First results of a superconducting undulator on the ACO storage ring

Transcription

1 First results of a superconducting undulator on the ACO storage ring C. Bazin, M. Billardon, D. Deacon, Y. Farge, J.M. Ortega, J. Pérot, Y. Petroff, M. Velghe To cite this version: C. Bazin, M. Billardon, D. Deacon, Y. Farge, J.M. Ortega, et al.. First results of a superconducting undulator on the ACO storage ring. Journal de Physique Lettres, 1980, 41 (23), pp < /jphyslet: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1980 HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 LETTRES Un A Tome 41 No 23 ler DÉCEMBRE 1980 LE JOURNAL DE PHYSIQUE LETTRES J. Physique 41 (1980) L547 L550 ler DÉCEMBRE 1980 L547 Classification Physics Abstracts First results of a superconducting undulator on the ACO storage ring (*) (**) C. Bazin (1), M. Billardon (2), D. Deacon (3) (~), Y. Farge, J. M. Ortega (2), J. Pérot (4), Y. Petroff and M. Velghe (5) LURE, Bât. 209 C, Université de ParisSud, Orsay, France. (1) Laboratoire de l Accélérateur Linéaire, Bât. 200, Université de ParisSud, Orsay, France. (2) Ecole Supérieure de Physique et Chimie, 10, rue Vauquelin, Paris Cedex 05, France. (3) Département de PhysicoChimie, Service de Photóphysique, CEN Saclay, Gif sur Yvette, France. (4) DPhPE, CEN Saclay, Gif sur Yvette, France. (5) Laboratoire de Photophysique Moléculaire, Bât. 210, Université de ParisSud, Orsay, France. (Reçu le 14 aofit 1980, accepte le 30 septembre 1980) 2014 Résumé. onduleur supraconducteur a été monté sur l anneau de stockage ACO. Le faisceau d électrons reste stable et n est pas perturbé par le champ magnétique de l onduleur. L émission lumineuse dans le visible et l ultraviolet a été observée pour des énergies de 140 et 240 MeV. Les premiers résultats montrent que sa distribution angulaire et sa répartition spectrale sont en accord avec les prévisions théoriques. Les légères différences observées proviennent certainement du fait que les électrons ne suivent pas exactement l axe de l onduleur Abstract. superconducting undulator has been fixed on the ACO storage ring. It has been observed that the electron beam is stable in the small gap of the vacuum chamber and unperturbed by the magnetic field of the undulator. Light emission has been observed at 140 and 240 MeV in the visible and ultraviolet. First results indicate that its geometrical as well as spectral distribution agree with theoretical predictions; small disagreements very probably arise from the fact that the electrons are not travelling exactly on the axis of the undulator. 1. Introduction. Since Ginzburg [1] proposed that relativistic electrons could emit intense, tunable and monochromatic radiation in a periodic electromagnetic structure, many theoretical works have been published on emission by an undulator (a helical or transverse periodic magnetic field) [2]. The main features of this emission are the following : i) small angular beam divergence (of the order of 1/7), ii) at a given angle of observation 0 (to the Z axis), emission at the fundamental wavelength and harmonics, the harmonic content increases with the magnetic field amplitude; the fundamental wavelength is given by (1) in the case of a planar undulator for electrons travelling parallel to its axis : where y E/mo c2, E is the electron energy, mo C2 its rest mass energy (0.511 MeV), Ao the period of the undulator, K 2 ebo Ào ~o Bo ~o. 2 nmo c Y o of o Bo maxi mum magnetic field on the trajectory, e and mo charge and mass of the electron, c speed of the light (Bo is expressed in tesla and Ao in mm), 0 angle of observation to the Z axis,. P maximum angle between the velocity of the particle and the Z axis. iii) One electron is emitting waves having the same phase (locked by the undulator itself) and the total intensity is expected to increase as N 2, the number of undulations, if the electron beam aperture is smaller than (1 IN) 1 at 0 0. Because of this interference effect, the spectral width of the peak emitted at 0 0 is expected to be near I/N, and to decrease with increasing 0. Such predictions have been roughly verified with electron beams accelerated by linear accelerators [3, 4] or by synchrotrons [5]. However the number Article published online by EDP Sciences and available at

