Structure Change in Toroidal Plasma Due to Nonlinear Evolution of Pressure Driven Mode

从压力驱动模式非线性演化引起的压力剖面结构变化和磁组态的角度，全面理解环形等离子体的MHD稳定性。氦离子等离子体被聚焦是因为交换模式是最危险的不稳定性。特别是在LHD实验中，在交换模式的线性不稳定区域观察到良好的约束，因此，研究了稳定机制。为了进行分析，在简化MHD方程的基础上开发了非线性数值代码NORM。首先，研究了低β等离子体的非线性演化，该等离子体对线性理想交换模式具有轻微的不稳定性，在这种情况下，模式是轻度饱和的，没有任何破坏现象。压力分布在谐振表面周围有局部平坦区域。这种结构变化降低了驱动力，导致了轻度饱和。其次，研究了具有相同压力分布和高β值的等离子体的非线性演化。在这种情况下，将获得破坏性现象。这是因为驱动力的增强导致了涡流的重叠。然而，由于实际实验中压力分布是连续变化的，所以对低β值和高β值的饱和压力分布的等离子体进行了实验。在这种情况下，等离子体是饱和的，没有破坏现象。这些结果表明，LHD等离子体是自组织的，在β值的增加中可以实现稳定的压力分布。当压力剖面产生局部平坦区时，同时产生磁岛。这主要是由于交换模式的涡旋引起的驱动重连。岛的数目是驱动模式的极向模数的两倍。

The comprehensive understanding of the MHD stability of the toroidal plasma has been aimed in the view point of the structure change in the profiles of pressure and the magnetic configuration induced in the nonlinear evolution of the pressure driven mode. The heliotron plasma is focused because the interchange mode is the most dangerous instability. Particularly, in the LHD experiments, good confinement is observed in the region linearly unstable for the interchange mode, and therefore, the stabilizing mechanism is pursued.In order to carry out the analysis, a nonlinear numerical code, NORM, is developed based on the reduced MHD equations. At first, the nonlinear evolution of a low beta plasma which is slightly unstable for the linear ideal interchange mode is examined In this case, the mode is saturated mildly without any disruptive phenomenon. The pressure profile has local flat regions around the resonant surfaces. This structure change reduces the driving force and leads to the mild saturation. Next, the nonlinear evolution of the plasma with the same pressure profile and high beta value is examined. In this case, a disruptive phenomenon is obtained. This is because the enhancement of the driving force causes an overlap of the vortices. However, since the pressure profile has to change continuously in the actual experiments, the plasma with the saturated pressure profile at the lower beta value and the high beta value is examined. In this case, the plasma is saturated without the disruptive phenomenon. These results indicate that the LHD plasma is self-organized so that the stable pressure profile can be achieved in the increase of the beta value.When the local flat region is generated in the pressure profile, the magnetic islands are generated simultaneously. The generation is attributed to the driven reconnection which is induced by the vortices of the interchange mode. The number of the islands is twice of the poloidal mode number of the driving mode.