Density limits in toroidal plasmas
Density limits in toroidal plasmas
复制标题
DOI：
10.1088/0741-3335/44/8/201
复制
发表时间：
2002-07
影响因子：
2.2
通讯作者：
M. Greenwald
中科院分区：
文献类型：
--
作者：
M. Greenwald
除了MHD稳定性对等离子体电流和压力施加的操作限制外，在受限环形等离子体中观察到对等离子体密度的独立限制。本文综述了密度极限的唯象学和物理学研究进展。也许最令人惊讶的结果是，所有的环形约束装置考虑在类似的范围内（适当归一化）的密度。托卡马克和反向场箍缩独立得出的经验标度基本上是相同的，而仿星器似乎以不同的标度在更高的密度下工作。专门的密度极限实验尚未进行spheromak和场反转配置，然而，在这些设备中的“优化”放电也很好地表征了相同的经验法则。在托卡马克中，已经进行了最广泛的研究，有强有力的证据将限制与等离子体边界附近的物理学联系起来：因此，可以通过使用峰值密度分布来扩展线平均密度的操作范围。等离子体核心中的额外粒子显然对密度极限物理学没有影响。虽然没有被广泛接受的密度极限的第一原理模型，但这一领域的研究集中在导致强边缘冷却的机制上。理论工作集中在增加的杂质辐射的后果，这可能会主导功率平衡在高密度和低温。这些理论并不完全令人满意，因为它们需要关于边缘输运的假设，并且对功率和杂质标度的预测可能与实验结果不一致。一个独立的研究线索寻找碰撞增强湍流传输的原因。虽然这种方法有实验和理论支持，但对潜在机制的理解仅处于初步阶段，尚未提供预测能力。
In addition to the operational limits imposed by MHD stability on plasma current and pressure, an independent limit on plasma density is observed in confined toroidal plasmas. This review attempts to summarize recent work on the phenomenology and physics of the density limit. Perhaps the most surprising result is that all of the toroidal confinement devices considered operate in similar ranges of (suitably normalized) densities. The empirical scalings derived independently for tokamaks and reversed-field pinches are essentially identical, while stellarators appear to operate at somewhat higher densities with a different scaling. Dedicated density limit experiments have not been carried out for spheromaks and field-reversed configurations, however, `optimized' discharges in these devices are also well characterized by the same empirical law. In tokamaks, where the most extensive studies have been conducted, there is strong evidence linking the limit to physics near the plasma boundary: thus, it is possible to extend the operational range for line-averaged density by operating with peaked density profiles. Additional particles in the plasma core apparently have no effect on density limit physics. While there is no widely accepted, first principles model for the density limit, research in this area has focussed on mechanisms which lead to strong edge cooling. Theoretical work has concentrated on the consequences of increased impurity radiation which may dominate power balance at high densities and low temperatures. These theories are not entirely satisfactory as they require assumptions about edge transport and make predictions for power and impurity scaling that may not be consistent with experimental results. A separate thread of research looks for the cause in collisionality enhanced turbulent transport. While there is experimental and theoretical support for this approach, understanding of the underlying mechanisms is only at a rudimentary stage and no predictive capability is yet available.