Mitigation of RF sheaths through design and implementation of magnetic field-aligned ICRF antenna
Mitigation of RF sheaths through design and implementation of magnetic field-aligned ICRF antenna
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发表时间：
2012
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通讯作者：
M. Garrett
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作者：
M. Garrett
在ITER和最终的反应堆中，由于内部传输势垒（ITB）和H模式操作而增强的杂质限制建立了对高Z杂质的非常低的耐受性[1]。实验表明，随着离子回旋频率范围（ICRF）功率的增加，杂质积累会增加[2]。因此，ICRF加热的主要挑战之一是减少或消除在ICRF操作期间引入等离子体中的杂质，特别是对于具有高Z等离子体面对部件（PFC）的托卡马克。普遍观察到与ICRF辅助加热相关的血浆杂质[3，4，5，6]。然而，ICRF特定的杂质产生的基本物理还没有得到很好的理解，杂质特性的观察结果在不同的托卡马克实验中是不同的。已经提出了几种方法来减少ICRF特定的杂质特性：低Z PFC涂层，如硼化[7];天线带的环形相位[3];以及天线法拉第屏元件与总磁场的对准[8]。在Alcator C-Mod上，我们设计了一种新的磁场对准ICRF天线，以最大限度地减少ICRF特定的杂质特性。场对准天线从水平方向旋转100 °，使得天线带垂直于典型等离子体放电（BT ~ 5.4 T，1，~ 1 MA）的边缘处的总磁场。ICRFinduced E-平行是一个可能的候选人产生增强的鞘层电压，导致大大增加溅射的材料表面和增强杂质的边缘传输。使用平板和圆柱形几何形状进行的初始模拟表明，对于某些环形定相，在天线结构的前面几乎完全消除了E平行。使用环形模型，取消E-平行是更温和的，这表明3-D几何效应是重要的。采用三维环形冷等离子体模型，利用有限元法对场对准天线的多天线相位进行了分析。在每种情况下，场对准天线相对于现有的非对准天线几何结构具有减小的集成E-平行，其中最大的减小是针对n [0，0，0，0]定相。初步结果表明，场对准天线操作的结果在更少的杂质比传统的天线在等离子体。
In ITER and in eventual reactors, enhanced impurity confinement due to internal transport barriers (ITBs) and H-mode operation establishes a very low tolerance for high-Z impurities [1]. Experiments have shown that impurity accumulation increases as power in the ion cyclotron range of frequencies (ICRF) is increased [2]. As a result, one of the primary challenges of ICRF heating is the reduction or elimination of impurities introduced into the plasma during ICRF operation, particularly for tokamaks with high-Z plasma facing components (PFCs). Plasma impurities associated with ICRF auxiliary heating are universally observed [3, 4, 5, 6]. However, the underlying physics of ICRF-specific impurity generation is not well understood, and observations of impurity characteristics differ among various tokamak experiments. Several methods have been proposed to reduce ICRF-specific impurity characteristics: low-Z PFC coatings such as boronization [7]; toroidal phasing of antenna straps [3]; and alignment of antenna Faraday screen elements with the total magnetic field [8]. On Alcator C-Mod we have designed a new magnetic field-aligned ICRF antenna to minimize ICRF-specific impurity characteristics. The field-aligned antenna is rotated 100 from horizontal, such that the antenna straps are perpendicular to the total magnetic field at the edge for a typical plasma discharge (BT ~ 5.4 T, 1, ~ 1 MA). ICRFinduced E-parallel is a likely candidate for producing enhanced sheath voltages that lead to greatly increased sputtering of material surfaces and enhanced impurity edge transport. Initial simulations performed using both slab and cylindrical geometry suggested nearly complete cancellation of E-parallel in front of the antenna structure for certain toroidal phasings. Using toroidal models, the cancellation of E-parallel is more modest, suggesting 3-D geometrical effects are important. Multiple antenna phases were analyzed for the field-aligned antenna using finite element method with a 3-D toroidal cold plasma model. In each case, the field-aligned antenna had reduced integrated E-parallel relative to the existing non-aligned antenna geometry, with the greatest reduction for monopole [0, 0, 0, 0] phasing. Initial results suggest that the field-aligned antenna operation results in fewer impurities in the plasma than conventional antennas.