| Budget Amount *help |
¥3,500,000 (Direct Cost: ¥3,500,000)
Fiscal Year 1988: ¥500,000 (Direct Cost: ¥500,000)
Fiscal Year 1987: ¥3,000,000 (Direct Cost: ¥3,000,000)
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| Research Abstract |
The neutral beam injection(NBI) based on positive ions is now widely used for efficient plasma heating not only in theworld four big tokamaks, but also almost in the major facilities for plasma researches. With the rapid increase of demanding beam energy, however, the neutralization fraction is decreasing to the unacceptable level, and efficient energy recovery of unneutralized ion energy becomes important for the improvement of NBI system efficiency, and also for reduction of beaminduced D-D neutrons. In the near future, negative-ion based NBI system is to be used capable of producing even l MeV neutral beam. When a gas neutralizer is used, as is now planned, 40% of beam remains as ions, consisting of approzimately, half D^+ , and half D^- with enoumous power. Efficient beam direct energy conversion (BDC) will be also essential in such system. The BDC developed at Kyoto Univ. has characteristics of secondary electron suppressors and has shown drastic improvement of energy recovery efficiency from 60% to 85% for 15.8 keV, 100 mA, 100 msec. helium ion beam. Also the operational pressure regime of BDC can be extended to 6.5x10^<-2> pa by the addition of secondary electron suppressors. Based on these improvements, long pulse energy recovery experiments are made in this study to examine the engineering issues associated with the future steady state NBI system. For this purpose, a differential pummping system was set in between an ion source and the BDC to reduce the gas pressure, and reducing the neutralization fraction of the beam. In the experiments using 2 sec. helium beam, the differential pumping was found to work well as designed, and the energy recovery efficiency is found to depend not on the time, but on the background gas pressure, exceeding more than 80% with good reproducibility. Nospecial technological, or thermal issues associated with long pulse BDC operation were not recognized.
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