3 Transverse A After 3.1 L548 JOURNAL DE PHYSIQUE LETTRES of electrons stored in bunches by such machines is very small compared with what can be achieved with electron storage rings. With such machines, undulators should constitute extremely powerful monochromatic and tunable sources of radiation which could reach the Xray range by raising the electron energy [6]. However, stability constraints are stronger on storage rings and we have decided to make an undulator to test it on the ACO storage ring at Orsay. This paper presents the first results of this system. While performing this first experiment we have been informed that a similar one has been done on VEPP 3 in Novossibirsk [7]. 2. The undulator. description of this superconducting undulator has been given in a previous paper [8]. It has been designed to be fixed on the straight section of the ACO ring, to have the shortest possible period to reach short wavelengths, and to give a maximum value of K equal to 1.5. Several requirements have to be considered, which are related to the parameters of the storage ring as well as the undulator. During "the injection of the ring, when pulsed magnets induce a transverse displacement of the e beam by several centimeters, the vacuum chamber must have a horizontal dimension of 12 cm. After injection, the beam is very stable and the vacuum chamber can have a gap of 12 mm. To obtain the largest field and the shortest period, it is necessary not only to reduce the magnetic gap of the undulator as much as possible, but also to reduce the space taken by the coils producing the magnetic field in the iron poles. Fig. 1. view of the undulator. For these reasons, a periodic superconducting magnet has been developed (Fig. 1) with a vacuum chamber having an inverse T shape. During injection, the large part of the beam chamber is used. After injection, the undulator is lowered to place the e beam in the middle of the pole pieces. The main characteristics of this undulator are the following : Period Number of periods 23 Effective length 40 mm 0.96 mm Maximum field Bo 0.45 T (K 1.68). Such a design is very similar to the VEPP 3 undulator [9]. A special vacuum chamber has been fixed on the ingoing bending magnet of the ring after the undulator, with an Alz03 window on the axis of the undulator to extract the emitted light. 3. Experimental results. ELECTRON BEAM STABILITY. injection at 240 MeV, the undulator has been partly lowered to perform a rough alignment using normal synchrotron radiation emitted by the outgoing and ingoing bending magnets on each side of the undulator axis. Then, when lowering totally the undulator, we have observed that it was possible to store a 100 ma current at 240 MeV. However, it happened several times that a large fraction of the electrons was lost during this operation, corresponding clearly to an excitation of the beam. When the beam was in the gap, we were able to lower the energy down to 140 MeV, but with a large loss of electrons, only a few ma were remaining in the ring. In both experiments, after alignment of the undulator to obtain longer lifetimes, we have observed that it was reduced by about a factor three when the undulator was lowered. Further work is necessary to study these effects in more detail and to improve the stability of the beam. The magnetic field had no effect on the electron stability. After compensation by the correcting coils of the first and last halfpole of the undulator, we were able to keep the beam at the same position in the ring using, for these measurements, TV cameras on bending magnets as well as light emission by the undulator LIGHT EMISSION BY THE UNDULATOR. Figure 2 represents a colour picture on a white screen of the light emitted by the undulator with E 150 MeV, a few ma in the ring and K 1.20 (Bo G). The lower and upper parts of the emission are limited by the vacuum chamber. One can see clearly the expected blue emission (À A) at the centre and coloured rings which correspond well to formula (1). Figure 3 is a similar coloured picture of the emission for E 240 MeV; two black regions can be observed symmetric to the horizontal plane of the magnetic field (in the direction of oscillation of electrons). They correspond to + y0 1 ; in a pseudorest frame travelling in the undulator with the mean longitudinal electron speed, these directions are the transverse vertical directions where no

