Clarke-Gayther MA

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A HIGH VOLTAGE SEMICONDUCTOR SWITCHED E-FIELD BEAM  KICKER FOR THE ISIS SURFACE MUON FACILITYMichael A. Clarke-Gayther, Rutherford Appleton Laboratory, Didcot, United KingdomAbstractA proposal to upgrade the Isis Surface Muon Facility atthe Rutherford Appleton Laboratory calls for a new largeaperture muon beam kicker and associated pulsed powersupply. A description is given of a fast switched E-fieldkicker, and an associated bipolar, ± 40 kV, pulsed powersupply. The high voltage, discharge mode power supplydesign is simplified by the utilisation of novel fastswitching high voltage MOSFET stacks.1  INTRODUCTIONThe ISIS Surface Muon Facility, as commissioned in1987 , delivered a  beam of positive muons with a meanmomentum of  26.5 MeV/c, and a double pulse timestructure of 82 ns pulse width at half maximum (fwhm),and 320 ns peak separation, to a single user area, at apulse repetition frequency (prf) of 50 Hz.In 1989, the periodic gaps in the range of availablefrequencies for muon spin resonance (µSR) measurement,arising from the double pulse structure, were eliminated,(with a consequent loss of  beam intensity), by  theaddition of a fast E-field beam chopper (Uppset), thatremoved the interfering second beam pulse [1].A major facility upgrade, funded by the CEC largefacilities program and commissioned in 1994, wasprompted  by a three-fold over-subscription for beamtime, and utilised a fast switching E-field kicker-chopperto distribute the muon pulse pair to three user areas, eachcapable of simultaneous single pulse operation at a prf of50 Hz [2].A recent proposal [3], prompted by the need to addressthe impact on the existing facility of the planned 200 to300µA ISIS upgrade , and the current interest in the use oflow energy muons for the study of thin films and multi-layers , identified the following objectives and the actionsrequired to achieve them:(a) To provide a new source of low energy (slow)muons  (10eV - 20 keV), by adding a large apertureE-field kicker, septum magnet, beam-line, and user area tothe existing facility.(b) To maintain or increase the muon transmission tothe three existing user areas (‘MuSR’, ‘EMu’, and‘DeVA’) by replacing selected beam-line quadrupoleswith a larger aperture design (CERN type M9).(c) To increase the upper cut-off frequency for µSRmeasurements in the existing user areas by using theexisting, but slightly modified EC muon kicker, in a newchopper-kicker mode.Figure 1: ISIS Surface Muon FacilityProceedings of EPAC 2000, Vienna, Austria23582  MODE OF OPERATIONA plan view, and schematic of the upgraded muonfacility are shown in figures 1, and 2(a), with the new, andexisting kickers labelled as K1 and K2, respectively.Operation of the facility is shown schematically infigure 2(b). Note that beam transit time has been ignored,to simplify the figure. Figure 2: Facility schematic and mode of operationAt t = 0ns, K1 and K2 are at full field level. During theperiod t = 0 to 120ns, the first muon pulse is deflected byK1, and transported by beam-line B1 to the ‘NEW’ slowmuon area. At t = 120ns, the field at K1 starts to decay,and reaches ~ 0.2% of  the initial value at t = 320ns.During the period t = 320 to 450ns, the second muon pulsepasses through K1 un-deflected. During the period  t =320 to 360ns, the first half of the second muon pulse isbisected and deflected by K2, and transported by beam-lines B2 and B3 to the ‘EMu’ and ‘DeVA’ areas. Duringthe period t = 360 to 380ns, the field at K2 decays, and thelast half of the second muon pulse passes through K2 un-deflected, via beam-line B4 to the uSR area. A recent testin the ‘uSR’ user area, with K2 operating in the newchopper-kicker mode, showed that the upper frequencylimit for µSR measurements was increased from ~8 to~14 MHz. Existing and upgraded ISIS facility µ+ beamcharacteristics  are shown in table 1.Table 1: ISIS facility µ+ beam characteristicsPulse Widthfwhm (ns)µSR cut-offFreq. (MHz)µ+ / pulse(@ 50 Hz)ISISUserArea Now Upgd Now Upgd Now UpgdNEW - 80 - 8 - 3.5e4EMu 80 30 8 20 0.5e4 0.7e4DeVA 80 30 8 20 0.5e4 0.7e4uSR 80 45 8 14 1.0e4 1.4e43  KICKER DESIGN The key specifications shown in table 2 were obtainedafter optimisation of the muon facility beam line design,to provide a minimised beam width at the input and outputof the kicker and at the downstream septum magnet, andapplication of the standard equations for field (E) andtransverse displacement (y), where q is the electroniccharge, V is the muon velocity, m0 is the muon rest