Theoretical Study on Neoclassical Tearing Mode

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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.