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Production of high-current large-area H- beams by a bucket-type ion source equipped with a magnetic filter

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8 Author(s)
Tanaka, M. ; Power & Ind. Syst. R&D Div., Hitachi Ltd., Ibaraki, Japan ; Takeiri, Yasuhiko ; Asano, E. ; Kaneko, Osamu
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A negative-ion-based neutral beam injection (NBI) system is planned for plasma heating of the Large Helical Device (LHD). We have developed a negative ion source, which is 1/3 the scale of the source for the NBI. A magnetic filter held was generated by external permanent magnets to lower the electron temperature in a large-area bucket plasma source (35 cm×62 cm) for efficient H- production. We investigated the magnetic field configuration and found a low electron temperature high density plasma (<1 eV, 1012/cm3) could be achieved with an optimized configuration, The filter strength (Bmax=70 G, line-integral flux=780 G cm at the center axis of the source) was proved to be enough to lower the electron temperature below 1 eV at high arc discharge power (100 kW) and low pressure (0.4 Pa). We injected cesium vapor into the plasma source to enhance H- production efficiency and obtained a 16.2-A H- beam current (31 mA/cm2, 47 kV) using a large-area, four-grid electrostatic extraction system (25 cm×50 cm). This satisfied the development target (>15 A: 1/3 current of LHD ion source). Based on the results, we are designing a negative ion source for the LHD

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Plasma Science, IEEE Transactions on  (Volume:25 ,  Issue: 6 )