Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› Preliminary studies for fast chopper - Analysis of the existing projects and translating for EURISOL
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› PLAN 1.Short Introduction 2.First Studies 3.SNS / ESS fast choppers 4.SPL fast chopper 5.Conclusion and Perspectives
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› Photograph [ Kurennoy et al., PAC 1997 ] and Scheme of the coax-plate chopper structure used at Los Alamos Neutron Science Center for PSR (Proton Storage Ring). The voltages on the upper and lower plates are synchronized and have opposite signs. SHORT INTRODUCTION All choppers are based on the following well- known phenomena : an electrical field established by two symmetrical electrodes with opposite high voltages deflects the beam of its initial path. conductor plate shorter coaxial cable Beam
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› Straight-strip meander-folded line C 0 = capacitance L 0 = inductance Z c = (L 0 / C 0 ) 1/2 v φ = 1/(L 0 ∙C 0 ) 1/2 Beam TEM wave FIRST STUDIES according to Kurennoy (1) - Limitations of the coax-plate chopper : mutual coupling between adjacent strips and the coax-to- plate transitions. - Kurennoy et al. proposed a new design to reduce parasitic coupling : a meander microstrip line at 50 Ohm. - Such a structure has got no coaxial transitions and the chamfers avoid pulse reflexions. - The phase velocity depends on the product of the lineic capacitance C 0 and the lineic inductance L 0. The meander line matches the phase velocity of the TEM wave and the beam velocity along the beam path. - structure efficiency η : ratio of the deflecting field to that produced by continous plates under the same voltage. Match point
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› FIRST STUDIES according to Kurennoy (2) Notched meander-folded line C’ = capacitance L’ = inductance Z c ’ = (L’ / C’) 1/2 v φ ’ = 1/(L’∙C’) 1/2 Match point A new structure, the notched meander line, to : - decrease the strip-to-strip magnetic coupling, - increase of the efficiency η, - slow TEM wave down to the beam velocity : v φ’ < v φ (The notches add an inductance and modify the inductance par unit length) Nevertheless it adds « dispersion that can worsen the rise and the fall times ».
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› (Kurennoy et al. in LINAC 2000) (Hardekopf et al. in PAC 2003) SNS and ESS FAST CHOPPER (1) ParametersSNS (Los Alamos)ESS Beam energy2,5 MeV2 MeV β0,0730,0652 Meander line length / width350 mm / 98 mm240 mm / 78,2 mm Plate length / width500 mm / 130 mm388 mm / 91 mm Gap between plates18 mm16 mm Chopper : two symmetrical plates synchronized. On each a notched meander- folded line fed by synchronized opposite voltages to establish a electrical field deflects the beam.
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› (ESS - LINAC 2000) (Kurennoy et al. - EPAC 2000) Part of MAFIA model : - Notched conductor meander-folded strip (darkblue) - RT Duroid 6002 (contains Teflon) dielectric substrate (green) [ε r = 2,96] - Metal ridges (red) to reduce the coupling between adjacent strips. - Ground plate (light-blue). - Characteristic impedance : 50 Ω. SNS structure : - A T-shaped suspended substrate : lower dielectric constant, weaker losses. Decay of dispersion losses (due to the evolution of the relative permittivity with the frequency) SNS and ESS FAST CHOPPERS (2)
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› ParametersSNS (Los Alamos)ESS Beam energy2,5 MeV2 MeV β0,0730,0652 Pulser voltage±2,5 kV±950 V Rise / fall times1,5 ns< 2 ns v φ / c0,680,498 Deflection angle18 mrad20 mrad Fabrication based on the microwave integrate circuit technology. Vacuum test of RT duroid [ Kurennoy et al. - PAC 2001 ] : the steady-state out- gassing rate was estimated to be about 2∙10 -5 Torr∙liter∙sec -1. According to Kurennoy et al. this rate is a low value for their special cleaning procedures in the process of manufacturing the prototype. They expect even lower out-gassing rate after rigorous cleaning during fabrications steps. SNS and ESS FAST CHOPPER (3)
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› Translating the ESS – structure for EURISOL (1) Estimation of parameters and dimensions ! L = 107 mm RT Duroid [ h = 3 mm - ε r = 2,96 ] 10 mm First simulations based on requirements for Spiral 2 : β = 0,04 between the RFQ and the sc linac (beam current > 5 mA).
