Hiramatsu, S

Kishiro, J

Ninomiya, S

Takano, S

Yoshii, M

CERN Document Server

Particle Accelerators, 1990, Vol. 32, pp. 63-68Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaThe Simulation of the Bunch Compressionin the JHP 1 GeV Compressor RingM. Yoshii, J. Kishiro, S. Hiramatsu, S. Ninomiyaand S. TakanoNational Laboratory for High Energy Physics1-1 Oho, Tsukuba, Ibaraki-ken, 305, JapanAbstract The bunch compression process in the 1 GeV proton compressor/stretcherring for the Iapanese Hadron froject (JHP) has been simulated with a computercode including the longitudinal space charge effect.INTRODUCTIONThe production of a short bunch high intense proton beam is. required at the IapaneseHadron £roject 1. The required bunch length is a few tens nano-second. To produce such ashort bunch beam, a non-adiabatic bunch compression technique is employed. In thistechnique the rf-accelerating voltage is increased much faster than the synchrotronoscillation period. As a result, the bunch begins to rotate in the phase plane, and afterrotating a quarter of a period of synchrotron motion, a short bunch will be obtained. Thecompression rate is proportional to the square root of the ratio of initial and final rf-voltages.The space charge field continuously acts on the traveling beam and the rf-accelerating field intermittently acts at each rf-accelerating gap. Below the transition energy,the effects of both fields on the beam are opposite to one another.In a high intensity machine such as the JHP ring, analysis of the beam motion includingthe space charge effect is indispensable. By using a computer code we have estimated theeffect of the space charge field on the beam. This computer code consists of two steps. The1st step of the code simulates the stacking and storing processes. The·2nd step simulatesthe bunch compression process.SIMULATIONThe injected beam, which is chopped at the exit of the ion source, has a [me structure of200 nsec pulse width at a frequency of 3 MHz. Since the hannonic number of the ring is 2,[1505]/6364/[1506] M. YOSHII ET AL.there are two bunches, one of which is used for compression. The designed peak currentof the linac beam is 10 rnA, i.e. the number of l!rotons ner hunch (ppb) is 1.2Sx1013. Forbrevity in calculation, we used 2000 macro-particles substituted for 1.2Sx1013, and alsoinstead of 600 turns of multi-turn injection 200 macro-particles were stacked into the ringevery 60 turns. The initial distribution of the linac beam is assumed to be uniform in thephase plane and the momentum spread ~p/p to be ±0.13%. The tracking analysis duringthe stacking and storing periods were simulated assuming a constant rf-voltage (typically 7kV). In order to compress the beam, the rf-voltage was increased from 7 kV to 200 kV inan interval of SOJlsec, which is short enough in comparison with a synchrotron period. Themaximum rf-voltage is restricted by the number of rf-cavities.Ener~ ~ain and phase variation per turnFor simplicity of calculation, the many rf-cavities along the ring were replaced by alocalized single cavity. We define the following variables;En : kinetic energy of a particle at the nth turn<l>n : rf-phase of a particle at the nth turn .While a panicle is traveling in the ring, both the rf-voltage Vrf and the potential Vs due tothe space charge field act on it every turn. So we can write two difference equations withregard to the energy and the phase of each particle,En+1 =En + e [ Vrf sin( <l>n) - Vs ]<II =<II + 2 1t rt 'Y h d En+1n+1 n Y+ 1 EOwhere Eo is the kinetic energy of the synchronous particle; Eo = 1 GeV, h is the harmonicnumber, Yand Yt are the Lorentz factor of a particle and the transition energy, respectively,11 = 1/Yt 2 - 1/y 2 and dEn=En - Eo. Using these two equations we can easily track thebehavior of the particles in the phase plane.Estimation of the Ioncltudinal space char~ vQlta~eTo estimate the longitudinal space charge voltage Vs acting on the beam, we assumed around beam of radius a traveling in a circular beam duct of radius b. For a perfectlyconducting smooth wall, the space charge voltage is written by the following equation 2;SIMULATION OF BUNCH COMPRESSION [1507]/65C e 'YO dA(s)V s= - CIS41tEO'Y 2where C is the circumference of the ring and go = 1+2In(b/a). The go value is supposed tobe 2.5. In order to get a differential value dA(s)/ds, the A(S) distribution was replaced by adiscrete histogram in the phase plane. the bin width of the histogram was typically 10degree.RESULTS AND SUMMARYThe JHP ring will