Study of long pulse high efficiency beam direct energy conversion

基于正离子的中性束注入(NBI)目前不仅在世界四大托卡马克装置中被广泛用于有效的等离子体加热，而且几乎在主要的等离子体研究设施中也得到了应用。然而，随着需要的束流能量的迅速增加，中和率正在下降到不可接受的水平，有效地恢复未中和的离子能量对于提高NBI系统的效率以及减少束流诱导的D-D中子变得非常重要。在不久的将来，基于负离子的窄带注入系统将能够产生甚至L MeV的中性束。按照现在的计划，当使用气体中和器时，40%的束流仍然以离子的形式存在，大约由一半D^+和一半D^-组成，具有足够的能量。在这种系统中，高效的束流直接能量转换(BDC)也将是必不可少的。BDC是在京都大学开发的。具有二次电子抑制器的特性，在15.8keV、100 mA、100毫秒的能量回收效率从60%大幅提高到85%。氦离子束。通过增加二次电子抑制器，BDC的工作压力范围可以扩展到6.5×10~(-2)~(-2)pa。在这些改进的基础上，本研究进行了长脉冲能量回收实验，以检验与未来稳态NBI系统相关的工程问题。为此，在离子源和BDC之间设置了差动泵浦系统，以降低气体压力，并减少束流的中和率。在实验中使用了2秒。结果表明，差动抽运效果良好，能量回收效率不依赖于时间，而依赖于背景气体压力，超过80%，重复性好。与长脉冲BDC操作相关的非特殊技术或热问题未被识别。

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.