodz, the National Centre for Nuclear Research and the Institute of Physics of the Polish Academy of Science made significant contributions to the laser in Hamburg. From the beginning of experimental work on the beam, Polish scientists have engaged in the development of experimental methods, especially the optical components exposed to impulses of unprecedented power density. One of the first two research projects performed early on in the FLASH laser testing phase was carried out independently by a group from the Institute of Physics of the Polish Academy of Science. Tests and research on equipment changed rapidly into material and basic studies. On the one hand, the development of measurement methods was necessary to collect systematically data on the influence between impulses and materials from which FEL optical elements are built. On the other hand, a new device allowed the physical models describing hitherto unattainable phenomena to be verified. Figure 3. Electron beam profile obtained from a superconducting thin lead film photocathode in April 2011 in the HZB facility. Two Polish groups of electronics engineers, from the Technical Universities of Warsaw and Lodz, greatly contributed to the development of the FLASH laser accelerator control system. A system of diagnostic data analysis was invented by adapting the solutions used in telecommunications. LLRF circuits based on ATCA technology receive data from the beam diagnostic devices and then analyze hundreds of signals describing the accelerated high frequency electromagnetic field and the course of the electron beam. Based on this, they immediately send a control signal within a time comparable to the high frequency field period of the accelerator. The challenge for the designers is the speed of analysis and the extent of the entire system. The system developed is intended for use in the European laser XFEL. In the laboratory of the National Centre for Nuclear Research couplers of high frequency higher modes are being developed for this device. 10

12 Also in the National Centre for Nuclear Studies work on a superconducting electron source equipped with a superconducting photocathode is being carried out. In the first place, this was aimed at a prototype application in POLFEL, but in recent years it has attracted the interest of leading accelerator centres in the world, such as TJNAF, DESY and HZB. Within the jointly fulfilled research program, in 2011 a beam of electrons from a gun equipped with a thin lead photocathode was achieved for the first time (Figure 3). These studies include the improvement of the resonance cavity and will be continued so that the developed electron gun could be used in POLFEL. The Polish contribution to accelerator technology also includes the achievements of the Technical University of Wroclaw and Wroclaw Technology Park in the field of cryogenics and construction of superfluid helium cooling systems for superconducting devices. Appendix B contains a detailed list of publications on the Polish contribution to the development of free electron lasers, and studies on their usefulness. 11

13 4. Meaning of POLFEL for science and the economy The great importance of free electron lasers for many different fields of science and technology is demonstrated by the following list of participants in the Consortium European XFEL Poland: Institute of Physics, Polish Academy of Science; The Henryk Niewodniczański Institute of Nuclear Physics, Polish Academy of Science; Institute of Plasma Physics and Laser Microfusion; Institute of High Pressure Physics of the Polish Academy of Science; Faculty of Mechanical and Power Engineering, Wroclaw University of Technology; Wroclaw Technology Park, National Centre for Nuclear Research, Warsaw University of Technology, Tele and Radio Research Institute, Institute of Electronic Materials Technology, Lodz University of Technology, Military Academy of Technology, Faculty of Physics University of Warsaw, Faculty of Electrical Engineering, Automatics, Computer Science and Electronics, AGH University of Science and Technology, West Pomeranian University of Technology, PREVAC sp. z o.o. (Precision & Vacuum Technology), Institute of Electron Technology, Faculty of Physics and Applied Computer Science, AGH University of Science and Technology. The coordinator of the Consortium is the National Centre for Nuclear Research. The above-mentioned institutions are concerned, depending on the theme of their activities, in the scientific applications of or engineering challenges posed by POLFEL. Readiness to start design and R&D work in the implementation of POLFEL has been declared by the following Polish research centers: Institute of Physics, Polish Academy of Science, Institute of Optoelectronics, Military Academy of Technology, Warsaw University of Technology, Wroclaw University of Technology, National Centre for Nuclear Studies, for which they have made many applications for funding research projects, most recently an application for funding a project for the years , under the title Studies of selected components and elements of free electron laser (FEL) and its application in materials technology and a request for placing POLFEL on the Polish roadmap for Research Infrastructure submitted to the Ministry of Education in

14 4.1. Science The development of modern science and technology increasingly requires the use of large research facilities. Poland does not have even one such device. This is a significant obstacle in conducting advanced research and innovation. The importance of this class of devices goes far beyond the interests of one institute. The POLFEL research program is a continuation of research conducted on the synchrotron and in this area Polish scientists have extensive experience and a strong international position. Therefore, investments such as the National Synchrotron Radiation Facility (NCPS) SOLARIS at the Jagiellonian University in Krakow or POLFEL in Swierk provide an opportunity to promote research with the use of intense sources of radiation as a representative area of Polish science. Free Electron Lasers open up a whole new area of research, achievable through a combination of several desirable features: high-energy pulses of short duration and short wavelength. The intensity and quality of the radiation emitted by such devices is thousands of times higher than the current sources. In some cases, free electron lasers make it possible to obtain coherent radiation of high intensity and wavelength ranges in the terahertz region, the far ultraviolet and soft X rays. In physics, chemistry, biology, materials science, environmental science and medicine, free electron lasers will help us to understand processes in living cells, molecules and materials that enable the study of their structures and the reactions occurring there. Construction of a unique in Poland test apparatus for broad research, whose design is based on a fast-developing technology, opens up new opportunities, not only for potential experimenters but also for active involvement of technicians in international cooperation developing the technology itself. The interdisciplinary nature of the research, the development of new technology and the ability to train students and young scientists testifies the importance of the role that such a device couls play for many years, starting from the gebral concept of the preliminary technical design. A source of coherent light built in Swierk can and should play this role in Poland. It should be noted that Polish researchers conducting experiments in foreign centers contribute to the flow of information and the integration of the Polish community with the global community. For cooperation to be valuable and allow full use of the country's scientific potential and enhance its role in the European and world scientific community it is necessary to make a contribution to it with a unique experimental capability. In addition to the institutes and universities that have expressed an interest in a new investigative tool, an exponential growth of users in the country is to be expected, especially in the first years of operation of POLFEL. This is the experience of both users of the FLASH laser and the synchrotron community. The community of users of electromagnetic radiation for the research purposes in Poland is large. NCBJ is currently cooperating in the construction of the Polish SOLARIS synchrotron in Krakow. An important role, in terms of modernizing the country, which the proposed device will meet, is the creation of new workplaces which allow unique experience to be gained. This applies to all phases of construction, testing and commissioning and subsequent long-term use of the POLFEL device, which is cutting-edge technology in many areas electronics, optics, information technology, precision engineering and vacuum technology requiring specialized knowledge and providing a focus for Polish scientific and technical staff scattered in many centers abroad. Traditionally, research in Swierk has focused on nuclear physics, particle physics, astrophysics, plasma physics and the physics of particle acceleration. The development of 13

15 these areas has made it necessary to construct larger and more expensive facilities. Their construction is possible only with international cooperation by many laboratories. In practice, a model has been adopted where there is only one main center and one subsidiary per continent. In Europe, these are CERN and DESY in particle physics and GSI/FAIR in Darmstadt and GANIL/SPIRAL in Caen in nuclear physics. Participation in major international projects provides participation in research on the global level. However, a large scientific institute can not develop properly without its own large research unit. Possession of such a device is necessary for staff development, training and expertise in the field of construction of equipment and the maintainence of a high level of technical resources to enable international partnerships and thus impact on the development and implementation of advanced technologies. Therefore, when research facilities in smaller centers lost their significance, they are forced to seek other areas of research. The development of this area has led to a dozen synchrotron laboratories in the world including several in Europe. The need to extend the wavelength and intensity of the radiation used was the reason for the creation of a new generation of light sources. After the era of the synchrotron the coming era is of lasers using electrons accelerated to energies of billions of electronvolts (GeV). The experience gained by nuclear physics and particle physics laboratories is extremely valuable in this field. Such centers as SLAC (Palo Alto, USA), DESY (Hamburg) and PSI (Villigen, Switzerland) have for this reason been chosen for the construction of FELs in the coming years. Long-term cooperation with the DESY center creates an opportunity for NCBJ for the construction of a FEL in Swierk, based on technology developed for the TESLA electron and positron superconducting linear collider and recently developed for the FLASH free electron laser and the XFEL project in particular. Following the path already taken by one of the major international laboratories ensures that Swierk has a chance to become a research center of European importance. The idea of the POLFEL project is to create in Poland a research center based on a large, universal research device - a free electron laser. Such a center will meet the conditions required for large research facilities, which will make the scientific center in Swierk Poland's first and so far the only such center in the new EU member states. Research carried out using free electron lasers will have a very wide multidisciplinary range from nuclear physics and materials science to biological research, medical and environmental science. The device is the result of the combined experience of physicists and engineers coming from different scientific communities: opticians, electronics, accelerator physicists and technicians and a wide range of various specialized synchrotron users. The opportunity to conduct research using POLFEL will attract to Swierk many outstanding scientists. With such potential the research center in Swierk will become one of the most important places in Poland for integrating different scientific fields. The proximity of Warsaw, the largest academic center in Poland, as well as the extensive cooperation NCBJ has with many scientific institutions in the rest of Poland, is an important factor in facilitating such integration. 14

16 Figure 4. The location of research centres in Europe, which operate or plan to build a free electron laser. The enormous research capabilities of FELs means that such facilities are planned by almost all economically developed countries. Figure 4 shows the location of research centers in Europe, which operate or plan the construction of free electron lasers. In table 1 the basic parameters of selected devices of this type in the world are listed. Table 1. List of selected research centres in the world, which operate or plan to build a laser. Project name Method of photon generation Laboratory Country Electron beam energy [GeV] Wavelength [nm] Status LEUTL SASE APS USA 0, Operated from 2001 DUV-FEL HGHG BNL/NSLS USA 0, Operated from 2002 SCSS SASE Spring8 Japonia 0, Operated from 2005 FLASH SASE DESY Niemcy 1 6 (2) Operated from 2006 X-FEL SASE DESY Niemcy 17 0,1 Operated from 2012 LCLS SASE SLAC USA Operated from 2009 Soft X-ray FEL HGHG BESSY Niemcy 2,3 64-1,2 Project SPARC SASE ENEA/INFN Włochy 0,15 VUV Project SPARX SASE ENEA/INFN Włochy 2,5 1,5 Project VXFEL SASE ELETTRA Włochy 1,0 Project FERMI SASE ELETTRA Włochy 3,0 1,2 Project 4GLS SASE/HGHG Daresbury W.Brytania 0,6 VUV-XUV Project HGHG MIT-Bates USA 3,0 VUV-XUV Project LUX LBL USA 3,0 Project/ERL ERL-Linac Cornell USA 5-7 Project/ERL Arc-en-ciel Orsay Francja 0,7 Project MAX-IV MAX-Lab Szwecja 3,0 Project To avoid duplication of work and create a synergy of action the IRUVX FEL Network arose, which after receiving funding from the 7 th Framework Programme has evolved into the EuroFEL consortium. Its purpose is to exchange experience in the field of the physics and technology of free electron lasers, conducting joint development and coordination of projects in order to build devices that are mutually complementary. POLFEL is affiliated with EuroFEL, which ensures that it is one of the key elements of the European Research Area. 15

