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Marginal beta-limit for neoclassical tearing modes in JET H-mode discharges

机译：JET H模式放电中新古典撕裂模式的边际β极限

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摘要

A neoclassical tearing mode (NTM) requires a finite size seed island to become unstable. Usually the local pressure gradient is relatively large at the beta-values needed for these seed islands to destabilize the NTMs. Therefore, the island has a large growth rate at mode onset and grows rapidly to its saturated island width. This width is proportional to as long as it is well above the marginal beta-limit below which the mode is stable. The marginal beta-limit is independent of the seed island trigger mechanism and provides detailed information on the stabilizing terms in the modified Rutherford equation, which are not unambiguously determined theoretically. It is shown that in JET the marginal normalized beta-limit for the 3/2 mode, beta(N,marg), is of the order of 0.5-1 for magnetic fields between 3.3 and 1T, with q(95) approximate to 3.3, and near the H-L transition. Therefore, all H-modes with typical q-profiles (q(95) approximate to 3.3) are metastable in JET to 3/2 NTMs. In addition, the marginal island width is of the order of 2-4 cm and the stabilizing terms are such that they influence the saturated island width when it is smaller than 4-6 cm in these H-mode discharges. It is also shown that detailed analyses of the time evolution of the island width with slow beta ramp-down suggest that the convective form of the stabilization term due to the 'chi(perpendicular to) model' is more appropriate and can explain the island decay between 4 and 6 cm to the marginal island width, while the polarization current model can explain the rapid stabilization when beta < beta(marg). The range of values of the different stabilizing terms are discussed in detail. In particular, it is shown that the mode is stabilized and has a large negative growth rate, when the effect of the stabilizing terms is such as to reduce the saturated width by a factor of 2. [References: 23]
机译：新古典撕裂模式（NTM）需要有限大小的种子岛才能变得不稳定。通常，在这些种子岛破坏NTM稳定所需的β值处，局部压力梯度相对较大。因此，该岛在模式开始时具有较大的增长率，并迅速增长到其饱和岛宽。该宽度与之成正比，只要它远高于该模式稳定的边际β极限即可。边际β极限与种子岛触发机制无关，并提供了有关经修改的卢瑟福方程式中稳定项的详细信息，这些在理论上不会明确确定。结果表明，在JET中，对于3.3和1T之间的磁场，3/2模式的边际归一化beta极限beta（N，marg）约为0.5-1，q（95）约为3.3 ，并且接近HL过渡。因此，具有典型q轮廓（q（95）近似于3.3）的所有H模式在JET到3/2 NTM中都是亚稳态的。另外，边缘岛的宽度约为2-4cm，稳定项使得当它们在这些H模式放电中小于4-6cm时，它们会影响饱和岛的宽度。还显示，对岛宽度随贝塔斜率缓慢下降的时间演化进行的详细分析表明，由于“ chi（垂直于）模型”而导致的稳定项的对流形式更合适，并且可以解释岛的衰变边缘边缘宽度在4至6 cm之间，而极化电流模型可以解释β

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