This paper describes a design for the front end of a superconducting (SC) ion
linac which can accept and simultaneously accelerate two charge states of
uranium from an ECR ion source. This mode of operation increases the beam
current available for the heaviest ions by a factor of two. We discuss the 12
MeV/u prestripper section of the Rare Isotope Accelerator (RIA) driver linac
including the LEBT, RFQ, MEBT and SC sections, with a total voltage of 112 MV.
The LEBT consists of two bunchers and electrostatic quadrupoles. The
fundamental frequency of both bunchers is half of the RFQ frequency. The first
buncher is a multiharmonic buncher, designed to accept more than 80% of each
charge state and to form bunches of extremely low longitudinal emittance (rms
emittance is lower than 0.2 keV/u nsec) at the output of the RFQ. The second
buncher is located directly in front of the RFQ and matches the velocity of
each charge-state bunch to the design input velocity of the RFQ. We present
full 3D simulations of a two-charge-state uranium beam including space charge
forces in the LEBT and RFQ, realistic distributions of all electric and
magnetic fields along the whole prestripper linac, and the effects of errors,
evaluated for several design options for the prestripper linac. The results
indicate that it is possible to accelerate two charge states while keeping
emittance growth within tolerable limits.Comment: LINAC2000, MOD0

Aseev, V. N.

Kolomiets, A. A.

Ostroumov, P. N.

Shepard, K. W.

