Boundary modulation effects on MHD instabilities in heliotrons
Boundary modulation effects on MHD instabilities in heliotrons
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DOI：
10.1088/0029-5515/46/2/001
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发表时间：
2006-01
期刊：
影响因子：
3.3
通讯作者：
N. Nakajima;S. Hudson;C. Hegna;Y. Nakamura
中科院分区：
文献类型：
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作者：
N. Nakajima;S. Hudson;C. Hegna;Y. Nakamura
在三维位形中，禁闭区域被与磁岛或分离线相关的随机磁力线包围，导致等离子体-真空边界与托卡马克相比不是很明确，特别是在高β操作中，由于随机区域周围的输运和整个等离子体的大沙夫洛夫位移之间的协同作用，等离子体-真空边界的各种调制将被诱导在随机区域周围。为了考察等离子体边界对磁流体不稳定性的调制效应，在内移磁轴Rax为3.6m的LHD组态中，考虑了允许大沙夫洛夫位移或大普菲尔施-施吕特电流的高β等离子体，先前基于固定磁流体力学平衡的理论分析表明，与实验观测相比，压力驱动模更不稳定。引入了允许平衡等离子体运动到随机区域的平均通量表面的概念，它引入了边界调制，同时减小了实验所得的MHD平衡与理论所考虑的MHD平衡之间的偏差。结果表明，边界调制，即大的Pfisch-Schlüter电流引起的整个等离子体外移对理想MHD不稳定性有显著的稳定作用，从而部分地解决了实验结果与理论分析之间的差异。
In three-dimensional configurations, the confinement region is surrounded by the stochastic magnetic field lines related to magnetic islands or separatrix, leading to the fact that the plasma–vacuum boundary is not so definite compared with tokamaks that the various modulations of the plasma–vacuum boundary will be induced around the stochastic region by synergetic effects between a transport around the stochastic region and a large Shafranov shift of the whole plasma, in especially high-β operations. To examine such modulation effects of the plasma boundary on MHD instabilities, high-β plasmas allowing a large Shafranov shift or a large Pfirsch–Schlüter current are considered in the inward-shifted LHD configurations with the vacuum magnetic axis Rax of 3.6 m, for which previous theoretical analyses based on fixed MHD equilibria indicate that pressure-driven modes are significantly more unstable compared with experimental observations. The concept of the averaged flux surfaces allowing a movement of the equilibrium plasma into the stochastic region is introduced, which induces a boundary modulation and, at the same time, reduces the discrepancy on MHD equilibria between the experimentally obtained and theoretically considered. As a result, it is shown that the boundary modulation, namely, the whole plasma outward-shift due to a large Pfirsch–Schlüter current has significant stabilizing effects on ideal MHD instabilities, leading to partially resolving the discrepancy on MHD stability between experimental results and theoretical analyses.