17 4.2. Economy POLFEL will have a double significance for the development of Polish science and technology. First, in the process of construction many experts in accelerator techniques will be trained, such as is already the case for NCPs, where a group is creating a facility from scratch. If we add to this the project for the National Centre of Hadron Radiotherapy, accelerator technology could become a Polish specialty. Meanwhile, demand for these technologies in the world is enormous, because accelerators are coming into common use today. The importance of this subject confirms the rank of National Centre for the abovementioned institutions. Second, studies conducted using POLFEL may result not only in interesting results in science, but also in new technologies which could modernize the economy. POLFEL will be close to the companies manufacturing high-tech equipment that will soon start operating in the emerging Science Technology Park in Swierk. Particularly good conditions for development exist in the field of surface engineering and nano-fabrication, in which laboratory work that is being carried out at, among others, the Military Academy of Technology, the Institute of Electronic Materials Technology and Warsaw University of Technology could be developed and implemented. The importance for modern industry is confirmed by the participation of Wroclaw Technology Park and the vacuum equipment factory PREVAC in the European XFEL - Poland Consortium. POLFEL also creates opportunities to save the Polish microwave industry from collapse. Converting factories formerly working in military radar technology into accelerator technology is a noticeable trend in the world. It should be noted that the microwave industry is one of the few areas of original Polish technology POLFEL on the ESFRI Roadmap The place of the POLFEL project on the European Research Area roadmap was aptly described by Professor Dr. Jerzy Buzek, former Prime Minister of the Polish Government, current Member of Parliament and its former chairman, in a speech, which was published by Technical Review: We expect that the Polish scientific institutions engaged in nuclear technology and all others dealing with relevant issues to join intensively in the research issues currently undertaken in the European Union. Modern technology is increasingly coming down to the size of the order of nanometers. Paradoxically, research on this scale needs major research facilities such as synchrotrons and free electron lasers. We do not have such devices (their construction is planned in Cracow and Swierk).. ESFRI compiled a list of 35 major research facilities, the key for the European Research Area. It includes a network of free electron lasers, IRUVX FEL Network. The planned construction of one of the elements of the network at the National Centre for Nuclear Research was to bring a new dimension to the National Technology Park. The POLFEL ultraviolet laser, being the largest and most modern research machine in this part of Europe, has created a unique opportunity to develop the domestic high tech industry. Major research infrastructures built as a joint effort of several Member States and open to scientists from across the European Union, according to the European Commissioner for Science and Research, Janez Potočnik, play the role of highways, so that Europe can achieve goals such as a significant increase in the European innovation economy, stimulating the mobility of researchers, increasing the attractiveness of science to young people and opening 16

18 new horizons of research. The above-mentioned ESFRI list, commonly called the ESFRI Roadmap is a road map of these highways. ESFRI is an organization founded in April 2004, which includes representatives of 27 EU member states, 5 associated countries and a delegate of the European Commission. The primary objective of ESFRI is to develop long-term policies coordinated at the European level on major projects and facilities for research infrastructure. Work on the Road Map was launched in November This was led by 1000 experts from twenty-seven member states and five associated countries. Of the more than two hundred projects submitted to the map eventually 35 were retained. The majority of them are initiatives at the European level, and some require collaboration with partners from other continents. The task of the Roadmap published on 19 October 2006 is to identify courses of action. Implementation of some of the tasks set by the Roadmap ended as early as 2008, but the progressive realization of all tasks will strengthen the European Research Area. At the end of June 2007 the Board of the European Strategy Forum on Infrastructure Research described the schedule of work related to the upgrading of the European Road Map. The aim is first of all to take into account the aspirations and capabilities of the new EU member states in the field of international projects in a manner consistent with the overall regional and national vision. In March 2007 a time-limited working group was formed whose remit was to prepare materials for the autumn meeting of ESFRI on the initiatives so far not included national, regional and interregional initiatives meeting the requirements of sustainable development. The ESFRI Committee is currently working on the assessment of the impact of scientific infrastructure on the local community (according to one of the priorities of the European Union). These are huge devices, which employ several thousand highly qualified specialists. Their socio-economic importance for the region by stimulating a specific profile of research undertaken at the nearby universities or by the creation of demand for innovative companies is certainly not marginal. The priority areas identified by the Roadmap are: Social Science and Humanities, Environmental Sciences, Energy, Biomedical and Life Sciences, Materials Science, Astronomy, Astrophysics and Nuclear Physics, Computer Data Treatment, Particle and Space Physics. Among the seven projects in the field of materials science initiatives, four relate to the construction of intense beams of electromagnetic radiation, and two to neutron beams. This illustrates a significant trend observed in the field of science to determine the material properties it is increasingly necessary to use beams of radiation with parameters impossible to obtain in a classical laboratory. Pan-European projects in the field of materials science include: to create an FELs infrastructure, including the interaction of laser radiation with matter, sub-infrastructures defined in this project, Ultra High Field Science, Attosecond Laser Science and a High Energy Beam Facility, will be of great importance for medicine, materials research and the environmental sciences, modernization of the ESRF synchrotron, to ensure a high level of research in various fields of science using synchrotron beams, build the world s most modern spallation neutron source which structurally guarantees the possibility of further modernization, equipped with more than twenty measurement lines, construction of the European XFEL in Hamburg, modernization of the ILL reactor with the aim of producing beams of slow neutrons, create the IRUVX consortium aimed at developing of free electron lasers in Europe, the establishment of a Pan-European Research Infrastructure for Nanostructures (PRINS), which, under the ENIAC European Technology Platform, is to create a bridge 17

19 between science and the market to ensure the transition from microelectronics to nanoelectronics. Poland participates on the basis of full membership in two of these projects: the first is the European XFEL and the second is the FAIR project (Facility for Antiproton and Ion Research) a unique global group of particle and heavy ion accelerators which is to be built by extending the existing GSI Helmholtz Centre for Heavy Ion Research in Darmstadt for research in nuclear physics and condensed matter in fields and medicine. Scientists from Poland, including scientists from the center in Swierk, are involved in the work on these two projects. The POLFEL project gives Poland the chance to participate in the implementation of the ESFRI Roadmap on a qualitatively new level. It would see the construction of major research equipmentin Poland as part of IRUVX FEL Network, instead of a contribution to building a device in another country. POLFEL is also strongly associated with the European XFEL project. It is planned to use many elements developed for the European XFEL in the construction of POLFEL. The advantage is to rely on cutting-edge, yet proven technology while reducing costs. In this way a long and costly design and prototype phaseis avoided, and items can be ordered at the price of series production rather than individual. This approach will make Swierk associated with DESY by a joint research program and by a FEL technology development program. Construction of a device similar in concept to the already operating FLASH laser (DESY, Germany) and, in the course of implementation, the European XFEL (also at DESY) will allow the exchange of technical experience with many international research centers working on the two experiments. In the Technical Design Report for the European XFEL as many as 72 institutions cooperating in the design of the device are listed. It is planned to use POLFEL as an experimental facility and for training engineers involved in the European XFEL project. After the end of construction of the two lasers, the new accelerator, laser and detector technologies can be developed in parallel on both devices. This solution will strengthen technical cooperation between Polish and worldwide engineering staff Connections of the POLFEL project with science development programs in the European Union The Polish free electron laser project POLFEL is closely related to European Union research programs. These can be divided into two areas: design and development of a free electron laser, use of lasers in research projects. One may expect that in both these areas the POLFEL project would require as yet nonexistent technologies and research opportunities. CARE, EUCARD and EUCARD 2 particle acceleration physics development programs In the field of the construction and technology of free electron lasers, the project is related to the CARE program (realized in the years ) and the similar EUCARD program (in the years ). The aim of both programs is the construction and upgrading of accelerator laboratories in Europe, conducted in concert by a number of research centres and industry. 18

20 NCBJ has been involved from 2004 in the research conducted under CARE and in the years in EUCARD. The aim of this first program is to develop techniques of deposition of thin superconducting niobium layers on the inner surfaces of the walls of cavities for accelerators with a high electric field gradient. The work is carried out within the framework of Work Package 4 Thin film cavity production directed by Prof. M. Sadowski of NCBJ. Work has also been carried out on the deposition of superconducting layers of lead in the resonant cavity of an electron source. This last topic of research is continued in the framework of EUCARD. It is especially important for the POLFEL project, because the superconducting electron source is an important innovation that will be developed within this project for future FEL designs. It is planned to continue this work as part of the EUCARD 2 project. There are plans for NCBJ to fulfill the role of task coordinator of exploring new types of photocathodes and electron guns for FELs and ERL conducted jointly by DESY, HZB, HZDR and STFC. The EuroFEL program (formerly IRUVX-PP) NCBJ is a member of the EuroFEL consortium that has received support from the 7th Framework Program project "IRUVX Preparatory Phase". The consortium was created as a result of the enormous growth of interest in free electron lasers, which took place after the commencement of the FLASH laser. It unites FEL projects in Europe which are either already in progress or planned. The aim of EuroFEL is to coordinate cooperation between European centers working on the development of fourth generation light sources. The consortium is to ensure the flow of scientific and technical information and experience from the individual participants to increase the efficiency of their work to develop technologies and applications of FEL. By participating in EuroFEL, NCBJ has gained access to technical assistance and training of academic and engineering-technical staff, through access of workers and PhD candidates to wide scholarship programs, such as those implemented under the Marie Curie program. On the other hand, POLFEL's participation in the EuroFEL program includes the use of a future Polish laser in the preparatory work and to test solutions for the European XFEL and other planned free electron lasers. Within the EuroFEL consortium it is expected to create common criteria and procedures for access to FELs for all affiliated centers. The mode of collecting applications for research projects, criteria, evaluation, implementation and evaluation of results should be uniform. The Polish free electron laser, by acting in such conditions, would be part of a European network of laboratories. The European XFEL Project Besides cooperation within IRUVX, begun in 2008, there is a particularly strong link between POLFEL and the European XFEL project relating to the optics of ultra-strong and ultra-short light beams and the high frequency synchronization system, and it provides a significant contribution to the implementation of these elements. High compatibility with the two facilities already mentioned and others planned in the same way, will enable trial implementation and testing of new design solutions such as the superconducting electron gun and work on the accelerating system in continuous wave and long pulses modes. This link is reflected by DESY in bilateral agreements and written statements concerning the POLFEL project. To return to the currently implemented European XFEL project, the National Centre for Nuclear Research has undertaken to make the High Order Modes (HOM) damping systems for high frequency fields for the entire linear accelerator. The Work Package WP6 (Higher Order Modes damping system) includes: 19

