Effect of the aspect ratio on the stability of tokamak edge MHD modes

Aiba, Nobuyuki; Tokuda, Shinji; Ozeki, Takahisa

In tokamaks, since small aspect ratio makes the magnetic well deeper and increases the magnetic shear, the pressure driven MHD modes become stable. Near the edge pedestal, the edge pedestal pressure gradient can be improved due to accessing to the second stability region of a ballooning mode, and the maximum pressure gradient is restricted by the stability of a peeling-ballooning mode, which depends on not only the pressure gradient but also the amount of the edge current density. Since the stability threshold value of the edge current density hardly depend on the aspect ratio, the lower stability boundary value of the magnetic shear increases as the aspect ratio decreases, and the second stability region becomes narrower. From the viewpoint of the accessibility to the second stability region, we investigate the effect of the aspect ratio on the stability of edge MHD modes, and reveal that the maximum pressure gradient sometimes decreases as the aspect ratio becomes smaller.