Optimization of Cs deposition in the 1/3 scale hydrogen negative ion source for LHD-NBI 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 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 have 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 hours/60 shot interval, produced a similar temporary increase of H-beam, but reached a larger H-yield steady-state value. Deposition of larger 0.1-0.2 g Cs portions with a 20-120 hours/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 an 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 8 filaments discharge operation, as compared to the 12 filaments operation at the same discharge power. (author)