Anami, S

Asami, A

Enomoto, A

Honma, H

Nakahara, K

Ogawa, Y

Ohsawa, S

Sato, I

Shitara, T

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Particle Accelerators, 1990, Vol. 27, pp. 125-131Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaSIMULATION OF HIGH INTENSITY ELECTRON BEAMMODULAETED BY DOUBLE SHCI. SATO,K.N AKAHARA,A. ENOMOTO, S. OHSAWA, Y. OGAWAA.ASAMI,H.HONMA,T.SHITARA and S.ANAMINational Laboratory for High Energy PhysicsAbstract A system of double subharmonic cavity (DSHC) wasdesigned to compensate the modulated velocity by SHB. Thefirst SHC in this system modulates the uniform velocity of theelectron beam from the gun and the 2nd SHC compensates themodulated velocity in the electron beam. Intensive electronbeams modulated by DSHC was simulated by a computer. Thesimulation include a space charge effect. It's result was givento compress the dispersed energies in the multi-bunched beam.INTRODUCTIONThe SHB(sub-harmonic buncher) was devised for the purpose of obtainingof a single bunch beam from an electron linac. 1-3) After that, the dou-ble SHB was designed for the purpose of obtaining a higher intensitybeam of the single bunch, it combined with a SHB operated at a lowersub-harmonic frequency. The double SHB is succeeding in an accelerationof a single bunch beam of about 56 nC. It is reported that the energy dis-persion in the intensive beams from the linac with the SHB expanded atboth of a single bunch operation and a multi bunch operation. 4-5) Thisenergy dispersion can decrease by using ECS (energy compression sys-tem). ECS device over the GeV energies becomes with large-scale andcomes expensive. Therefore, we devised a new double SHC (DSHCnamely) for the energy dispersion decreasing in the final electron beams.A simulation for DSHC was done by using a computer. The result givesthis method is effective to the decreasing of the energy dispersion.SHB AND DSHC SYSTEM[371]1125126/[372] I. SATO ET AL.A popular electron linac with a SHB system is composed of an electrongun, subharmonic buncher, a prebuncher section, a buncher section andregular accelerator sections. The formation of a bunch beam in this linac isprepared through the next process. On positive voltage of a short pulsebeing added to a grid of the gun, an electron beam like a cord is emittedfrom the cathode. If the definite negative voltage adds to the electron gun,electrons in the beam is accelerated to the uniform velocity. When theelectron beam passes through the SHB, it's velocity is modulated by theelectric field of the SHB' s cavity. The electron beam is compressed by themodulation during it advancing a drift space. The bunched electron beamis compressed tightly again by both rf field of the prebuncher and thebuncher. The electron beam is whether a single bunch or multi bunches, itis due to whether shorter bunch length or longer bunch length than a wave-length of operating frequency on prebuncher. Figure 1 shows a block dia-gram of an ordinary linac with a single SHB system.The DSHC system is located between the electron gun and the pre-buncher as well as the linac with a single SHB system. The system con-sists of two cavities. The cavities is operated individually by the samesubharmonic frequency or the different. Each electric fields in the firstcavity and the 2nd cavity are excited quite the opposite phase angle for theelectron beam frame. The first cavity modulates an uniform velocity ofelectron beam from an electron gun and the 2nd cavity compensates themodulated velocity in the electron beam. Figure 2 shows a block diagramof a linac with the DSHC system.DISCUSSION AND SIMULATIONOn operation of high intensity beams from a linac with a single SHB, theenergy dispersion in electron beam expands. It shows a similar tendency ineither beams of a single bunch and multi bunches. We thought thatspread energies in the beam at the single bunch operation is caused by theheavy beam loading in the accelerator guide, and that the spread energies atthe multi bunch operation is caused by two factors. The one is due to thevelocity spread in the electron beams modulated by the SHB, the other isdue to the beam loading as described. Velocity modulation in electronbeam by SHB is similar