This preliminary feasibility study is based on the availability of the CERN LEP2 superconducting RF system after LEP de-commissioning. The option that is explored is to use this system as part of a high energy H- linac injecting at 2 GeV into the CERN PS, with the aim of reliably providing at its output twice the presently foreseen transverse beam brightness at the ultimate intensity envisaged for LHC. This requires the linac to be pulsed at the PS repetition rate of 0.8 Hz with a mean beam current of 10 mA which is sufficient for filling the PS in 240 ms (i.e. about 100 turns) with the ultimate intensity foreseen for injection for the LHC. The linac is composed of two RFQs with a chopping section, a room temperature DTL, a superconducting section with reduced beta cavities up to 1 GeV, and a section of LEP2 cavities up to 2 GeV. This study deals, in particular, with the problems inherent in H- acceleration up to high energy and in the pulsed operation of SC cavities. Means for compensating microphonic vibrations in the SC cavities are considered, with the aim of reducing the final overall energy spread to the tight requirements for injection into a synchrotron. Other possible applications of such a machine are also briefly reviewed, that make use of its potential for working at a higher duty cycle than required for LHC alone

Garoby, R

Haseroth, H

Hill, C E

Lombardi, A M

Ostroumov, P N

Tessier, J M

Vretenar, Maurizio

Vretenar, M

CERN Document Server

Feasibility study of a 2 GeV superconducting H- Linac as injector for the CERN PS

