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Numerical study on toroidal mode coupling and triggering of neoclassical tearing modes by sawteeth

机译：环形锯齿耦合和触发新古典撕裂模式的数值研究

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Numerical simulations have been carried out to understand the seeding of neoclassical tearing modes (NTMs), using a single fluid model and the four-field equations. The paper starts by considering a simplified physics situation: the destabilization of a linearly stable m = 3/2 mode by an unstable m = 2/2 mode (m being the poloidal/toroidal mode number). Within the single fluid model the mode coupling is found to be weakened by relative mode rotation and enhanced by finite plasma beta value. For the four-field equations in case of a sufficiently low electron diamagnetic drift frequency, the results are similar to those from the single fluid equations. For a sufficiently large diamagnetic drift frequency, three regimes are found: (a) suppression regime for moderate plasma beta values, in which the originally unstable 2/2 mode is stabilized by its coupling to the originally stable 3/2 mode; (b) oscillation regime for a sufficiently large beta value, in which the 3/2 mode has the feature of an ideal mode, and the diamagnetic drift frequency reaches the shear Alfven frequency outside the tearing layer; (c) NTM regime for sufficiently large beta value and bootstrap current density. Consistent with these findings, in our simulations 3/2 modes are triggered by sufficiently strong sawtooth crashes at high beta value and/or a low electron diamagnetic drift frequency. A sufficiently large bootstrap current density is then required for the 3/2 island to grow into the NTM regime.
机译：使用单个流体模型和四场方程进行了数值模拟，以了解新古典撕裂模式（NTM）的产生。本文首先考虑一种简化的物理情况：线性m / n = 3/2模的线性稳定因不稳定的m / n = 2/2模（m / n是极/环模数）而变得不稳定。在单一流体模型中，模式耦合被相对模式旋转削弱，而有限的血浆β值增强。对于电子反磁漂移频率足够低的四场方程，其结果与单个流体方程的结果相似。对于足够大的反磁漂移频率，发现了三种状态：（a）中度血浆β值的抑制状态，其中原始不稳定的2/2模式通过耦合到原始稳定的3/2模式而得以稳定； （b）β值足够大的振荡状态，其中3/2模式具有理想模式的特征，并且反磁漂移频率达到撕裂层外部的剪切Alfven频率； （c）使用NTM机制获得足够大的β值和自举电流密度。与这些发现一致，在我们的仿真中，3/2模式是由在高β值和/或低电子抗磁漂移频率下足够强的锯齿波碰撞触发的。然后，需要足够大的自举电流密度，以使3/2岛增长为NTM机制。

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