Theoretical Study on Neoclassical Tearing Mode

在高β托卡马克等离子体中经常观察到磁岛，其中等离子体约束退化，或者有时由于岛的形成而使放电变得破坏性。为了在聚变等离子体中获得高性能，有必要了解高β等离子体中孤岛形成的物理机制和伴随的崩塌现象。用四场简化的新经典MHD方程研究了新经典撕裂模(NTM)的线性稳定性，其中考虑了起伏的离子平行流和离子新经典粘性。结果发现：(1)即使在四场模型中，NTM的稳定区也是存在的，这意味着实验解释需要仔细的检验；(2)在香蕉高原区，离子新经典粘性使NTM稳定，而不是离子平行可压缩性使NTM稳定。结果表明，离子新古典主义粘性效应与离子效应和电子抗磁效应共同作用，使NTM趋于稳定。为了弄清高β实验中磁岛或亚临界分叉的开始，需要对NTM进行非线性模拟。我们开发了用于非线性模拟的光谱代码。结果表明，碰撞区存在漂移波(DW)湍流，导致NTM生长的非线性增强。然而，北半球湍流对NTM饱和振幅的影响不大。NTM的饱和机理有待于进一步研究。

The magnetic islands are often observed in high beta tokamak plasmas, where the plasma confinement is degraded or sometimes discharges become disruptive due to the formation of the island. It is necessary to understand the physical mechanism of island formation in high beta plasma and the associate collapse phenomena for the achievement of high performance in fusion plasmas. We investigate linear stability of neoclassical tearing mode (NTM) using the four-field reduced neoclassical MHD equations, in which the fluctuating ion parallel flow and ion neoclassical viscosity are taken into account. It js found that (1)the stable regime of the NTM exists even if $VDelta'>0$ in four field model which implies the careful examinations are necessary for experimental interpretations, (2)the ion neoclassical viscosity stabilizes the NTM rather than the ion parallel compressibility does in the banana-plateau regime. It is concluded that the synergetic effects of ion neoclassical viscosity and both ion and electron diamagnetic effects stabilize the NTM. To clarify the onset of magnetic island or sub-critical bifurcation in high beta experiments, the nonlinear simulation of the NTM is necessary. We have developed spectral code for nonlinear simulation. It is shown that there exists the drift wave (DW) turbulence in the collisional regime, which leads to the nonlinear enhancement of NTM growth. However, the saturation amplitude of NTM is not so much affected by DW turbulence. The saturation mechanism of NTM should be investigated as a future work.