to modulate an accelerating voltage of electrongun. It is popular that a variation of electron gun voltage shift a phase an-gle on an accelerating field frame of electron beam from the end of linac.HIGH INTENSITY ELECTRON BEAM [373]/127The fact can be proved easily by simulation as a beam trace on a linac.We infer as velocity spread in electron beams on a beginning section in alinac may disperse phase angles of electron beams from the final section.Accelerating phase angle dispersion in electron beam from a linac isequal to energy dispersion in the final electron beam. The first SHC per-forms a part of a bunch compression for an electron beam and the 2ndSHC performs a part of a compensation for modulated velocity in the elec-tron beam. Accordingly, the bunch intervals among electron beams be-comes equal by this method.When each bunch in electron beam is at a low charge density re-spectively, the beam trace formation is very simple as given in (1)-(4)(GV+VSHBsin8o)'Y - +1 .0ffioC2~ = (y-asin81)d~dd~ = 21t(_1 __1 )\:) ~ro ~e(1 )(2)(3 )(4 )where GV and VSHB are the gun voltage and the RF voltage in the SHB,eo is a phase angle in the SHB as given by 80= 21tz/'Ao and 10 is a wavelength of the SHB frequency fo as given by Ao=c/fo, moc2 is the electronrest mass energy, a is electric field strength normalized by electron restmass energy as given by a. = Eo'A / moc2 , ql is a phase angle of accelerat-ing field as given by 81 =21t z/'A, A is a wave length of accelerating fre-quency, ~ is the length normalized by the wave length as given ~ =Z/A, Eois the electric field strength in an accelerator guide, and ~w and be giverespectively a phase velocity and an electron velocity normalized by thelight velocity as ~w=vp/c and ~e= Ve/C.Whenever each bunch in electron beam is at a high charge density respec-tively, the beam trace formation complicates because of the space chargerepulsion. The effect of this repulsion is expressed as (5) by use of thedisk model. 6) It's effect adds to (2).128/[374] I. SATO ET AL.where Oi is phase of ith disk, 'Yi is energy of disk, N is number of disks Inis equivalent DC current of beam (I=Nq), a and b are pipe and beam radiirespectivivly, and ~on is successive zeroes of the zero order Bessel func-tion of firs t kind (J 1).The first beam trace was performed in the section from the entranceof single SHB to the exit of buncher' s section. This trace is simulated byuse of the equation (1)-(4). The result is shown in figure 3. The tracerefered to the parameters of injection section in the pos itron generatorlinac in the PF injector, and the parameters show in table 1. The trace inthe figure 3 shows that the intervals of the final phase angle do not uni-form for individual bunches.The next trace was performed for searching the effect of DSHC sys-tem. It's trace was done in the section from the entrance in the system tothe exit of buncher' s section by use of the same parameter. This result isshown in figure 4. The trace in the figure 4 shows that interval of eachphase angle uniform in the multi-bunches.Table 1 Design Parameters of Injection Section in KEKPositron GeneratorGun Voltage 200 KVSHB Freguncy 119 MHzHarmonic numbers 24Prebuncher Buncher sectionCavity Cavitynumbers a. ~ numbers a. ~1 0.036 0.7 1 1.37 0.782 0.038 0.7 2 1.43 0.803 0.040 0.7 3 1.50 0.834 0.042 0.7 4 1.58 0.875 0.044 0.7 5 1.66 0.916 0.046 0.7 6 1.72 0.9457 0.048 0.7 7 1.77 0.9658 0.050 0.7 8 1.82 0.9839 1.86 1.010-18 2.0 1.0Intensive electron's charge compressing in a short pulse beam inducesa strong repulsion force action in it's bunch. This force action prevents thebunch action of SHB. Those actions may balance themselves under theHIGH INTENSITY ELECTRON BEAM [375]/129some specfications in the linac's parameters. There may be a ballancepoint in the drift section between the single SHB and the prebuncher'sentrance. The energy dispersion in this electron beam becomes small at thispoint. If prebuncher's section is set up at the point, the interval of thephase angles will uniform for individual bunches.Accordingly, the beam trace mus t calculate under the cons iderationof the space charge repulsion. The trace was simulated by use of theequations (1)-(5). The simulation were