21 absorbers for HOM oscillations propagating through an electron beam (in 70K cryogenic environments), 103 pcs, high frequency power drainage system (the HOM couplers), for the 9-cavity superconducting accelerating structures (in 2K cryogenic environments), to the amount of 1616 sets. Tasks performed within the WP6 are: calculation of the scattered fields and resonant modes induced in different elements of the linear accelerator by a beam of accelerated electrons, electric and magnetic complex permittivity measurements of materials used for construction of HOM absorbers, development and manufacture of HOM absorbers, development and manufacture of non-propagating HOM damping into electron beam tracks. The European XFEL Poland Consortium received support for preparatory activities under FP7. In February 2010 a conference International Workshop on X-ray diagnostics and scientific application of the European XFEL, combined with a day-long seminar on the scientific program for the laser POLFEL was held. SOLARIS National Synchrotron Radiation Centre The themes and experimental technology of scientific-research projects implemented with FELs are largely an extension of research activities conducted using synchrotrons. The European environment is reflected in user participation in synchrotron experiments at the FLASH laser and in the development of research projects to take advantage of FELs as an extension of the experimental database. In Poland this is clearly visible in NCBJ cooperation with centers involved in the project for the National Synchrotron Radiation Facility (Item No. 49 on the Indicative List of Individual Projects of the Innovative Economy Operational Programme, Priority Axis II, R&D Infrastructure) within the consortium Polish Synchrotron. This cooperation covers the preparation phase, construction and subsequent use of the two test facilities. In particular, research projects prepared for POLFEL will benefit from the preparatory phase carried out at the NCPS synchrotron. Synchrotron users will have access to unique features of the FEL source: high time resolution and intensity. Implementation of both projects will make the environment of Polish FEL and synchrotron users largely overlap and complement research subjects showing strong synergistic effect. EU Framework Programs One example of the use of intense ultraviolet radiation is laser ablation, that is the removal of a thin layer of material, often organic, and the deposition of this material on other substrates (laser deposition). Topics of this nature were developed at the National Centre for Nuclear Research and were the subject of European cooperation in the Copernicus project in the 6 th Framework Program. Cooperation completed so far concerns the method of formation of corrosion-resistant titanium layers in the windows of the reaction chamber, used in the high-energy electron beam irradiation method of removing SO 2 and NO x pollutants from flue gases of coal power plants. This method was used in two contracts of the Copernicus project, which developed methods for removing common pollutants and removing pollutants such as volatile organic compounds (VOCs). The first topic ( Window to the development of technology for industrial flue gas cleaning system of NO x and SO 2 using an electron beam ) was carried out under INCO- COPERNICUS Contract No. IC15-CT ). 20

22 The second topic was carried out under contract ICA2-CT As a result, an effective method of corrosion-resistant windows was worked out. The method involved the implantation of Pd at a dose of about cm -2, and melting nitrogen plasma pulses with an energy of about J/cm 2. Such windows show no signs of corrosion after work for about 2000 hours in the reaction chamber electron dry scrubber. Work to improve the corrosion properties of materials is underway. 21

23 5. Characteristics of POLFEL laser The POLFEL free Electron Laser will be built in two stages. In the first stage called THz-POLFEL a terahertz and infrared radiation source will be built, at first with a single undulator U1, emitting radiation of microns wavelength, then the emitting wavelength will be extended down to 6 microns by adding a second undulator U2. In the second phase POLFEL will be extended by adding accelerating modules, an elliptical undulator and experimental stations, to achieve a range of XUV radiation available for experiments. In this way, a full version of the proposed Free Electron Laser device called XUV-POLFEL (to distinguish it from the first stage version) will be completed. Table 2. Selected parameters of the POLFEL laser Stage 1 Stage 2 THz-POLFEL XUV-POLFEL The maximum energy of the electron beam: Accelerating field frequency: 5 50 MeV > 800 MeV 1,3 GHz (TESLA) Mode of operation of the high frequency source: Repetition frequency of the UV laser initiating emission from the photocathode: pulse with pulse duration ranging from 25 ms to continuous wave (CW) 50 khz 100 khz The primary wavelength of light: µm < 10 nm Maximum power of the beam in one pulse: 0,2 GW Pulse energy: 50 µj > 10 µj Pulse duration: 1 ps 10 fs Average power: 250 W > 10 W Length of the device: 25 m m The electron beam energy depends on the number of accelerating modules and the intensity of the high frequency electromagnetic field in them. The POLFEL accelerator will work in pulse mode with a high field intensity and in continuous mode, for which the field intensity will be lower. Accordingly, the electrons will reach energies of the ranges shown in Table 2, emitting radiation at the appropriate wavelengths. The main working modes will be long pulse mode and continuous mode. This represents the value of the device for scientific and applications purposes. The pulse mode will be used for generating light with a shorter wavelength. A choice between high intensity and short-wavelength is the second advantage of the POLFEL laser. The frequency of the electromagnetic field of 1.3 GHz is the frequency of accelerators based on the TESLA technology. Compliance with this technology enabling collaboration with European XFEL and FLASH is one of the main objectives of the project. The frequency of the laser initiating photocathode and the energy of the pulse determine the electron beam current, and hence the radiation power produced by the FEL. POLFEL's 22

24 high average power is obtained by using long pulsed high frequency fields and highfrequency of the initiating laser. The wavelength of light produced is dependent on the electron beam energy, and on other beam parameters. The first stage will produce terahertz and infrared radiation. After the accelerator extension, XUV radiation will be produced. In both stages, the radiation produced by the POLFEL free electron laser will have desirable unique properties. In addition to being tuneable, the pulse energy will be many times greater than in other known sources of radiation. Pulse duration could be extremely short, of the order of fs. When operating in continuous mode, a beam of emitting light pulses depends on the laser initiating photocathode. It will be possible to adapt it according to the needs of the experiment, and the emission of pulses in successive groups, after a time and containing a specified number of pulses. Examples are shown in Figure 5. Figure 5. Examples of emission timing of light pulses by POLFEL. From the top: continuous, pulsed (work packages pulse emission), packages emission of growing number of pulses as an example of arbitrarily shaped waveform. These characteristics will allow tests impossible to perform using other radiation sources, including work in the field of atomic physics, analysis and modification of materials, the study of biological structures and dynamics of chemical reactions. Thanks to these properties POLFEL will complement the existing capabilities of light sources: conventional lasers, by extending the spectrum of radiation to shorter wavelengths (about ten times shorter) synchrotrons, for the transverse and longitudinal coherence of the light, intensity higher by several orders of magnitude and for about a thousand times shorter pulse duration, existing free electron laser FLASH at DESY, for the opportunity to work in a quasicontinuous mode, and - ultimately - in continuous mode. The main elements of the device are: a source of electrons, cryogenic acceleration modules, the related HF power source, undulator, optical path and measurement systems. The whole unit will measure approximately 400 meters and will be the largest and most technologically advanced research device in Poland. The entire project includes construction of the laser, the necessary premises, measuring systems, power supply installations, laboratories and office space. The POLFEL accelerator will be built with superconducting TESLA type cavities. Similar designs are employed in, among others, the planned ILC accelerator, the currently under construction European Free Electron Laser (European XFEL), the FLASH laser, and for other lasers in Europe. Thanks to innovative solutions in the field of HF power, and of construction of an electron gun, POLFEL will make a significant and original contribution to the development of the physics of particle acceleration. Part of the planned laser concept is based on the achievements of Polish researchers collaborating in international projects, 23

25 mainly in the areas of dynamics and diagnostics of relativistic electron beams, control and synchronization of accelerators, cryogenics and optics. 24

26 6. FEL sources in science and technology 6.1. Introduction The addition of a THz-FEL laser, followed by XUV-FEL sources to the national research capacity will significantly influence the development of many scientific fields such as: condensed matter physics, materials science, chemistry, biochemistry, biology and medicine. The use of new powerful sources of radiation with unique properties, including the infrastructure, will enable the development of the new methods and technologies of tomorrow, leading to profound transformations in many areas of life. In particular, new sources will allow for: research and development of new materials necessary to develop faster electronics with a higher degree of compactness, based on new concepts such as spintronics and quantum computing, work on the development of new alternative energy sources, such as fuel cells, photovoltaic cells, sources based on photochemical conversion (analogous to photosynthesis), study of the dynamics of chemical reactions, catalysis processes and how to influence their course, identification of spectral signatures of substances in the THz regime with high sensitivity and resolution. Applications in the detection of contents and composition (including potentially hazardous materials such as explosives and inflammable materials) on the basis of signature analysis, for example at airports and other locations in need of protection, development of efficient methods of quality control for a wide variety of products from microelectronics to food using spectroscopy and imaging in the THz range, research that will lead to cleaner and more efficient ways of refining and use of fossil fuels such as coal, petroleum and natural / shale gas, and impact on reducing greenhouse gas emissions, research on superconductivity, which could change the transmission and storage of electricity, studies on the structure and dynamics of biomolecules and biologically active molecules (important for biology), which will contribute to an understanding of the relationship between structure and biological functions, including disease processes and interactions of medicines, development of new accelerator techniques based on the latest achievements of the science and technology of the generation, acceleration and formation of electron and photon beams, which can be used in medical diagnosis and therapy, quality control, or environment protection, and many others. The wide range of applications is due to the exceptional parameters of the proposed source: a very large volume, coherence, tunable capabilities and structure of generated radiation pulses, as well as the extraordinary properties of the emitted range of radiation and its effects on condensed matter interactions. This study focuses primarily on scientific and technological applications of terahertz radiation, a source of which will be built in the first phase of construction. For several years now the observed rapid growth of interest in science and technology related to the scope of terahertz radiation has been associated on the one hand with the 25

27 development of laboratory sources of low power, and on the other hand with the introduction of powerful tunable sources based on synchrotron radiation, culminating in the terahertz free electron laser Terahertz radiation range, the basic features and a bit of history The concept of terahertz radiation, also called T-rays or sub-millimeter waves, usually refers to the range of frequency range ν = THz (λ = microns). For the purposes of this study we will use this definition of the scope of terahertz radiation. There are other common definitions, lowering the upper limit to 10 THz, or extending it up to 100 THz, or even up to 300 THz. This higher value is historically justified by the fact that in 1947 the International Telecommunication Union (ITU) defined the highest official band of radio frequency (EHF) as the band in the frequency range from 0.3 to 300 THz. These included the whole range of terahertz waves in the modern sense. Sometimes the THz range extends down to 0.1 THz, or even lower, most often for commercial purposes. The Oxford English Dictionary dates the first use of the term "terahertz" as no later than 1970, when it was used to denote the emission line of a He-Ne laser located in the far infrared. In spectroscopy it had been used previously to denote the frequency of emissions below the infrared range. The boundaries between the terahertz range (submillimetre) and the middle (MIR) and far infrared (FIR, reaching, respectively up to 15 microns and from the MIR border down to the millimeter-wave range) are also not clearly defined. The assigning of a specific frequency to this or that spectral range is sometimes a matter of methodology, local custom or convention relating to the construction of a particular instrument. In the case of tunable sources such as synchrotrons and free electron lasers the terms source infrared and far infrared are used when all or part of the spectral range of emission is in the range of terahertz radiation. Alternative IR ranges are defined as: MIR: µm, cm -1 and FIR: µm, cm -1. In the figure below the range of terahertz radiation as defined in this section is marked in grey. Vertical lines indicate the position of the individual ranges of infrared radiation compared to the terahertz range. A description of the sources presented in this figure is given later in this chapter. 26

