Alrutz-Ziemssen, K

Flasche, D

Gräf, H D

Huck, V

Knirsch, M

Lotz, W

Richter, A

Rietdorf, T

Schardt, P

Spamer, E

Stascheck, A

Voigt, W

Weise, H

Ziegler, W

CERN Document Server

Particle Accelerators, 1990, Vol. 29, pp. 53-58Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaLATEST RESULTS FROM THE S-BAND SUPERCONDUCTING ACCELERATORAT DARMSTADT-)KLAUS ALRUTZ-ZIEMSSEN. DIETER FLASCHE. HANS-DIETER GRAF.VOLKER HUCK. MARTIN KNIRSCH, WILFRIED LOTZ. ACHIM RICHTER,THOMAS RIETDORF, PETER SCHARDT, ERNST SPAMER.ANDREAS STASCHECK, WOLFGANG VOIGT, HANS WEISE ANDWOLFGANG ZIEGLERInstitut fur Kernphysik, Technische Hochschule Darmstadt.Schlossgartenstrasse 9. 0-6100 Darmstadt. GermanyAbstract In the course of the last two years the 130 MeV superconductingelectron accelerator in its different states of construction has produced some1500 hours of beamtime. Atomic and nuclear physics experiments providedvery valuable information about the performance and reliability. Due to animproved field gradient of more than 6 MV/m in one accelerating cavity theinjector energy was increased almost up to the design value of 10 MeV andan upper limit for the beam emittance with less than 0.2 7r -rnrn -rnrad couldbe achieved. Investigations for improving the cavity inspection facilities andextensive calculations that predict cavity field profiles and power distribu-tions were made.1. INTRODUCTIONThe superconducting electron accelerator presently under construction at the nuclearphysics institute of the Technische Hochschule Darmstadt is designed to produce acw beam for inelastic electron scattering coincidence experiments. This is reflectedin its main parameters listed in Tab. I.TABLE I Design parameters of the acceleratorBeam energy/MeVEnergy spread /keVcw current /tJ-AOperating frequency / MHzNumber of structuresCapture section1.00 m long0.25 m long10 - 130± 13~ 202997101.) Work supported by the Bundesministerium fiir Forschung und Technologie ofthe Federal Republic of Germany ( contract number 06 DA 184I )[755]/5354/[756] K. ALRUTZ-ZIEMSSEN ET AL.The superconducting 10 MeV injector produced its first beam in August 87 [ 1 ] andstatus reports [2,3] have been given following the step by step installation of themain linac. Therefore only a brief update of the present status is presented in Sect. II.Main emphasis is laid on the experience with the superconducting cavities andassociated equipment like rf couplers and tuners ( Sect. III ) which we have gainedduring some 550 hours of machine time for accelerator tests and from 950 hours ofbeamtime for atomic and nuclear physics experiments ( Sect. IV). Finally wegive an outlook on developments presently still in progress. The use of the acceleratoras a driver for a free electron laser (FEL) has been discussed elsewhere [4-6}II. STATUS OF INSTALLATIONThe photograph of Fig. 1 gives an impression of the present accelerator installation.The electron gun at a potential of - 250 kV ( right upper part of the figure ) producesa de beam which is chopped and prebunched in the room temperature part of theFIGURE 1 View of accelerator from the extraction sideinjection ( left of the gun ). The beam then enters the superconducting injector whichconsists of a 0.25 m long 5-cell capture section and two standard 1 m long 20- cellaccelerating structures. The 10 MeV beam from the injector linac is then bent to theleft by an isochronous 1800 beam transport system and injected into the main linac( center position) where it gains another 40 MeV. The linac consists of four cryogenicS-BAND ACCELERATOR AT DARMSTADT [757]155modules with two 20 - cell structures each. Three of them are presently installed. Twobeam transport systems each consisting of one straight section and two 1800 bendsallow for recirculation and therefore reinjection of the beam into the main linac. Aftergaining three times 40 MeV the 130 MeV beam can be finally extracted into the experi-mental area. Behind the injector two experiments, one for the investigation of channel-ing radiation and one for nuclear resonance fluorescence studies have been set up.III. SUPERCONDUCTING ACCELERATING STRUCTURESThe gradients of the cavities summarized in Tab. II have been obtained in two waysby rf measurements following the techniques described in [7] and by measureingthe