SUMMARY OF THE 4TH LHC CRAB CAVITY WORKSHOP LHC-CC10

Transcription

1 EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH CERN ACCELERATORS AND TECHNOLOGY SECTOR CERN-ATS SUMMARY OF THE 4TH LHC CRAB CAVITY WORKSHOP LHC-CC10 Editors : Rama Calaga, Steve Myers, Frank Zimmermann Abstract The 4 th workshop on crab cavities for the LHC high-luminosity upgrade project HL-LHC (LHC- CC10) was held from 15 to 17 December 2010 at CERN, Geneva, Switzerland. This document outlines the key conclusions from the LHC-CC10 workshop and summarizes each individual session based on the final presentations by the session conveners and on the subsequent discussions. At LHC-CC10, local crab crossing with the aid of 400 MHz deflecting SRF cavities was identified as the baseline tool for geometric luminosity-loss compensation and luminosity levelling for the HL-LHC. Associated key issues such as machine protection and robust operational scenarios still have to be studied in detail, so as to identify and mitigate potential beam-dynamics or technology limits on implementing and fully exploiting crab crossing at the LHC. Also at LHC-CC10, the decision was taken not to install a KEK-B crab cavity in the SPS. However, it was recommended that all measures to test future LHC prototype cavities with and without beam outside the LHC be taken in order to ensure robust operation of the crab RF structures in the LHC. The 4 th workshop on the crab cavities (LHC-CC10) for the LHC luminosity upgrade project (HL-LHC) was held from 15 to 17 December 2010 at CERN, Geneva, Switzerland. Geneva, Switzerland, March 2011

2 TABLE OF CONTENTS 1 EXECUTIVE SUMMARY INTRODUCTION WORKSHOP CHARGE AND OBJECTIVES SESSION SUMMARIES LHC-CC STATUS & REVIEW OPTICS & BEAM PHYSICS ASPECTS CAVITY TECHNOLOGY AND CRYOMODULES CRAB CAVITY BEAM STUDIES AND INSTRUMENTATION MACHINE PROTECTION RF CONTROLS & TESTING SCHEDULE & FUTURE OUTLOOK ADVISORY BOARD MEETINGS AND COMMENTS PARTICIPANTS MEETING COMMENTS OPEN ISSUES AND ACTION ITEMS

3 1 EXECUTIVE SUMMARY 1. Crabs are the *baseline* tool for geometric compensation and luminosity levelling for the HL-LHC. The nominal scheme for HL-LHC is a local crab crossing scheme with 400 MHz superconducting cavities independent of each IP (IP1 and IP5). Other options are pursued as backup if technology limits or machine protection issues are identified. 2. A refined roadmap should be sketched by the RF group and presented at the LMC before summer 2011 showing the overall effort and resources needed, including the prototyping. 3. No cavity down selection will be made now. A cavity down selection should be foreseen after the results from prototyping yield conclusive reasons to pick the best candidate for the LHC, which is expected by the end of The RF group will identify available resources within CERN to support prototyping effort while synchronizing with the US and UK efforts. 4. Identical systems for both IPs are preferable but this is not an absolute requirement. Therefore, one should pursue the best candidate satisfying the appropriate crossing scheme. 5. KEK-B crab cavities will not be installed in the SPS due to several reasons such as cost, difficulty, extrapolation to LHC, risk etc. 6. No synergies with CLIC crab cavities have been identified except for the low-level RF and cavity controls. 7. Machine protection with crab cavities needs detailed investigation with realistic RF failure signals in conjunction with the upgraded collimation system to arrive at a clear conclusion. 8. No show stoppers have been identified for the compact cavity designs presented. Three to four promising designs compatible with the LHC footprint exist. Cavity engineering design and prototyping should follow. A common platform for couplers, tuner systems and cryomodule equipment should be studied. 9. Detailed specifications for the cavities should be given, addressing the requirements on transparency, impedance, field quality, fundamental mode damping, trip rate, synchrobetatron resonances, plane of crabbing, etc. 10. A clear timeline for cryomodule development and prototype testing needs to be put in place. A generic cryostat could be used for testing all compact candidates. 11. A clear program for beam tests in the SPS and LHC, associated with their benefits, needs to be defined with a consistent timeline, taking into the account the regular operation and shutdowns of the LHC and its injectors. Critical issues such as machine protection, cavity transparency, impact on collimation system, emittance growth, low-level RF and cavity controls, and cavity impedance should be prioritized and addressed with a detailed experimental program 12. Other applications of crab cavities in the LHC or SPS such as the compensation of beamloading effects, momentum cleaning, bunch compression, etc.. should be investigated. 3

