Propagation of radio frequency waves through turbulent plasmas

The practical and economic viability of tokamak fusion reactors depends, in a significant way, on the efficiency of radio frequency (RF) waves to deliver energy and momentum to the plasma in the core of the reactor. The RF electromagnetic waves, excited by antenna structures placed near the wall of a tokamak, have to propagate through the turbulent edge plasma along their path to the core of the fusion device. In present day experiments, the radial width of the edge region and scrape-off layer is of the order of a few centimeters. In ITER, and in future fusion reactors, this width will be of the order of tens of centimeters. Any effects on RF waves due to plasma turbulence have to be properly understood in order to optimize the delivery of RF energy and momentum into the core. This paper is on a multi-pronged, theoretical and computational, approach that is being pursued to quantify the effect of edge plasma turbulence on the propagation of RF waves. The theoretical and analytical models are based on solutions of the Faraday-Ampere equation in a magnetized plasma and on the Kirchhoff tangent plane approximation. An effective medium approach has been developed so as to approximate the permittivity of a turbulent plasma by analytical expressions. The computations are being carried out with a newly developed code ScaRF that is based on the finite difference finite domain technique for solving Maxwell's equations. The plasma permittivity can be assigned as desired. The code is being used to validate the analytical and theoretical models and to evaluate their limitations.托卡马克核聚变反应堆的实用化与经济可行性，在很大程度上取决于射频（Radio Frequency，RF）波向反应堆核心等离子体传递能量与动量的效率。由布置在托卡马克内壁附近的天线结构激发的射频电磁波，需沿传播路径穿过湍流边缘等离子体，方能抵达核聚变装置的核心区域。在当前的实验中，边缘区域与刮削层（Scrape-off Layer）的径向宽度约为数厘米量级；而在国际热核聚变实验堆（ITER）及未来的聚变反应堆中，该宽度将达到数十厘米量级。为优化射频波向核心区域的能量与动量输送效率，必须充分厘清等离子体湍流对射频波产生的各类影响。本文采用多维度理论与计算相结合的研究方法，旨在量化边缘等离子体湍流对射频波传播的影响。该理论与解析模型基于磁化等离子体中的法拉第-安培方程求解，以及基尔霍夫切平面近似。研究人员已开发出有效介质法，可通过解析表达式近似得到湍流等离子体的介电常数。计算工作依托全新开发的代码ScaRF完成，该代码基于求解麦克斯韦方程组的有限差分有限域技术，可按需指定等离子体的介电常数。目前，该代码正用于验证上述解析与理论模型，并评估其适用局限。