Clarke-Gayther MA

Eaton GH

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A Fast Switching E-Field Kicker, For The Temporal and Spatial Division of a 26.6 MeV/c Surface Muon Beam. h/l.A.Clarke-Gayther and G.H. Eaton Rutherford Appleton Laboratory Chilton, Didcot, UI(. A hsf ract A description is given of the design and commissioning of a fast, switching E-field kicker system, for the temporal and spatial division of a 26.6 MeV/c muon beam. The system is an important component in a recent CEC funded up- grade of the surface muon experimental facility at ISIS, the high intensity neutron source, at the Rutherford Appleton I,aborat,ory, Chilt,on, Oxfordshire, linited Kingdom. 1 INTRODUCTION Thr ISIS pulsed muon facility as built in 1987 [l], deliv- ered a pair of 82 ns (fwhm) pulses, with a peak to peak separation of 340 ns, to a single experiment area. at a prf of 50 Hz. This double pulse time structure limited the available frequency range for /LSR. (muon spin resonance) to discrete frequencies up to a maximum of approximately 10 MHz. This disadvantage was removed in 1989 by the addition of a fast E-field kicker [2] t.hat, removed the sec- ond muon pulse The successful operation of this kicker increased the scientific utilisation of the facility at a cost of a factor of two in intensity. This was a tolerable loss, as the majority of experiments typically used only 30% of the available double pulsed muons, being limit.4 by high data acquisition rates at early times with the consequent severe distortions induced in t,he measured spectra. With a single experiment area, there was a three-fold over-subscription for beam time. To solve this problem, a proposal was made in 1989. to the CEC large facilities pro- gram, to extend the facility, and provide three experiment areas with the capability of simultaneous op(,ration wit.h single pulses at a prf of 50 Hz. This wa to be achieved with a temporal and spatial division of the muon pulsc~ pair by a fast switching E-field septum kicker. 2 MODE OF OPERATION A plan view of the CEC funded muon beam line is shown in Fig. 1, with the position of the kicker indicated by the letter K. Operation of the CEC’ kicker is illust,ratrd in Fig. 2. A simplified plan view shows the 70@mm long central anode, equidistant from t,hl> t,wo outer grounded cathodes. At the start of the cycle, the central anode is at a potential of 32.5 kV, producing an elect,ric field of about 0.6 MV/m in the gaps. The first muon bunch is bisected and subsequently deflected through angles of 66.5 mr by KEY K: ELECTROSTATIC KICKER SYl: SYITCHYARD HAGNET (0 14T) SY2: SUITCHYARD MAGNET (0.07T) SEP: SEPTUII BENOING MAGNETS ES: BEAM SLITS 6: BEAM BLOCKERS GV: GATE VALVES 94: STEERING MAGNETS IlIOA/C: NIKHEF OUAORUPOLES OIlA/C, NIKHEF OUAORUPOLES 0108: QUAORUPOLE MAGNET tM9) 0118: QUAORUPOLE HAGNET tH9) 012: CERN QUADRUPOLE HAGNETS K 09 09 i J--p1 -, z~-j-“;‘- 4wfl*=-- - -- 3 tl I Figure 1: Plan view of the CEC muon beam lines 2414 EC KICKER VOLTAGE .IZkV l uonpulrr (c-q L- worn .73anr 700.. r” -. pJ l uon puIa* I 1ocms Figure 2: *Mode of operation the electric field, to provide two simultaneous muon pulses to the outer (EMu and DeVA) areas. Just after the passage of the first muon pulse the kicker field is rapidly reduced to zero. The second muon pulse is consequently undeflected, and travels on to the MuSR area. 3 M.ECHANICAL CONSTRUCTION OF THE CEC KICKER The mechanical construction of the E field kic.ker is shown in Fig. 3. This design produces the required 66.3 millira- dian deflection, for 26.61 hleV/c posit,ive muons, with an electrical Irngt,h of 0.7 metres, and an applied voltage of 32.5 kV. Maximum deflection for a given electrical length, and applied voltage, has been achieved by matching the cur- vature of the outer electrodes to the path of the deflected muons. The finite element code PE2D [3] was used to model and optimise the electric field design. Components subject to high electric fields are manufac- tured from 304 grade stainless steel, and have been electro- polished to achieve a snitable surface finish [4]. The selec- tion of a 2.0 mm thick central electrode was dictated by the need to maintain adequate stiffness and flatness, a field level of not more than 50 kV/cm at the fully radiused elec- trode edge, and minimum muon loss (-5%). Other features of the design are the commercial high voltage feedthrough, the use of canted beryllium/copper spring rings, and fingerstrip to provide low impedance elec- trical connections between the ‘in vacuum’ components. The high voltage electrode support insulators are made from ‘PEEK’ [5], a strong, high temperature, advanced polymer, and have been designed using specialised tech- niques [6], 4 THE CEC KICKER DRIVE SYSTEM. A block diagram of the kicker system is shown in Fig. 4. This low power