4 2 INTRODUCTION The 4 th workshop on the crab cavities (LHC-CC10) for the LHC luminosity upgrade project (HL- LHC) was held from 15 to 17 December 2010 at CERN, Geneva, Switzerland. The workshop was organized by a joint collaboration of CERN, EUCARD, KEK and US-LARP. Approximately 50 participants from 3 continents and several institutions participated in the workshop to discuss the future implementation of the crab cavities in the LHC and related issues. The detailed participation list, scientific program along with the associated contributions are available at: WORKSHOP CHARGE AND OBJECTIVES The charge of the workshop was as follows: 1. Can compact cavities for the LHC be realized and made robust with the complex damping schemes? 2. Are crab cavities compatible with LHC machine protection, or can they be made to be so? 3. Should a KEKB crab cavity be installed in the SPS for test purposes? A scientific program committee of 17 members from 10 institutes around the world compiled a comprehensive two-day program spanning various subjects related to the need, technologies, beam aspects and precise implementation of crab crossing in the LHC. The program was divided into 7 sessions with dedicated session conveners. The session conveners prepared individual summaries which were presented to an advisory board chaired by S. Myers with representatives from most of the major collaborating institutes. The session conveners were specifically requested to focus on the open issues and to trigger discussions addressing the charge of the workshop. 4

5 3 SESSION SUMMARIES Notes from the session summaries prepared by their respective conveners and additional material from the discussions during the workshop are detailed in this section. 3.1 LHC-CC STATUS & REVIEW Session Convener: Jean-Pierre Koutchouk As an initial disclaimer, it was mentioned that this session was not geared to address the charge, but to set the scene for the need of crab crossing in the LHC. KEK-B has established the proof of principle for crab crossing in a short period, but the expected performance of factor 2 increase in beam-beam parameter was not reached. This issue is specific to the strong beam-beam regime at which KEK-B operated, with very large dynamic beta changes, and less relevant to the LHC. Approximately 13 years for R&D, 2.5 years for commissioning and 1 year to achieve improvement was spent. In addition to crab cavities, skew sextupoles were needed to realize the full gain in beam-beam parameter that was finally achieved with crab crossing at KEK-B. A 4- mm orbit bump and a 10 degree RF phase shift were introduced to stabilize an observed instability which was caused by a coupling of the crab cavities of the two rings through the beam-beam interaction. The KEK-B crab cavities were sufficiently mature for operational purposes. However, the voltage performance was 30% lower than on the test bench. A high trip observed was mainly related to either the cavity fields or the coaxial couplers. Reducing the surface field had a strong impact on reducing the trip rate. No strong dependence of the trip rate on the beam current was evident. The KEK experience justifies using crab cavities for LHC. Strong performance during 2010 run was the highlight for the LHC. It was indicated that the speed of the LHC performance increase had been dictated by MPS considerations. Beam instrumentation was working extremely well. The transverse emittance had been 30% to 50% lower than the design value, which was promising for the future. The beam-beam tune shift had been twice the nominal value. No evidence had been observed of any issues related to operating at such a high beam-beam tune shift, which also bodes well for the upgrade. However, operation with much less than the nominal emittance would be very difficult for collimation. A common scenario and a coordinated shutdown calendar for the accelerator and the detectors are essential to reaching the LHC physics goals. It is extremely difficult if not impossible to integrate accelerator elements into the detector and such elements will clearly limit the physics reach. Since CCs do not interfere with the detector, they could be very important as upgrade ingredients which do not to perturb the physics reach of the experiments. HL-LHC is a project with a clear target for the upgrade to reach the LHC performance goal including luminosity levelling. Validation of a 400-MHz crab cavity with beam in the LHC circa 2016 is envisioned, which leaves a total 5 years for R&D and prototyping. This challenging time scale calls for vigorous R&D rather immediately. LHC challenges could be very different from 5

