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

从压力驱动模的非线性演化引起的压强分布的结构变化和磁组态的角度，对环形等离子体的MHD稳定性有了全面的认识。由于交换模是最危险的不稳定性，所以对氦加速器等离子体进行聚焦。特别是，在LHD实验中，交换模式的线性不稳定区域观察到了良好的约束，因此，对稳定机制进行了探索。为了进行分析，基于简化的MHD方程，开发了一个非线性数值程序NORM。首先，研究了线性理想交换模式下略不稳定的低β等离子体的非线性演化，在这种情况下，模式是温和饱和的，没有任何扰动现象。压力分布在共振面周围有局部平坦区域。这种结构变化降低了驱动力，导致了温和的饱和。其次，研究了具有相同压强分布和高β值的等离子体的非线性演化。在这种情况下，就得到了一种破坏现象。这是因为驱动力的增强导致了涡旋的重叠。然而，由于实际实验中压力分布必须连续变化，所以我们考察了压强分布在低贝塔值和高贝塔值的饱和等离子体。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，当β值增大时，可以获得稳定的压强分布，当压强分布中产生局部平坦区域时，同时产生磁岛。这种产生归因于由交换模式的涡旋引起的驱动重联。岛的数目是驱动模式的极向模式数目的两倍。

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.