We have designed a 100 MHz RFQ to accelerate Pb25+ ions from 9.6 keV/u to 250 keV/u for the LHC ion program. We assume an input beam from a laser ion source with a total beam current of 90 mA, out of which 9 mA is Pb25+. The main challenge of the design is to match the tight longitudinal acceptance of the downstream Interdigital H structure while dealing with a high intensity beam composed of a variety of charge states. In this paper, we present a baseline setup optimized for nominal conditions, and show the sensitivity of the RFQ performance to varying input beam characteristics and rf parameters. Further studies will cover the compatibility of this design with an upgraded ECR source under investigation at CERN

Hanke, K

Lombardi, A M

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH      CERN/PS/2002-055 (PP)      DESIGN OF A HIGH-INTENSITY RFQ FOR A POSSIBLE LHC LASER ION SOURCE   K. Hanke and A. Lombardi     Abstract    We have designed a 100 MHz RFQ to accelerate Pb25+ ions from 9.6 keV/u to 250 keV/u for the LHC ion program. We assume an input beam from a laser ion source with a total beam current of 90 mA, out of which 9 mA is Pb25+. The main challenge of the design is to match the tight longitudinal acceptance of the downstream Interdigital H structure while dealing with a high intensity beam composed of a variety of charge states. In this paper, we present a baseline set-up optimized for nominal conditions, and show the sensitivity of the RFQ performance to varying input beam characteristics and rf parameters. Further studies will cover the compatibility of this design with an upgraded ECR source under investigation at CERN.        Paper presented at the XXIth International Linear Accelerator Conference August 19-23, 2002,Gyeongiu (KYONGIU), Korea    Geneva, Switzerland 27 August 2002 DESIGN OF A HIGH-INTENSITY RFQ FOR A POSSIBLE LHC LASERION SOURCEK. Hanke, A. Lombardi, CERN, Geneva, SwitzerlandAbstractWe have designed a 100 MHz RFQ to accelerate Pb25+ions from 9.6 keV/u to 250 keV/u for the LHC ion pro-gram. We assume an input beam from a laser ion sourcewith a total beam current of 90 mA, out of which 9 mA isPb25+. The main challenge of the design is to match thetight longitudinal acceptance of the downstream Interdig-ital H structure while dealing with a high intensity beamcomposed of a variety of charge states. In this paper, wepresent a baseline set-up optimised for nominal conditions,and show the sensitivity of the RFQ performance to vary-ing input beam characteristics and RF parameters. Furtherstudies will cover the compatibility of this design with anupgraded ECR source under investigation at CERN.1 INTRODUCTIONThe LHC heavy ion program calls for a new, high-intensity ion source. A laser ion source (LIS), presently un-der development at CERN, is a possible candidate. In orderto match the beam parameters from the source to the accep-tance limits of the CERN ion linac, a dedicated injectionscheme consisting of extraction system, low-energy beamtransport line (LEBT), radio-frequency quadrupole (RFQ)and beam transfer lines is needed. While the extractionscheme and the design of the LEBT are discussed in [1]and [2], we present here a possible design for the RFQ. Wehave based our simulations on estimated source parame-ters (beam emittance, current) and present a baseline solu-tion for these nominal conditions. Furthermore, we haveperformed a parameter scan where we investigate the per-formance of the RFQ for beam parameters other than thenominal ones and for a multi charge-state beam.2 BEAM CHARACTERISTICS ANDREQUIREMENTSAt this time experimental data from the LIS are notavailable. We have therefore based the design of the RFQon estimated beam parameters as well as on extrapolationsfrom experiments done with a prototype of the final source.The constraints at injection into the CERN ion linac havebeen assumed to be as in [3].The total beam current is estimated in [1] as 92 mA for theproposed extraction system. This total current consists ofa variety of charge states, out of which 9 mA are estimatedto be 208Pb25+. We have based the baseline RFQ designon a current of 10.0 mA 208Pb25+.The beam emittance from the source can be extrapolatedfrom measurements, which indicate a total normalisedemittance of about 2 π mm mrad. Preliminary simulationshave shown, that an RFQ at 101.28 MHz having such alarge transverse acceptance requires large