Clarke-Gayther MA

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A FAST CHOPPER FOR THE ESS 2.5 MeV BEAM TRANSPORT LINEMichael A. Clarke-Gayther, CLRC RAL, Didcot, United Kingdom.AbstractBeam chopping requirements in the proposed EuropeanSpallation Source (ESS) 2.5 MeV beam transport lines arechallenging. Stringent beam loss restrictions in thedownstream linacs and accumulator rings dictate thatpartial chopping of the high current 280 MHz bunchedH- beam must be minimised. A description is given of amodulation scheme where the required time dependentchopped beam structure is generated by a fast transition,short duration, pulsed E-field in a distributed elementslow-wave chopper, followed by a slower transition, longduration, pulsed E-field in a lumped element slow-wavechopper. Candidate modulator systems are identified.1 INTRODUCTIONThe European Spallation Source (ESS) [1], is the mostambitious of the existing proposals for the next generationof accelerator driven pulsed neutron sources [2].Designed to address the rapid expansion in the field ofcondensed matter research, the ESS accelerator willgenerate intense, fast pulsed, beams of neutrons bydelivering up to 10 MW of protons to short pulse(5 MW, ~ 1.2 us), and long pulse (5 MW, ~ 2.0 ms) liquidmercury targets.Figure 1: ESS block schematicComponents of the ESS, are shown in schematic formin Figure 1. The low beam loss (< 1 nA/m @ 1.334 MeV)design features a high current H- linac, and twinaccumulator rings with sequential charge exchangeinjection and sequential fast extraction.The time dependent function of the fast choppers,switching and fast extraction magnets, is shown inFigures 2 and 3, where the ‘history’ of a beam pulse istraced from RFQ to target, over a linac cycle. The sevenshort pulse chopping regimes listed in Figure 2, aredescribed in Table 1, and the four long pulse choppingregimes listed in Figure 3, are described in Table 2.Figure 2: Short pulse beam chopping and switchingTable 1: ESS short pulse chopping regimesChopper Duty CycleLabel( fig.2)Chopping Regime Dura--tion On time Off timeNo. ofTurns*FunctionA Ion source transition 50µs 50 µs 0 60 Gates low intensity beamB Beam duty cycleramping50µs 803.57 nsto 241.1 ns0 to563.8 ns 60Limits linac beam loadingtransientC Ring 1 stacking 0.5 ms 241.1 ns 563.8 ns 583 Gaps for fast extractionD Ring switching 0.1 ms 0.1 ms 0 120 Gap for ring switchingE Beam duty cycleramping50µs 803.57 nsto 241.1 ns0 to563.8 ns 60Limits linac beam loadingtransientF Ring 2 stacking 0.5 ms 241.1 ns 563.8 ns 583 Gaps for fast extractionG Ion source transition 50µs 50 µs 0 60 Gates low intensity beam* One accumulator ring revolution period = 803.57 nsStringent beam loss requirements dictate that thechopping field in the 2.5 MeV transport line rises andfalls within the beam bunch interval of 2.9 ns (1 - 90%).In addition, the field duration must be rapidly program-mable in the range 240 ns - 0.1 ms (see Tables 1, 2).Proceedings of EPAC 2002, Paris, France2136Figure 3: Long pulse chopping and switchingTable 2: ESS long pulse chopping regimesChopper Duty CycleLabel( fig.2)Chopping Regime Dura--tion On time Off timeNo. ofTurns*FunctionA Ion source transition 50µs 50 µs 0 - Gates low intensity beamB Beam duty cycleramping50µs 805.15 nsto 241.1 ns0 to563.8 ns -Limits linac beam loadingtransientC Beam to long pulsetarget2.0ms 241.1 ns 563.ns - Maintains short pulselinac beam loadingD Ion source transition 50µs 50 µs 0 - Gates low intensity beam2 MODULATION SCHEMESlow wave (E-field) transmission line structures havedemonstrated field transition times in the nanosecondregime [3, 4], and an ESS chopping scheme utilising thesestructures has been identified [5]. The scheme has beenrefined, following the recent addition of the long pulsetarget option to the ESS specification, and a blockschematic of the proposed chopper beam line is shown inFigure 4. The new beam line design provides space for anadditional chopper sub-system that permits a halving insub-system pulse repetition frequency, and beam dumpdissipation.Figure 4: 2.5 MeV Chopper beam line layoutBeam trajectories, and a timing schematic for theproposed scheme are shown in figures 5 and 6 respect-ively, where slow wave chopper 1, produces a bipolar,pulsed field that deflects just two adjacent bunchesthrough ± 16 mr, to beam dumps 1 and 2 (BD1, BD2),creating two ~ 10 ns duration gaps in the bunch train, atthe beginning and end of each chopped beam interval.Modulator peak power is ~ 39 kW, but the low duty cycleresults in a mean power requirement of only 25 W.Figure 5: Beam trajectories(a) Fast transition pre-chopping - beam to BD1&2.