Optimization of Cs deposition in the 1/3 scale hydrogen negative ion source for the large helical device-neutral beam injection system

A compact cesium deposition system was used for direct deposition of cesium atoms and ions onto the inner surface of the 1/3 scale hydrogen negative ion source for the large helical device-neutral beam injection (LHD-NBI), system. A small, well defined amount of cesium deposition in the range of 3-200 mg was tested. Negative ion extraction and acceleration were carried out both in the pure hydrogen operation mode and in the cesium mode. Single Cs deposition of 3-30 mg to the plasma chamber has produced temporary 2-5 times increases of H- yield, but the yield was decreased within several discharge pulses to the previous steady-state value. Two consecutive 30 mg depositions done within a 3-5 h/60 shot interval, produced a similar temporary increase of H- beam, but reached a large H- yield steady-state value. Deposition of larger 0.1-0.2 g Cs portions with a 20-120 h/150-270 shot interval improved the H- yield for a long (2-5 days) period of operation. Directed depositions of Cs to the various walls of the plasma chamber showed approximately the same H- increase. Deposition of 0.13 g Cs to a surface polluted by a water leak, produced a temporary increase, and a H- steady-state level similar to that from a single 30 mg cesium deposition. Deposition of 0.1 g with a cesium plasma produced one half the H- yield obtained by deposition of the same amount of cesium atoms. A higher steady-state H- current value and a smaller rate of H- yield decrease was recorded during the eight filament discharge operation, as compared to the 12 filament operation at the same discharge power. (C) 2000 American Institute of Physics. [S0034-6748(00)62102-0].