aperture andhigh vane voltage, and is disadvantageous from an rf powerpoint of view. As the normalised transverse acceptance ofthe CERN ion linac is only 1 π mm mrad, we assume thisvalue as nominal input parameter.The longitudinal acceptance of the IH linac is0.018 deg MeV/u r.m.s. (0.09 deg MeV/u total) at101.28 MHz. We have aimed at a longitudinal emittanceat the exit of the RFQ which is 10-15% below this value toallow for some emittance dilution in the transfer lines.The longitudinal emittance which can be achieved atthe exit of an RFQ is essentially determined by theinput beam energy which, in turn, is given by the sourceextraction voltage Vextr. In experiments with the LIStest set-up, Vextr had typically values between 60 kVand 110 kV. We discuss in this paper a baseline scenariowith Vextr= 80 kV (9.6 keV/u) as well as a solution forVextr= 100 kV (12.0 keV/u).3 BASELINE RFQThe design of the RFQ was done using the codes CURLI,RFQUICK, PARI and PARMTEQM [4]. The input beamparameters are those discussed in Section 2, with an inputbeam energy of 9.6 keV/u. The main design goals were tomatch the acceptance of the IH (in particular in the longi-tudinal plane) while at the same time dealing with the highbeam current and rigidity. In order to obtain a small longi-tudinal emittance at the exit of the RFQ, two design choiceshave already been made in this first design stage:a) The synchronous phase is ramped to only -40 deg at theend of the gentle buncher.b) The starting point of the ramping is set to 1/3 of theshaper length rather than to the beginning of the shaper.We have found a solution with good transmission but an un-acceptable longitudinal emittance, which is more than 30%larger than the IH acceptance. An analysis of the beamevolution in the longitudinal phase space reveals that theproblem is due to space charge effects: particles in the cen-tre of the bunch rotate at a different speed than those furtheroutside. In order to fight space charge effects, we have cho-sen the following design principles:a) Delay the ramping of the synchronous phase in theshaper where the longitudinal emittance is formed.b) Quickly ramp the phase after the shaper in order to ac-celerate quickly, thus reducing space charge effects whilekeeping the longitudinal emittance constant.c) Increase the transverse beam size along the RFQ by re-ducing the focusing parameter.In practice this can be accomplished by adjusting the vanemodulation in the first part of the RFQ, where the lon-gitudinal emittance is formed. We find an RFQ withgood transmission (97.5%) and a longitudinal emittance ofεl = 0.015 MeV deg/u r.m.s. (0.075 MeV deg/u total). Thisis 20% below the IH acceptance and hence leaves somemargin for blow-up in the injection lines. The transverseemittance is essentially unchanged. The vane voltage is90 kV and the length of the RFQ is 396 cm. The radiusof curvature of the vane tips was chosen to ρ/r0=0.65 inorder to reduce the maximum electric field. The main RFQparameters are listed in Table 1. Figure 1 shows the in-put and output beam as computed with PARMTEQM andFig. 2 shows the evolution of beam parameters versus cellnumber in the RFQ.design ion 208Pb25+current [mA] 10.0input energy [keV/u] 9.6output energy [keV/u] 250.0design emittance (tot., norm) [mm mrad] 1.0frequency [MHz] 101.28vane voltage [kV] 90.0max. electric field [MV/m] 26.0RFQ length [cm] 396number of cells 310power loss (for 60 kΩm) [kW] 540minimum aperture [cm] 0.333max. modulation factor 1.7transmission1 [%] 97.5mid-cell radial aperture ro [cm] 0.458vane tip radius of curvature ρ/ro 0.651) For 1 π mm mrad input emittance (tot., norm.) and 10 mA, simulationwith 1000 particles.Table 1: Important parameters of baseline RFQ.Figure 1: Input (upper plots) and output (lower plots) beamof the RFQ as simulated with PARMTEQM.Figure 2: Evolution of beam parameters versus cell num-ber as simulated with PARMTEQM. The lines indicates thephysical aperture in the upper two plots and the separatrixin the lower two plots.4 PARAMETER SCAN4.1 Beam EnergyThe input beam energy is a trade-off between extractedbeam current and achievable longitudinal emittance. Thehigher the input beam energy, the longer the RFQ and themore difficult it is to form a small longitudinal emittance.We have found a solution for Vextr=80 kV. However, wehave investigated the possibility of designing an RFQ forVextr=100 kV (12.0 keV/u). We find a solution