4 Color Color Spectral Angular It SUPERCONDUCTING UNDULATOR ON THE ACO STORAGE RING L549 Fig. 2. picture on a white screen of the light emitted by the undulator (E 150 MeV, K 1.2). Fig. 4. distribution of the emitted light (E 150 MeV, K 1.2) through a very small pinhole in the horizontal plane. Experiment : full lines; Theory : dashed lines. Fig. 3. and K picture similar to figure 2 with E 240 MeV Fig. 5. distribution of light emitted in the horizontal plane, at A with E 150 MeV and K 1.5. emission is expected, since electrons are oscillating along this direction. The spectral distribution of the emission at a given angle 0 has been measured with an angular resolution of A6 0.2 mrad. Figure 4 represents such a distribution with E 150 MeV and K 1.2 at 0 0. It looks very much like a (sin X/X)2 function, as expected (see dashed line). Positions of the principal maximum and the secondary maxima, as well their amplitudes agree well with theoretical predictions : for example, the wavelength of the principal maximum has been observed at A when it was expected at A. In several measurements, a strong dissymetry of the coloured rings was measured, as can be seen on figure 5. It probably comes from a lateral displacement of the e beam toward the undulator axis : in such a case, the magnetic field is expected to present a very strong horizontal dissymetry where the electrons are travelling. Computer simulations are under way to test this hypothesis quantitatively. If it is verified, it would be very easy to align an undulator optically : perfect alignment occurs when the emitted spectrum corresponds to theory, after small translations and rotations. Acknowledgments. is a pleasure to thank B. Moussalam, C. Depautex, M. Lemonnier, M. Sommer and P. Marin, for their decisive help. References [1] GINZBURG, V. L., Bull. Acad. Sci., USSR, Sov. Phys. 11 (1947) 165. [2] HOFMANN, A., Nucl. Instrum. Methods 152 (1978) 17 and references included. COÏSSON, R., Phys. Rev. A 20 (1979) 524. [3] MOTZ, H., THON, W. and WHITEHORST, R. N., J. Appl. Phys. 24 (1953) 826. [4] EL IAS, L. R., FAIRBANKS, N. M., MADEY, J. M. J., SCHWETT MAN, H. A. and SMITH, T. I., Phys. Rev. Lett. 36 (1976) 717.

5 L550 JOURNAL DE PHYSIQUE LETTRES [5] ALFEROV, D. F., BASHMAKOV, Yu. A., BELOVINTSEV, K. A., BES SONOV, E. G. and CHERENKOV, P. A., Part. Accel. 9 (1979) 223. DIDENKO, A. N., KOZHEVNIKOV, A. V., MEDVEDEV, A. F., NIKITIN, M. M., Pis ma Zh. Tekhn. Fiz. 4 (12) (1978) [6] European Synchrotron Radiation Facility. Suppl. II, The Machine, ed. by D. J. Thomson and M. W. Poole (European Science Foundation, Strasbourg, France) p. 52. [7] ARTAMONOV, A. S., BAROV, L. M., BARYSHEV, V. B., BASH TOVOY, N. S., VINOKUROV, N. A., GLUSKIN, E. S., KOR NIUKHIN, G. A., KOCHUBEI, V. A., KULIPANOV, G. N., MEZENTSEV, N. A., PINDIURIN, V. F., SKRINSKY, A. N. and KHOREV, V. M., to be published. [8] BAZIN, C., FARGE, Y., LEMONNIER, M., PEROT, J. and PETROFF, Y., Nucl. Instrum. Methods 172 (1980) 61. [9] BARKOV, L. M., BARYSHEV, V. B., KULIPANOV, G. N., MET ZENTSEV, N. A., PINOYURIN, V. F., SKRINSKY, A. N. and KHOREV, V. M., Nucl. Instrum. Methods 152 (1978) 23.