English

arXiv

UNT Digital Library

● ✍✍✎ ☞Rtik/pBy\cP-lol?qy ‘4))‘d~%) e~HEAVY-ION BEAM ACCELERATION OF TWO-CHARGE STATES FRO/fjAN ECR ION SOURCE +*’ 4$)P.N. Ostroumov, K.W. Shepard, Physics Division, ANL, 9700 S. Cass Av., Argonne, IL, 60439 ‘V.N, Aseev, Institute for Nuclear Research, Moscow 117312, RussiaA.A. Kolomiets, Institute of Theoretical and Experimental Physics, Moscow 117259, RussiaAbstractThis paper describes a design for the front end of asuperconducting (SC) ion linac which can accept andsimultaneously accelerate two charge states of uraniumfrom an ECR ion source. This mode of operationincreases the beam current available for the heaviest ionsby a factor of two. We discuss the 12 MeV/u prestrippersection of the Rare Isotope Accelerator (HA) driverlinac including the LEBT, RFQ, MEBT and SC sections,with a total voltageof112 MV.Tle LEBT consists of two bunchers and electrostaticquadruples. The fundamental frequency of bothbunchers is half of the RFQ frequency. The f~st buncheris a mukiharmonic buncher, designed to accept morethan 80% of each charge state and to form bunches ofextremely low longitudinal emittrmce (rms emittance islower than 0.2 mkeV/wnsec) at the output of the RFQ.The second buncher is located directly in front of theRFQ and matches the velocity of each charge-statebunch to the design input velocity of the RFQ. Wepresent full 3D simulations of a two-eharge-stateuranium beam including space charge forces in theLEBT and RFQ, realistic distributions of all eleernc andmagnetic fields along the whole prestripper linac, andthe effects of errors, evaluated for several design optionsfor the prestripper Iinac. The results indicate that it ispossible to accelerate two charge states while keepingemittance growth within tolerable limits.I INTRODUCTIONTke Rare Isotope Accelerator (RI& Facility requires a1.3 GeV Iinac which would accelerate the full massrange of ions and would deliver -400 kW of uraniumbeam at an energy of 400 MeV per nucleon [1,2]. Thedriver would consist of an ECR ion source and a shornnormally-conducting RFQ injector section which wouldfeed beams of virtually any ion into the major portion ofthe accelerator an array of more than 400superconducting (SC) cavities of seven different types,ranging in frequency from 57.5 to 805 MHz. The linaccontains two stripping targets, at 12 MeV/u and 85MeV/u, for the uranium beam. A novel feature of theIinac is the acceleration of beams containing more thanone charge state [3,4]. The front end of the RIA driverIinac consists of a ECR ion source, a LEBT, a 57.5 MHzRFQ, a MEBT and a section of SRF drift-tube Iinac.The present-day performance of ECR ion sources, andconsiderations based on fundamental limiting processesin the formation of high-charge state uranium ions insuch sources, indicate that uranium beam intensities ashigh as 7 ppA in a single charge state of 29+ or 30+ areunlikely to be obtained in the near future. Such a highcurrent is required in order to produce the RIA driverlinac design goal of a 400 kW uranium beam, even if weassume multiple charge state beam accelerationfollowing the fwst srnpper.This paper discusses in detail a solution to thislimitation. It doubles the heavy-ion beam intensityaccepting two charge states from the ion source.2 DESIGN OF THE FRONT END2.1 LEBTbyThe LEBT is designed for the selection and separationof the required ion species and the acceptance of single-or two-charge states by the following RFQ structure. Thefmt portion of the LEBT is an achromatic bendingmagnet seetion for charge analysis and selection. For theheaviest ions, such as uranium ions, the transport systemmust deliver to the entrance of the fwst buncher a two-charge-state beam with similar Twiss parameters forboth charge states. The design features of the two-chargeselector will be discussed elsewhere. The ECR is placedon a high voltage platform. The voltage VO=IOOkV isadequate to avoid space charge effects in the LEBT andRFQ and to keep the RFQ length to less than 4 m.A simplified layout of the second part of the LEBT isshown in Fig. 1. This part of the LEBT solves thefollowing tasks: a) Beam bunching by a four-harmonicexternal buncher B, (the fundamental frequency is 28.75MHz); b) Velocity equalization of two different chargestates by the buncher B,, operating at 28.75 MI@ c)Charge-insensitive transverse focusing of the 2-chargestate beam and matching to the RFQ acceptance by theelectrostatic quadruples Q,-Q.The reference charge state for the design of the LEBT,RFQ and MEBT is 29.5. The RFQ injector is designed toaccelerate any beam from protons to uranium to avelocity v/c = 0.01893 at the exit of the RFQ.The computer code CC)SY [5] was used in order