28 Figure 6. Qualitative comparison of the average spectral brightness of selected THz radiation sources. The dashed line indicated the conventional sources, continuous synchrotron and FEL sources. It can be seen that for the latter there is no alternative in the terahertz break and at higher frequencies. The curve marked thermal source corresponds to the emission spectrum of Global source (silicon carbide) heated to a temperature of 2000 K. Other indications: RTD: resonant tunneling diodes. IMPATT: IMPATT diode (IMPact ionization Avalanche Transit-Time diode). Gunn: Gunn lasers. QCL: quantum cascade lasers (called Quantum-Cascade Laser). Laser III-V: semiconductor lasers using compounds of elements of groups III and V. Sources producing synchrotron radiation synchrotrons and FEL lasers, indicated by solid lines in the figure. Radiation power is given in units similar to W/cm-1 (based on. [ 1, 2 ]). Also shown are the two emission ranges of projected THz- POLFEL source: (a) corresponding to undulator U2 (red line) and (b) that corresponds to undulator U1. 27

29 In Table 3 the limiting values of the terahertz range and the scope of the proposed undulator in phase Ia, in four scales most commonly used in the literature and in the temperature scale are shown. Table 3. Terahertz range Undulator Ia Frequency THz 0,3 1,0 30 1,499 37,47 Wavelength µm 999,31 299,79 9, Wave number cm -1 10,01 33, , Photon energy mev 1,24 4, ,2 154,98 Temperature K 14,39 47, ,8 71, ,3 By the end of the 1980s publicly available sources of terahertz emitters and low-cost, easy-to-use detectors had not been constructed. As a result, positioned between the traditional microwave and optical technologies, commercial technologies of terahertz systems were not developed. Despite the great interest of scientists and engineers, at least since the 1920s, the terahertz frequency range has remained the least studied among all ranges of electromagnetic radiation and today is one of the least understood [ 3, 4 ]. Figure 7. Distribution of radiation emission in the THz range as a function of wavelength for the radiation of a black body at a temperature of 30 K, the spectrum of typical interstellar dust, together with the main emission lines of the labeled molecules in the submillimeter range, as well as the distribution of the microwave background radiation at a temperature of 2.7 K (from [ 5 ]). Currently, the range is managed by the innovative use of technologies of tomorrow. For many years, the only niches of use of terahertz technology were high resolution spectroscopy using remote sensing instruments placed on satellites, which allowed astronomers, chemists, 28

30 planetary and space scientists, to measure, catalogue and map the spectrum of thermal emission of a wide variety of molecules with low molecular masses (Fig. 8). As it turned out, in any other electromagnetic spectrum such a wealth of information about the structural properties of chemical compounds, including macromolecules is not available. The universe is in fact full of terahertz radiation, of which only a small part can be recorded from the surface of the Earth. More than half of the energy of electromagnetic waves in space and as much as 98% of the photons from the Big Bang are in the spectral range λ = microns [ 6 ]. Recently, interest in the terahertz range has increased rapidly due to the fact that numerous physical phenomena are revealed, which on the one hand require a broad multidisciplinary knowledge for their explanation and on the other hand allow one to penetrate and explore many other phenomena and processes in condensed matter that are fundamental to the development of the future of science and technology. An overview of current applications of terahertz bandwidth can be found in numerous works dedicated to this subject (e.g. [ 5, 7, 8 ]). The phenomena discovered in the THz band proved to be so important that a few years ago, in February 2004, in the influential journal published by MIT Technology Review, terahertz radiation was included in the top ten technologies that will have a revolutionary impact on our lives in a decade [ 9 ]. The impetus for exploration in this area is progress in the construction of sources. The intense work carried out during the last quarter of a century has produced a number of new types of sources, based on a variety of phenomena and materials. Fig. 6 shows the emission spectral ranges of selected laboratory THz radiation sources (indicated by dotted lines). Detailed information about these sources, as well as others not included in this comparison, can be found in the extensive literature devoted to this subject [ 10 ]. However, most of these sources have a low power emission, particularly at low frequencies around THz. At a frequency of about 1.2 THz emission power exceeds all of these sources of several µw. This is an area called the terahertz break [ 11 ]. Only for recently available devices TECSEL lasers, does the radiation power reach a few milliwatts in the terahertz break range [ 12 ]. Despite the lack of strong laboratory sources, scientists were able to identify the basic properties and phenomena of the interaction of THz radiation with matter. THz radiation easily penetrates plastic, fiber, ceramic, brick, non-polar solvents, and materials with a minimum of tissue water content (e.g. bone, fat, teeth, tendon), but is not passed through metal. It is important in many applications that the radiation penetrates the most commonly encountered materials in packaging and clothing made of typical fabric, allowing hidden dangerous objects to be detected and potentially dangerous materials to be indentified [ 13 ]. This phenomenon is used in modern traveler and baggage scanners at airports for example. In contrast to penetrating radiation, applying higher photon energies, in the THz range, these are of the order of mev and are far from any material ionization thresholds. In comparison with infrared radiation, which is also observed to have a high spectral sensitivity in many chemical and biological agents, THz radiation penetrates many more materials [ 14 ]. A great part of research focuses on spectroscopy. A number of chemical and biological agents (such as bacteria) exhibit characteristic spectral signatures in the THz range, making it easy to detect their presence and to map the distribution of the sample with resolution comparable to the wavelength. Identification of materials in the THz range can be made on the basis of the refractive index n(ω). For a given substance, n(ω) defines the time delay of a terahertz range beam with a given frequency. TDS spectroscopy works on this principle. Resolution imaging in the far field is limited by diffraction (Rayleigh criterion). In the THz range, where the wavelength reaches the order of several to several hundreds of micrometers, it is far from being encountered in the visible range. For imaging in the near- 29

31 field using the SNOM technique, resolution much less than the wavelength is achieved. In the case of the so-called Apertureless SNOM, with scattering from a tungsten blade and reistration AFM using a Michelson interferometer, a resolution of more than a dozen to a few dozen nm can be achieved. It is expected that these techniques will be used with the THz- POLFEL source. Appendix C contains a list of publications on terahertz radiation by Polish authors The use of FEL-type light sources in the terahertz range (THz- FEL) in science and technology. Particular attention is paid to the applicability of this non-ionizing but penetrating radiation in biology and medicine. Techniques such as X-ray crystallography, NMR, or High Resolution Electron Microscopy (HR-TEM) provide static images of proteins, enzymes and biological membranes. However, linking the structure of biological objects with their function requires the use of spectroscopic methods, of which the best suited to this kind of research are those in the middle (MID) and far (FIR) infrared (including, of course, the THz range). Application of these methods to biological problems increased dramatically after the introduction of an FT-IR spectrometer, typically operating in the cm -1 range [ 15 ]. They can be used to explore the transformation of individual bonds, even in large protein complexes, which allows structural and conformational changes to be monitored in detail and the details of biological reactions to be explained. The functionally important spectral range for proteins and enzymes extends up to 10 cm -1, thus including the whole terahertz range up to 0.3 THz, making it necessary to use strong, tunable sources free electron lasers. Współczynnik absorpcji H O 300 K 2-1 liczba falowa (cm ) Figure 8. The absorption spectrum of water at room temperature (according to [ 16 ]). The terahertz radiation range is highlighted in gray. The importance of strong sources of THz radiation is increased in research in the aquatic environment, typical of biological organisms. Due to the rotational movement of electric dipoles, the absorption of polar solvents in the terahertz wave range is very high, with maxima observed for water at 6 THz and 19.5 THz (see Figure 8). Although the solidification of water (ice transition) strongly limits the mobility of the dipoles, so that it becomes much more clear, this procedure prevents the study of the majority of tissue or cells in their natural liquid medium. Intense THz sources can effectively penetrate even relatively thick 30

32 preparations and dramatically improve the detection sensitivity for small concentrations of the test substances. They are the foundation for efficient use of the methods of spectroscopy and imaging in the THz frequency range for biomedical issues. Anther important application is biomedical imaging in the THz band safe in the absence of ionization and with resolution sufficient e.g. for images of teeth in dentistry or imaging of skin or tissue to detect early pathological changes [ 17 ]. It is also used in conjunction with the spectroscopy of proteins and DNA [ 18, 19 ], and in the measurement of water content in tissue and in food studies [ 20 ]. Figure 9. Photo (left) and an image (right) of a tooth in the THz range (from [ 21 ]) The issue of radiation safety in the terahertz range has been thoroughly investigated in the course of THz-BRIDGE project funded by the EU under the Quality of Life Program, Key action 4 [ 22 ]. The main conclusion of the research carried out on both the laboratory sources and synchrotron sources (including free electron lasers) is formulated carefully and confirm the biological safety of THz radiation, at least in single doses needed for medical imaging *. It should be noted, however, that at high energy densities THz radiation can cause destructive processes in the ablation of the material, which is used, among others to develop new techniques and medical instruments. It is one of the first issues for which a FEL laser operating in the infrared range was used, the interaction of an intense laser beam with tissue. The use of a fully tunable source of high intensity and controlled time structure of pulses significantly contributes to the development of medical laser technology by enabling systematic work on the optimization of the parameters of the ablation [ 23 ]. The condition of optimal photoablation is among others the selection of an appropriate wavelength at which the penetration of tissue by the radiation is as shallow as possible. This confines the deposition of energy to a small volume, which increases the precision of the cut. Equally important is thermal insulation of the exposed volume in order to limit the spatial extent of heat diffusion from the volume and to limit the increase of temperature around it. The extent of the stress * It is worth quoting the original wording of the summary report on the matter: The results indicate that under various exposure conditions no biological effects could be detected. However, under some specific conditions of exposure, change in membrane permeability of liposomes was detected and an induction of genetoxicity was observed to occur in lymphocytes. These studies suggest that medical imaging employing appropriate exposure parameters is probably unharmful at least for single exposures. Moreover, since some effects were observed to be induced by the THz radiation at a relatively low intensity when compared to the limits set by the ICNIRP for exposure, these studies should be extended to establish more accurate dose-response relationships. This is expected to provide in future improved guidelines of exposure. (op. cit. p. 61) 31

