Analysis of Magnetic Helicity Injection by RF Waves

从解析和数值两个方面研究了高频波的磁螺旋度注入和电流驱动机制。在稳态下研究了射频波与等离子体之间的非共振相互作用的电流驱动。这种非共振相互作用除了产生通常保守的有质动力之外，还产生一种非保守力。我们关注的力主要作为等离子体物种之间的内力；在这个过程中，从波到等离子体的净动量输入很小。这一过程产生的电流驱动不同于传统的基于共振波-粒子相互作用的射频电流驱动方案，它与射频波螺旋度的变化有关。扩展了一维波动程序来计算与非谐振力相关的驱动电流。该分析适用于大型托卡马克中的ICRF波。得到了驱动电流和螺旋度吸收曲线。结果表明，局域电流驱动效率与等离子体密度无关，强烈依赖于温度。然而，由于传播和吸收结构的变化，整体电流驱动效率依赖于中心密度。通过等离子体表面的螺旋度注入所驱动的电流需要电流的空间扩散。为了研究托卡马克中的电流扩散机制，我们发展了一个三维弹跳平均福克-普朗克程序。利用该程序对电子回旋电流驱动实验进行了模拟。通过考虑快电子的空间扩散，观察到电流分布的展宽和电流驱动效率的降低。

The mechanism of magnetic helicity injection and current drive by RF waves was studied both analytically and numerically.1. Current drive via non-resonant interaction between RF waves and plasma studied in a stationary state. The non-resonant interaction generates a non-conservative force, in addition to the usual conservative ponderomotive force. The force of our concern mainly acts as an internal force among plasma species ; the net momentum input from the wave to the plasma is small in this process. The current drive by this process is different from the conventional RF current drive schemes based on resonant wave-particle interactions, and is associated with the change of RF wave helicity.2. A one-dimensional wave code was extended to calculate the driven current associated with the nonresonant force. The analysis is applied to ICRF waves in a large tokamak. The profiles of driven current and helicity absorption were obtained. It was confirmed that the local current drive efficiency is independent of the plasma density and strongly depends on the temperature. The global current drive efficiency, however, depends on central density owing to the change of propagation and absorption structure.3. Current drive by the helicity injection via a plasma surface requires spatial diffusion of the current. In order to study the mechanism of the current diffusion in a tokamak, we have developed a three-dimensional bounce-averaged Fokker-Planck code. This code was used to simulate the electron cyclotron current drive experiment. The broadening of the current profile and the reduction of the current drive efficiency have been observed by including spatial diffusion of fast electrons.