6 those at KEKB. Therefore detailed simulations and experiments are vital to the development of LHC crab hardware, but the hardware may not be the only challenge. 3.2 OPTICS & BEAM PHYSICS ASPECTS Session Convener: Massimo Giovannozzi An optics with beta* of 15cm based on the slhcv3 layout was designed. A crossing angle of 0.58 mrad is required with this optics to retain the 10sigma separation in the common aperture. The principle of the optics design is based on the off-momentum beat-beating correction using adjacent IR insertion regions resulting in larger than nominal beta s in the arc. A scheme of alternating crossing at IR1 and IR5 is still considered as nominal separation scheme for HL-LHC. Therefore, the crab cavity designs should envision providing dual plane kicks but with the same horizontal constraint for the beam pipe separation. Two cavities per beam per side of the IP placed after the D2 starting from the IP is ideal to profit from the large beta functions. About 10 MV of total voltage for 400-MHz crab cavities is required per side, per beam and per IP at 15 cm beta* and the current baseline crab location. Studies are ongoing to determine the dynamic aperture and field quality tolerance at top energy. Synchro-betatron resonances and the nonlinearity of the RF kick voltage as a function of transverse offset should be carefully considered. The local bump between the cavities is not absolutely closed due to imperfect phase advance between the cavities, but this non-closure is at the 2% level. Large orbit variations may be needed for luminosity scans. However, the crab cavities themselves could also be used to perform the luminosity scans, at least in the plane of crossing. The crab cavities may require transverse re-alignment in case of systematic large crossing angle changes requested by the experiments. Mitigation of static offsets for operational flexibility with remote alignment needs to be studied. In addition, the placement of the cavities on the IP side of the D2 in order to increase the beta function at the crab cavities has to be studied and compared with the non-ip side placement. An 8-mrad optics was initially proposed in 2006, triggered by a large crossing angle scheme. The present separation dipoles based on Nb-Ti magnets are relatively moderate from the aperture field plots. Therefore one solution for twin aperture magnets with smallest crossing would be to shorten D1. Additional gain in strength and in space between the dipole twin apertures is possible. For quadrupoles, there is less flexibility with regards to larger field gradient or larger aperture. Exotic designs are available, but need to be investigated more closely. Luminosity leveling in the presence of IBS and proton-burn off was studied for the nominal LHC and for LHC upgrade options. The assumptions on the machine availability used the simulations were optimistic, but the observed scaling is useful. Cross checks with one fill were performed and more cross checks are needed, e.g. benchmarking the model with a number of fills. We are far from the integrated luminosity goals set by the experiments even at ultimate intensities. Adding crab crossing significantly increases the reach of the upgrade. Adding luminosity leveling will help towards better integrated luminosity and reach the expected goal of 3000 fb -1. There is 6

