大功率射频离子源调频模式下射频系统可靠性分析与优化

High-power RF-driven ion source is a key component of the neutral beam injection (NBI) system of magnetic confinement fusion (MCF) device. The source is comprised of multiple systems, among which the RF system plays an important role in providing energy for plasma ignition and sustaining, and as well as impedance matching between the RF generator (RFG) and the driver. The matching is crucial for improving the efficiency of power transmission, but found difficult to be achieved since it needs to follow the equivalent driver impedance, which varies significantly during the plasma ignition. Hereby various matching methods have been proposed. The matching method based on self-oscillation has disadvantages of poor matching and low efficiency. Another method of capacitance adjustment needs to control mechanical devices remotely, which has poor reliability, moreover, faces lots of limitations such as vacuum compatible and radiation resistant, etc. In recent years, a frequency tuning method according the standing wave ratio is proposed and found to be of good performance. However, the frequency variation during RFG running changes not only the equivalent load condition but also the characteristics of RFG. To guarantee the reliable operation of RFG under the frequency tuning mode, an overall analysis and investigation of RF system is required. Thus, solutions for obtaining the equivalent driver impedance under complicated EM circumstances and selecting optimized parameters of RF transformer for high-power operation is crucial. After these are achieved, an overall simulation model would be built with taking accounts of all components of RF system which includes the RF antenna, matching network and RFG. With this model, focused research on the power loading limitation during plasma ignition could be implemented and a method for evaluating the reliability of frequency tuning could be provided as well. In the meanwhile, by studying the variation of equivalent anode impedance of the vacuum tube, a simple but effective anode circuitry could be provided for improving the reliability of the tube oscillator.

大功率射频离子源是磁约束核聚变装置中性束注入系统的核心部件，其中射频系统为离子源提供能量以激发和维持所需的等离子体，在运行过程中需根据负载的变化进行阻抗匹配调节。基于自激振荡的匹配方法存在匹配效果不佳、效率低等问题，而基于电容调节的匹配方法需要安装远程控制的机械调节装置，降低了可靠性，而且还存在诸多限制。根据驻波比主动调节频率是近些年国内外积极探索的一种新匹配方法。然而频率的变化不仅改变了等效负载，也改变了RF功率源的特性。为保证调频模式下射频系统可靠运行，需对射频系统进行一体化分析和研究。为此必须解决复杂电磁场下的离子源激励器等效负载阻抗和大功率条件下匹配用RF变压器参数的求解问题，建立一体化仿真模型，重点研究激发过程中射频系统可靠运行的功率加载要求，为大功率射频离子源调频工作的可靠性分析提供一种研究手段。同时，通过研究电子管阳极等效阻抗的变化规律，提出一种简单有效的阳极电路优化方案。

大功率射频负离子源是未来受控核聚变堆高能量长寿命中性束注入系统的必然选择，其中射频系统为射频负离子源提供能量以产生等离子体。射频系统主要由功率源、阻抗匹配电路和离子源激励器负载组成，在等离子体激发过程中激励器负载等效阻抗会发生变化，为了实现射频功率最大效率传输、减小反射功率，功率源可采用主动调节运行频率的方式以实现阻抗匹配。本项目针对基于电子四极管放大器的功率源主动调频工作方式可能存在的阳极电压振幅偏大进而导致帘栅极过流保护的问题开展了研究。1）首先针对带法拉第屏蔽的激励器负载建立了三维电磁场模型，视法拉第屏蔽为良导体，在其表面施加有限电导率边界条件，电磁场分析时不会在其内部求解场，大大减小计算量。法拉第屏蔽的涡流损耗采用表面阻抗的方法计算。三维模型进行电磁场分析后，依据坡印亭定律计算单位体积内的能流密度，然后对整个求解域进行积分，得到相应的有功损耗和无功储能，计算得到线圈等效阻抗。小型激励器的计算值与实测值吻合较好。基于实验数据和理论分析对仿真模型进行进一步改进后，可为射频系统的设计与分析提供可用的激励器阻抗参数；2）针对匹配电路，建立了1kW 匹配电路中的RF变压器有限元模型，在原边与副边端口采用场-路耦合的方法，对变压器在不同工作条件下进行瞬态分析研究，分析结果与测试结果吻合较好；3）分析计算得到电子四极管的电流模型以及各电极之间的电容，完成四极管的Pspice模型。建立了包含射频功率源、匹配电路、激励器等效负载在内的射频系统一体化仿真分析模型，研究了不同驱动频率、RF线圈等效阻抗的电路工作状态；4)针对现有电子四极管放大器输出电路，计算调频过程中不同激励器等效负载情况下的电子管阳极等效阻抗随频率的关系。针对等离子体产生过程中整体负载可能感性分量较大的问题，采用集总参数元件设计并实现了一种90°移相电路，用以等效于1MHz频率下调节λ/4传输线，实现感-容性变换。