be operated during the stacking and storing processes with aharmonic number of h=2, and after ejecting the first bunch, the harmonic number of rf-system will be switched to unity for the compression. Since the stationary bucket lengthfor the h=2 rf-system is 'V 333 nsec and the initial bunch length is 200 nsec, there is notenough marginal phase in the bucket area. Therefore, particles with a negative product of~p/p and ~<t> do not stay long inside the bunch area. Figure-l (A) shows the bunch shapeat 1 msec from beam injection. Some particles have already spilled out of the bucket area.In this case the beam intensity was 1.25x 1013 ppb.For compression, the harmonic number is switched from h=2 to h=1 by means ofchanging the rf-frequency 3 MHz to 1.5 MHz. The reason is to have enough longitudinalbucket area so that the non-linear effect of synchrotron motion will be reduced. In Figure-l(B) and (C), the phase space distributions of the bunch during the compression process areshown, where compression is started at 1.0 msec from beam injection. As seen in bothfigures, the filamentation is already observed at the beginning of compression. Thisfilamentation caused by the diluted beam becomes conspicuous as the intensity exceedsIx 1013 ppb. As shown in Figure-2 (A), the results of our simulation are summarized. Thesolid line (a) in Figure-2 (A) shows that the filamentation arrested the bunch compression.When compression is started at 0.5 msec from beam injection, the dilution does not growso much. Therefore, in spite of higher intensity, the bunch is successfully compressed (thedashed line (b) in Figure-2 (A)). Finally, in the case that the harmonic number ofrf-systemis switched before and after the compression, the bunch length of 200 nsec linac beam iscompressed to 'V50 nsec. Figure-2 (B) shows the results of the case that the linac beam iscaptured with the rf-bucket made by the harmonic number h = 1. In this case, especially,when the linac beam is chopped into 100 nsec pulses, a short bunch below 30 nsec isobtained.In order to minimize the space charge effect, an optimized rf-voltage pattern during thestacking and storing processes is essential and is currently under developement.66/[1508]ACKNOWLEDGEMENTSM. YOSHII ET AL.It is a pleasure to acknowledge the contributions of the KEK computer center group whohelped us to develope the computer code. Special thanks goes to Professor M. Kihara whogave us the opportunity for this work.References1. Y. Kamiya, Report of the desi~n study on the compressor/stretcher rin~ of theJapanese Hadron Project,JHP-11,KEK Internal 88-9 (1988) p.12. A. Hofmann, Proc, of Erice School, CERN 77-13 (1977) p,139300-180-120-60060120180-120-60060120180)1.3msec(1550turns)2.0msec(1650turns)~ ~ ~ :> ::j o z o ~ to ~ ::r:: n ~ ~ ~ ~ o z(C)(B)RF-Phase(Degree)r!!;;:~T~'!I"!'I'\T''j''/lY'~':'~'!I!~.;"10JJI':"~I~...;.:....j_.:~1~..~~;.~...J 1,~~f~~\t\..r/~--.I'1-r~fr0!I0J0I!,!!I!!0!I!!0I00!of!00I!!0010'"0looo!I"!,~o00!1.0msec(1500turns0.02~h=2...-...0.01~ ~~~~....0.00''"':.u:''~...'';.~W'Y~..~.::.:~.>•.•.,---- ~ <:]-0.01-0.02!!I!r!0,J!,I!III-300-1500150(A)FIGURE-I.Simulationofbunchcompression;theinitiallengthandthebunchintensityare200nsecand1.25x1013ppb,respectively.(A)Bunchshapeat1msecfrombeaminjection Gustbeforebunchcompression).Theh=2rf-systemisused.(B)&(C)Bunchshapesduringcompression.Theh=lrf-systemisusedforthispurpose.~ VIo ::g 0\ -.....l68/[1510] M. YOSHII ET AL.(A)~ 70 ....---------.......---....o<L)~= 60 .......-lUUUUUUU.........................................................-lUUUUUUU..............................~...............lUUUUUUU..............."4'-'"'5 50 -r--..............~==~---~~~--.,bO .......~ (b) ~•..J 40 ....-------------..........dg 30::s~ 20 ~--.- .........r"P"I'"'I""I"I'I"'-..............."'I""I"I"'....__._......""I"'9""I''I'''I'I10 11 10 12 10 13 10 14Intensity (particles per bunch)(B)..............(a) ........-.. ....•~ ~-v--.n...n. "'-(b) - v""",," vI 'I302010 11 10 12 10 13Intensity (particles per bunch)70..d 50~bO=<L) 40~FIGURE-2. Obtained short bunch length vs beam intensity;(A) The h=2 rf-system is switched to the h=l rf-system at (a) 1msec and (b) 0.5 msec from beam injection. The initial bunch lengthis 200 nsec.(B) The h=l rf-system is only used and the bunch compressionstarts at 0.5msec from beam injection. The initial bunch length is (a)200nsec and (b) 100 nsec

The Simulation Of The Bunch Compression In The Jhp 1-gev compressor Ring

Abstract

Similar works

Full text

Available Versions

CERN Document Server