33 fields is also limited, leading to an increase in the efficiency of the ablation. This results in a reduction in the volumetric energy density required for removal of the material. A review of these issues can be found for example in [ 24 ]. The complexity of real processes of absorption and energy transport associated with photoablation necessitates experimental optimization of all relevant parameters, not only the wavelength. Applying the IR-FEL laser, absorption bands in the range 2-12 microns, potentially relevant to the optimization of surgical ablation, have been identified in various tissues, hard and soft. The mechanisms of ablation and the extent of ablation related damage was examined, too [ 25, 26, 27, 28, 29 ]. Among others the performance and characteristics of eye tissue and nerve tissue ablation have been studied, as well as photothermolysis of lipid-rich tissues. Ablation experiments on the cerebral cortex of rats allowed the wavelength of the mid-ir range to be determined as optimal for neurosurgical procedures [ 30 ]. Due to their unique properties FEL laserswill gain a detailed knowledge of the real processes of tissue ablation as a function of beam parameters such as wavelength, fluency, and the length and structure of the time pulse. As a result, development of new and optimized techniques of laser surgery based on optimized conventional lasers will be performed. Direct use of FEL laser surgery in daily practice is, as yet, impractical due to the large scale and the specific requirements of these devices that do not allow for cheap and easy implementation in an operating room. Terahertz radiation provides a powerful tool for insight into the fundamental excitations in semiconductors and semiconductor nanostructures. Studies using terahertz radiation are needed not only to set the basic parameters of the materials, but it is also necessary for the characterization of materials for different applications. In semiconductors many characteristic resonance frequencies are in the range of THz (phonons, charge states of hydrogen related to impurities, internal excitonic transitions, volume plasmons in doped materials). Quantum wells are structures in which carriers are confined by a potential whose shape can be controlled with high accuracy. Advancement of linear and nonlinear terahertz spectroscopy of semiconductor materials is now greater than any other type of materials. In addition to the study of excited states, THz radiation can be used to manipulate and control the quantum states of matter. Here one may expect completely new applications in areas such as spintronics and coherency control. A THz-FEL source with appropriate instrumentation will have a strong impact on the following areas of the science and technology of semiconductors: fundamental properties of semiconductor nanostructures, fundamental limitations of electronic devices, control of coherence and nonlinear THz spectroscopy, quantum optics, THz spectroscopy of supershort pulses (below the wavelength), quantum computing, spintronics, physics of strong fields in the THz range and nonlinear quantum dynamics. Metals perfectly reflect THz radiation. The reflectance coefficient of simple metals in the terahertz frequency range is determined by the DC conductivity and the frequency with a simple equation derived empirically by Hagen and Rubens in Derogations from this dependence point to processes which can not be described by the Drude model. Linear THz spectroscopy under conditions far from equilibrium, such as an extremely strong magnetic 32

34 field, low temperature or high pressure. It is a great scientific challenge, which is expected to give birth to many revolutionary applications. A related and fascinating research issue is the phenomenon of pumping materials with strong light pulses or magnetic pulses and the investigation of the response with THz spectroscopy. Recently, experiments of this kind were carried out on insulators, semiconductors and superconductors, yielding unexpected results. Strong pulses generated by a THz-FEL can also be used to induce structural phase transitions in superconductors by accelerating the condensate to the speed at which there is a burst of superconducting electron pairs. This can also change the orientation of the ferromagnetic or ferroelectric domains on the order of a single picosecond, and produce new metastable structures. One of the early applications of THz spectroscopy was the study of superconductivity. The existence of the energy gap in the superconducting state was demonstrated by FTIR, ahead of the BCS theory prediction. Low and high temperature superconductivity are examples of a number of exciting phenomena that occur in materials with strongly correlated electrons. These include giant magnetoresistance, magnetism, spin and charge density waves, and the dynamics of heavy fermions. The use of a strong coherent THz-FEL source emitting both short picosecond pulses, as well as a continuous wave (CW) opens a great opportunity to examine and use a new class of phenomena associated with the dynamics of correlated electron (charge) systems in metals and semiconductors. We should mention the following research questions: quantum critical processes, phase rigidity and Jospehson plasma resonance, submicrometric inhomogeneities of superfluid density, single-molecule magnets, vapor-condensate dynamics in superconducting magnesium diboride, antiferromagnetic resonance, THz loss and dynamics of vortices in high temperature superconductors, non-linear optical effects in the THz frequency range. The strong and focused light beams emitted by conventional lasers have been used for various types of surface modification and are now part of industrial technology. An industrial activity, in which cost is an important factor in the choice of methods and tools, requires great efficiency. The high average power of the beam and the ability to change the wavelength determine the use of free electron lasers in this field. This was recognized by a group of industrial corporations in the United States, which created the Laser Processing Consortium. Together with Thomas Jefferson National Accelerator (TJNAF), they adapted the CEBAF accelerator to industrial tasks and built on it a free electron laser working in continuous mode [ 31 ], as proposed for POLFEL. Participants in the consortium, which included market leading companies in their respective fields, such as Newport, IBM, Xerox, 3M, General Motors, Armco, Lucent, Aerospace, Ford and others, formulated a comprehensive program of applications including, but not limited to: amorphization of layers in order to improve their adhesion to the base and to improve the surface conductivity, surface micromachining to manufacture, for example, high-density data recording layers and set them to the corresponding optical properties, as well as the manufacture of micron-sized electromechanical systems, so-called MEMS, recrystallization of the metal surface to improve corrosion resistance, conducting laser deposition and etching on large surfaces. 33

35 The Government of the United States gave high financial support to these plans. Interesting from the point of view of the proposed POLFEL project, is the opinion of the committee of independent reviewers appointed by the U.S. government to evaluate the submitted proposal: "Potential commercial value of the technology is significant, impacting several multibillion-dollar markets" Associated equipment and prospects for development of the POLFEL center Many of the above mentioned applications of a THz-FEL source, require the installation of associated equipment. Particularly important for the optimal use of the POLFEL center are: the ability to use strong magnetic fields in several key spectroscopy techniques to study the dynamics of correlated spins and charge systems and equipping the experimental stations with optical laser sources for experiments of the pump and probe type, and for the diagnosis and synchronization of THz sources and experimental stations. The magnetic fields required for the use of different measurement techniques are within the range from a few T, generated by permanent magnets (regulated), to impulse magnets producing strong magnetic fields of several tens (40 70) T, with a pulse duration of about 100 ms and a possible large rise time. Strong pulsed magnets are placed at such a distance from the FEL that the magnetic field does not interfere with the device. The distance from the tip of the undulator (and from the accelerator) is usually about 60 m. Due to the cost of a good pulse magnet with its associated power supply it is expected that the installation will be the subject of a separate project financed by external users. However, the target design of the POLFEL center infrastructure has to take into account the location of the strong magnet laboratory at appropriate distances from other EM field sensitive FEL laser system components, preferably in a separate building. A sample set of external lasers could consist of the following items: picosecond Nd:YLF and Nd:YAG lasers, nonosecond dye lasers pumped by Nd:YAG, femtosecond Ti:Al2O3 lasers, and optical parametric generators using lasers operating in the UV and IR range. Since obtaining the best possible time resolution is an essential condition for 'two-color' experiments, all lasers emitting short pulses should be synchronized with the THz-FEL clock (jitter H 1 ps). Most of the proposed optical sources will be the subject of separate projects. The system, however, must be planned in the design phase of the POLFEL center infrastructure. Installation of magnets and supporting optical lasers, significantly expands the utility of THz-FEL sources and allows using measurement techniques both high field and test the dynamics and spectroscopy in multiphotons modes. This will allow the performance of experiments such as: nuclear magnetic resonance (NMR), high-resolution mass spectrometry using a cyclotron resonance ion analyzer with Fourier transformation of the results (FTICR), used in recording infrared spectra, and thus the structural information of large (bio) molecules and clusters by monitoring their dissociation after irradiation with the THz-FEL beam, measurements in the field of quantum physics condensed matter: resonant spectroscopy of low-energy correlated states, electron spin resonance of d- and f- electron systems, cyclotron resonance at high magnetic fields (about 40 T), ESR to 1.25 THz, characterization and manipulation of qbits, nanosciences and nanotechnologies: scanning optical nanoscopy and imaging of nanoscale phase separation and phenomena in new superconducting materials, 34

36 energy: solid state chemistry for catalysis, energy storage, chemistry of f-shell electrons to reduce radioactive waste, soft materials (liquids): selective bond breaking using IR range multiphoton dissociation, mass spectroscopy analysis of complex systems with large numbers of components (such as petroleum), macromolecules: multiphoton dynamics (multi-frequency) with spin labeling, vibration modes coupling study using Raman spectroscopy, biomedicine: dynamic tissue imaging at the nanoscale using coherent Anti-Stokes scattering. Other supporting devices include: flow and flood cryostats and superconducting coil cryostats (producing a magnetic field of a few-several T) for spectroscopic studies, equipment for molecular beam generation enabling the study (in the infrared range) of the spectra of narrow molecular weight fractions of molecules, clusters and complex clusters in the gas phase, and a system for pump and probe experiments, allowing a resolution of 10-4 in measuring a transmission rate thanks to the use of a special noise compensation method. Detectors are essential pieces of equipment. Due to the fact that the energy of the photon in the THz range is small compared to the thermal energy at room temperature (see Table 3), the detectors for this range of the electromagnetic spectrum have to meet specific requirements. Extensive discussion of this issue can be found for example in the review works [ 32, 33 ] and in the literature cited there. In the first phase of POLFEL it is expected to purchase low-noise cryogenic bolometers, bolometric array detectors and IR detectors with different structures operating in the shorter wavelengths. The purchase of most detectors will be included in projects related to specific measuring apparatus. Much of the research in the field of THz is performed at low temperatures. An experimental hall with preparation rooms will be equipped with systems for liquid helium, which will be linked to the main cryogenic installation for cooling the superconducting accelerator of the FEL. In the later stage of construction of the POLFEL center it is expected to link it to the "Maria" nuclear reactor operating at NCBJ. Nuclear radiation produced in the reactor, especially the thermal neutron stream, can be used for both research (in particular spectroscopy and neutron diffraction) and to modify the structure and properties of materials for purposes of research and technology. The inclusion in the POLFEL program of techniques based on nuclear radiation would put this facility among the most advanced in Europe and in the world. The possibilities and conditions of linking the reactor and POLFEL require further detailed analysis. However, the proposed infrastructure for the POLFEL project includes this capability; in particular the location of the POLFEL center infrastructure relative to the "Maria" reactor. It is envisaged that the beam from the POLFEL source will be transported by waveguides for a distance of several meters both to the strong magnetic fields laboratory, and to the apparatus placed close to core of the "Maria" reactor. In the second stage of construction, the possibility of using an electron beam to carry a spallation neutron source that could supply an experimental system of subcritical transmutation of long-lived radiotoxic isotopes in spent nuclear fuel is being considered Selected applications of a XUV-FEL source (Vis - XUV) Construction of devices capable of producing a coherent beam of radiation with a wavelength tuned in the nanometer band opens a new area of research previously unavailable because of the shortcomings of the instruments. Pioneering research in a newly shared 35

