Acceleration of 1 MeV H- ion beams at ITER NB relevant high ...

Acceleration of 1 MeV H- ion beams at ITER NB relevant high ...

FTP/1-2 24 th IAEA Fusion Energy Conference San Diego, CA, USA, October 8-12, 2012 Acceleration of 1 MeV H- ion beams at ITER NB relevant high current density Takashi INOUE, M. Taniguchi, M. Kashiwagi, N. Umeda, H. Tobari, M. Dairaku, J. Takemoto, K. Tsuchida, K. Watanabe, T. Yamanaka, A. Kojima, M. Hanada and K. Sakamoto Japan Atomic Energy Agency (JAEA) NBTF / ITER NB procurement by JA DC generators -200kV -400kV -600kV -800kV -1MV 1 MV insulating trans. HV bushing Feedthrough for bus bars and water pipes Bulkhead for SF6 and vacuum. Double layers: f1.56 m ceramic rings Large FRP rings MeV accelerator for ITER R&D HVD2 HVD1 (EU) The MeV accelerator have been developed at JAEA since 1994. Vacuum insulation, instead of SF6 gas, for 1 MV high voltage to avoid RIC. MeV accelerator Metal flange Five rings in series for 1 MV insulation. Transmission line 90m ITER NB Injector 16.5 MW D0 at 1 MeV for 1 hour Large stress ring Triple junction (cathode) 1 MV dc power supply MAMuG accelerator Large electrostatic accelerator Multiaperture Multigrid 1 MeV, 40 A D- ion beams at 200A/m2 Procurement arrangement for NBTF signed in Feb. 2012. FRP insulator ring The stress rings suppress surface flashover on FRP. Issues tackled in 2010 - 2012 Improvement of voltage holding by mitigation of local electric field, Compensation of beamlet deflection by a 3D trajectory analysis, Gird heat load reduction by secondary particle suppression. Voltage holding improvement Mockup test of HV bushing Single stage test rated voltage: 200 kV Two stage test (rated voltage: 400 kV) with electrostatic screens inside From 1 MV PS Insulation of high voltage (160-200 kV) in long vacuum gap (50-100 mm) (a) Before (b) After Accelerators show lower voltage holding than equal field. SF6 gas 6.3 5.8 6.4 -1 MV 1.56 m FRP (outer layer) Ceramic ring (inner layer) Vacuum 0V Rogowski electrodes Cathode Gap: 0~50 mm To ion source, accelerator Anode Small grooves simulating high local field in gaps HV test interrupted by the 3.11. Reduction of secondary particles Breakdowns triggered by local stress in wide vacuum gaps. Vacuum insulation improved by lowering local stress, 1 MV for 4,000 s at 2 x 10-4 Pa. Beam parameters increased: 879 keV, 157 A/m2 from previous: 796 keV, 140 A/m2 Experimental and EAMCC results EAMCC analyzes behavior of secondary particle in accelerators. (EAMCC: Electrostatic Accelerator Monte-Carlo Code) PG An example of EAMCC analysis for MeV accelerator Stripped electrons EXG MeV accelerator in 2007 geometry 4.2 (a)Discharge marks at the opposite sides of edges/steps, where local stress was higher than 3-4 kV/mm. (b)The gaps were expanded to mitigate the local field. ~2.5 m Vacuum insulation test at HV (1)-240 kV 120% of rated voltage) for 2 hours. (2)Long period stability test (-220 kV, 5 hours) demonstrated without breakdown. 2.8 3.7 4.2 3.9 Measured heat load by water calorimetry Comparison of heat loads Aperture dia. 14 generated in and around extractor

A1G Stripped electrons deflected by EXG magnet A2G Secondary electron emission by collision of H0 on grid side wall A3G A4G Direct interception of H- ions GRG He- Grid heat loads are mainly caused by: 1) Stripped electron from extraction gap, 2) Secondary electrons generated from grid side wall, 3) Direct interception of H- ions at grid aperture H0 H+ Comparison of heat load by stripping & ionization between EAMCC and experiment. (32 kW by EAMCC and 31 kW by experiment) Tendency of heat loads in each grids are well represented in EAMCC, such as heat loads high in A2G and low in A3G. Heat load on A1G is twice higher in exp., suggesting contribution of stray electron/ H- ion or backstreaming, which are not taken into account properly in EAMCC. Conclusions Voltage holding improvement, and compensation of beam deflections have been applied to the MeV accelerator. Reduction of beam direct interception at grids has brought improvement in voltage holding during beam acceleration at around 1 MV. 0.98 MeV, 185 A/m H ion beam acceleration achieved. This almost satisfies the ITER NB requirements (1 MeV, 200 A/m 2 D-). 2 - EAMCC analysis has started and followings has examined: Smaller aperture (f14 mm) in upstream grids (A1G and A2G) to stop electrons before acceleration to high energy, Thinner grid to suppress secondary electron generation. An extrapolation of EAMCC analysis shows that highest grid heat load is to be decreased to 0.84 MW in ITER, which meets the design criteria since ITER EDA (1 MW). The long pulse test of the MeV accelerator will be restarted in 2013 after recovery of the MeV test facility from damages of the 3.11. Beamlet deflections Extractor 3D beam trajectory analysis Extractor Measured beam footprint at 500 keV A1G A3G A4G H- beam A2G Magnetic filter and electron suppression magnets Current reduction by stripping loss Aperture offset at ESG: 0.8 mm Kerb at ESG backside 1 mmt Grid support structure GRG Deflection direction of the beamlets due to magnetic field. Peripheral beamlets were much deflected. The beamlets in each row are deflected in the alternative directions line by line due to the magnetic field in the extractor. The peripheral beamlets are deflected outward more due to superposition of i) and ii) space charge repulsion. Compensation techniques; Aperture offset and kerb Aperture offset against magnetic deflection Kerb against space charge repulsion Beam footprint before compensation 4.7 mrad 9.5 mrad OPERA-3D code (Vector Fields software) At 0.98 MeV After compensations Application to the experiment For compensation of the beamlet deflection of 1 MeV beam; Aperture offset = 0.8 mm Kerb with 1 mm in thickness at dk = 16 mm. The beamlet deflections were compensated in the beam acceleration test as the 3D multi beam analysis. The compensation techniques were successfully demonstrated using 1 MeV beams. Achievement of 0.98 MeV H- ion beam Suppression of secondary electrons Modification of grid structure: MeV accelerator in 2010 geometry Geometry 2007 2010 Thickness 20 mm 10 mm Aperture dia. f16 mm A1-A2G: f14 A3G-GRG: f16 Grid heatload load EAMCC shows reduction in total heat load from 49 kW to 35 kW by reducing thickness and aperture diameter. By reduced grid thickness, heat loads by secondary electrons decreased in all grids, direct interception of H- ions decreased in A4G, GRG. By reduced aperture dia. stripped electrons were intercepted in A1G and A2G, and subsequently heat loads on A3G- GRG were decreased. Maximum heat load is expected to be 9.8 kW on GRG. Extrapolating 9.8 kW on 15 apertures in MeV accelerator to ITER (1280 apertures), total heat load is expected to be 0.84 MW. This is below design target of 1 MW. Compensation of beam deflection resulted in reduction of grid heat load (17% 10%). Improvement in voltage holding during beam acceleration, Consequently the achievement of 0.98 MeV 185 A/m2 H- ion beam.

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