energy gain of the electron beam with a deflection magnet.TABLE II Fields of superconducting cavitiestt Type Location RRR Eacc ICMV /rri)Cexp ) Crf)I 5 cell Injector 100 5.5 5.52 20 cell Injector 30 2.0 2.13 20 cell Injector 100 6.3 7.54 20 cell Linac 30 2.25 20 cell Linac 30 0.9 0.96 20 cell Linac 30 4.5 4.97 20 cell Linac 30 2.0 2.78 20 cell Linac 30 1.49 20 cell Linac 100 3.4FIGURE 2 Defect near the equatorweld inside the 9 th cellof a 20 - cell cavity56/[758] K. ALRUTZ-ZIEMSSEN ET AL.On a first glance the gradients look rather disappointing compared with modern stan-dards but one has to recall the fact that the cavities have been fabricated almost 6years ago, most of them from reactor grade ( RRR ~ 30 ) material. The average gradientof these cavities amounts to 2.2 MV1m and is much lower than the correspondingfigure for the three samples made from RRR =100 niobium which amounts to 5.1 MV1m.The third cavity in the injector is the first one of the 20 - cell structures thatexceeds the design figure of 5 MeV1m. This has been achieved after an intermediatechemical polishing ( ~ 10 urn ) of the cavity at the University of Wuppertal.The reason for an extremely poor performance of a cavity is very probably a "de-fect" like the one shown in Fig. 2. This "bad spot" is located inside the 9 th cell of a20 - cell structure which has been limited to 1 MV1m. "Defects" of similar appearancehave been found occasionally in reactor grade niobium. In the meantime we haveordered six cavities manufactured from high purity ( RRR ~ 280 ) material to replacethe cavities with the lowest gradients presently installed.IV ACCELERATOR OPERATIONIn the course of the last two years the accelerator in its different states of constructionhas produced some 1500 hours of beamtime for accelerator testing and developmentbut mainly for the two experiments set up behind the injector linac ( Tab. III).TABLE III Beam for experimentsExperiment Energy I MeV Current I tlA TimelhAccelerator Test and 1.0 - 9.1 0.1 - 40 550DevelopmentNuclear Resonance 2.5 - 8.5 20 - 40 700FluorescenceChanneling Radiation 3.0 - 7.7 0.001 - 30 250It has to be noted that the time when the beam. was used for atomic and nuclearphysics experiments provided very valuable information about the performance andreliability of the accelerator. The nuclear resonance fluorescence experiment requiredrather high beam currents because of the small cross sections involved. Currents of20 - 40 tlA could be obtained without major difficulties. The beam spot size at the exitwindow ( about 5 m downstream the bearnline ) was only 4 - 5 mm in diameterand remained that way even for running periods of several days. The channelingradiation experiment [8 ] requires excellent beam quality because of the smallacceptance angle of the crystals used as targets. The beam always met theS-BAND ACCELERATOR AT DARMSTADT [759]/57requirements without any collimation or focusing between the accelerator andthe target. By increasing the beam current from several nA up to 30 tlA nomeasurable degradiation of the beam quality could be observed.During the most recent test period this summer again a leak in the main linacdid not allow a phase locked operation of its accelerating structures. Nevertheless someimportant results could be obtained: i ) Due to an improved gradient of the second20 - cell structure the energy of the injector increased to 9.1 MeV. ii ) A 6.8 MeV beamfrom the injector was injected into the main linac and transmitted through all of itssix cavities. Without further acceleration and using three viewscreens, one in frontof the main Iinac. one at its exit ( 12.2 m downstream) and one in front of the dipolemagnet of the extraction ( another 3.3 m downstream ), to determine the beamsizeenabled us to calculate an upper limit of the beam emittance. In horizontaldirection we achieved an emittance of Ex = O.ll± 0.03 n-rnrn-rnrad. in thevertical direction beam spot diameters give a maximum emittance of Ey =0.21 ± 0.03n .mm ° mrad This number contains some contribution from a small vertical ac deflec-tion of the beam, the origin of which could not be localized yet. iii) The beam wasalso sent through the first 1800 bent of the first recirculation and transported backalong the straight section of this beamline. By proper