Physics of the density limit in the W7-AS stellarator
Physics of the density limit in the W7-AS stellarator
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W7-AS 仿星器中密度极限的物理学
DOI：
10.1088/0741-3335/42/6/301
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发表时间：
2000
影响因子：
2.2
通讯作者：
S. Itoh
中科院分区：
文献类型：
--
作者：
L. Giannone;J. Baldzuhn;R. Burhenn;P. Grigull;U. Stroth;F. Wagner;R. Brakel;C. Fuchs;H. Hartfuss;K. Mccormick;A. Weller;C. Wendland;K. Itoh;S. Itoh
本文研究了W7-AS仿星器在三种环向磁场（B = 0.8，1.25和2.5 T）下的密度极限放电，其线积分密度为常数，放电持续时间为2 s。控制仿星器中密度极限的物理学的中心因素被证明是当中心峰值辐射功率密度分布超过沉积功率密度时，等离子体的净功率减小。在这些条件下，电子密度的峰值进一步加速了这一过程。在放电与B = 2.5 T，模拟的中心峰值辐射功率密度分布可以被证明是由于峰值杂质密度分布。激光吹离测量清楚地推断出注入的铝的向内收缩。这些放电具有在W7-AS上的改进约束H-NBI模式中发现的电子密度分布形式。在仿星器中以尽可能高的密度产生稳态放电的目标自然对反应堆操作特别感兴趣。这样的情况下已经最好地实现了H模式放电，其中ELMs限制杂质流入等离子体和平衡的等离子体参数与适当低的辐射功率水平是可能的。ECRH放电中的密度扫描突出了控制杂质源和选择远低于密度极限的电子密度的需要，以便在没有ELMs的放电中可以尝试稳态操作。一个简单的整体辐射模型预测，极限密度应取决于加热功率的平方根，这是实验证实。在这个简单的模型中，实验发现的极限密度的磁场标度将部分取决于关于杂质密度的径向分布的项，这又是杂质离子的扩散系数和向内箍缩的函数。理论研究表明，假设导热系数与B有关，则导出了具有明确B依赖性的密度极限标度律。
Density-limit discharges in the W7-AS stellarator, with constant line-integrated density and a duration of up to 2 s, were studied at three values of the toroidal magnetic field (B = 0.8, 1.25 and 2.5 T). The central factor governing the physics of the density limit in stellarators was demonstrated to be the decreasing net power to the plasma when the centrally peaked radiated power density profile exceeds that of the deposited power density. The process was further accelerated by the peaking of electron density under these conditions. In discharges with B = 2.5 T, simulations of the centrally peaked radiation power density profiles could be shown to be due to peaked impurity density profiles. Laser blow off measurements clearly inferred an inward pinch of the injected aluminium. These discharges had the electron density profile form found in the improved confinement H-NBI mode on W7-AS. The aim of producing steady-state discharges at the highest possible density in stellarators is naturally of special interest for reactor operation. Such a scenario has been best achieved in H-mode discharges, in which ELMs restricted the impurity influx to the plasma and an equilibrium in the plasma parameters with suitably low radiation power levels was possible. A density scan in ECRH discharges highlights the need to control impurity sources and choose electron densities well below the density limit in order that steady-state operation can be attempted in discharges without ELMs. A simple model of bulk radiation predicted that the limiting density should depend on the square root of heating power and this was experimentally confirmed. The magnetic field scaling of the limiting density found experimentally in this simple model will partly depend on the term concerning the radial profile of the impurity density, which in turn is a function of the diffusion coefficient and inward pinch of the impurity ions. Theoretical studies have shown that an assumption about the B dependence of the thermal conductivity leads to density limit scaling laws with an explicit B dependence.