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

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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.