7 also one scenario with crab crossing and 50-ns bunch spacing which comes close to the goal set by the experiments. RF feedback and feed-forward at 400 MHz with 1 MHz bandwidth available are currently available in the LHC. Im Z/n leads to loss of Landau damping. Controlled longitudinal emittance blow up during the ramp is performed to make the beams stable. For stability, the longitudinal shunt impedance is limited to a total of 60 kohm at 0.45 TeV determined for the anticipated 200 MHz RF system at nominal intensity. At 7 TeV the shunt impedance is limited to 300 kohm determined for the 400 MHz RF system assuming a longitudinal emittance of 2.5 evs. The exact impedance budget is strongly dependent on the frequency of the HOMs. Transverse feedback with 20 MHz bandwidth is also available with a damping time down to less than 1.8 ms achieved already in the 2010 run. The damping specification at 0.45 TeV is 3.6 ms which corresponds to 40 turns. At 7 TeV the damping specification is expected to be about 60 ms. The transverse impedance budget for crab cavities is limited to 0.8 MOhm/m/#cc at 800 MHz for ultimate intensities using a pessimistic scenario. Rama Calaga remarked that one might need updated impedance curve for 400 (or 800) MHz crab cavities that can be used by the cavity designers. Elena Shaposhnikova replied that there were two curves: the second, lower curve assumed that the beam was captured by a 200-MHz system; if the 200 MHz capture system was not installed the crab-cavity designers could use the upper curve. Summary: The 2010 experience is already folded into some of the studies. A promising first HL- LHC optics solution for a low-beta lattice with local crab cavities is available. Some margin exists in the parameters of the separation dipoles. The impact of leveling on overall performance was shown. The impedance bounds were discussed. The question was raised whether these bounds are realistic. Steve Myers asked how the field quality would be specified. Massimo Giovannozzi responded that the specification would come from simulations. 3.3 CAVITY TECHNOLOGY AND CRYOMODULES Session Convener: Edmond Ciapala Several talks spanning compact cavity designs, cryomodule, tuners and ancillaries were given. The abstracts for the talks had been geared to move forward on realizing the four major contenders from LHC-CC09. The recommendations in 2009 had been to develop compact cavities suitable for both local and global schemes. Complete conceptual designs, followed by a down select to 1 or 2 candidates with full specifications and engineering drawings were requested in This is now foreseen for the end of In the HL-LHC plan, 2 major milestones are defined, 1) compact cavity technology validation and 2) later implementation into a prototype cryomodule by Tuning systems are not yet ready for design specification as they are highly dependent on the cavity topology. A common platform might be impossible. An initial layout of the cryostat has 7

8 started for some of the designs. Baseline for local CC is to accommodate the second beam inside the He tank. At present four compact contenders satisfying the LHC constraints at 400 MHz are available, and a new one has been proposed at the LHC-CC10 workshop. The SLAC design is well advanced and robust with some complicated coupler designs. The JLab-ODU parallel bar cavity with an improved design is much more efficient than the previous version, with fewer higher order modes and a larger separation from the operating mode. The 4-rod design is cell optimized for surface fields, and higher order mode damping concepts are also well advanced with LEP type hook couplers on the top and bottom of the cavities. Kota cavities from KEK were not discussed. A new idea on a ¼-wave cavity was presented by BNL. The possible side effect of residual acceleration needs to be addressed. A slim cavity at 800 MHz also seems as a viable option. Cavity characteristics are well advanced and defined. The non-linearity of the deflecting voltage is below the percent level as has been calculated from a numerical fit at 17mm reference radius for atleast two of the designs. Further optimization is possible when specifications are given. Multipacting and tuning concepts need to be studied in greater detail. Other issues such as fabrication, cleaning, assembly, He vessel etc... also need detailed studies. No fundamental show stopper for embarking on compact cavity technology and hardware prototypes were identified. Excellent progress since the goals set from the last workshop. Fairly advanced engineering designs exist for some of the designs. Therefore, the plan to reach HL-LHC milestone 1 is defined and awaiting cavity prototype validation. Designs of the cryomodules and ancillaries should continue to be finalized after the 1 st milestone is reached. In summary, clear set of specifications are needed for both vertical and horizontal cavities. Engineering complications arising from the need to crab in either plane need to be addressed immediately. Couplers were discussed see later. Steve Myers asked how one would decide which of the proposed designs should be prototyped, or if several designs should all be prototyped. Ed Ciapala commented that most likely three designs would be prototyped by the respective promoters. Frank Zimmermann suggested to also include the QWR of Ilan Ben-Zvi in this list. Ilan Ben-Zvi commented that the QWR cavity had already been tested with beam in the other direction. Important action items were identified: Establish precise impedance budget for beam stability with the current baseline of 8 cavities per beam. Experimental verification of dynamic failures (multipacting and fast quenches). Early construction of a full prototype CC for early test in the SPS? 8

