NIFS-620

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Author(s):

Y. Oka, Y. Takeiri, Yu.I.Belchenko, M. Hamabe, O. Kaneko, K. Tsumori, M. Osakabe, E. Asano, T. Kawamoto and R. Akiyama

Title:

Optimization of Cs Deposition in the 1/3 Scale Hydrogen Negative Ion Source for LHD-NBI System

Date of publication:

Dec. 1999

Key words:

hydrogen negative ion, ion surce, cesium deposition, NBI

Abstract:

A compact cesium deposition system was used for direct deposition of cesim atoms and ion 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-yielad steady-state value. Deposition of arger 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.

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