Bhatia, T S

Guy, F W

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Particle Accelerators, 1990, Vol. 29, pp. 105-110Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaRF DEFLECTOR-CHOPPER FOR sse INJECTOR*F.W. GUY and T.S. BHATIALos Alamos National Laboratory, MS H817, Los Alamos, NM 87545USAAbstract In a proposed SSC linac injector, the Low-Energy Boosterlattice painting scheme requires a 50-MHz microbunch structure withtransverse and longitudinal normalized rms emittances of less than0.45 n-mm-mrad and 1.7X 10-5 n-eV's, respectively, at 600 MeV.A 50-MHz RFQ design does not meet the longitudinal emittancerequirements; a l50-MHz RFQ can do so, but requires a choppingscheme that produces a clean 50-MHz beam structure withoutdegrading emittance. We present an RF deflector-chopper conceptualdesign that converts a l50-MHz RFQ beam into a 50-MHz microbunchbeam structure while matching transversely and longitudinally to afollowing 450-MHz DTL. Multiparticle simulation with 3-D spacecharge shows negligible emittance growth. Output beam emittancesare factors of3 below requirements; further acceleration to 600 MeVshould produce only small additional growth. The chopper is 54 emlong, comprising an RF deflector, two rebunchers, five smallpermanent magnet quadrupoles, and a beam dump. Similar schemescould use a 200- or 250-MHz RFQ for even smaller longitudinalemittance.INTRODUCTIONA proposed lattice painting scheme1,2 for the Superconducting SuperCollider (SSC) Low-Energy Booster (LEB) uses a 3-rnA H- beam (3.8 X108 H- per bunch) with a 50-MHz microbunch structure. Transverse andlongitudinal emittance requirements are, respectively, 0.45 n-mm-mradand 1.7 X 10-5 n·eV·s (rms normalized). A 50-MHz, 2.5-MeVradio-frequency quadrupole (RFQ) beam dynamics design has been completed",but simulations of this RFQ predict transverse and longitudinal emittancesof 0.18 n-mm-mrad and 2.6 X 10-5 n-eVvs, respectively; longitudinalemittance is too large. However, a l50-MHz RFQ design easily satisfiesemittance requirements with transverse and longitudinal emittances of0.14 n-mm-mrad and 0.44 X 10-5 n-eV·s. Higher-frequency RFQs canhave even smaller emittances.We present a conceptual design of an RF deflector-chopper in which ahigh-frequency beam is converted to a lower-frequency beam structure with*Work supported by Los Alamos National Laboratory Program Development, under theauspices ofthe United States Department of Energy.[807]/105106/[808] F. W. GUY AND T. S. BHATIAlittle emittance growth. This system selects every third bunch from thel50-MHz, 9-mA, 2.5-MeV RFQ output beam for transmission, convertingthe beam to a 50-MHz, 3-mA structure. The system also matches the beamfrom the RFQ into the following 450-MHz DTL.CHOPPER DESIGNFunneling, using an RF deflector, has been proposed as a method ofincreasing brightness in high-current beams by interlacing the bunches oftwo converging bunched beams to form a single beam of twice the frequencyand current." Such a funnel is the subject of an experimental program atLos Alamos.P The RF deflector-chopper is a modification of the funnelconcept. The direction of travel is opposite to that of a funnel, and the beamis split into separate beams of lower frequency. In this design, the frequencyis reduced by a factor of3 as shown in Figure 1.-! -!- +-- 150 MHz Beam+-- Keep These+-- Discard These+-- 50 MHz BeamFIGURE 1 Beam structure modification by a 200-MHz deflector.For beams with appreciable space charge in which emittance growth isof concern, transport-line design should minimize charge-redistributionemittance growth" by keeping constant (or changing only slowly) bothtransverse and longitudinal focusing strength and by proper matchingbetween different accelerator structures. These considerations were kept inmind in the present design (Figure 2). However, because space-chargeeffects are not of major importance here, the line can be shorter and