9 Steve Myers observed that the cryomodule was not independent of the cavity. Ed Ciapala commented that a first stage test of all cavities could be performed in the same vertical test stand, and that some work could go on in parallel for the different designs. Steve Myers expressed a concern about the cavity field quality. Stephane Fartoukh highlighted the question of cavity alignment. Ali Al-Nassiri reported that 100 micron cavity alignment had been achieved for light sources; some pertinent work was ongoing in collaboration with JLAB. Jean Delayen pointed out that the crab-cavity design had progressed beyond the point that had been specified, and that, therefore, a more complete set of specifications was required. Steve Myers asked if the frequency had been decided. The answer was yes. The frequency agreed upon for all compact designs is 400 MHz. Graeme Burt clarified that there was no overlap of the LHC crab-cavity design with the CLIC crab cavity. Riccardo De Maria observed that not all the cavities presented could be integrated for both vertical and horizontal crossing. Philippe Lebrun inquired if an extended specification would favor one or the other design. Erk Jensen replied that the cavity designers had not yet taken into account the field linearity requirements, but could do so if required. Ali Al-Nassiri suggested publishing complete specification in order to arrive at a final design. Frank Zimmermann commented that the maximum voltage and the trip rate should be an important part of the specification, though the trip rate might not be predictable in the design process. Tor Raubenheimer suggested that the engineering demonstration would be very important. Steve Myers reiterated his earlier question if 3 cavities were too many. Graeme Burt answered that two of these designs would move forward anyway. Steve Myers added that Ilan Ben-Zvi would build one, too. CERN could construct the fourth one. Ed Ciapala stressed that there was significant synergy with other projects. 3.4 CRAB CAVITY BEAM STUDIES AND INSTRUMENTATION Session Convener: Paul Collier Results from the KEK noise experiments are very interesting. Little or no problem was observed when the noise spectrum remained outside the beam spectrum. Small amounts of noise close to the betatron tunes could be catastrophic, on the other hand. An instability driven by beambeam in the presence of noise excitation at frequencies between the sigma and pi mode was observed. Approximately deg of phase noise control seemed ok from preliminary simulations for the LHC using a rigid model. The LHC hump is 10 times smaller than this number. Concerning the installation of a KEK cavity in the SPS, the available space in LSS4 is sufficient but integration appears complicated. In addition the frequency is non-ideal and therefore forcing a special configuration in the SPS for beam test. The full installation including the refurbishment 9

10 and transport to CERN is not cheap. In 2010, two beam experiments had been performed in the SPS in coast to prepare for a possible experiment with a KEK cavity. The main observation was a significant emittance growth at least in the horizontal plane without any external excitation. To study the effects of crab cavities on emittance growth in the SPS, the natural emittance growth needs more investigation and mitigation measures. Else it may require a vertical crab cavity for an SPS test and potentially higher voltages to kick at higher energy coasts. Synchronization between main RF and KEK cavity frequency may be difficult and may introduce noise which will be difficult to separate from the actual crab cavity phase noise. Measuring all relevant quantities will be challenging with KEK cavities and will need a lot of resources. Validity tests of the LHC prototype compact crab-cavities in the SPS should be performed at the latest by the end of 2016 before the final decision on the construction for the HL-LHC crab cavities is taken. An SPS test cannot access beam-beam driven noise instabilities. Therefore, an LHC test before a full installation in the LHC is obligatory. In addition the LHC has better instrumentation, for example, a head-tail monitor with 4 GHz bandwidth (versus 2.5 GHz in the SPS). It is recommended to test crab cavities that are as close as possible to final design as the schedule towards final implementation is tight. Therefore, the idea of testing a KEKB crab cavity at the SPS has been abandoned and it was suggested to focus all efforts on a real LHC cavity. A decision on an elliptical or compact cavity for the SPS tests should be taken soon. Measures to address the SPS emittance growth problem should be pursued in the coming years, in order to enhance the usefulness of future crab-cavity tests. Steve Myers commented on specifications with respect to the possible impact of the crab-cavity prototype on the LHC. In the case of the SPS test, the proposed bypass should solve this problem. Ilan Ben-Zvi quoted the example of the 56 MHz frequency cavity for RHIC, where an FOM damper makes it look like the cavity is not there (Q value reduced down to 300). In the meantime one will need to spend time understanding SPS without crab cavities Elena Shaposhnikova commented that at the time of ppbar collisions there had been a single bunch operation, while the SPS crab-cavity tests were performed with bunch trains. Bunches of smaller intensity and higher energy should be tried. Specifications on crab-cavity field quality will be provided by Massimo Giovannozzi. Rama Calaga suggested that the solution proposed by Philippe Baudrenghien for injection & ramp should be tested in the SPS Wolfgang Hofle commented that regarding LLRF, feedback, etc., many items could be tested in the SPS. An SPS test would reduce the risk of a long commissioning in the LHC. 10