todesign and optimize the LEBT taking into account termsthrough third order. The final beam dynamics simulationhas been performed by the DYNAMION code [6] wherethe equations of motion are solved in a generalapproximation using realistic 3D electrostatic fields ofthe quadruples and rf bunchers, including space chargeforces for the multi-charged ion beams. Realistic 3Dfields for the electrostatic quadn.qdes were calculatedDISCLAIMERThis report was prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best available originaldocument.Bh SlbIIIll,[ Elani ❑ IEIUQ1Q2Q3❑ ID❑ 0Q4 Q5Figure I: Layout of the LEBT.X ,mmw1..Q1 Q2 Q3 Q4 Q5 Q6 Q7Lluu UuUuw,--..L //~\Q8,0 ... .,, ___ ,/ \“‘. z ,m002 04 06 O* 70 ?2 i. ?. 7>.20 22.,0 I ,,-....e. _ ‘“”’’’......./’n.204 nnn nn— 3t-d order nnY ,mm 7st orderFigure 2: Beam envelopes in the LEBT.j=!~E-200-103 0 lW 2LVJm m 5Figure 3: Longitudinal phase space plots of a two-chargestate beam along the LEBT: a) after B], b) before Bz, c)after B,, c) RFQ entrance with scale changed to RFQfrequency.by the SIMION code [7]. The 3D field distributions havebeen used with both the COSY and DYNAMION codes.l@re 2 shows beam envelopes along the LEBToptimized by COSY. The total normalized emittance atthe exit assumes an ion source emittance of 0.5. mnrnmmad.. After careful optimization, including thirdorder terms, the rms emittance growth is less than 7% inthe horizontal plane. In the vertical plane there is noobservable rms emittance growth. Figure 3 shows thetmnsformation of the beam image in longitudinal phasespace. The fxst multi-harmonic btmcher modulates thebeam velocity of the two charge states as shown in Fig.3a. The drift space between the two bunchers (see Fig. 3)is chosen from the expression——where 2 is the wavelength of the RFQ frequency, m< isthe atomic unit mass. This drift space separates in timethe bunches of different charge states q, and q,- 1 by 360°at the RFQ frequency. The voltages of the multi -harmonic buncher have been optimized together withthe RFQ parameters, in order to obtain a total efficiencyabove 80%, while minimizing the longitudinal emittancefor each charge state. The second buncher is used toequalize the velocities of the 2 charge states (see Fig.3C).2.2 RFQThe formation of heavy-ion beams of low longitudinalemittance has been discussed in ref. [8], in which thelowest beam emittance was obtained by prebunching andusing a drift space inside the RFQ. This procedure workswell for a single charge state beam, but cannotaccommodate two charge states because of the differentvelocities of difTerent charge states coming from thesame ion source.We describe below a new design for a low currentRFQ and injector system which can provide very lowlongitudinal emittance for operation with both singlecharge state and two-charge state beams. Low rms andtotal longitudinal emittance are achieved by using anexternal multi-harmonic buncher and an RFQ ofmodified design. The RFQ has three main sections 1)the standard radial matcher, 2) the transition section and3) the acceleration section.The radial matcher transforms the RFQ acceptance toa set of Twiss parameters that avoids large beam sizes inthe LEBT quadruples.The transition section is a part of the RFQ with alinear variation of the synchronous phase. The RFQparameters A, w a and q, are calculated self-consistently in order to fflter the longitudinal emittancein such a way that the low populated area will be losteither inside the RFQ or in the MEBT. It is performed byan iterative procedure of the whole RFQ design and byobserving the emittance formation in the longitudinalphase space. The desired result is achieved if theseparatrix size is slightly larger than the denselypopulated area of beam emittance. The design goal is anrms emittance below 0.2 mkeV/unset, required formultiple charge state operation through the rest of thelinac.The acceleration section is a portion of the RFQ withconstant synchronous phase which is equal to –24°. TheRFQ forms a beam crossover at the exit in bothtransverse planes. This results in better matching of thetwo-charge state beam in the MEBT.In this way, the RFQ was designed for acceleration ofone- or two-charge state heavy-ion beams from 12.4keV/u to 167 keV/u over a length of 4 m. The phasespace plots from numerical simt.dations of a two-chargestate uranium beam exiting the RFQ are shown in Fig. 4.2.3 Beam simulation in the h4EBT and SRFlinacBetween the RFQ and SRF lAac there is a matchingsection - the MEBT. We found that SC solenoidsplaced in individual cryostats are the best option to focusa two-charge state beam.. ,.’.X (mrad) Y’ (mrad)40 ):., I 401::: I-40~ :w’:/40Ji;! 1-0.3 cm 0.3 -0.3 0.0 0.3dW/W (%)4::c1::2 ..