1 / 12

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

This paper discusses the acceleration of 1 MeV H- ion beams at ITER NB with a high current density. It focuses on the procurement arrangement for NBTF, the development of the MeV accelerator, and progress in voltage holding and beamlet deflection compensation. The achievement of a 0.98 MeV H- ion beam and reduction in grid heat load are also presented.

bridgesf
Télécharger la présentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. FTP/1-2 Acceleration of 1 MeV H- ion beamsat 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 24th IAEA Fusion Energy Conference San Diego,CA, USA October 8th, 2012

  2. NBTF/ITER NB procurement by JA DC generators 1 MV insulating trans. -200kV • HV bushing • Feedthrough for bus bars and water pipes • Bulkhead for SF6and vacuum. • Double layers: • f1.56 mceramic rings • Large FRP rings -400kV -600kV HVD2 -800kV -1MV HVD1 (EU) • Five rings in series for 1 MV insulation. Transmission line 90m ITER NB Injector 16.5 MW D0 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/m2 Procurement arrangement for NBTF signed in Feb. 2012.

  3. MeV accelerator for ITER R&D The MeV accelerator has been developed at JAEA since 1994. Due to RIC in ITER, vacuum insulation for 1 MV high voltage Metal flange MeV accelerator Large stress ring Triple junction FRP insulator ring The stress rings suppress surface flashover on FRP. 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.

  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 . • 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. • The gaps were extended to mitigate the local field. Accelerators show lower voltage holding than equal field. Gaps extension to mitigate local stress, 1 MV for more than 1 hour, Beam energy increased: 880 keV (a) Before (b) After 3.7 6.3 4.2 3.9 5.8 6.4 4.2 2.8

  5.  Beamlet deflections Extractor Measured beam footprint 500 keV S N S N S N Beamlets in each row deflected by magnetic field in alternative directions Outermost beamlets deflected by magnetic deflection + space charge repulsion The outermost beamlets are deflected with largerer angle, intercepted at grids giving high heat load, Beam current reducedand target temperature rise are lower.

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

  7. Aperture offset and kerb Footprint obtained by 3D analysis Before compensation Kerb against space charge repulsion Aperture offset against magnetic deflection Magnetic deflection: 4.7mrad Magnetic deflection + space charge repulsion: 9.5 mrad • 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 dk = 16 mm.

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

  9. Achievement of 0.98 MeV H- ion beam Measured grid heat load Summary of beam progress Compensation resulted in reduction of grid heat load (17% -> 10%). Compensation of beam deflection led to improvement of voltage holding Achievement of 0.98 MeV 185 A/m2 H- ion beam. World first achievement of ITER-relevant negative ion beams.

  10. EAMCC: secondary particle analysis • Further reduction in heat load , further improvement in voltage holding, and so, Long pulse 1 MeV beam. PG EXG Stripped electrons generated in and around extractor Another source of grid heat load is accelerated secondary particles. A1G MeV accelerator in 2007 geometry 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 • 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. H- H0 e- H+

  11. Suppression of electron acceleration 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. • 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. • Grid heat load in ITER can be reduced further for long pulses.

  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/m2 H- ion beam acceleration achieved. • This almost satisfies the ITER NB requirements (1 MeV, 200 A/m2 D-). • For further reduction of grid heat load, EAMCC analysis has run: • 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. • 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 facilityfrom damages of the 3.11.

More Related