11 3.5 MACHINE PROTECTION Session Convener: Jörg Wenninger Machine protection and criticality are strongly correlated to the luminosity gain. Up to 4 sigma kicks with 15 cm beta* can be expected due to a crab-cavity failure with the proposed optics. Simulations have been only very preliminary, and more detailed studies are needed. Realistic models of the beam distribution and of the RF failure dynamics are essential for correctly modeling and understanding the impact on machine protection. Tail distributions could/should be adjusted to what had been measured in 2010 (1-3% of the beam in last beam sigma). In addition, the upgrade collimation with cryo-collimators should be included in the simulations for a complete understanding. MP considerations also need as input the maximum acceptable losses on specific collimators (latest) for a worst case scenario with beam loss between 1-10 turns. No show stoppers from the collimation simulations using halo tracking have been found for the nominal LHC, but the same analysis still has to be done with the SLHC optics. As a reference, all electrical circuits in the LHC had been analyzed for criticality to adjust interlocking. It is crucial to understand the quench and multipacting processes that were highlighted as possible failure modes of interest. KEK has observed a wide range of failures. Are these failures specific to each cavity design? One should agree on reference failure scenarios for simulations of realistic failures. Input from RF experts for this topic is highly desired. A decision on using multiple cavities to help alleviate MP should be taken. The final frequency choice is also important in order to focus the simulations. There still exists an 800 MHz option. The collimation system needs further optimization for the best possible protection and performance in conjunction with crabbed beams. Possible issues along the LHC cycle of operation need to be studied. Operation with a prototype crab-cavity in the SPS will be extremely interesting for MP studies. First simulations indicate that a test in the LHC with the nominal optics cannot lead to catastrophic failures, which was an interesting outcome. Several possible paths have been proposed for optimizing machine protection with crab cavities. Critical is the operation with crab cavities all along the cycle. Installing proto-type cavity in the SPS for beam tests should be given adequate priority Oliver Brüning suggested that when optimizing the collimation system the absolute collimator gaps in number of mm should be maintained. Eric Prebys did not see how testing a crab cavity different from the one ultimately installed in the LHC, such as the KEKB crab cavity, could provide any information. Jean-Pierre Koutchouk recommended establishing a calendar of failure scenarios. 11

12 3.6 RF CONTROLS & TESTING Session convener: Jean Delayen The scheme developed at KEKB to determine the cavity center is innovative and will also be useful for cavity alignment in the LHC. Several discussions on the instabilities at KEK took place. For example a large betatron amplitude oscillation at high currents with colliding beam was observed.. The instability was only seen only in the presence of beam-beam collisions, and depended on the crabbing phase and was concluded to be beam loading related effect. It was remedied with static shift of the crabbing phase. Cavity trips were reduced to the order of 1/day. It appeared that the trips were caused predominantly by the couplers and not by the cavities proper. A weak correlation of the trip rate with the beam current, but a strong correlation with the cavity voltage has been observed. Preparation and assembly are well known. Facilities suitable for crab-cavity power tests are available at a number of places. In particular, high-power tests are possible and the required facilities are available at CERN. A cryogenic upgrade of SM18 is necessary for both SPL R&D and crab cavities. 400 MHz at 4.5K is a logical combination, but 4.5 K would be difficult at 800 MHz. However, 2K is difficult in the LHC tunnel at 30 mbar. A temperature decision needs to be finalized soon. The design of the cavity, couplers and ancillaries should include different cooling circuits at different operating temperatures. An operational scenario for making the crab cavities invisible during injection and ramp should be defined and tested. No show stoppers were seen, but several issues related to failure scenarios need more detailed investigation. Loaded Q specifications must be determined taking into account technology and machine protection constraints. A complete set of specifications and requirements are needed as soon as possible including crossing planes, field quality, impedance and other relevant information. A real LHC cavity is preferable for the tests. Vertical cryostats are also available at other places around the world. For the LLRF operational scenario a number of open questions need to be addressed. Philippe Lebrun commented on the SPS test, highlighting space constraints and cryogenics limitations. 3.7 SCHEDULE & FUTURE OUTLOOK Session Convener: Roland Garoby The summary of this session started with a review of the HL-LHC project, which is aiming at improving the LHC luminosity beyond the nominal reach. The high-luminosity LHC design study is only a small part of the HL-LHC project. The total cost of project and study are estimated at 500 ME and 27 ME respectively. An overall cost estimate of 80 MCHF for the crab project and an associated 150 FTE man years was given. Other applications and projects requiring crab cavities were also discussed. For example, crab-cavity can removing the effect of the transient beam loading at the collision point, and momentum cleaning were two LHC 12