done for the both of single SHBsystem and DSHC system. These results are shown in figure 5 and 6. Thebeam loading effect within this simulationsin was excluded from the tracesbecause of a simple comparision for the space charge effects.ACKNOWLEDGEMENTSWe are appreciated to Dr. M.Oyamada for his effort programing acalculation code with space charge effects.G : Electron GunD : Drift SpaceB : BuncherSHBPRRASub-harmonic BuncherPre-BuncherRegular Acceleratior Guide200KV 40KV@] ISHB 1 Dl G 02 0~_RA _Figure 1. Block diagram of injection section in an electron linac with asingle SHB system.SHe : Sub-hannonic cavity200KV 40KV -30KVo 1SHC IDll SHC102~ 03 0 __RA _Figure 2. Block diagram of injection system in an electron linac withdouble SHC systems.BUNCH SHAPE ENERGY max=0.24MeV1.0 r---.,----,.----rw---,--__--.,Or..---""---.....I.-.--I...-........_...&.._---J-1[/2 0 1tI2 -1[/2 0 1[/2Figure 3(a). Simulated bunch shape and energy disparsion in electronbeam which is modulated by a single sub-harmonic buncher. The beamtrace 1,~as done from the gun to the drift space in a single SHB.130/[376] I. SATO ET AL.BUNCH SHAPE ENERGY max=O.292MeVi .,/l,......! i H M1.00.0-8x -4x 0 4x -8x -4x 0 4xFigure 3 (b). Simulated energy dispersion in a buncing electron beamwhich is modulated by a single sub-harmonic buncher.BUNCH SHAPE ENERGY max=~.7MeVII I I J1.00.0-8x o 4n -8n.:1: ,''1. :;1i~ 1\-4x oFigure 3 (c). The bunch shapes and the energy dispersions in electronbeams which is traced from the gun to the buncher section with the singlesub-harmonic buncher.BUNCH SHAPE ENERGY max=O.24MeVOL..---J~---1----L._~---'----'-1CI2 0 1CI2 -1CI2 0 1t/2Figure 4(a). Simulated bunch shape in electron beam which is modulatedby double subharmonic cavities.BUNCH SHAPE ENERGY max=O.278MeV0.0-8x_ ..I :: ~ ~ V VJ ,,-L/ -./;' I ,I~· l/".,t'-1.0I ~ ( Vo 4x -8n oFigure 4(b). Simulated energy dispersion in a buncing electron beamwhich is modulated by a single sub-harmonic buncher.BUNOI SHAPE ENERGY max=S.7MeV0.0-8x1.0,I ~ ~o 4n -8x o 4nFigure 4 (C). The bunch shapes and the energy dispersions in electronbeams which is traced from the gun to the buncher section with the doublesub-harmonic cavities.HIGH INTENSITY ELECTRON BEAM [377]/131Figure 5. Bunchshapes and energydispersions in theelectron beamwhich was modu-lated by a singleSHB. This pro-files were tracedfrom the gun to thedrift space end inthe prebunchersection by thesimulation codewith the spacecharge effect.BUNCH SHAPE.1 I1\ !l I ~I I,..~.,...-8 n - 4 n 0 4 n BnENERGY max=O.2I(MeV)\;.:-8 n - 4 n 0 4n 8nBUNCH SHAPEI1\, ,1\ i~, , ,~,max=O.203(MeV)Figure 6. Bunchshapes and energydis pers ion in theelectron beamwhich was modu-lated by doublesubhartnonic cavi-ties. This profileswere traced by thesame simulation inthe figure 5.REFERENCE-8 n- B n- 4 n- 4 nENERGY4n4n.. -.:BnBn1. Y. Tabata, S. Tagawa,H. Kobayashi, Y.Katsumura, T. Uedaand J. Tanaka,Linac Project at Nuclear Enginnerin Reseach Laboratory Proccedingof the 3th Meeting on Linear Acclerator in Japan, 15 (Tokai 1978).2. K. Tsumori,N .Kimura, T. Yamamoto, T.Hori, S. Takeda,J. Ohkuma,T. Sawai and M. Kawanishi Present Status of The Osaka UniversitySingle bunch Electron Linac 4th Symposium on AcceleratorScience and Technology, 43 (RIKEN 1982)3. A. Asami, S .Ohsawa, Y.Ogawa,A.Enomoto, S .Anami,M. Yokota, I. Sato,H.Iwata, and O.Azuma Accelerator Characteristics of The PositronGenerator Injector Proceedings of the 12th Linear AcceleratorMeeting in Japan, 162 (Tokai 1987)4. K. Tsumori, S. Takeda, T. Yamamoto, T. Hori, N. Kimura, J. Ohkuma,T. Sawai, S. Suemine and M. Kawanishi Present Status of The OsakaUniversity Single bunch Electron Linac 5th Symposium onAccelerator Science and Technology, 77 (KEK 1984)5. S.Takeda,N.Kimura,K.Tsumori and M.KawanishiThe Wake Potintial and the Energy Spread of a High -Current SingleBunch 5th Symposium on Accelerator Science and Technology,80 (KEK 1984)6. G.Mavrogents,W.Ramler,W.Wesolowski,K.Johnson and G.clifft,Subnanosecond high-intensity Beam Pulse IEEE Trans. Nuc. Sci.NS-20, No3. 919 (1973).

Simulation Of High Intensity Electron Beam Modulated By double Shc

Simulation Of High Intensity Electron Beam Modulated By double Shc

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