+-... -..+..-.,o ~:1-----------2 -.7---.-+------4’:0 200 400X (cm) Y (cm) Phase (degree)Figure 4: Phase space plots of beams with charge state29+ (blue dots) and 30+ (red dots) exiting the RFQ.1.5r-o-! ,0 10 20 30 40 50 60 70Dis18nc4(m)@gure 5: Two-charge state beam envelopes (rms andtotal) along the MEBT and SRF Linac.05-.#QCL25-g4..-cm --0s ~-lo -5 0 5 10Pha. &gl15MHzFigure 6: Phase space plots of a two-charge state beamjust prior to the fwst stripper.l%e portion of the Iinac prior to the fwst stripper(prestripper) contains 96 cavities of four different types.Prior to ray tracing a multiple charge state beamthrough the prestripper Iinac, the transverse beam motionwas matched carefully using fitting codes for a trialbeam of charge state q=29.5. A particularly criticalaspect of fitting was to avoid beam mismatch at thetransitions between focusing periods of differing lengths.‘Ihe focusing lattice length is different for each of thefour types of SRF cavities. The phase advance perfocusing period was set at 60°. As mentioned in ref. [3],the correct choice of phase advance is crucial foreffective steering of the multiple charge state beam. Thedesign and simulation of 3D beam dynamics in the SRFlinac was performed by the LANA code [9]. The pre-processor code generates the phase setting for a uraniumbeam with average charge state q=29.5. The rf phase isset to –30° with respect to the maximum energy gain ineach SRF cavity. Realistic field distributions for the SRFcavities were generated using an axially-symmetricapproximation of the actual drift tube cavities. Thesefields are used both for the design procedure and for thebeam dynamics simulation. The initial phase spacedktribution used for each charge state was the beam atthe exit of the RFQ, as simulated with the DYNAMIONcode. The particle coordhates were then tracked throughthe SRF linac. Figure 5 shows the transverse beamenvelopes (rms and total) along the MEBT and SRFIinac. Despite the slight mismatch of the two-chargestate beam along the linac, there is no rms emittancegrowth in the transverse plane for an ideal Iinac withoutany errors.In longitudinal phase space the emittance of the twocharge state beam is always larger than for a singlecharge state beam. Growth in effective emittance occursdue to the oscillations caused by the slightly differentoff-tune synchronous phases for charge states 29+ and30+. However, the very low longitudinal emittanceachieved by the RFQ injector ensures that the totalemittance of the two-charge state beam remains wellinside the stable area in longitudinal phase space. Figure6 shows the longitudinal phase space just prior to thefnt stripper. Note that the energy and phase acceptanceof the reminder of the SRF Iinac are &3Yoand MOO,respectively.3 CONCLUSIONThe problem of acceptance and acceleration of twocharge states of a heavy-ion beam from a single ECR ionsource was successfully solved. A front end has beendesigned for a driver linac for RIA that accelerates atwo-charge state uranium beam. The use of a two-chargestate beam is a powerful tool to double the total beampower produced by the heavy ion driver linac.This work was supported by the U. S. Department ofEnergy under contract W-3 1-109-ENG-38[1][2][3][4][5][6][7][8]4 REFERENCESK. W. ShePar& et al., in the Proceedings of the 9thInternational Workshop on RF Superconductivity,Santa Fe, New Mexico, 1999, to be published.C. Leemann, Paper TU103, these Proceedings.P.N. Ostroumov and K. W. Shepara Phys. Rew. STAccel. Beams, 3,030101 (2000).P.N. Ostroumov, K. W. Shepard and J.A. Nolen,Paper FR1O1, these Proceedings.M. Berz. COSY INFINITY. Version 8. User’s Guideand Reference Manual, MSU, 1999.A.A. Kolomiets, et al, in the Proc. of the Sixth Eur.PAC. Stockholm, Sweden, June 22-26, 1998.SIMION 3D. Version 6.0, User’s Manual, INEL-95/0403 (Idaho Nat. Eng. Laboratory, 1995).J. Staples, in Proceedings of the XVIII Inter. LinacConf., V. 2, Tsukuba, Japan, 1994.[9] D.V. Gorelov and P.N. C)stroumov, Proc. of the FifthEur. PAC, p. 1271. Sitges, Spain, June 10-14, 1996.: The submitted manuscript has been createdby the University of Chicago as Operator ofArgonne National Laboratory (“Argonne”)~ under Contract No. W-31-109-ENG-38 with~the U.S. Department of Energy. The U.S.; Government retains for itself, and others act-f ing on its behalf, a paid-up, IIOneXChJSiVe,\ irrevocable worldwide license in said articleI to reproduce, prepare derivative worka, dis-~tribute copies to the public, and perform pub-~Iicly and display publicly, by or on behalf of./he Government._ .-.. e.. —-. .—. —. . . .

Heavy-Ion Beam Acceleration of Two-Charge States from an Ecr Ion Source

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