37 research area always brings unique and important scientific results and sets new trends. The research subjects of the POLFEL laser can be determined by extrapolating previos experience with third and fourth generation light sources, primarily on the basis of the achievements of the FLASH laser. However, FEL technology is so new that the applications given below should be regarded as examples only, as an exponential development of new applications can be expected, as in the case of synchrotrons. The range of scientific issues related to XUV radiation is extremely broad. For brevity, we present it in Table 4, supported by examples of research carried out in recent years with the FLASH and FERMI lasers. It is expected that future research with POLFEL will be a development of these themes. Examples of experiments using XUV lasers: Research the energy levels of highly charged ions, SW Epp et al., Physical Review Letters, vol 98, entry no , 2007 [ 34 ], Examine the process of ablation of silicon and gallium arsenide with a time resolution, N. Stojanovic et al., Applied Physics Letters, 89, entry no , 2006 [ 35 ], Two-photon absorption, P. Radcliff et al., Applied Physic Letters 90, entry no , 2007 [ 36 ], Imaging based on a single pulse of radiation, HN Chapman et al., Nature Physics, 2, page 839 (2006) [ 37 ]. Table 4. Research areas of the Vis XUV range Engineering of supershort light pulses The physical basis for the production of attosecond light pulses Atomic and molecular physics Multiphoton and multiple ionization under the influence in the field of EUV radiation Interaction of atoms and molecules with two synchronized beams with different wavelengths The impact of EUV light pulses of trapped ions Spectroscopy of highly charged ions Physics of plasma and hot matter Interaction of strong electromagnetic fields and matter Basic research in the physical sciences Research of compton scattering on protons and neutrons will determine the electric and magnetic polarizability of nucleons Physics of ultrafast phenomena Time distribution spectroscopy Imaging of nanostructures Time distribution holography Coherent light scattering on single molecules or clusters Physics of plasma and hot matter Strong VUV pulse absorption, mechanisms of energy dissipation, production of plasma, plasma absorption, studies of properties of the emerging of plasma Biophysics Spectroscopy and imaging of biological molecules Condensed matter physics Structural changes caused by a strong impulse of EUV range light Materials engineering Research of changes the physical properties of the surface under illumination Nanotechnology Preparation of nano-scale structures, photoetching, lithography Applications in medical science Diagnosis and therapy 36

38 6.3. Final remarks The POLFEL project is a response to a long-term vision of scientific development, including new areas of research. Its aim is to provide academic and industrial users with a comprehensive selection of new and improved techniques specific to two different ranges of wavelength: THz and XUV. For sources of terahertz spectroscopic methods, these are: research of the pump-probe type, picosecond time resolution, absorption and nonlinear multiphoton techniques. Many of these newly available methods will be used to research and construct entirely new types of materials and can be applied to new areas of knowledge, including a detailed understanding of key biological processes and chemical reactions. Others will take advantage of spin and nuclear quantization caused by magnetic fields and a strong photon source. In many cases, the development of new measurement techniques will build on extensive experience in microscopy, magnetic resonances (EMR, NMR and FTICR) and advanced methods of multiphoton scattering. The research centre created around the measurement infrastructure will combine the use of a world-class radiation source intended to carry out fundamental research in the world of modern science and technology with programs to increase to the next quality level innovation in technology and economy. The program of activities of the POLFEL center will cover a wide range of applications, including research, development of new experimental techniques, and industrial applications using sources and other research infrastructure. Research and development work of an industrial character will cover a wide range of issues discussed in this chapter, which are characterized by the following groups: new materials, traditional and new energy sources, safety, the environment, biomedical issues, chemical reaction kinetics, catalysis, spintronics, superconductivity, new technologies, fundamental processes in semiconductors, metals and insulators. The placement of the POLFEL center which will have a THz source, and also in the later phase XUV-FEL sources emitting in the range reaching soft X-ray radiation at the NCBJ in Swierk increases the science and technology importance of this center in areas critical for the economic and scientific future of the country. The convenience of the location is not limited to the possibility of using the extensive infrastructure and scientific expertise which the centre already has. The direct vicinity of the proposed device to the nuclear reactor "Maria" is unique on a European scale. This will, after the addition of the instrumentation of the POLFEL center for the position of strong pulsed magnets generating fields of about T, create a strong center bringing together in one place the study, modification and characterization of materials and phenomena with numerous complementary measurement techniques. This will allow effective use of radiation methods in materials science and determine the properties of materials exposed to extreme environmental conditions, such as high pressures and temperatures, radiation, strong electromagnetic fields, mechanical shock or aggressive atmospheres. This kind of research and technology center, possessing both 4th generation strong radiation sources and nuclear sources, including neutron sources and generators of strong magnetic fields will play a key role in mastering the revolutionary technology of tomorrow. 37

39 One of the important tasks of the centre, which is essential given due consideration and appropriate resources, already in the preparatory and construction stages, is to organize and educate the POLFEL user community. The primary method should be by providing and facilitating the dissemination of information on the research potential and uses of THz sources, as well as support of both academic and industry users. The most efficient way to build such a community is to reduce the technical and financial barriers for access to the new source. This has been proven many times during the construction of similar projects in Europe and the USA. An important element of this program is also involvement in education at the university level and higher, inspiring an introduction to educational and training programs in issues related to POLFEL, co-organization of the educational process coupled with the research program, for example by performing research in theses and dissertations, as well as the organization of conferences, symposia and workshops on topics related to POLFEL. Eventually, after the end of first phase of construction, the POLFEL center will have a THz-FEL source that will emit in the full terahertz range, THz, and in adjacent areas of the middle and far infrared. It will also be equipped with a basic device for THz spectroscopy in the time domain (TDS) and for imaging in the far field. At the same time, through the design of the infrastructure, conditions will be created for the installation at a later date of multifunctional measuring devices that allow the generated beam to be used extensively. Because of the considerable cost of some of these systems, it is expected that they will be the subject of separate projects formulated jointly with future users. 38

40 7. Technical description 7.1. Conceptual work In developing the concept of POLFEL the following assumptions were made: POLFEL will be a free electron laser based on a superconducting accelerator. A FEL device with a superconducting accelerator is more complex than a device using a conventional accelerator, but with the possibility of continuous operation it allows the achievement of higher average power and a much wider range of experiments, POLFEL will be built using as many as possible, components compatible with the XFEL [ 38 ] and ELBE [ 39 ]. This solution has the following advantages: o a modern, yet proven technology, o a costly and long-term R & D phase can be avoided, o possibility to purchase items at "mass production" rather than single item prices, o possibility to involve Polish specialists working on XFEL, o the use of the know-how of the entire international XFEL and TTC consortium [ 40 ], a two-stage project: first a THz-IR radiation source and then one in the Vis-XUV range. Because of the need to optimize the innovations planned in the POLFEL accelerator a two-stage implementation is proposed. The advantage of a phased schedule is that, whenever possible, further accelerator sections may be installed and thus the energy of the generated beam of light may be raised to get shorter wavelength, increasing the wavelength range of the generated radiation. The separation into two stages of construction allows for a modification of the second phase plans and implementation of solutions that are not available at the beginning of the project. Moreover, years of experience taken from the construction of large research facilities such as HERA and TTF at DESY have shown that such a gradual and staged improvement of technical complexity keeps the device in good condition. Presented below are descriptions of the elements from which a free electron laser is made, the technical project of the planned THz-POLFEL first phase of construction, and an outline of further progress possibilities Free Electron Laser elements Electron source The first element of a free electron laser is an electron source. It consists of an electron gun which is a resonance cavity with a photocathode mounted on its rear wall, a coupler connecting the resonance cavity with a high frequency waveguide, configuration of the optical path of the photocathode initiating laser and coils which are components of the electron optics. Under the influence of an ultraviolet laser impulse bunches of electrons are emitted from the cathode. When this happens, because of synchronization between the laser and the klistrode, they face a strong component of the high frequency electric field, which accelerates them [ 41 ]. 39

41 Figure 10. Simplified diagram of the electron source Acceleration sections The acceleration sections (also called modules or cryomodules) are an essential element of the electron accelerator, as in these parts the electrons are accelerated (Figs ). In the case of the POLFEL accelerator, 9-cavity superconducting accelerating structures (NSP) will be used, which work on the L-band frequency of 1.3 GHz [ 42 ]. Due to the choice of superconducting technology, NSP will be placed separately in titanium tanks with liquid helium. One cryomodule will contain from two to eight accelerating structures, depending on the chosen technology. A typical cryomodule consists of: superconducting accelerating structures made of highly pure Nb (RRR 300) maintained at temperature of 1.8 K, coaxial couplers (FMC - the fundamental mode coupler), 1300 MHz of microwave power working at the border of the cryogenic zone at temperatures of 2K and 80K, bifunctional tuners located one on each accelerating structure, higher-order modes (HOM) couplers (HOMC) placed two on each accelerating structure, HOM absorber for the propagating modes from the accelerating structures into the electron beam chamber, the main vacuum ion pump, located on the absorber chamber, corrective superconducting magnets and magnetic quadrupole lenses placed at the end of the module, accelerated electron beam position monitor (Beam Position Monitor) superfluid helium filling and refilling system (helium vessel) of each accelerating structure casing and the removal of helium in the gas phase, supporting structure allowing fine adjustment of the structure location and thermally insulating the cold structures from their surroundings, measurement and control of the electrical wiring system (including microwave). 40

42 Figure 11. Cross section of a TESLA type cryomodule [ 43 ]. Figure 12. A TESLA-type cryomodule with 8 accelerating structures [ 43 ]. Figure 13. An ELBE type cryomodule with two accelerating structures [ 44 ]. 41

43 Electron bunch compressor Electrons emitted from the photocathode of the source are not at one point, but have a certain spatial distribution, creating a small electron cloud, the size of which depends on the length of time and the cross-section of the laser impulse. The consequence of this is that electrons in different cloud places (especially along the direction of flight) are accelerated by a varying intensity electric field, since it is sinusoidal as a function of time (Fig. 14). Figure 14. Uneven acceleration of electrons in bunch with the high frequency field at the cathode. For example, in FLASH a bunch that leaves the source has a length of 6 mm and a circular cross section with a diameter of 2.2 mm. For proper functioning of the undulator it is necessary to reduce this length by magnetic compression. The rate of the efficiency of undulator is transformation of electron beam energy into coherent radiation and the degree of this coherence. To obtain the best efficiency of generating radiation in the undulator it is required to: maximize the impulse currents of the beam (range of kiloamperes) have minimal scatter of the electrons energy in the bunch have the smallest transverse bunch emittance. It is worth saying that the uneven acceleration of individual electrons within the bunch does not directly affect the change of their position in the bunch, because they move with relativistic speeds (which is why acceleration of electrons with low energies has the greatest influence on bunch deformation). If the bunch moves at relativistic speed, then the scattered electrons within it are increasing their kinetic energy in an uneven manner, but this does not affect their speed. This can be interpreted as the increase of their relativistic mass. To be able to shorten the bunch length (to compress it), the bunch needs to be placed relatively to the phase of the HF field in the structures so that the electrons at the end of the bunch have more energy than those at the beginning. In order to obtain the correct compression, the proper correlation between position and energy in the bunch is provided by a set of deflection magnets called a bunch compressor [45]. The POLFEL compressor will have four identical electromagnets arranged like a streamer (magnetic chicane) in the beam path. The bunch is compressed, because higher energy electrons travel a shorter distance than electrons with lower energy. The compression process is shown in Figure