mass,x is the electrode length, and θ is the required deflectionangle,xqVmE⋅⋅⋅=20tanθ  ,  and  2022 VmxEqy⋅⋅⋅⋅=Table 2: Kicker specificationx (cm) y (cm)Beam size at kicker input      (100%) 14 24Beam size at kicker output    (100%) 9 22Beam size at septum input     (100%) 8 8Deflection (mrad) 70 Electrode length (m) 0.9Displacement (cm) 3.2 Electrode gap (m) 0.15Field       (MV/m) 0.49 Electrode height (m) 0.3Potential   (kV) 73.0 Capacitance  (pF) 53  A preliminary mechanical layout for the proposed largeaperture kicker is shown in figure 3. The parallel platedesign is configured for bipolar drive at ± 36.5 kV, andpresents a load capacitance of ~ 53 pF on each electrode.In-vacuum components, subject to high electric fieldswill be designed to reduce field level to less than 5 MV/m.Base mounted discharge mode power supplies will limitload capacitance to the level required for successful highvoltage MOSFET switch application.Figure 3: Large aperture kicker (end and side views)2359Proceedings of EPAC 2000, Vienna, Austria4  PULSED POWER SUPPLY DESIGNFigure 4: Discharge mode pulsed power supplyThe basic elements of the discharge mode circuit areshown in figure 4. The circuit is suitable for low dutycycle kicker applications [4], and uses a ‘stock’, constantvoltage mode, high voltage power supply (HVPS). The‘close coupled’ configuration minimises high voltageswitch power dissipation  and is hence, most suitable for alow power MOSFET switch  application [5].Figure 5: Discharge mode circuit modelA circuit schematic for the discharge mode powersupply model is shown in figure 5. The electrode voltagewaveform v(3), and switch current I(R9) duringdischarge, are shown in figure 6. At t = 0ns, the electrodecapacitance C1 is charged to ~ V1, and SW1, 2, and 3 areopen. At t = 30ns, electrode discharge is initiated byclosure of SW1 and 2. The voltage at node 5, initially at ~V1, decays with MOSFET switch turn-on  time constantC6*R6 (15ns). This voltage, mirrored at the output of thedependent source E1, sinks current via the MOSFETseries on resistance R9, inductance L1 and current limitF1, from electrode capacitance C1 via external currentlimiting resistor R12 and inductance L2. At t = 230ns,v(3) has decayed to ~ 0.2% of V1.At t = 300ns, SW1 and 2 open, SW3 closes,  and C1charges via R4, C2, and R7, back to V1. Power loss in theMOSFET switch is ~ 4 W at a prf of 50 Hz .Figure 6: Electrode voltage and switch currentCharging waveforms are shown in figure 7.Figure 7:  Electrode voltage and  HVPS current5  SUMMARYA large aperture E-field kicker and discharge modepower supply have been designed to provide a new sourceof low energy muons at ISIS. The use of commerciallyavailable high voltage MOSFET switches in the powersupply design, is expected to reduce circuit complexityand cost [6]. A test rig, to determine long term switchreliability, has been designed and will be run in the nearfuture.In conclusion, it is clear that E-field kicker-choppertechnology will make a key contribution to the economicrealisation of the next generation of multi-station surfacemuon facilities [7]. ACKNOWLEDGEMENTThe author would like to thank Gordon Eaton, forproviding  support and encouragement during the designphase of this project.REFERENCES[1] A. Borden, A. Carne, M. Clarke-Gayther, G. Eaton,H. Jones, and G. Thomas, ‘Uppset - A pulsedelectrostatic kicker to improve the µSR frequencyresponse in the ISIS pulsed muon beam’, Nucl. Inst.and Meth. A292 (1990) p.21-29.[2] M. A. Clarke-Gayther and G. H. Eaton, ‘A fastswitching e-field kicker, for the temporal and spatialdivision of a 26.5 MeV/c surface muon beam’, Proc.of the 4th European Particle Accelerator Conference,London, UK, 1994, p.2414-6.[3] G. H. Eaton, ‘A proposed upgrade for the ISISEuropean Muon Facility’, ISIS internal document,February 1999.[4] Curtis B. Figley, ‘A high speed electrostatic kickerfor the pulse stretcher ring at the SaskatchewanAccelerator Laboratory’, Nucl. Inst. and Meth. A273(1988) p.59-62.[5] A. Saab,  M. Kogan, and T. Russ, ‘Fast TEM kickerwith MOSFET solid state driver’, Proc. of the 1991IEEE Particle Accelerator Conference, SanFrancisco, USA, 1991, p.3174-6.[6] ‘HTS series’ MOSFET switch, from ‘BehlkeElectronic GmbH’, Frankfurt am Main, Germany.[7] J. E. Sonier (Editor), ‘A Next Generation U.S. MuonFacility’,  website:  http://musr.physics.ubc.ca./~sns/Proceedings of EPAC 2000, Vienna, Austria2360

A high voltage semiconductor switched e-field beam kicker for the ISIS Surface Muon Facility

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