Clarke-Gayther MA

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SLOW-WAVE ELECTRODE STRUCTURES FOR THE  ESS 2.5 MeV FAST CHOPPER Michael  A. Clarke-Gayther, CLRC RAL, Didcot, United Kingdom, Yi-Nong Rao†, Dag Reistad, TSL Uppsala, Sweden.                                                             † Now at TRIUMPH, Vancouver, BC, Canada. Abstract Slow-wave electrode designs for the European Spallation Source (ESS) 2.5 MeV fast beam chopper are described. The proposed meander folded strip-line structures feature a composite ceramic pillar / vacuum dielectric with excellent radiation hard and vacuum compatible properties. Broad-band transmission line properties have been optimised by modelling time dependent electromagnetic fields in the three dimensional, finite element frequency domain (FEFD) MAFIA, and HFSS codes, and in the finite difference time domain (FDTD) CONCERTO code.  1 INTRODUCTION The European Spallation Source (ESS) [1], is the most ambitious of the existing proposals for the next generation of accelerator driven pulsed neutron sources. Stringent beam loss requirements dictate that the chopping field in the 2.5 MeV medium energy transport line (MEBT), rise and fall (1 to 90%, 90 to 1%) within the beam bunch interval of 2.9 ns. Slow wave (E-field) transmission line structures have demonstrated field transition times in the nanosecond regime [2, 3], and ESS chopping schemes utilising these structures, have been identified [4] and refined [5].  The structures are designed using finite element field modelling codes, where complex geometry, electrical length, and inter-electrode coupling set a practical limit on the computational accuracy of the broad-band pro-perties. Speed of computation and / or accuracy of the computed properties for the proposed structures, have been enhanced, by identifying small repetitive modules, and modelling their properties in the three dimensional finite element frequency domain (FEFD) HFSS, and MAFIA codes, and the finite difference time domain (FDTD) CONCERTO code. Time domain characteristics for the complete structures were analysed in a SPICE based circuit simulator where the complete structures were modelled by linking the repetitive two-port modules in large series arrays.      2 SLOW-WAVE ELECTRODE DESIGN  Basic design parameters for the proposed slow wave electrode structures are shown in Figure 1, where partial chopping of beam bunches is avoided by ensuring that the deflecting E-field propagates at the beam bunch velocity.  Figure 1: Slow-wave electrode design  In Figure 1:  L2 is the transverse extent of the beam. T(L1) is the beam transit time for distance L1. T(L2) is the pulse transit time in vacuum for distance L2. T(L3) is the pulse transit time in dielectric media (A) for distance L3 and L4 is the electrode width.  For the generalised slow wave structure: Maximum value for L1 = V1 (T3 - T1) / 2 Minimum Value for L1 = L2 (V1/ V2) T(L1) = L1/V1 = T(L2) + T(L3)  The relationships for field (E), and transverse displace-ment (x), where q is the electronic charge, ν is the beam velocity, m0 is the rest mass, z is the effective electrode length, θ is the required deflection angle, V is the deflecting potential, and d is the electrode gap, are:   zqmE⋅⋅⋅=20tanνθ , dVE = , 2022 ν⋅⋅⋅⋅=mzEqx   Inspection shows that for given values of m0, ν, and V, large θ and x are obtained when z is large and d is small. The inter-electrode gap, shown as L1-L4 in Figure 1, must be made significant if pulse distortion (dispersion) due to inter-electrode coupling is to be minimised.   For a given overall structure length, the effective length (z) will therefore, be maximised, by maximising the electrode width (L4).  Proceedings of EPAC 2002, Paris, France21333 CANDIDATE ELECTRODE STRUCTURES Two, candidate, slow-wave structures are presented. Both structures feature a vacuum dielectric, strip-line configuration near the beam axis. The dispersive effect of inter-electrode coupling has been minimised in these designs by ensuring that coupling occurs only in a region near to the beam axis. Key parameters, common to both designs, are listed in table 1.   