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

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10 Author(s)
Oka, Y. ; National Institute for Fusion Science, Toki-city, 509-5292, Japan ; Takeiri, Y. ; Belchenko, Yu.I. ; Hamabe, M.
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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. © 2000 American Institute of Physics.

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Review of Scientific Instruments  (Volume:71 ,  Issue: 3 )