thechanges more abrupt than if a high-current beam were being transported.The transport line at 2.5 MeV consists of three FOnO focusing periodsof different lengths. Permanent-magnet quadrupoles provide increasingtransverse focusing strength, matching transverse beam parameters to thefollowing DTL. The first and third periods contain 150-MHz rebunchersthat match the longitudinal beam parameters into the DTL. The secondfocusing period contains the RF deflector. Table I lists the beam elementsand their dimensions and field gradients or voltages. Beam apertures arenot critical but have been set at l.G-cm diameters in the quadrupoles Ql,Q2, and Q4, 3 em in Q3, and 1.2 em in the deflector.The bend-plane (x-z plane) cross section of the RF deflector is shown inFigure 2. The two oval electrodes extend in the y-direction (perpendicular toRF DEFLECTOR-CHOPPER FOR SSC INJECTOR [809]/107450 MHz DTL ---+150 MHz RFQ7\ ~. -_ .._--; Bunches at ~ = O·200 MHz RF Deflector LBeam DumpBunches at rp = +1200 and -120°+------------ 54.0 cm ----------------+FIGURE 2. The RF deflector-chopper for the sse injector.Table I Beamline elements.Element Type Length FieldD1 Drift 3cmR1 150-MHz rebuncher 4cm 241KVQ1 Defocusing (in x) quad 3cm 76.5BT/MD2 Drift 6cmQ2 Focusing quad 3cm 92.19 TIMDef 200-MHz RF deflector Bcm 12.5 MV/MQ3 Defocusing and deflecting quad 4cm 70.00 TIMD3 Drift BcmQ4 Focusing quad 4cm 99.37 TIMR2 150-MHz rebuncher 4cm 314KVQ5 Defocusing quad 4cm 131.81 TIMD4 Drift 3cmthe plane of the bend) near the beamline whereas the noses on the end wallsare cylindrically symmetric around the beamline. In a similar device beingconstructed at Los Alamos, electrodes are mounted on half-wavelength rods,and one nose contains a permanent-magnet quadrupole. The electrodes areslightly less than 13A/2 in length along the beamline. If the deflector couldproduce a uniform transverse field of length 13A/2 with a sintcot) timedependence, and if the synchronous particle with energy K were at thecenter of the deflector at the time of maximum field strength E, then thedeflection of the synchronous particle from the electric field alone would be108/[810] F. W. GUY AND T. S. BHATIAe = (13AJ2n)(eE/K). It can be shown that the force from the RF magnetic fieldgoes roughly as {32; this was neglected in our calculations.A real deflector cannot produce a uniform transverse field, of course;the electric field components from the deflector are shown in Figure 3. The0.6ow,..:.>ti..JW0::~~ ~iaIIIii~0.6FIGURE 3 The RF deflector electric fields.nonuniformity of the transverse (Ex) field, the presence of the longitudinal(Ez) field coupled with the changing transverse coordinates of the particles,and the phase spread of the bunch coupled with the time dependence of thefield, all contribute to emittance growth in the RF deflector. This emittancegrowth can be reduced by shaping the deflector plates and noses for the bestfield uniformity, making the bunch envelope small in the plane of the bendto avoid regions offield nonuniformity, shortening the bunch to lessen theeffect of the time-dependent field, and keeping the angle of deflection small.In the present design, all these techniques were used to some extent. Thereare other emittance-growth-reducing techniques such as introducing a thirdharmonic to flatten the field time-dependent wave-shape, using a seconddeflector to bend the beam in the opposite direction thereby cancelling someof the aberrations, and using a lower-frequency deflector; these were notnecessary in this design. The first quadrupole downstream from thedeflector is defocusing in x and gives further deflection to the beam becausethe beam enters it off-axis. In this design, the RF deflector bends the beamthrough 4.4° and the defocusing quadrupole provides 3.6° for a total bendofBo.A modified version of the Los Alamos PARMILA particle-followingcode was used for these beam dynamics calculations. Approximately 4000macroparticles