EUROPEAN ORGANISATION FOR NUCLEAR RESEARCHCERN - PS DIVISIONCERN/PS 98-049 (RF/HP)FEASIBILITY STUDY OF A 2 GEV SUPERCONDUCTING H- LINACAS INJECTOR FOR THE CERN PSR. Garoby, H. Haseroth, C.E. Hill, A.M. Lombardi, P.N. Ostroumov*,J.M. Tessier**, M. VretenarAbstractThis preliminary feasibility study is based on the availability of the CERN LEP2 superconducting RF systemafter LEP de-commissioning. The option that is explored is to use this system as part of a high energy H– linacinjecting at 2 GeV into the CERN PS, with the aim of reliably providing at its output twice the presently foreseentransverse beam brightness at the ultimate intensity envisaged for LHC. This requires the linac to be pulsed at thePS repetition rate of 0.8 Hz with a mean beam current of 10 mA which is sufficient for filling the PS in 240 ms(i.e. about 100 turns) with the ultimate intensity foreseen for injection for the LHC.The linac is composed of two RFQs with a chopping section, a room temperature DTL, a superconductingsection with reduced beta cavities up to 1 GeV, and a section of LEP2 cavities up to 2 GeV. This study deals, inparticular, with the problems inherent in H– acceleration up to high energy and in the pulsed operation of SCcavities. Means for compensating microphonic vibrations in the SC cavities are considered, with the aim ofreducing the final overall energy spread to the tight requirements for injection into a synchrotron. Other possibleapplications of such a machine are also briefly reviewed, that make use of its potential for working at a higherduty cycle than required for LHC alone.*)   INR, Moscow, Russia**) SL DivisionXIX International Linac Conference, August 23-28, 1998, Chicago, USA,Geneva, SwitzerlandOctober 1998FEASIBILITY STUDY OF A 2 GEV SUPERCONDUCTING H– LINAC ASINJECTOR FOR THE CERN PSR. Garoby, H. Haseroth, C.E. Hill, A.M. Lombardi, P.N. Ostroumov*, J.M. Tessier**, M. VretenarPS Division, CERN, CH 1211 Geneva 23, Switzerland(*: on leave of absence from INR, Moscow, Russia   **: SL Division)AbstractThis preliminary feasibility study is based on theavailability of the CERN LEP2 superconducting RFsystem after LEP de-commissioning. The option that isexplored is to use this system as part of a high energy H–linac injecting at 2 GeV into the CERN PS, with the aimof reliably providing at its output twice the presentlyforeseen transverse beam brightness at the ultimateintensity envisaged for LHC. This requires the linac to bepulsed at the PS repetition rate of 0.8 Hz with a meanbeam current of 10 mA which is sufficient for filling thePS in 240 ms (i.e. about 100 turns) with the ultimateintensity foreseen for injection for the LHC.The linac is composed of two RFQs with a choppingsection, a room temperature DTL, a superconductingsection with reduced beta cavities up to 1 GeV, and asection of LEP2 cavities up to 2 GeV. This study deals, inparticular, with the problems inherent in H– accelerationup to high energy and in the pulsed operation of SCcavities. Means for compensating microphonic vibrationsin the SC cavities are considered, with the aim ofreducing the final overall energy spread to the tightrequirements for injection into a synchrotron. Otherpossible applications of such a machine are also brieflyreviewed, that make use of its potential for working at ahigher duty cycle than required for LHC alone.1  INTRODUCTIONMost of the RF equipment of the CERN LEP-2 will beavailable after the year2000. Among the possible re-usesof this valuable hardware [1-4] the realisation of a 2 GeVLinac injector for the PS is an attractive option with manybenefits with respect to the present scheme for LHCinjection [5].As a result of the smaller emittance of the Linac beamand of the higher injection energy into the PS (at present1.4 GeV), the LHC would profit from an increasedbrightness of the proton beam delivered by the PS injectorcomplex. The peak beam intensity in the PS could beimproved as well by filling the entire aperture. Beamlosses would be reduced by the efficient charge exchangeinjection in the transverse planes, and by the choppedbeam in the longitudinal phase plane. The injectors of thePS could be modernised and re-built with standardisedequipment, with advantages in terms of reliability andmaintenance.Other potential applications of this facility at a higherduty cycle justify the use of SC cavities. They include:1) neutron production with a spallation target, using thePS as an accumulator ring; 2) feeding a second generationISOL facility for the production of radioactive ion beams;and 3) any physics application requiring intensesecondary beams.A small study group has concentrated on the mainaccelerator technology topics and on the most promisingscenario. A first report indicating the feasibility of such afacility is being prepared [6].2  PARAMETERS AND LAYOUTThe Superconducting Proton Linac, SPL, (Figure 1) ismade of an H– source, two RFQs with a chopper inbetween, a Drift Tube Linac up to 100 MeV and asuperconducting section up to 2 GeV. The main designparameters are given in Tables 1 and 2.Table 1: Linac Beam ParametersNumber of Particles / PS Pulse 1.5 1013Mean Linac Current during Pulse 10 mAPulse Length 250 msRepetition Rate 0.83 HzFilling Factor of Linac Buckets ½N. of Linac Bunches per PS Bucket 11SPL Micropulse (11 bunches) 59.6 nsChopping Factor 46 %Mean Bunch Current(in an RF period, for a full bucket)37 mASource Current 20 mABeam Duty Cycle (for PS filling) 0.021 %Maximum Design Duty Cycle 5 %Maximum Average Current 500 mATransverse Emittance, source exit, rms0.2 mmTransverse Emittance, PS input, rms0.6 mmLongitudinal Emittance (5 rms) 3 °MeVFigure 1: Schematic layout of the Linac.H- sourceRFQ1chopperRFQ2 DTL SC- reduced b SC - LEP2to PS0.05         2                  7                100                1000                    2000  Energy [MeV]      8m                        100m              372m                   407m                   208mTable 2: Linac structure parameters.Wout[MeV]Freq.