中性束注入器大型负离子源磁滤器特性研究与优化设计

Neutral beam injection is a major auxiliary heating method for large magnetic confinement fusion device. The high-energy and high-power neutral beam injector for future fusion experimental reactors will be equipped the giant negative ion source, in order to attain an acceptable neutralization efficiency. Although, the production of negative ions is not high in the source, and the negative ions are easily destroyed by hot electrons. In order to reduce the negative ions loss in the extraction region, the current negative ion sources were installed a magnetic filter to lower electron temperature in the extraction region, and the co-extracted electrons were also decreased. However, the transverse magnetic field caused non-uniformity of plasma production and negative ion beam deflection, leaded to a significant deterioration of the beam quality, resulted in the limitation the beam power and pulse duration time of negative ion source. Thus optimizing the magnetic filter to improve the performance of giant negative ion source becomes an important subject in the research field of neutral beam injection. This project will base on the designing and developing prototype negative ion source, and develop an electron tracing simulation program. By this simulation program, the influence of magnetic filter field on the electrons behavior and plasma distribution will be investigated, and the performance of current magnetic filters will be evaluated and compared. Based on these research results, a new magnetic filter field will be built which has less negative ions loss and co-extracted electrons, more uniform plasma and less influence on the beam extraction & acceleration. And then, an advanced magnetic filter design will be proposed for the giant negative ion source.

中性束注入是大型磁约束聚变装置上最主要的辅助加热手段。未来聚变实验反应堆的高能量、高功率中性束注入器必须装备大型负离子源，以获得较高的中性化效率。负离子源中的负离子产额不高，而且极易被高能电子碰撞损失。为了减少引出区的负离子损失，负离子源均采用磁滤器产生横向磁场，以降低引出区的电子温度，同时还能减少被引出的电子。但过滤磁场会引发等离子体不均匀性和负离子束偏转，导致引出束流品质变差，限制了负离子源的束功率和束脉宽。如何优化磁滤器以提升大型负离子源性能，成为中性束注入研究领域的重点课题。本项目依托正在设计研制的负离子源样机，自主开发电子追踪模拟程序，用于研究过滤磁场对电子集体行为和等离子体分布的影响，评估和比较现有的各种过滤磁场位形的物理性能。在此基础上，构造一种负离子损失和电子引出更少、等离子体分布更均匀、对负离子引出与加速影响更小的过滤磁场位形，提出一种面向大型负离子源的先进磁滤器设计。

中性束注入器是将高能中性粒子束注入到聚变装置内，用于加热等离子体、驱动等离子体电流的装置。由于高束能量条件下的中性化效率较高，基于大型负离子源的中性束注入系统成为磁约束聚变反应堆的必然选择。磁滤器是负离子源必不可少的部件，它在等离子体发生器内形成横向过滤磁场，用以降低引出区的电子温度和电子密度，从而能够有效的减小负离子的损失、降低引出电子数量、提高负离子源的整体引出效率。但过滤磁场会引发等离子体不均匀性和负离子束偏转，进而导致负离子束流品质变差，限制了负离子源的性能表现。为了研究过滤磁场对等离子体的作用机制，本项目自主开发了任意电磁场环境的三维电子追踪模拟程序。首先，它可以计算常见的永磁体或电流组合产生的三维电磁场位形；然后，可以计算不同的发射电子在该电磁场中的运动轨迹，以及电子与气体碰撞反应引起的散射和能量损失；最终，通过大数量的电子追踪模拟，统计分析出电子密度和电子速度在该电磁场中的分布情况。该电子追踪模拟程序首先用于EAST-NBI离子源的优化工作。通过对电极系统出射反向电子流和灯丝发射电子的模拟分析，解释了EAST-NBI离子源背板发生烧蚀融化的原因；预先评估了应急处理方案的效果，指出其能够解决背板热负荷集中的问题，但会导致约束磁场性能的下降；该结论与实验结果相符。电子追踪模拟程序还用于研究不同磁滤器条件下的电子集体行为，揭示了过滤磁场导致等离子体不均匀的原因是磁场梯度引起的电子漂移，明确了磁滤器的优化改进目标。依托ASIPP负离子源样机，开展了外置磁滤器、“PG电流”磁滤器、组合磁滤器的物理设计和工程设计，并进行了负离子源样机的初步实验。这为未来大型负离子源的磁滤器设计提供了必要的设计工具、积累了丰富的设计经验。