generated by the PARMTEQ calculation of the l50-MHzRFQ were followed through the chopper line. Space-charge effects werecalculated using 3-D charge-cloud interactions. Particles were integratedthrough the deflector in small steps using the electric field maps shown inFigure 3.RESULTS AND DISCUSSIONBeam rms-radius envelopes are shown along the beamline in Figure 4. The90% beam radius is everywhere below 3 mm. In one simulation, theRF DEFLECTOR-CHOPPER FOR SSC INJECTOR [811]/109apertures were set at a 5-mm radius throughout the line and the resultingtransmission was 99.970/0.0.15E 0.10UX 0.050.00-.0.15E 0.10u~ 0.050.000.4Eu 0.2N0.01010202030304040505010 20 30 40 50BEAMLINE LENGTH, em~--t~-m=~=t~--fm~~-LINE DIAGRAM: R=Rebuncher, Q=magnetic Quadrupole, C=Coord transform, D=DeflectorFIGURE 4 RMS beam envelopes.Table IT Emittances, rms normalizedRFQoutputChopper outputex( n-mm·mrad)0.140.16ey( n-mm-rnrad)0.150.15ez(n-e'V-s)0.44 X 10-50.44 X 10-5A transverse particle plot (Figure 5) at the position of the beam dump,1 em upstream from the fourth quad, shows a pattern at about x = + 1.7 em(the initial axis is at x =0). This is the bunch that enters the deflector at theproper phase (0°) for maximum positive x-deflection. The other two bunches,at phases of +120° and -120°, are deflected in the negative x-direction andare smeared in the x-direction because of the time-dependence of thedeflector field. If the beam dump can accept a smaller separation of thedesired and undesired beams than that shown in Figure 5, the beamdeflection angle and thus the emittance growth of the desired beam can bereduced.This RF deflector-chopper design, combined with the 150-MHz RFQand the 450-MHz DTL designs, can produce a beam that satisfies110/[812] F. W. GUY AND T. S. BHATIA0.5Eo 0.0~2.01.51.00.50.0X, em-0.5-1.0-1.5-0.5-r------,----,-----r-------,-----,------,---~--_______,-2.0FIGURE 5 Particle plots at beam dump.requirements for the lattice-painting scheme proposed in Ref. 1. Similardesigns could provide beam-structure modifications for other frequencyratios. If desired, bending magnets could be incorporated in drifts D2 andD4 to align the input and output beams to the same beamline.Chopper designs such as the present example will allow low-emittancebeams of high-frequency RFQs to be used when lower-frequency beamstructures are desired. Ongoing funneling tests will provide experience foroptimum design of such choppers.ACKNOWLEDGMENTSWe thank James Stovall for his support in preparing this design.REFERENCES1. E.P. Colton and H.A. Thiessen, ttH_ Injection into the Low Energy Booster of the SSC,"Los Alamos National Laboratory report LA-UR-88-2532.2. M. Leo, R.A. Leo, G. Mancarella, and G. Soliani, "The Problem of Painting in theInjection into Circular Accelerators; A General Analytical Approach," Nucl. Instrum.& Methods in Physics Research, A278 (1989),629-642.3. T.S. Bhatia, F. W. Guy, G. H. Neuschaefer, M. Pabst, S. O. Schriber, J. E. Stovall, T. P.Wangler, M. T. Wilson, and G. T. Worth, ttSSC Linac Injector," Presented at the 1988Summer Study on High Energy Physics in the 1990s, Snowmass, Colorado, June 1988,Los Alamos National Laboratory report LA-UR-88-3909.4. J.E. Stovall, F.W. Guy, R.H. Stokes, and T.P. Wangler, "Beam Funneling Studies at LosAlamos," Nucl.lnstrum. & Methods in Physics Research A278 (1989), 143-147.5. R.J. Kashuba, G.E. Taylor, F.W. Guy, and K.R.Crandall, ttThe Magnetic Optics Designof the ATS Funneling Experiment," presented at the Symposium on Neutral ParticleBeam Technology, Monterey, California, July 1989, Los Alamos National Laboratoryreport LA-UR-89-277.6. T.P. Wangler, K.R. Crandall, and R.S. Mills, "Emittance Growth from Charge DensityChanges in High-Current Beams," Proc. International Symposium on Heavy IonFusion, Washington D.C., AlP Conf Proc. 152 (1986), p. 166.

Rf Deflector - Chopper For Ssc Injector

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