[MHz]#ofcav.Power[MW]# ofklystLength[m]RFQ1 2 176.1 1 0.45 - 2.3RFQ2 7 352.2 1 0.5 1 4DTL 100 352.2 29 5.8 6 99SC -red. b1027 352.2 152 13 19 372SC -LEP22000 352.2 136 14.2 17 407Line 2000 352.2 1 - - 208Total 43 1094The facility is designed to provide 1.4 1013 particles atthe exit of the PS, corresponding to the LHC beam-beamlimit (“ultimate beam”). For a mean linac current of10 mA, this number of particles can be obtained byinjecting 110 turns into the PS, with a linac pulse lengthof ~250 ms. For the PS repetition period of 1.2 sec, theresulting linac beam duty cycle is only 0.021%. Theinjection energy into the PS, 2 GeV, has been chosen touse most of the existing LEP equipment, to improvetransverse beam stability in the PS and to profit from thehigh accelerating efficiency of the LEP cavities at highenergies.The LEP RF frequency of 352.2 MHz is also used formost of the room temperature section. A significantnumber of klystrons with their power distribution systemscan therefore be recovered, and a standard RF system canbe used throughout the linac.Due to the low duty cycle, the SC cavities need to bepulsed to minimise heat dissipation and wall plug power.The linac is foreseen for a beam duty cycle of 5%: up tothis value the cryogenic system is dimensioned mainly tohandle static losses and RF pulsing has no impact on thecryoplant. The main additional investment for this dutycycle comes from the shielding needed to cope with thehigher activation due to losses in the linac.The relative particle loss at 2 GeV and 5% duty mustbe smaller than 10–6/m to allow hands-on maintenance;this is not a strong design constraint as a large fraction ofthe halo particles are transported through the largeaperture of the SC cavities (>20 cm) and can be properlyremoved before PS injection.An important design constraint is the high beambrightness needed by the LHC: this requires an emittanceof 0.2 mm from the source because a factor 3 blow-upbetween the source and the PS has been conservativelyassumed to account for space-charge, mismatch, andmisalignment effects.3  ROOM TEMPERATURE SECTIONThe room temperature section is composed of a front-end (source, RFQs, chopper) injecting into a Drift TubeLinac (DTL). The 20 mA beam coming from the source isaccelerated to 2 MeV by a 176.1 MHz RFQ. The beam isthen chopped and injected, filling every other bucket, intoan RFQ at double frequency (352.2 MHz), which bringsthe beam energy to 7 MeV. Matching to and from thechopper is performed by dedicated sections integrated inthe first and second RFQ respectively.A distance of 1.6 m is provided between the RFQs tohouse a wide-band electrostatic chopper of the BNLdesign [7] and some diagnostics. The chopper voltagerequired is 1.7 kV, and, to avoid partially filled buckets inthe Linac, a 4.2 ns rise time is required: should it be toochallenging, a chopper/antichopper line will be chosen.The DTL has been divided in two sections. The firstone (7-20 MeV) consists of one standard Alvarez tank,with FODO focusing. The second, up to 100 MeV, is ofthe separated-focusing DTL type, made of 28 8-cell tanksseparated by 3 bl drifts containing a quadrupole triplet.This structure offers higher shunt impedance and simplerm chanical construction than a standard DTL. Tripletfocusing is preferred because of the resulting round beaminside the tank, which minimises the emittance growthdu  to RF defocusing. The transmission of the roomtemperature part is 99% (without taking into accountstripping losses after the source) and the transverseemittance increase is 10%.4  SUPERCONDUCTING SECTIONThe superconducting part of the Linac consists of fourdifferent sections, with cavities optimised for beta 0.48,0.6, 0.8 and 1. LEP-2 standard cavities (b=1) andcryostats are used between 1 and 2 GeV, while 5-cellcavities optimised for b=0.8 would be built and installedin the existing LEP-2 cryostats to cover the energy rangebetween 450 MeV and 1 GeV [3]. Two additionalsections of 4-cell cavities optimised for b=0.48 andb=0.625, arranged in shorter cryostats, cover the energyrange between 100 MeV and 200 MeV, and between 200and 450 MeV respectively. A development program isunderway at CERN for the production of reduced-b(b=0.5 to 0.8) cavities with the niobium on coppertechnique [8]. In case the sputtering is not be feasible, thelowest beta cavities would be made of bulk niobium andthe DTL energy increased up to 150 MeV. The layout ofthe