44 Figure 15. Construction and operation of an bunch compressor. In addition, the compressor will be equipped with correction coils at each solenoid and a system of three quadrupole electromagnets to ensure beam re-focusing Undulator An undulator (Figs ) is a device in which laser action occurs [ 46 ]. An electron bunch accelerated and shaped in the accelerator emits light as a result of interaction with the magnetic field. The undulator is a system of pairs of magnets arranged periodically and symmetrically about the longitudinal axis. Alternately compound of many areas where are uniform magnetic induction fields of a parallel directions but opposite turns leads to a periodically varying in space magnetic induction field. A field shaped in this way repeatedly bends the path of the electron bunch during the process of acceleration. As a result of the cyclic bending of the bunch track radiative emission occurs in the undulator. Radiation emitted in subsequent periods interferes, which causes the creation of a beam with a high intensity and focused in a narrow solid angle around the direction of emission along the undulator axis. Figure 16. The general scheme of an undulator. 43

45 The parameters which determine the length and conditions of interference are the period and amplitude of the magnetic structure. The amplitude can be varied by changing the width of the gap, and thus the magnetic field induction intensity. Figure 16 shows schematically an undulator section. The idea of the SASE free electron laser is to have a strong interaction between the electron bunch and its self-generated field during one pass through the undulator. Then there is bunch density modulation and consequently coherent light emission by all electrons. Spontaneous modulation of the bunch density develops as it moves through the undulator magnetic field. To obtain coherent emission the undulator should be long enough that the bunch is able to split into separate microbunches as it travels through it. Free electron lasers are capable of producing induction fields of around 1 T. Such a field can be obtained using constant magnets. Magnets are mounted in aluminum holders fixed to a longitudinal steel bench. Two benches are placed on the frame. An undulator is a device with the precision of micrometers, while the elements are subjected to magnetic forces of many kilonewtons. The key to the functioning of the device is a stable frame, precise movement of the benches when changing the gap width, precise setting of magnet positions in the holders on the bench and the homogeneity and repeatability of the magnetic properties of the magnets. Figure 17 shows the process of radiation generation in an undulator. Figure 17. Radiation emission in an undulator Collector Right after the undulator there is a strong magnet which bends the electron flight path to separate it from the radiation generated by the FEL. Electrons used to generate the radiation in the undulator must be disposed of by directing them to a beam collector. This device is designed to absorb the electron bunche energy and dissipate its charge to the ground. An important aspect in the selection of materials and design of the collector is to take into account temperature changes and the resulting deformation of individual elements. These changes result from the absorption of beam energy by these elements. This is very important due to the fact that a single continuous tunnel (called the beam line) in which high vacuum is maintained runs from the source of the electron beam to the collector. Uncontrolled temperature changes can lead to deformation of collector elements, which in turn can lead to leakage of the beam channel. Another important aspect of the collector is radiation resistance. Although charge values of single nanocoulombs do not appear to be dangerous, one should remember that electrons at these charges have very high energies, and are highly concentrated (peak currents reach values of kiloamperes). Deceleration of such a bunch in the collector associated with 44

46 returning all its energy into a very small area results in its activation (in FLASH, the beam collector is one of the most activated components). Significant in this regard is that the collector distributes the energy as well as possible, so it would not lose itself spot by spot and that the collector is properly shielded. The FLASH collector is shown in Figure 18 [ 47 ]. Figure 18. Beam collector in the FLASH accelerator Electron beam diagnostics The electron beam diagnostics is a system consisting of detectors placed along the beam track for measuring the parameters of bunches and their tracks. This system provides the information necessary to maintain the best possible beam parameters, which corresponds to the quality of the radiation generated in the undulator. Furthermore, this system monitors the electron bunches in different spots and when it detects an invalid operation (such as a move away from the assumed trajectory), it turns off the electron source thanks to an accelerator safety system called Technical Interlock. Key elements of the system (according to the nomenclature used in the FLASH accelerator) are: toroid the simplest cylindrical detector, in which a moving bunch induces a small electrical current. It allows an initial measurement of the time of the flight of an electron bunch and its intensity, Beam Position Monitor (BPM) monitors the position of the beam. It allows the deviation of the beam from the intended track in a plane perpendicular to the axis of the machine to be measured. A device of this type is needed for each element through which the beam passes (accelerator section, undulator, compressor, etc.), Optical Transition Radiation (OTR) a screen inserted into the beam line and a camera. This device allows the bunch intensity distribution in the plane perpendicular to the direction of acceleration to be viewed as a two-dimensional image. In this way one can estimate the quality of the beam (stability over time, focus, intensity and spread in one of the planes, etc.), Beam Loss Monitor (BLM) detects loss of bunch charge. Detectors of this type will be placed on any element that may be exposed to beam absorption when it deviates 45

47 from its trajectory (like the undulator). Information from the BLMs is used by safety systems such as the BIS and Interlock, Beam Inhibit System (BIS) monitors selected parts of the machine and in the event of an unsafe situation it blocks the beam generation. This system is closely connected with the main security system - Interlock, collimator a magnetic structure placed in front of the undulator, which adjust the geometry of the beam before it enters the undulator, quadrupole magnets they focus the beam Cryogenics As mentioned in the description of the electron source and accelerating sections, both of these components will be built using superconducting technology to enable long pulses and continuous working for the highest possible accelerating field gradient. In contrast to the copper structures the superconducting ones are characterized by small heat losses in the walls. To get the effect of superconductivity and minimize losses in the walls the electromagnetic resonators, made of niobium, are cooled to about 2 degrees kelvin. A method for maintaining the structures at this low temperature is to put them in liquid helium. To do so they are placed in special cryostats. The use of superfluid helium II brings the very best conditions of heat exchange resulting from the excellent heat conductivity of helium II, but it gives rise to the need to maintain low pressure in the helium pipe system, around 30 mbar. For this reason it is necessary to install an appropriate refrigerator system for the POLFEL project with a capacity ranging from 100 W to 500 W (depending on the variant and size of the system). Its main task will be to receive helium in a gaseous state from the cryomodules and then cool and condense it. Helium in the refrigeration system will be stored in a closed cycle because of ecological and economical reasons. Due to the very low pressure of the helium vapor leaving the cryomodules, cold compressors will be used for compressing the helium vapor at low temperature, when the vapor's specific density is relatively large. The idea of using cold compressors it to avoid very large volume flows of helium at ambient temperature. The cooler itself will be made with the technology verified during the operation of the linear accelerators TTF1 and FLASH [ 47 ]. In the refrigerator after being compressed in warm compressors and initially cooled in the heat exchangers the helium will be expanded in turbine expanders and Joule-Thomson throttle valves. In this way enough cooling power will be generated to support cryomodules at a temperature of 2 K and radiation screens at a temperature of 4.5 K High-frequency power supply Nowadays, elementary particles are accelerated in resonators (accelerating structures) using alternating electromagnetic fields (and more specifically their electric components). The ever-growing need to increase the energy of the accelerated particles makes the demand for higher intensity fields in the accelerating structures. The currently used free electron lasers have HF electric field gradient components reaching up to 30 MV/m. To obtain such field intensities in resonant cavities it is necessary to use proper high power microwave amplifiers. In particle accelerators klystrons and klystrodes are used. Klystrons are better in cases when a device is operating in impulse mode and they allow higher field intensities to be obtained with higher power consumption [ 48 ]. 46

48 IIn case of POLFEL, which will ultimately operate in continuous mode, klystrodes have two major advantages over klystrons [ 49 ]. First, they can work in both continuous and impulse modes, for any chosen length of the impulse duration and for any interval between impulses. Second, the amplifiers are characterized by minimal power consumption in the time between the impulses, which makes their use much more economical than an alternative solution based on klystrons working in continuous mode. Output power impulses from klystrons designed for work in the continuous regime can be achieved by modulating the control input signal. This way of obtaining high frequency impulses to power the accelerating structures is possible with constant maximum power consumption, even during the time between pulses LLRF System The main task of the LLRF system (low power radio frequency signals control) is the generation (modulation) of the controlling signal of the high power amplifier (klystrons or klystrodes). The efficiency of the entire accelerator depends greatly on the quality of this control, the higher the quality the greater the efficiency. Control of the high frequency amplifiers stabilizes the electromagnetic field in the accelerating cavities. The stabilization of the field in the cavities is based on a control in a closed feedback loop. The first element of the track (just after the coupler) are electronic microwave systems, whose task is to change the carrier frequency signal from the cavity resonance frequency (1.3 GHz) to the desired intermediate frequency. Another part of the system are the digital electronics circuits, which feature the analog-digital converters that sample the intermediate frequency signal and on that basis calculate new control values. The required value is transmitted by the digital-analog converter to an analog circuit that modulates the reference signal (1.3 GHz) from the exemplary generator. The modulated signal is supplied by several degrees of preamplifiers to the input of a high frequency power source klystrode - which amplifies the signal to higher powers, after which it is distributed through waveguides to the accelerating structures, thereby closing the feedback loop. The primary determinant of the control quality is the delay in the feedback loop. To get the best quality, the following parameters are minimized: the processing time of the control algorithms in the digital part, by applying the most effective available technology (like FPGA), and delays caused by long signal leads to and from the control systems. The last technique requires placement of the electronic control systems in close proximity to the accelerator, in many cases in the tunnel, right after basic radiation shields (which of course does not eliminate the local shielding). This requires additional safety conditions for the control electronics, such as a complete or partial (increased) resistance to ionizing radiation. This requirement forces the electronic control systems to be characterized by high reliability and to have the possibility ofremote diagnostics, as the devices are located in an area exposed to radiation which might not be accessible by the staff. For this reason, repairs may be difficult to make. An additional task of the LLRF system is the generation of the exemplary frequency. This task requires precise microwave circuits, ensuring the maintenance of stable signal parameters. Maintaining the stability of the generated signal parameters requires that adequate conditions such as temperature stabilization of key elements of the system be ensured. An issue related to the exemplary signal generation is synchronization, i.e. its distribution across the whole experiment area. It should be taken into account that the distances mentioned are hundreds of meters long, with a phase stability having single picoseconds accuracy. 47