with alength of 435 cm (390 cells) and an output longitudinalemittance of εl=0.016 MeV deg/u, which is 12% belowthe IH acceptance. However, this solution is unstable andalready minor variations of the modulation lead to unac-ceptable blow-up of the longitudinal emittance. We willtherefore in the following sections only consider in moredetail the reference case with Vextr=80 kV.4.2 Energy SpreadThe simulations shown in Section 3 have been done withzero energy spread. As from the real source we expect asignificant energy spread, we have changed in the simu-lation the input beam accordingly. Figure 3 shows trans-mission and longitudinal emittance versus energy spreadfor the baseline RFQ. It can be seen that the transmissiondrops down to about 85% for 10% energy spread. More im-portant, the longitudinal emittance increases significantlyalready for small values of∆E.4.3 Beam EmittanceFigure 4 shows transmission versus input transverseemittance. For values smaller than the nominal value of1 π mm mrad, the transmission is constant around 98%.0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 28486889092949698transmission [%]0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20.0150.020.0250.030.0350.040.045total energy spread [keV/u]rms long. emit. [MeV deg/u]Figure 3: Transmission (upper plot) and longitudinal emit-tance (lower plot) versus input energy spread. The dashedline represents the longitudinal acceptance of the IH.No higher transmission can be achieved, as the losses havetheir origin in the longitudinal plane. For higher input emit-tance than the nominal one, the transmission drops downuntil about 87% for twice the value of the nominal inputemittance. However, the acceptance of the RFQ is suchthat even for 1.5 π mm mrad the transmission is still about94%.0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.286889092949698100emittance (tot.,norm.) [mm mrad]transmission [%]Figure 4: Transmission versus input beam emittance.4.4 Beam Current and CompositionThe charge state distribution expected from the CERNLIS is shown in Fig. 5. It peaks at Pb25+ but there is asignificant contribution of neighboring charge states to thetotal beam current. The total beam current is here 92.0 mA,which is distributed among the various species accord-ingly. We have tracked several charge states simultane-ously through the RFQ using the code TOUTATIS [5]. Fortechnical reasons we can only track a total of 10 ion speciesin parallel, so we have selected charge states between +21and +30 with their corresponding current and a total beamcurrent of 73 mA. The transmission is 47.9% of the fullcurrent and 70.0% for 208Pb25+. The reduced transmis-sion of 208Pb25+ can be explained by the increased spacecharge. The longitudinal parameters for the design ion aswell as for all 10 charge states are summarised in Table 2.Figure 5: Charge state distribution as expected from CERNlaser ion source.208Pb25+ total beamαl 0.057 1.014βl [deg/MeV] 588.66 752.03εl (r.m.s.) [MeV deg] 0.24942 335.48εl (r.m.s.) [MeV deg/u] 0.030 -Table 2: Longitudinal beam parameters for baseline RFQfor a beam composed of 10 different ion species.5 CONCLUSIONSWe have found a beam dynamics solution for an RFQwhich matches the assumed beam parameters of the CERNlaser ion source and the constraints given by the CERNion linac. The transverse acceptance is the one given bythe linac, leaving sufficient margin. The longitudinal emit-tance formed in the RFQ is well below the one requiredby the IH. Simulations of a multi-charge state beam havebeen performed. The design can be finalised as soon as thedefinite source parameters are known.6 ACKNOWLEDGMENTSThe authors would like to thank R. Duperrier for provid-ing the code TOUTATIS.7 REFERENCES[1] R. Scrivens, Extraction Geometry for the Future LIS Plasma,CERN PS/PP Note 2001-003 (2001).[2] K. Hanke, S. Heising, R. Scrivens, Comparison of the BeamDynamic Solutions for Low Energy Beam Transport Systemsfor a Laser Ion Source at CERN, CERN PS/PP Note 2001-004 (2001).[3] D. Warner (ed.), CERN Heavy-Ion Facility Design Report,CERN yellow report 93-01 (1993).[4] K. Crandall, T. Wangler, L. Young, J. Billen, G. Neuschaefer,D. Schrage, RFQ Design Codes, LA-UR-96-1836 (1998).[5] R. Duperrier, Intense Beam Dynamics in RFQs, thesis Univ.Paris XI Orsay (2000).

Design of a high-intensity RFQ for a possible LHC laser ion source

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