(b) Slower transition chopping - beam to BD3.The chopper 2, slow wave, lumped element array(6 pairs of 6 cm long electrodes), produces a unipolar,pulsed field that deflects the beam through 47 mr to awater-cooled electrode / beam dump array (BD3).The chopper 2 modulators (12 x switch modules) arelimited to transition times of ~ 8 ns. Pre-chopping inchopper 1 ensures that no partially chopped bunchesresult from the slower field transition time of chopper 2.Each switch module will dissipate ~ 60 W mean, withchopper 1 electrodes, BD1, 2, and 3 dissipating beampowers of ~ 27, ~ 78, ~ 27, and ~ 3530 W, respectively.Figure 6: Timing schematicProceedings of EPAC 2002, Paris, France21373 MODULATOR DESIGNA block schematic of the proposed fast choppermodulator system, is shown in Figure 7. Systems 1A and1B drive chopper 1A and 1B distributed slow waveelectrodes (see Figure 6), and output fast transition(~ 2 ns), quasi-trapezoidal, bipolar high voltage pulses(± 1.4 kV) into a 50 Ω load. The modular configurationmakes extensive use of high power transmission linetransformers (TLT) to match impedance, and combine,the outputs from 32 solid-state high voltage pulsegenerators.Figure 7: Modulator block schematicA block schematic of the pulse generator module isshown in Figure 8. The class-D, push-pull, current switch-ing design, outputs ~ ± 245 V (peak) from a 52 V supply,and utilises eight RF power MOSFETs (f t ~ 2 GHz)Figure 8: System 1 / ModuleFigure 9: System 2 / Moduleand a step up TLT. The module can be very compact, asthe average duty cycle for system 1 is ~ 0.12 %, and meanload power per module is only ~ 1 W.Systems 2A and 2B drive chopper 2A and 2B lumpedelement, slow wave electrodes, and output ~ 8 nstransition, unipolar, trapezoidal pulses (+3.0 and -3.0 kV)into a 40 pF load. A block schematic of the module isshown in figure 9. The high voltage MOSFET switch hasthe standard dc-coupled, totem-pole configuration. The60 W modules will be close-coupled (< λ /10 = < 50 cm)to individual electrodes to preserve signal integrity.4 SUMMARYA modulation scheme and a modulator design for theESS 2.5 MeV fast beam chopper have been identified.The scheme makes use of a new modulation techniquethat enables the implementation of an elegant, lowaverage power, modulator.5 ACKNOWLEDGEMENTThe author would like to thank Chris Prior forproviding data for the beam envelope trajectories,presented in figure 5, and also Grahame Rees and IanGardner for their support and encouragement.6 REFERENCES[1] ‘The ESS Project Volume III, ’, Technical Report,2002, ISBN 3-89336-303-3.[2] Spallation Neutron Source, (SNS), ORNL, USA;Japanese Hadron Facility, (JHF), KEK, Japan;Austron, Austria / CERN Switzerland.[3] S.S. Kurennoy and J.F. Power, ‘Development ofMeander-Line current structure for SNS fast 2.5 MeVBeam Chopper’, Proc. of the 7th European ParticleAccelerator Conference, Vienna, Austria, 2000,p.336-338.[4] J.S. Lunsford and R.A. Hardekopf, ‘Pulsed beamchopper for the PSR at LAMPF’, IEEE Trans. Nucl.Sci., NS-30, 1983.[5] M. A. Clarke-Gayther, ‘Modulator Systems for theESS 2.5 MeV Fast Chopper’, Proc. of the 2001Particle Accelerator Conference, Chicago, USA,p. 4062-4065.[6] C.R. Prior, ‘Transverse image effects in the SNS andESS beam choppers’, RAL internal communication.Proceedings of EPAC 2002, Paris, France2138

A Fast Chopper for the ESS 2.5 MeV beam transport line

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