49 Monitoring and Diagnostics In addition to the stabilization of the electron accelerating field in the cavities, the control must also include support for the entire infrastructure of the experiment. Issues related to monitoring and control of everything except the basic tasks of the accelerator, which in the case of FEL experiments is the electron acceleration and radiation generation, are often named "Slow Control". For POLFEL, the monitoring system will include issues such as: control and monitoring of cooling, vacuum, high voltage, the monitoring of temperature and pressure in particular systems and monitoring of the radiation which is generated by the machine during its operation. The monitoring of machine status is related to diagnostics, which is a verification of the correctness of the experiment, and in the case of any problems localization and determination of the cause. Because the diagnostic systems are used to detect failures in other systems, they must be independent of them and more reliable The security system The security system has the highest degree of reliability (commonly called Interlock), and is a low-level security system of the device, whose purpose is to protect against any damage or destruction of the individual elements of the accelerator and to prevent the exposure of personnel and the environment to any risk. The security system monitors selected parameters and, if the value of one of them goes out of the acceptable range the whole subsystem associated with a given size is switched off, which is usually associated with stopping the experiment. Continuation is possible only after removing the cause of failure. In addition to monitoring operating signals, Interlock monitors the doors to the tunnel and locks with keys for people entering the experimentatal area. Unauthorized opening of the tunnel door during accelerator operation generates a signal with the highest priority (staff at risk), which involves immediate shutdown of the machine and the need to re-run the search procedure and closure of the tunnel. During short breaks between work any person entering the tunnel gets the key from the lock and places it back into place after leaving the tunnel. This prevents accidental operation of individual systems in the machine when there are people in the tunnel - an experiment can be carried on only if all monitored doors are locked and all keys are placed in the locks. For the person entering the tunnel to have an interlock system key is a guarantee of safety Computer control system The primary structure to control all of the subsystems described above is a computer control system - control - monitoring scattered software called SCADA. The system must provide: remote monitoring and control of individual devices that are installed on the device in real time, visualization of measurement data, define the operational procedures which by making the running of the experiment more automatic simplify the operators' work, save and restore machine status (setup and calibration parameters), so as to easily control the activity of several installations at once, archive important operating parameters in order to be able to recreate the device working history. 48

50 7.3. Teststand for the Superconducting Photoinjector In the preliminary phase (stage zero ) of constructing POLFEL Free Electron Laser, standalone superconducting electron source will be built. This small facility will be used to prepare and develop technologies needed later for liniac contruction, including RF frequency generation, low porer RF, cryrogenics, vacum, laser, interlock and electron beam diagnostics. The electron source will be equipped with a thin superconducting photocatode made of lead, which will be placed on the back wall of the 1.6 cell superconducting accelerator structure made of pure niobium [ 50 ]. For electron emission, a UV laser beam of 206 nm wavelength will be focused on the photocathode. An IOT, which may work in continuous and long pulse modes, will be used as a RF power source. Figure 19. Main components of the electron source. The electron source is made up of the following components: superconducting accelerating niobium structure of TESLA type (1.6 cell), with a resonance frequency of 1.3 GHz and a micrometer thin lead photocathode placed on the back wall, 1.3 GHz clock signal source, IOT RF power amplifier of min. power of 30 kw, working in the long pulses (>100ms) or continous mode, cryostat containing accelerating structure, closed cryogenic system, with a minimum power of 30 W, equipped with ajoule Thomson valve and a helium transfer line, able to cool the superconducting structure to 2K, laser with a wavelength not longer than 266 nm, pulse energy not less than 10 µj, pulse duration of 10ps and a repetition rate of 100 khz, electronic control system employing the same technologies used in FLASH and X-FEL beam diagnostics system, which will be composed of: o dipole bending magnet (made by NCBJ, do electron energy measurement), 49

51 o Ce:YAG monitors, for beam position measurement, o OTR monitors (bunch size), o Faraday cup (bunch charge and beam current measurement), o photodiodes, MCP, and other (electron and radiation detection), o adjustable slots, o fast oscilloscope (>2 GHz), o vacuum system mbar, (ion pumps, turbo, etc.). The electron source facility (Figure 20) will be built as the first part of the POLFEL Free Electron Laser. This device will work in standalone mode, to perform R&D studies on the new type of electron source, including following tasks: diagnostics and optimization of the outgoing electron beam, development and testing of new diagnostic methods, testing RF power source (IOT) in long pulse and continuous modes of operation, testing and optimization of the synchronization and control systems, cryogenic system optimization, photocathode UV laser testing, performing experiments with THz radiation generated from the 5 MeV electron beam. First calculations of the emitance, bunch size and average final kinetic energy for the 1.6 cell superconducting structure based electron source have been made. For the cavity model considered, with an amplitude of the electric component of the RF field of 40 MV/m, bunch charge of 1nC and laser pulse duration of 4 ps, the calculated emitance was εx,y = 2.7 π mm mrad mm, energy dispersion was E/E = 10-2, bunch length was around 1 mm and maximum average kinetic energy was 3.8 MeV Photoinjector teststand cost 0 stage: Test Photoinjector Element Cost in k TDR 50 Feasibility study 50 Electron gun in cryo-module 1000 RF Power, synchronisation, LLRF 350 Cryogenics 50 W 2500 El. Beam diagnostics, control, safety sys. 300 Bunker 60 m 2 and technical building 1000 Personal 1000 Management 150 Collaboration and assistance from other FELs 500 Commissioning 200 Contingency 10% 710 Total

52 0 stage: Test Photoinjector Y Investment in k Total without contingency 7100 I stage: Test Photoinjector Year Quarter Permissions and certificates TDR Civil engineering Cryogenics Injector Diagnostic line Commissioning Y expenses procurement - activity at the vendor site - activity at site End of cryomodule production for XFEL 7.4. Thz-POLFEL technical design In the first stage of the Polish free electron laser POLFEL, a facility containing the following components (Figure 19) will be built: electron source with laser for the photocathode, two accelerating sections, ELBE type criomodules (Figure 13), undulator, beam dump (together with the magnet), experimental and measurement station 51

53 Figure 20. Thz-POLFEL main components scheme The main components of the Thz-POLFEL are shown in Figure 19. For the sake of clarity, the following components and sub systems have been omitted from the Figure: RF power source, LLRF control, master oscillator and the synchronization system, electron beam diagnostics, cryogenics, safety system (Interlock), monitoring and diagnostics. THz-POLFEL will use Test Photoinjector described above, as an electron source for the main part of liniac. The device will reach ae maximum electron bunch energy of 50 MeV and a range of wavelengths generated in the undulator from 4 µm to 500 µm. As an additional feature, usage of the neutron stream from the Maria nuclear reactor, together with THz-POLFEL radiation in measurements is under consideration Accelerating sections In the first period of POLFEL construction, the accelerator will be equipped with 2 ELBE type accelerating modules, each module will contain two 9-cell accelerator structures of the TESLA type (Figures 21 22). The main parameters of such structure are given in Table 5. Figure 21. Picture of the 9-cell TESLA superconducting accelerating structure 52

54 Figure 22. Dimensions of the 9-cell TESLA superconducting accelerating structure Table 5. Parameters of the 9-cell TESLA superconducting accelerating structure. Frequency Accelerating mode Maximum gradient of the accelerating field Parameter Value Remarks 1,3 GHz TM010-π 30 MV/m Unloaded goodness Cryostat temperature Cryogenic load Length (including FMC i HOMC) Diameter of the hole between cavities 1,8 K ok. 3 W 1,256 m 0,07 m Material Niob #2,8mm RRR > 300 Cavities in the structure RF power sources For providing RF power to the accelerating structures, IOT microwave amplifiers will be used [ 49 ]. At selected values of accelerating field and beam current, each accelerating structure should be powered with at least 15kW. To fulfill this requirement, IOTs available on the market can be used, a 16kW IOT for powering a single structure or 32kW for powering each structure pair. High voltage power supplies for both types of IOTs should have stabilized voltages in the range from kv. The effiency of IOTs is about 50-60%, so peak power should not exceed 30kW and 64 kw for the types of IOTs described. The accelerating structure used in the electron source (1.6 cell), can be powered from a 4kW semiconductor amplifier. IOTs are shown in Figure

55 Figure 23. IOTs produced by CPI (left) and Thales (right). Excluding the IOTs, the RF power system will contain: preamplifiers (1 st i 2 nd stage), high voltage power supplies (25-50kV), safety devices, a waveguide RF distribution system, which will provide power to the accelerating sections and to the electron source. As the IOT production technology will develop in the future, it may be possible that during the realization of the project, more powerful IOTs will become available on the market. In such a case, several smaller IOTs could be replaced by a stronger one. In the ideal case, one 100kW IOT could be used to power all the accelerating sections and the electron source. Such a solution would significantly reduce the cost of the whole installation and would simplify the control scheme for the machine, because the LLRF system would only need to drive one RF amplifier. Possible solutions for the RF powering system of THz- POLFEL are show in Figures Figure 24. Structures powered by separate IOT amplifiers. 54

56 Figure 25. Each module powered by a 32 kw IOT. Figure 26. Installation powered by a single 100 kw IOT Cryogenics THz-POLFEL will be equipped with a cooling system which is able to convert helium back to liquid form, keeping it in a closed installation. Liquid helium will be delivered to all subsystems which need low temperatures, such as the electron source accelerating sections, etc. The cooling power provided will be enough to operate in the continuous mode at low gradients (at least 5MV/m). In the currently considered project a cooling system of 100W total power will be enough to cover all needs. However, requirements for cooling will grow with future machine development Undulators In the POLFEL Free Electron Laser, self amplified spontaneous emission (SASE) will be used. Coherent light generated in the undulator will interact with the electron bunch from which it was emitted, and this will cause charge modulation within a single bunch. This process will start with the first pair of magnets and it will develop over the whole length of the undulator. To achieve coherent emission and maximum radiation power, the undulator must be long enough to cause charge micromodulation in the traveling bunch. Magnets in the undulator will be attached to aluminum support arms, which will be mounted on a steel bar placed on the ground, two such bars will be placed in the frame. The Undulator is a device which on the one hand must be made with micrometer precision, but on the other hand its components will be exposed to magnetic forces of many kilonewtons. Most important for the undulator are such aspects as stability of the frame, precision positioning of the ground bars during slot width adjustment, precision positioning of the magnets in the support arms and repetition of magnetic values of subsequent magnets. In the first stage of the POLFEL project, two flat undulators will be designed and built. The first one (called U1), will have constant slot width, a magnetic structure period of 12cm, and will be used for generation of radiation in the wavelength range from 100 µm to 1000 µm. Wavelength tuning will require changing the electron energy delivered by the accelerator. A second undulator (called U2) will be designed to generate radiation in the wavelength range 55