中性束中性化效率影响因素与优化研究

For the neutral beam injection (NBI) heating experiments on magnetic confinement device, the neutralization efficiency of the ion beam from ion source is one of the key factors that affect NBI power into plasma. In order to improve the neutralization efficiency, gas puffing in the neutraliser may be carried out for increasing the gas target thickness. However, it is difficult to get optimal gas target thickness with simple gas puffing in neutralizer. With small gas flow, the neutralization efficiency can not reach idea value. With large gas flow, the.vaccum pumping load and reionization loss are increased. Both of above gas puffing status will limit NBI heating power into plsama. The gas target thickness and its characteristic ,such as the dependence of gas target thickness on gas puffing flow and gas puffing position in neutralizer, the formation mechanism of average thickness will be studied in detail under the state of gas molecular and transitional gas flow  in this project. The main study contents in the project are as follow: At first, the average gas target thickness and pressure.distribution will be investigated with Monte Carlo simulation and section iterative methods. Secondly, the test feeback complementary gas puffing system in neutralizer will be developed in ion source test bed and NBI beamline of HL-2A tokamak. Thirdly,the correaltion and dependence of particle cross section on the gas target thickness will be researched experimentally by scaning operation parameters of ion sources and the fixed step gas flow. The research objective is to study the molecular distribution at two different flow status in the neutraliser and obtain the relationship between the optimal neutralization efficiency and the gas puffing flow under the condition of different discharge parameters, at the same time, optimization and improvement of the diagnosis method of the neutral efficiency. In a word, the foundation of theory model and  experimental technology for optimizing the ion beam neutralization efficiency and developing integrated feedback gas puffing system will be established with the preforming this project.

在磁约束装置的中性束注入加热实验中，离子束中性化效率是决定中性束注入功率的关键因素之一，为提高中性化效率，可在中性化器中采用补充送气，增加气体靶厚，但是当补充送气偏小时达不到理想的中性化效率,偏大时增加真空负载和再电离损失，从而影响实际的有效注入中性束功率。本项目以中性化器中处于过渡流和分子状态下的气体靶分布为研究对象，对补充送气位置及送气流量、平均靶厚形成机理和最佳中性化效率相关性进行基础研究。具体内容包括：采用蒙特卡洛模拟方法和分段迭代法研究平均靶厚分布；研究不同放电参数下中性化碰撞截面与气体靶厚的相关性；通过定步长扫描补充送气实验，建立基于放电参数反馈程序控制的补充送气系统，同时优化完善中性化效率诊断方法。本项目旨在研究中性化器中两种流态下气体分子分布特性，得到不同放电参数下最佳中性化效率和补充送气流量的相互关系，研制反馈补充送气系统和提高中性束有效加热功率。

为提升中性效率，需在添加中性化送气以实现最佳气体靶厚，但这些气体负载会增加剥离损失和再电离损失，从而降低了注入功率。针对上述问题，本项目主要研究的是气体靶厚对中性化效率的作用机制，明确气体靶厚形成过程和影响因素，通过设计、优化真空送气以及控制方案使得束流既实现了最佳中性化效率，又降低传输过程的损失。.研究方法是基于蒙特卡洛方法的稀薄气体流动数值模拟，得到不同条件下中性化器气体分子分布云图，通过四个措施实现靶厚的优化。首先，设计了150°倾斜中性化送气机构。第二，采用中性束低温泵出口的冷却剂冷却中性化器壁。第三，补偿物理响应时间。第四，设计气体分子反射阵列。上述优化措施能有效地降低中性化送气，减少束线真空室的气体负载，使得高能中性粒子的再电离损失从6.8%降到2.5%左右，提升中性束注入功率。