superconducting part is given in Table3.Table 3: Layout of the superconducting section.Sec. CryostatsklystronsCavitiescells/cav.outputenergy[MeV]length[m]RFpower[MW]1 8 4 32 4 191 58 1.32 14 7 56 4 452 126 3.73 16 8 64 5 1027 188 8.04 34 17 136 4 2041 407 14.2tot. 72 36 288 779 27.2This layout makes use of 34 LEP2 4-cavity moduleswith their cryostats, i.e. 53% of the 68 installed in LEP.Including the cryostats used for the b=0.8 cavities, only50 cryostats would be re-used, leaving some margin forreaching a higher linac energy if needed.Due to the pulsed mode of operation, static cryogeniclosses will dominate. Assuming a static loss of 180 W per4-cavity module as in LEP [9], the 72 cryostats of theSPL would have an overall static loss of 13 kW, i.e.slightly more than the cooling capacity of a LEP-typecryoplant (12 kW).The mean field used in this design is 6 MV/m althoughoperation at a higher gradient should be possible inpulsed mode. The focusing for the superconductingsection is provided by a doublet (two 400-mm longquadrupoles spaced by 100 mm) placed outside eachcryostat. It has so far been optimised for zero current. For40 mA the emittance increase is 45%, coming from thelong focusing period at low (<1 GeV) energy and frommismatches between the different sections. A new layoutfor the low energy part and a more accurate matchingshould reduce the emittance blow-up.5  ENERGY STABILITYMechanical vibrations in the SC cavities change theirresonant frequency, leading to oscillations of the bunch inthe longitudinal phase plane and finally to a pulse-to-pulse jitter in the mean bunch output energy. The effect ofthe vibrations can be greatly reduced by a self-excitedloop and an RF feedback of the cavity voltage. For theSPL, a feedback scheme and calculation tools originallydeveloped for the TESLA project [10] have been adaptedfor a proton beam. Since the correction is applied at theklystron input, the beam motion cannot be compensatedcompletely when the klystron feeds several cavities as isthe case in the SPL (8 cavities per klystron). In thesimulations, the gains of the regulation loops are set to100 and 500 respectively for the amplitude and for thephase. 20 % extra power is required for the amplitudeloop and 20% for the phase loop.The effect of the Lorentz detuning at 6 MV/m field isvery small: the cavity phase can be cancelled by thefeedback loops when the beam is injected, and thecorresponding peak-to-peak energy error at 2 GeV is only0.006 MeV.The effect of mechanical vibrations has been studiedassuming a pessimistic maximum cavity-to-cavityvariation in resonant frequency of ± 40 Hz. The motionsof the beam centre and the energy and phase errors atlinac exit have been calculated for 500 uniform randomdistributions of frequency errors. The scatter in theposition of the beam centre in the longitudinal plane at2 GeV is shown in Figure 2.Inside the single pulses, energy and phase are verystable (the period of the mechanical vibrations is muchlonger than the pulse length), while from pulse to pulsethe rms energy variation is 0.3% (6 MeV). The energyjitter can be reduced to ± 3 MeV (total), by an energy-correcting cavity placed 200 m downstream, resulting in agood match to the PS bucket.Figure 2: Relative position of the bunch centre in thelongitudinal plane at 2 GeV for 500 random errordistributions.6  REFERENCES[1] R. Corsini, A. Hoffmann, “Considerations on an FELbased on LEP superconducting cavities”, CERN/PS96-04.[2] C.Rubbia, J.A.Rubio, “A tentative program towards afull scale energy amplifier”, CERN/LHC 96-11.[3] D. Boussard, E. Chiaveri, G. Geschonke,J. Tuckmantel, “Preliminary Parameters of a ProtonLinac using the LEP 2 RF System whenDecommissioned”, SL-RF Technical Note 96-4.[4] C. Pagani, G. Bellomo, P. Pierini, “A high CurrentLinac with 352 MHz SC Cavities”, Proceedings ofthe 1996 Linac Conference, Geneva, p. 107.[5] R. Garoby, M. Vretenar, “Proposal for a 2 GeVLinac Injector for the CERN PS”, PS/RF/Note 96-27.[6] G. Bollen, D. Boussard, R. Cappi, R. Garoby,H. Haseroth, C.E. Hill, P. Knaus, A. Lombardi,M. Martini, P.N. Ostroumov, J.M. Tessier,M. Vretenar, “Report of the Study Group on theSuperconducting Proton Linac as a PS Injector”,CERN/PS, in preparation.[7] J.M. Brennan, L. Ahrens, J. Alessi, J. Brodowski, J.Kats, “A Fast Chopper for Programmed Populationof the Longitudinal Phase Space of the AGSBooster”, Proc. of the 1988 EPAC, Rome, p. 1003.[8] C.Benvenuti, D.Boussard, S.Calatroni, E.Chiaveri,J. Tückmantel, “Production and Test of 352 MHzNiobium Sputtered Reduced-Beta Cavities”, CERN-SL 97-63 RF.[9] G.Geschonke, “Superconducting Structures for HighIntensity Linac Applications”, Proceedings of the1996 Linac Conference, Geneva, p. 910.[10]J.M. Tessier, “Field Stabilisation in SuperconductingCavities in Pulsed Mode”, Ph.D. Thesis, Paris XI,Orsay 1996.-40-30-20-10010203040-40 -30 -20 -10 0 10 20 30 40Df (deg)DW (MeV)

Feasibility study of a 2 GeV superconducting $H^{-}$ linac as injector for the CERN PS

Feasibility study of a 2 GeV superconducting $H^{-}$ linac as injector for the CERN PS

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Feasibility study of a 2 GeV superconducting $H^{-}$ linac as injector for the CERN PS