1. 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 24 th IAEA Fusion Energy Conference San Diego, CA, USA October 8 th, 2012 FTP/1-2

2. -200kV -400kV -600kV -800kV -1MV 1 MV insulating trans. DC generators HVD1 (EU) HVD2 Procurement arrangement for NBTF signed in Feb. 2012. NBTF/ITER NB procurement by JA HV bushing Feedthrough for bus bars and water pipes Bulkhead for SF 6 and vacuum. Double layers: −  1.56 m ceramic rings − Large FRP rings ITER NB Injector 16.5 MW D 0 at 1 MeV for 1 hour 1 MV dc power supply MAMuG accelerator Large electrostatic accelerator Multiaperture Multigrid 1 MeV, 40 A D - ion beams at 200 A/m 2 Five rings in series for 1 MV insulation.

3. MeV accelerator for ITER R&D MeV accelerator The MeV accelerator has been developed at JAEA since 1994. Due to RIC in ITER, vacuum insulation for 1 MV high voltage Progress in 2010 - 2012  Improvement of voltage holding by mitigation of local electric field,  Compensation of beamlet deflection using a 3D trajectory analysis, Analysis of gird heat load reduction for long pulse operations. Large stress ring FRP insulator ring Triple junction Metal flange The stress rings suppress surface flashover on FRP. FTP/1-2 INOUE 3/12

4.  Voltage holding improvement Insulation of high voltage (-200 kV) in long vacuum gap (50-100 mm) Vacuum insulation in MeV accelerator is “low average electric field”, different parameter ranges than positive ion sources. Gaps extension to mitigate local stress, 1 MV for more than 1 hour, Beam energy increased: 880 keV (a)Discharge marks at the opposite sides of edges/steps, where local stress was higher than several kV/mm. Breakdowns triggered by local high stress in long vacuum gap. (b)The gaps were extended to mitigate the local field. 6.3 5.8 6.4 3.7 4.2 3.9 4.2 2.8 (a) Before (b) After Accelerators show lower voltage holding than equal field. FTP/1-2 INOUE 4/12

5.  Beamlet deflections The outermost beamlets are deflected with largerer angle, intercepted at grids giving high heat load, Beam current reduced and target temperature rise are lower. ExtractorMeasured beam footprint Outermost beamlets deflected by magnetic deflection + space charge repulsion Beamlets in each row deflected by magnetic field in alternative directions 500 keV S S S N N N FTP/1-2 INOUE 5/12

6. 3D beam trajectory analysis OPERA-3D code (Vector Fields software) H - beam ①Magnetic filter and electron suppression magnets ②Current reduction by stripping loss A1G A3G A2G GRG A4G Extractor ③Aperture offset at ESG for magnetic deflection: 0.8 mm ④Kerb at ESG backside for space charge repulsion: 1 mm t ⑤Grid support structure    FTP/1-2 INOUE 6/12

7. Aperture offset and kerb Aperture offset against magnetic deflection For compensation of the beamlet deflection due to: Magnetic field, Aperture offset δ = 0.8 mm Space charge repulsion, Kerb with 1 mm in thickness at d k = 16 mm. Kerb against space charge repulsion Magnetic deflection: 4.7 mrad Magnetic deflection + space charge repulsion: 9.5 mrad Footprint obtained by 3D analysis Before compensation FTP/1-2 INOUE 7/12

8. Compensation of beamlet deflections Measured at 0.98 MeV Intensity is high even in outermost beamlets, Less interception of beams at grids, and hence, low grid heat load Calculated at 1 MeV Footprint envelope closer to square shape Smaller distance between beamlets Aperture offset Kerb are effective for compensation FTP/1-2 INOUE 8/12

9. Achievement of 0.98 MeV H - ion beam Compensation of beam deflection led to improvement of voltage holding Achievement of 0.98 MeV 185 A/m 2 H - ion beam. World first achievement of ITER-relevant negative ion beams. Measured grid heat loadSummary of beam progress Compensation resulted in reduction of grid heat load (17% -> 10%). FTP/1-2 INOUE 9/12

10. Further reduction in heat load, further improvement in voltage holding, and so, Long pulse 1 MeV beam.  EAMCC: secondary particle analysis Another source of grid heat load is accelerated secondary particles. Grid heat loads are mainly caused by: 1) Electrons stripped from H - ions in extractor, 2) Direct interception of H - ions at grid aperture, 3) Secondary electrons emitted at grid aperture. MeV accelerator in 2007 geometry Stripped electrons generated in and around extractor Secondary electron emission by collision of H 0 on grid side wall Stripped electrons deflected by EXG magnet PG EXG A1G A2G A3G A4G GRG H-H- e-e- H0H0 H+H+ Direct interception of H - ions FTP/1-2 INOUE 10/12

11. Suppression of electron acceleration Geometry20072010 Thickness20 mm10 mm Aperture dia.  16 mm A1-A2G:  14 A3G-GRG:  16 By reduced aperture dia. stripped electrons were intercepted in A1G and A2G, and subsequently heat loads on A3G- GRG were decreased. Total heat load was decreased from 49 kW to 35 kW. Modification of grid geometry: MeV accelerator in 2010 geometry A test of MeV accelerator showed: Higher heat load in A1G, Similar tendencies of heat load distribution among grids. By reduced grid thickness, area for ion collision is reduced inside aperture − heat loads by secondary electrons decreased in all grids, − direct interception of H - ions decreased in A4G, GRG. Grid heat load in ITER can be reduced further for long pulses. FTP/1-2 INOUE 11/12

12. Conclusion Voltage holding improvement, and compensation of beam deflections have been applied to the MeV accelerator. Reduction of beam direct interception at grids has brought further improvement in voltage holding during beam acceleration. 0.98 MeV, 185 A/m 2 H - ion beam acceleration achieved. This almost satisfies the ITER NB requirements (1 MeV, 200 A/m 2 D - ). For further reduction of grid heat load, EAMCC analysis has run: −Smaller aperture (  14 mm) in upstream grids (A1G and A2G) to stop electrons before acceleration to high energy, −Thinner grid to suppress secondary electron generation. Preliminary results of the MeV accelerator and EAMCC show possible further reduction of grid heat load to an acceptable level. Long pulse test of the MeV accelerator will be restarted in 2013 after recovery of the facility from damages of the 3.11. FTP/1-2 INOUE 12/12