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› Translating the ESS – structure for EURISOL (3) Remarks : - Propagation time between two successive snapshots : 0,84 ns. - Distance covered by the beam : β∙c∙Δt = 0,04∙300∙0,84 = 10 mm ! - the shape of the pulse is distorted by developing overshoots caused by dispersion losses, parasitic coupling, HF filters, … - rise and fall times : 0,5ns, - hold time : 1,5 ns, - voltage : 1000 V Voltage versus time in 3 different points on the beam path in the chopper Input pulse Δt = 0,84 ns 10 mm
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› SPL FAST CHOPPER (1) Caspers asked if Roger’s glass microfibre PFTE composite (RT Duroid substrate) used by the SNS / ESS structures involves radiation aspects because it contains Teflon. If Caspers translates the ESS-design, the dimensions of the SPL chopper would be too large for his specifications. Caspers has suggested a new solution : a meander structure on aluminium oxide. According to him, using alumina on a massive metallic support increases the heat transfer capability and has “infinite” radiation resistance. The coupling separators are hard to implement for a ceramic substrate. Without separators the mutual coupling between adjacent strip increases. He had to find an optimum for the thickness of the substrate into SNS / ESS structure. But it reaches two limits ! If the substrate is too thin it will become too fragile and will involve a growth of conductor losses. If it is too thick the mutual coupling between adjacent strips increases. “For 2 ns rise and fall times (10% - 90%) the external drivers could become very costly for a deflection voltage of 0-1 kV (peak, unipolar) per deflecting plate. (1 kV into 50 Ohm requires 20 A !).” (Caspers)
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› After having tested straight meanders on a thick substrate he has proposed a symmetric double meander-folded structure in order to decrease the mutual coupling and to reach a feasible solution : SPL FAST CHOPPER (2) β = 0,08 Gap between plate = 20 mm Pulser voltage = 1kV Chopper length = 400 mm Rise / fall time < 2 ns Deflection angle = 6,8 mrad [ Caspers, LINAC 2004 ] BEAM 42,5 mm 6 mm 0,45 mm (100 Ω) 3 mm (50 Ω)
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› Translating the SPL–structure for EURISOL (1) Alumina (ε r = 9,6) 63,75 mm (=1,5*42,5) Δt = 0,5 ns Voltage versus time in 3 differents points on the beam path in the chopper (β∙c∙Δt = 0,08∙300∙0,5 = 6 mm) 6 mm
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› Conclusion and Perspectives (1) Fast power pulser : the main difficulty ! Necessity of beam requirements along the EURISOL MEBT Calendar : by June : Analysis of RIA project, Studies of power from existing project to translate for Eurisol chopper Continue of preliminary studies by July : Definition of the Prototype Specifications. by August : first prototype development. Estimated parametersESS designSPL design Width107 mm64 mm
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#›
Gwenal Le Dem – EURISOL DS Heavy Ion AcceleratorTask 6 - Meeting 03/05/2006‹#› Conclusion and Perspectives (2) ParameterPresent ValueDepending onFinal Choice date/ comments Linac frequency MHz Available length650 mmMEBT Beam dynamics Available widthMEBT Beam dynamics Required voltage3 kVAvailable length and gap height Switching time6 nsRise and fall time between bunches Pulse duration99 T Repetition rate100 T T modeCWLinac mode Beam velocity0.04cRFQ output Energy Beam intensity5 mA Beam q/m ratio0.5 to 0.33 Beam emittance Emittance enlargement allowed Security modeNo pulse = no bunch Power consumption/ complexity Not implemented on other choppers