MHD Equilibrium and Stability of Spherical Tokamak Plasma with Current Hole

N.Mizuguchi, T.Hayashi

National Institute for Fusion Science, Toki, Gifu 509-5292, Japan

The spherical tokamak (ST), which is defined as tokamaks with low aspect ratio less than 2, has been recently received much attention in that high beta plasma can be obtained with good confinement and stability. Especially, more advanced confinement style for potential economical fusion reactor is one of the most important problems in recent ST research. In conventional large tokamak experiments, an extreme high confinement state, which is called the current hole, is observed. In such state, the plasma density and temperature gradient exists only in the edge region, which forms an internal transport barrier, and an area with no plasma current is formed in the core region. In this paper, we study numerically the MHD properties of ST plasma with a current hole. The equilibria of ST with current hole can be obtained numerically by using conventional Grad-Shafranov equation with appropriate assumptions on the pressure and the current profile. The linear and nonlinear MHD stability is investigated by means of MHD simulations. The simulation results show the growth of destructive internal kink modes with low wavenumbers such as m=3/n=1 mode (m and n are the poloidal and toroidal wave numbers). However, it is necessary for better understandings to take account of the effect of plasma flow, because strong plasma shear flows are formed in the internal transport barrier. Here, we also make discussions on the stability problem for ST equilibria with strong plasma flows by using the modified Grad-Shafranov equation.