13 applications being highlighted. Several electron ion colliders anticipate the use of crabs with required crab voltages between 2 MV and 200 MV. To address the field quality for the RF cavity a specification is needed from optics and beam simulations. Development and implementation of LHC crab cavities needs a consistent global planning. The precise purpose of the SPS tests should be outlined. Requirements such as deflecting field, direction of kick, dimensions, acceptable non-linearities, impedance, and constraints from machine protection need all be specified The question was raised whether one could run for physics if one cavity was not operational. 4 ADVISORY BOARD MEETINGS AND COMMENTS 4.1 PARTICIPANTS T. Baer, I. Ben Zvi, C. Bhat, O. Bruning, G. Burt, J. Byrd, R. Calaga, E. Ciapala, P. Collier, J. Delayen, R. De Maria, S. De Silva, T. Ekelof, S. Fartoukh, Y. Funakoshi, L. Ficcadenti, R. Garoby, M. Giovannozzi, W. Hofle, K. Hosoyama, E. Jensen, H. Kim, J.-P. Koutchouk, P. Lebrun, Z. Li, E. Metral, S. Myers (Chair), K. Nakanishi, A. Nassiri, V. Parma, E. Prebys, T. Raubenheimer, E. Shaposhnikova, B. Strauss, J. Tuckmantel, W. Weingarten, J. Wenninger, A. Yamamoto, F. Zimmermann 4.2 MEETING COMMENTS Steve Myers summarized the main conclusions of the LHC-CC 10 workshop and the subsequent advisory board session. 1. For the KEK-B crab-cavity test, the answer was no. Such test was discarded for a number of reasons. 2. The final LHC crab-cavity scheme should be the local scheme only. 3. The crab-cavity frequency was set to be 400 MHz. 4. Whether machine protection could be a possible showstopper was still not absolutely clear. No hard obstacle had been found in preliminary studies, but a lot more work was required on this topic. 5. There was almost no synergy with CLIC crab-cavity technology, the sole exception being the LLRF. 6. No show stoppers had been encountered for compact crab cavities. At least 3-4 mature designs were available now. Stephane Fartoukh asked for the consequences if no local solution would be found. Steve Myers answered that the choice of local scheme could be reconsidered later if the development of compact crab cavities or the associated optics were unsuccessful, and/or if strong new arguments would favour a global or hybrid scheme. The original workshop charge had been threefold: 13

14 1. Can compact cavities for the LHC be realized and made robust with the complex damping schemes? 2. Are crab cavities compatible with LHC machine protection, or can they be made to be so? 3. Should a KEKB crab cavity be installed in the SPS for test purposes? The answers for points 1 and 3 had been given. There had been progress on point 2, but this was still an open issue. Complete specifications for further optimization were stressed to be of uttermost importance to the crab-cavity designers. 4.3 OPEN ISSUES AND ACTION ITEMS Steve Myers and the other advisory board members plus meeting participants compiled the following points for further follow up. (1) Specifications One wants to be able to make cavity invisible for many reasons (ramping, testing, when not operational) this possibility was a sine qua non. Specifications could come from Massimo Giovannozzi and his team; the crab cavities should be transparent with regard to optics and beam operation. Impedance and field quality had to be specified. Could one damp the fundamental mode? The need for a dynamic alignment of the crab cavities in-situ has be first clarified and appropriate specification should be derived. Is this dynamic alignment easily feasible at cold temperature and to what level? The trip rate should be part of the specification, as a function of voltage, and its effect on impedance on the beam has to be studied. Horizontal and vertical crossing might need two types of cavities. A single system is preferable for ease of technical maintenance among other reasons but not a requirement if specific cavities if different cavities are more efficient for different planes while maintaining the same horizontal space constraint of 192 mm separation. (2) Testing, SPS and/or LHC What do we need to test? Machine protection, making the cavities invisible, etc The final collimation system, different from the present one, may have an impact on the cavity qualification. Concerning a test with an LHC prototype cavity in the SPS, what could be learnt from an SPS test? 14

