Improvement of Accelerator of NNBI on LHD M
Improvement of Accelerator of N-NBI on LHD M. Kisaki 1, K. Ikeda 1, M. Osakabe 1, 2, K. Tsumori 1, 2, H. Nakano 1, 2, S. Geng 2, K. Nagaoka 1, 2, O. Kaneko 1, 2, Y. Takeiri 1, 2 and LHD-NBI group 1 National Institute for Fusion Science, Japan 2 SOKENDAI (The Graduate University for Advanced Studies), Japan 1 This study is supported by NIFS (NIFS 14 ULRR 015, NIFS 14 ULRR 702) and KAKENHI Grant No. 25249134.
NBIs on LHD and performance of N-NBI 2
Large Helical Device (LHD) Plasma major radius 3. 42 -4. 1 m Plasma minor radius 0. 6 m Plasma volume 30 m 3 Toroidal magnetic field strength 0. 4~2. 9 T One of the world largest magnetically confined plasma devices with superconducting magnet. All of the confining magnetic fields are externally applied on LHD. Ideal for the steady state operation ~2 m and free from disruptions. Illustration of LHD 3
Neutral Beam Injection systems on LHD Two types of Neutral Beam Injection systems are now under operation on LHD 1. 2. High energy (180 ke. V (H), 5 MW) tangential NBI based on negative-ion sources (filament arc) • The injection power and energies are expanded. • Further R&D activity to improve the performance. Low energy (40 ke. V (H), 9 MW) radial NBI based on positive-ion sources (filament arc) BL 5 (positive NBI) BL 2 (negative NBI) BL 1 (negative NBI) LHD BL 3 (negative NBI) BL 4 (positive NBI) 4
Performance of NIFS N-NBI BL 1 Injection power obtained with three N-NBIs has reached and kept 15 MW every year by optimizing caesium dose and beam control. The H- current density has reached 340 A/m 2, and the value exceeds the target current density of ITER NBI. 5
D plasma experiment on LHD D plasma experiment will start soon. High power beam injection is demanded with higher stability. BL 2, BL 3 BL 1 2, 3 No. of grids 4 4 Type of grounded grid MSGG (slot) MAGG (aperture) Injection power [MW] ~6 ~5 Voltage holding capability 〇 ▲ Heat load on GG 〇 ▲ 1. Improvement of voltage holding capability 2. Reduction of grid heat load on GG Modification of accelerator to achieve these issues cross-section of accelerator of NIFS N-NBI MSGG (Multi-Slot Grounded Grid) MAGG (Multi-Aperture Grounded Grid) 6
Improvement of voltage holding capability for BL 3 accelerator 7
Electric field near GG support strong electric field (8. 14 k. V/mm) EG+SG R 3 GG Cross-sectional view of accelerator of BL 2 and BL 3 (EG, SG and GG) Map of electric field strength The edge of GG support has the relatively small curvature radius (3 mm). The electric field becomes strong. 8
Relaxation of electric field near GG support EG+SG To moderate the electric field near the edge of GG support, the field limiting ring with R 12 was installed. R 3 Previous removed GG Modified EG+SG R 12 GG Maximum electric field strength is reduced from 8. 14 k. V/mm to 7. 0 k. V/mm 9
Voltage holding capability Break-down rate = number of break-down / pulse duration Previous Modified The voltage holding capability is improved, especially at high acceleration voltage (> 160 k. V). 10
Application of slot GG for BL 3 accelerator 11
Structure of SG EG PG SG Slot GG Circular SG m a e B Racetrack SG Structure of SG has very important role for beam optics of the slot GG
Beam optics of slot GG with circular SG Experiment SG aperture Φ 13 GG Beam GG Potential map at slot GG (vertical) Beam width in horizontal and vertical directions are minimized at different voltage ratio. diverging lens only in vertical direction at slot GG. higher electric field to converge the beam. 13 Compensation of the lens effect at SG
Design of racetrack SG by simulation 12 13 WH/WV = 1. 2 11 WH/WV = 1. 3 10 10 13 WH/WV = 1. 18 To converge the beam more in vertical direction, the vertical width of SG aperture is decreased. Racetrack shape Beam trajectories were calculated for different aspect ratios of SG aperture with OPERA-3 d. designed the new SG with racetrack aperture (WH/WV = 1. 18) 14
Experimental verification of designed racetrack SG WH/WV = 1. 18 11 13 Typical operating range It was confirmed that the beam optics in horizontal and vertical directions become optimum at the same voltage ratio. 15
Improved performance of ion source using slot GG with racetrack SG 16
Reduction of heat load on GG Heat load on GG is decreased by 40%. 17
Improvement of arc efficiency Beam current = Heat load on dumps / Beam energy The arc efficiency is improved without any modification on the plasma source. 18
Foot-print on back plate Aperture GG Slot GG Foot-print of back-streaming ion is formed on the back plate of plasma chamber during experiments. contributes to the improvement of the arc efficiency? 19
Effect of back-streaming ion Aperture GG 23. 8 mm 2 Slot GG 28. 3 mm 2 Heat load on back plate by back-streaming ion (obtained by EAMCC 1, 2) The back-streaming ion impinges on the back plate in larger area with slot GG This could affect the Cs recycling from the wall and improve the arc efficiency. 1 G. Fubiani, et. al. , Phys. Rev. ST Accel. Beams 11, 014202 (2008), 2 N. Fonnesu, et. al. , AIP Conf. Proc. 1655, 050008 (2015). 20
Summary To enhance the injection power of N-NBI on LHD, the accelerator of BL 3 was modified. 1. The field limiting ring was installed to the upper frame of the GG support. ü The voltage holding capability is improved, especially at high acceleration voltage. 2. The slot GG was applied with the racetrack SG designed by OPERA simulation. ü The heat load on GG is decreased by 40%. ü The arc efficiency is significantly improved only by replacing the GG. è beneficial to filament lifetime, arcing avoidance, etc. !!! It is foreseen that BL 2 and BL 3 can inject the neutral beam of more than 5 MW with higher stability. 21
Analysis of particle trajectories by EAMCC: Electrostatic Accelerator Monte Calro Code In EAMCC, the heat load on GG is mainly caused by the electrons. The heat load on GG is reduced because of the higher transmission probability of the particles through GG. 22
Cross-sectional view of accelerator of BL 1 Cross-sectional view of accelerator of BL 2 and Bl 3
Co-extracted electron
Before Cs seeding
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