adjustment of the quadrupolesthe beam diameter could be kept to within 3 rom at all viewscreens.V. OUTLOOKBesides the development of new mechanical coarse tuners and magnetostrictive de-vices for fine tuning we are presently trying to improve the infrastructure for cavityinspection, treatment and repair. An optical inspection facility has been built.Figure 2 was obtained with this device. When the optical parts of the device are re-placed by a string with a bead aligned to the axis of the cavity precise measure-ments of field profiles can be obtained Reliable measurements of the n mode dohowever still present a problem because of its overlap with the 19n/20 mode.Using the full 20 - cell geometry as input to the URMEL code we have performedextensive calculations to obtain the field profiles and power distributions in the 20modes of the TMo l O passband and to study the effect of any single detuned cell ina 20 - cell accelerating structure. The results enable us to calculate field profiles andpower distributions for any combination of detuned cells. First calculations with twodetuned cells showed good agreement with measurements. The final goal of thesestudies is to invert the calculations in order to be able to determine the detuning ofindividual cells from measured field profiles.With the construction of the accelerator we will proceed as follows: The mainlinac will be disconnected cryogenically fro the injector because some repair work has58/[760] K. ALRUTZ-ZIEMSSEN ET AL.to be performed on each of the three modules presently installed. The injector willbe cooled down in order to deliver beam to the experiments while the modules ofthe main linac are worked on and while the fourth module will be equiped withtwo further structures. During the following test period with all cavities we hope notonly to recirculate the beam for the first time but also to extract it to the spectro-meter area for electron scattering experiments.ACKNOWLEDGMENTThe accelerator is the result of a very fruitful collaboration with H. Piel and his groupfrom the physics department of the University of WuppertaL The still continuing helpof H. Heinrichs and J. Pouryamont is gratefully acknowledged. We feel very obligedto H. Lengler for his continuous support by fruitful discussions and help with equip-ment. Stimulating discussions with J.M. Cavedon, 1. Delong, P. Leconte, A. Matheisen,D. Proch, A. Schwettman, 1. Secutovicz, K. Shepard and W. Weingarten have beenvery helpful in the course of the project. We feel indebted to B. Aune and A.Mosnierfor their generous help with ideas and equipment. Without T. Weiland and his groupthe very extensive use of their codes on our computers would have been impossible.We are very grateful for the tremendous help provided by the technical staff at theS - DALINAC and the mechanical and electronic workshops of the institutions ofWuppertal and Darmstadt.REFERENCES1. V. Aab, K. A1rutz-Ziemssen, R. Amend, D. Flasche, H. - D. Graf, V. Huck,K. D. Hummel, M. Knirsch, F. Lindqvist, W. Lotz, A. Richter, T. Rietdorf,U. Schaaf, S. Simrock, E. Spamer, A. Stiller, O. Titze, H. Weise, W. Ziegler,H. Heinrichs, H. Piel, J. Pouryamont,Proc. Third Worksh. on RF Superconductivity , ANL - PHY - 88 - 1,Argonne, III., USA ( 1988 ) 127.2. V. Aab et al.,Proc. 1988 Europ. Part. Ace. Conf., June 7 - 11. 1988, Rome, Italy3. H. - D. Graf, A. Richter, Proc. 1988 Lin. Ace. Conf., October 3 - 7, 1988,Williamsburg, VA, USA ( to be published ).4. V. Aab, K. Alrutz-Ziemssen, H.-D. Graf, A. Richter, A. Gaupp,Nucl. Instr. Methods A2T2 (1988) 53.5. V. Aab, K. Alrutz-Ziemssen, H. Genz, H.-D. Graf, A. Richter, H. Weise,A. Gaupp, Proc. lOth Int. Free - Electron - Laser Conf. 1988, Jerusalem, Israel.6. V. Aab, K. Alrutz-Ziemssen, H. Genz, H.-D. Graf, A. Richter, H. Weise,A. Gaupp, Proc. Int. Congr. on Opt. Sci. and Eng., April 24 - 28, 1989,Paris, France (to be published).7. H. Piel, Proc. First Workshop on RF Supercorductivity, KfK 3019, Karlsruhe,Fed. Rep. of Germany (1980) 85.8. W. Lotz, H. Genz, A. Richter, W. Knilpfer, J.P.F. Sellshop, [ourn. Phys., C9,(1987) 95.

Latest Results From The S Band Superconducting Accelerator At darmstadt

Latest Results From The S Band Superconducting Accelerator At darmstadt

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