15 (3) Prototypes Prototyping of compact crab cavities should continue Also power couplers should be tested. Each cavity needed a special power coupler, but a common platform for power coupler development is highly desirable to focus the R&D. Cryomodule time line should be defined. Paul Collier suggested using a generic cryomodule for the tests of all cavities. Erk Jensen confirmed that it would indeed be possible to perform the first test in identical cryostat for all cavities. Ed Ciapala, Erk Jensen and Wolfgang Weingarten reported that a suitable cryostat already existed at CERN. The development of the cryomodules for the final LHC compact crab cavities was included in the HL-LHC design study, which was considered to be sufficient. (4) Other issues Steve Myers stressed that other potential applications of crab cavities should be looked at, such as off-momentum cleaning, compensation of beam loading, emittance exchange, beam instrumentation and use towards future projects such as LHeC and HE- LHC. (5) Roadmap The CERN-RF group will sketch a roadmap with milestones and resources to be presented to LMC before the end of summer Erk Jensen reported that together with Ed Ciapala and Rama Calaga he had described the overall crab-cavity project. He proposed to distribute this draft document to the community. Concerning the machine-protection issues, Ilan Ben-Zvi suggested developing a dynamic model of cavity breakdown and the exact time dependence, based on the physics of cavities. Jean- Pierre Koutchouk added that this model should then be combined with a beam tracking model. Oliver Bruning remarked that the most pessimistic case simulation looked challenging from the machine-protection point of view. However, it is unclear if this is a realistic scenario. Frank Zimmermann commented that an early crab-cavity quench detection system was needed, in analogy to the quench protection system of the LHC SC magnets. Ilan Ben-Zvi reported that RHIC will install a 56 MHz SRF cavity which heavily damps the operating mode when not required. One response to a detected trip onset would be to turn off two cavities simultaneously. Philippe Baudrenghien observed that segmenting into several independent systems was another mitigation measure. 15

16 Protection might need to be balanced against reliability. However, more modules (of lower voltage each) could also lead to higher reliability. Eric Prebys inquired about the need of both horizontal and vertical crab cavities. The response was that the crab cavities would assist in leveling, and that the standard LHC had alternating planes of crossing. The proposed crab-cavity system should therefore work in both planes. Tor Raubenheimer commented that other activities besides the RF designers needed a list of final specifications to focus the development. Paul Collier asked if 1.9 K was needed to reach the target field, or if the cavities could be operated at 4.5 K as well, for the SPS test or LHC IP4 test. This was not excluded. One would need to know how the cavities behaved at 4.5 K. Philippe Lebrun commented that when operating the cavities in boiling gas one needed to get vapor out. Superfluid helium at 1.9 K did not only provide a higher field, but also cooling without bubbles. Rama Calaga suggested that higher temperature could be possible, at least for the tests. Jean Delayen pointed out that spoke cavities were running at higher temperature. Wolfgang Weingarten estimated that the power dissipation at 4.5 K would be W per cavity. The exact dynamic load should be calculated with appropriate cavity designs and heat loads, the KEK-B requires about 100 W of dynamic heat load. Paul Collier observed that this might possibly exclude the SPS tests. Responding to a question by Stephane Fartoukh, it was clarified that beam-beam simulations by Kazuhito Ohmi (KEK) had demonstrated that a perfect cavity was very good for the LHC (increasing the beam-beam lifetime by a factor of 10). 16