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

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 field was generated by external permanent magnets to lower the electron temperature in a large-area bucket plasma source (35 cm x 62 cm) for efficient H- production. We investigated the magnetic field configuration and found a low electron temperature high density plasma (<1 eV, 10(12)/cm(3)) could be achieved with an optimized configuration, The filter strength (B-max = 70 G, line-integral flux = 780 G cm at the renter axis of the source) was proved to be enough to lower the electron temperature below 1 eV at high are 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 16.9-A H- beam current (31 mA/cm(2), 37 kV) using a large-area, four-grid electrostatic extraction system (25 cm x 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.