configuration is capable of charging the kicker electrode to 32.5 kV (iO.25%) in less than 15 ms: and discharging to 0 V (&0.25X of 32.5 kV) in less than 200 ns. The CX1528 medium power, single gap t,hyratron [7], was selected for its good high voltage holdoff, convenient air cooling, and the protnise of a long operational life in this application. Other features of the design are t.he commercial high voltage capacitor charging supply with internal gating function, stabilised DC heater and reservoir supplies (for reduced thyratron trigger jitter), and a commercial high voltage power mosfet thyratron trigger unit. A timing waveform schematic is shown in Fig. 5. At t=O, V2 has settled to 32.5 kV (f0.25%), and the thgra- tron grid driver has just received a trigger pulse. At t=150 ns the first muon pulse enters the kicker, and is horizon- tally divided and deflected through equal and opposite an- gles of 66.6 mrad, by the electric field. These muons are subsequently transported through a septum magnet to the 0 100 100 3OO.m Figure 3: CEC’ kicker showing side and plan views 2415 I --- CONTROL AREA -, r- --ii%. BEAML I NE I 1 4. ii\,’ L ---\~!;.---j 2 +I --- _- J.^\ r- .--__ , -4 -___ --_ ,., \ ~_F’ \ \ --1 El ‘*! F---------- ii f-\ k&4 : 1) 4 1 i\~ -_?--------- --- / t t i hl 1 OOV' Ch2 500 VW M 100 pIB 500&4 Ch4 IO?" c ns Aux / I 50 V L-----J l------L Figure 4: Block diagram of the CEC kicker system Figure 6: Action of CEC kicker on muon pulses H”0” Iticnr~ly 0 3: 5 i? Yi .nme Tr’ggcr H” &I :I::: vI---’ Sld”S l”hlblf 1 -;1;T :- ;-J-y;Fj-f-f$@- t I, .I-- : : : : 2 a 1 0 z: - E N F :: Figure 5: Timing and waveform schematic left and right experimental areas. At t=250 ns, the thyratron fires, and V2 decays with a time constant of Cl*(R2+R3) (25 ns). The timing is set so that at t=250 ns the first muon pulse has (just) passed through the kicker. At t=450 ns the second muon pulse enters the kicker and passes through, undeflected, to the central experimental area. At t=250 ps the high voltage power supply responds to the low impedance load at its output, and switches from constant voltage to inhibited mode. After an internally generated delay of about 1 ms, the supply switches to con- stant current enabled mode (t=1.2 ms). At t=17 ms: the system capacitance (500 pF) is charged to 32.5 kV: and settles to 0.25% of 32.5 kV by t=19.5 ms, ready for the next cycle. 5 OPERATIONAL MEASUREMENTS At the time of writing, the kicker system has run wit,h beam for more than 4500 hours, with few problems. The operating voltage, predicted by electrostatic field com- putation [3] of 32.0 kV compares favourably with that obtained by the optimisation of muon intensity in the DeV,4 and EMu beam lines (35.5 kV). The voltage wnve- form, measured wit,h a wide-band high voltage probe (Tek P6015A), shows t,hr volt,agc settling to < 0.25% of 32.5 kV in 180 ns. Fig. 6 shows the action of the CEC kicker on the muon pulses. The top trace shows the Cerenkov pulses reveai- ing the time structure of the protons at the production target. The second and fourth traces are derived from the muon detectors in the MuSR and EMU beam lines respec- tively, and show the relationship with the kicker voltage waveform (third trace) in distributing the two muon pulses to the EMu/DEV.4 and MuSR beam lines. 6 ACKNOWLEDGEMENTS Thanks are due to D.J.hlcPhail (mechanical design), M.Krendler (alignment), T.P.Parry (vacuum), C.D.IJden (engineering administration), C.A.Scott (helpful discus- sions), W.G.Wil1iam.s (project coordinator), and many others. 7 REFERENCES [I] G.H.Eaton, A.Carne, S.F.J.Cox, J.D.Davies, R.De Renzi, O.Hartmann, A.Kratzer, C.Ristori, C.A.Srott, G.C.Stirling and T.Sundqvist. Nuclear Inst. and hleth. A269( 1988) p.453. [2] A.I.Borden, A.Carne, M.A.Clarke-Gayther, G.H. Eaton, H.J.Jones, G.Thomas, D.Hartmann and T. Sundqvist. Nucl. Inst. and Meth. A292(1990) p.21 [3] PE2D “A two dimensional static and time varying field computational package.” Vector Fields Ltd, 24 Bankside, Kidlington, Oxford OX5 1JE [4] F.McCoy, C.Coenraads, M.Thayer, “Some effects of electrode metallurgy and field emission on high voltage insulation strength in vacuum”, Proc. of the First Int. Symp. on Insulation of high voltages in vacuum, MIT, USA, Oct. 1964, Addenda No. 1 [5] PEEK Polyaryletherketone, ICI Materials, PO Box 90, Wilton, Middlesborough, Cleveland TS6 8JE, England [6] J.P.Shannon, S.F.Philp, J.G.Trump, “Insulation of high voltage across solid insulat,ors in vacuum”, Proc. of the First Int. Symp. on Insulation of high volt,ages in vacuum, MIT, USA, Oct. 1964, p.281 - 304 [7] English Electric Valve Company Ltd, Waterhouse Lane, Chelmsford, Essex, CM1 2QU 2416 

A fast switching E-field kicker, for the temporal and spatial division of a 26.5 MeV/c surface muon beam

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