Theoretical Study on Neoclassical Tearing Mode

磁岛通常在高β托卡马克等离子体中观察到，在那里，由于磁岛的形成，等离子体约束退化或有时放电变得破坏性。为了实现高β等离子体的高性能，有必要了解高β等离子体中岛屿形成的物理机制和相关的坍塌现象。利用四场简化的新经典MHD方程研究了新经典撕裂模式（NTM）的线性稳定性，其中考虑了波动离子平行流动和离子新经典粘度。结果表明：(1)在四场模型中，即使$VDelta'> $也存在NTM的稳定状态，这意味着实验解释需要仔细检查；(2)在香蕉高原状态中，离子新经典粘度比离子平行压缩率更能稳定NTM。结果表明，离子新经典黏度的协同作用以及离子和电子抗磁性的共同作用使NTM稳定。为了阐明高β实验中磁岛或亚临界分岔的发生，有必要对NTM进行非线性模拟。我们开发了用于非线性模拟的谱代码。结果表明，碰撞区存在漂移波湍流，导致NTM生长的非线性增强。而NTM的饱和幅值受DW湍流的影响不大。NTM的饱和机理是今后研究的重点。

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.