Characteristics of the MHD instabilities for high beta LHD (large helical device) plasmas in the inward shifted configurations have been investigated by numerical simulations. On the condition that the magnetic Reynolds number is lower than the experimental value, the time evolution of the MHD instabilities is as follows: (1) resistive ballooning modes, the toroidal mode number of which is higher than the LHD's toroidal pitch (), are destabilized in the plasma peripheral region; (2) low toroidal modes typified by are destabilized by the nonlinear mode coupling of the resistive ballooning modes the toroidal mode number of which is adjacent to each other where n is the toroidal mode number. In particular, the velocity of the mode is finite at the magnetic axis so that there is a large velocity directed to the magnetic axis; and (3) when the high n modes are saturated, the destabilized low n modes, which have a global mode structure and shift the magnetic axis, also begin to be saturated. The self mode coupling of the low n modes induces the core crush and transports the plasma from the core region to the peripheral region. Since the nonlinear MHD phenomena are dominated by the resistive modes, the MHD phenomena for the experimental high magnetic Reynolds number are considered to be milder than our numerical results. Thus, when the experimental high magnetic Reynolds number is taken in the simulation, the core crush may be suppressed so that the numerical results are expected to become close to the experimental results where the stable high beta plasmas are obtained.
Mode coupling and aspect ratio effects on low and high-n plasma instabilities