Table 1:Key Parameters  Beam velocity ν 2.184e7 m.s-1 Mechanical length   400.0 mm Effective length z 294 mm Electrode to beam axis gap d 6.0 mm Beam aperture  11.0 mm Deflection angle θ 16 mr Deflection potential  (±) V 1.6 kV Characteristic impedance Zo 50 ± 0.5 Ω Total structure delay  18.04 ns No. of sections  21  Section delay T(L1) 0.87 ns Pulse transition time (1-90, 90 –1%)  2.9 ns Pulse duration (fwhm)  8.0 ns Pulse aberration (droop / overshoot / reflections)   < 2 % pulse max. Structure bandwidth  0.2 - 500 MHz Section pitch L1 19.0 mm Strip-line width L4 14 mm Strip-line thickness  0.5 mm  3.1 Electrode structure A A mechanical schematic of electrode structure A is shown in Figure 2, where the planar strip-line structure is supported at intervals, on ceramic pillars in vacuum. Thin walled separators extend full height, and minimise inter-electrode coupling.  Strip-line bends and  Figure 2: Electrode structure A supports are capacitively compensated by mitre and notch dimensioning, respectively. A central grounded plate ext-ends full length, and minimises upper to lower electrode coupling. Finite element models for two modules, identified as MOD A1/A2 and B1 in figure 2, were analysed. Simulat-ed S-parameters for the modules are shown in Figure 3.   Figure 3: Structure A / Module S-Parameters  Time and frequency domain characteristics for the complete structure were computed by circuit simulation of series connected, two port modules, and lumped element inter-electrode, coupling capacitors, as shown in Figure 4.  Figure 4: Structure A / SPICE model   The simulated pulse transmission characteristics for the complete structure are shown in Figure 5. Comparison with the structure B response, as shown in Figure 9, suggests that structure A exhibits lower transmission loss and less pulse shape distortion, but that the characteristic impedance is not as tightly controlled. Overshoot and undershoot are shown to be a function of inter-electrode coupling capacitance. This effect is more pronounced for the higher bandwidth structure A, than for structure B.  Figure 5: Structure A / Transient Response  Proceedings of EPAC 2002, Paris, France21343.2 Electrode structure B A mechanical schematic of electrode structure B is shown in Figure 6, where the compact helical structure is formed by strip-line sections near the beam axis, linked by semi-circular sections of semi-rigid coaxial cable (UT-390). Coaxial to strip-line transitions, and ceramic strip-line supports, are capacitively compensated by structure and notch dimensioning, respectively.       Figure 6: Electrode structure B  Finite element models for two modules, identified as A1/A2 and B1/B2 in Figure 6, were analysed. Simulated S-parameters for the modules are shown in Figure 7, where the S21/A1/A2 for the coaxial cable shows good agreement with manufacturers’ data.     Figure 7: Structure B / Module S-Parameters  Figure 8: Structure B / SPICE model   Time and frequency domain characteristics for the complete structure were computed by circuit simulation of series-connected two-port modules and lumped element inter-electrode coupling capacitors, as shown in Figure 8. The simulated pulse transmission characteristics for the complete structure are shown in Figure 9.       Figure 9: Structure B / Transient Response  4 SUMMARY Slow-wave electrode structures for the ESS 2.5 MeV   beam chopper have been modelled using finite element and circuit simulation codes. The computed frequency and time domain characteristics indicate that the candidate structures may meet the required specifications for chopper pulse fidelity. Speed of computation and / or accuracy of computed properties have been enhanced by the analysis of repetitive modules and inter-electrode coupling elements in a circuit simulation code.   5 REFERENCES [1] The ESS Accelerator and Beam Lines with the 280/560 MHz Linac, Report No. ESS-01-1A, December 2001.  [2] S. S. Kurennoy, and J. F. Power, ‘Development of Meander-Line current structure for SNS fast 2.5 MeV Beam Chopper’, Proc. of the 7th European Particle Accelerator Conference, Vienna, Austria, June 2000, p.336-338.   [3] J. S. Lunsford and R. A. Hardekopf, ‘Pulsed beam chopper for the PSR at LAMPF’, IEEE Trans. Nucl.Sci., NS-30, 1983. [4] M. A. Clarke-Gayther, ‘Modulator Systems for the ESS 2.5 MeV Fast Chopper’, Proc. of  the 2001 Particle Accelerator Conference, Chicago, USA,  p. 4062-4065. [5]  M. A. Clarke-Gayther, ‘A Fast Chopper for the ESS 2.5 MeV Beam Transport Line’, this conference.  Proceedings of EPAC 2002, Paris, France2135

Slow-wave electrode structures for the ESS 2.5 MeV Fast Chopper

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