Scattering of Radio Frequency Waves by Density Fluctuations in Tokamak Plasmas

In tokamak fusion plasmas, coherent fluctuations in the form of blobs or filaments and incoherent fluctuations due to turbulence are routinely observed in the scrape-off layer. Radio frequency (RF) electromagnetic waves, excited by antenna structures placed near the wall of a tokamak, have to propagate through the scrape-off layer before reaching the core of the plasma. RF waves in the electron cyclotron and lower hybrid range of frequencies are commonly used to modify the current profile. In the International Thermonuclear Experimental Reactor (ITER), electron cyclotron waves are expected to stabilize the neoclassical tearing mode by providing current in the island region. While the effect of fluctuations on RF waves has not been quantified experimentally, there are telltale signs, arising from differences between results from simulations and from experiments, that fluctuations can modify the spectrum of RF waves. Consequently, pioneering theoretical studies and complementary computer simulations have been pursued to elucidate the impact of fluctuations on RF waves. These studies, using the full complement of Maxwell's equations for a cold, magnetized plasma, show that the Poynting flux in the wake of the filament develops spatial structure due to diffraction and shadowing. The uniformity of power flow into the plasma is affected by side-scattering, modifications to the wave spectrum, and coupling to plasma waves other than the incident RF wave. The Snell's law and the Fresnel equations have been reformulated within the context of magnetized plasmas. These are distinctly different from their counterparts in scalar dielectric media, and reveal new and important physical insight into the scattering of RF waves. All of these studies apply to the scattering of RF waves in any frequency range and for arbitrary variations in density.

在托卡马克（tokamak）聚变等离子体中，刮削层（scrape-off layer, SOL）内常规观测到两类涨落：一类为团块或丝状体形式的相干涨落，另一类为湍流引发的非相干涨落。由安置于托卡马克壁附近的天线结构激发的射频（Radio frequency, RF）电磁波，在抵达等离子体芯部之前，必须穿过刮削层。电子回旋（electron cyclotron）频段与低混杂（lower hybrid）频段的射频波常被用于调控等离子体电流分布。在国际热核聚变实验堆（International Thermonuclear Experimental Reactor, ITER）中，电子回旋波有望通过在磁岛区域注入电流，实现对新经典撕裂模（neoclassical tearing mode）的稳定作用。尽管涨落对射频波的影响尚未通过实验完成量化，但模拟结果与实验结果间的差异已显现出若干明确征兆，表明涨落能够改变射频波的频谱。为此，学界已开展开创性的理论研究与配套的计算机模拟，以阐明涨落对射频波的作用机制。这类研究基于冷磁化等离子体的完整麦克斯韦方程组（Maxwell's equations），结果显示：丝状体尾迹中的坡印廷通量（Poynting flux）会因衍射与遮蔽效应形成空间结构。入射至等离子体的功率流均匀性，会受到侧向散射、波谱畸变，以及与入射射频波之外的等离子体波耦合的影响。研究人员已在磁化等离子体的框架下重新推导了斯涅尔定律（Snell's law）与菲涅耳方程（Fresnel equations），其形式与标量介质（scalar dielectric media）中的对应法则存在显著差异，为射频波散射研究提供了全新且重要的物理认知。上述所有研究结论适用于任意频段的射频波散射，以及任意形式的密度扰动。