Scattering of radio frequency waves by cylindrical density filaments in tokamak plasmas
Scattering of radio frequency waves by cylindrical density filaments in tokamak plasmas
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DOI：
10.1063/1.4941588
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发表时间：
2016-02-01
影响因子：
2.2
通讯作者：
Hizanidis, Kyriakos
中科院分区：
文献类型：
--
作者：
Ram, Abhay K.;Hizanidis, Kyriakos
在托卡马克聚变等离子体中，在刮擦层中经常观察到斑点或细丝形式的相干波动。由放置在托卡马克壁附近的天线结构激发的射频（RF）电磁波，在到达等离子体的核心之前，必须通过刮擦层传播。虽然波动对射频波特性的影响尚未在实验上量化，但进行理论研究以确定波动是否会影响射频波的传播特性是有意义的。通常情况下，灯丝内部等离子体密度与背景等离子体密度之差相当大，其与背景密度之差为1阶。一般来说，这就排除了使用几何光学来确定波动的影响，因为相关比率必须远小于1，通常为10%或更小的数量级。本文建立了射频平面波被圆柱形等离子体细丝散射的全波解析模型。假设灯丝内外的等离子体是冷且均匀的，并且灯丝的长轴沿环向磁场排列。灯丝内部的密度与背景等离子体的密度之比不受限制。该理论框架适用于任何冷等离子体波的散射。为了满足灯丝与背景等离子体界面处的边界条件，灯丝内外的电磁场需要与入射平面波具有相同的平行于环境磁场的波矢量k（平行于）。因此，与球形斑点对射频波的散射相反[Ram等人，物理学家]。等离子体20,056110-1-056110-10(2013)]，场对准灯丝的散射不会使k（平行）谱变宽。然而，灯丝诱导侧散射导致表面波，也可以耦合一些功率的冷等离子体波不同于入射波。数值结果显示了波印亭矢量的性质，说明了在电子回旋波和低杂波传播过程中灯丝引起的变化。波印亭通量在灯丝的尾迹中，并指向等离子体的核心，由于衍射和阴影而形成空间结构。因此，波动影响了进入等离子体的功率的均匀性。(C) 2016 AIP出版有限责任公司
In tokamak fusion plasmas, coherent fluctuations in the form of blobs or filaments 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. While the effect of fluctuations on the properties of RF waves has not been quantified experimentally, it is of interest to carry out a theoretical study to determine if fluctuations can affect the propagation characteristics of RF waves. Usually, the difference between the plasma density inside the filament and the background plasma density is sizable, the ratio of the density difference to the background density being of order one. Generally, this precludes the use of geometrical optics in determining the effect of fluctuations, since the relevant ratio has to be much less than one, typically, of the order of 10% or less. In this paper, a full-wave, analytical model is developed for the scattering of a RF plane wave by a cylindrical plasma filament. It is assumed that the plasma inside and outside the filament is cold and uniform and that the major axis of the filament is aligned along the toroidal magnetic field. The ratio of the density inside the filament to the density of the background plasma is not restricted. The theoretical framework applies to the scattering of any cold plasma wave. In order to satisfy the boundary conditions at the interface between the filament and the background plasma, the electromagnetic fields inside and outside the filament need to have the same k(parallel to), the wave vector parallel to the ambient magnetic field, as the incident plane wave. Consequently, in contrast to the scattering of a RF wave by a spherical blob [Ram et al., Phys. Plasmas 20, 056110-1-056110-10 (2013)], the scattering by a field-aligned filament does not broaden the k(parallel to) spectrum. However, the filament induces side-scattering leading to surface waves and can also couple some power to the cold plasma wave different from the incident wave. The changes induced by a filament in the propagation of electron cyclotron waves and lower hybrid waves are illustrated by numerical results displaying the properties of the Poynting vector. The Poynting flux in the wake of the filament, and directed towards the core of the plasma, develops a spatial structure due to diffraction and shadowing. Thus, the fluctuations affect the uniformity of power flow into the plasma. (C) 2016 AIP Publishing LLC.