Study of long pulse high efficiency beam direct energy conversion

目前，基于正离子的中性束注入（NBI）不仅在世界四大托卡马克中被广泛用于等离子体的高效加热，而且几乎在主要的等离子体研究设施中也得到了广泛的应用。然而，随着所需束流能量的快速增加，中子中和率逐渐降低到不可接受的水平，对未中和离子能量的有效能量回收对于提高NBI系统效率和减少束流诱导的D-D中子至关重要。在不久的将来，以负离子为基础的NBI系统将能够产生甚至1mev的中性束。当使用气体中和剂时，如现在计划的那样，40%的束流仍然是离子，由大约一半的D^+和一半的D^-组成，具有足够的功率。高效的光束直接能量转换（BDC）在该系统中也是必不可少的。日本京都大学开发的BDC具有二次电子抑制的特性，在15.8 keV, 100 mA， 100 msec的条件下，能量回收效率从60%提高到85%。氦离子束。此外，通过添加二次电子抑制子，BDC的工作压力范围可以扩展到6.5x10^<-2> pa。基于这些改进，本研究进行了长脉冲能量回收实验，以探讨与未来稳态NBI系统相关的工程问题。为此，在离子源和BDC之间设置了一个差压泵送系统，以降低气体压力，并降低光束的中和分数。在2秒氦束流实验中，差速抽气效果良好，能量回收效率与背景气体压力无关，与时间无关，可达80%以上，重现性好。没有发现与长脉冲BDC操作相关的特殊技术或热问题。

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.