Nonlinear MHD analysis for LHD plasmas

For the large helical device (LHD), the nonlinear evolution of equilibria that are linearly unstable to ideal interchange modes is studied using the reduced MHD equations. At sufficiently low beta, each individual mode saturates without affecting directly the evolution of the other modes. They only couple through the modification of the averaged pressure profile. The change of the averaged pressure profile is limited to the local flattening near the resonant surfaces. At higher beta values and for the same initial pressure profile, a bursting phenomenon in the kinetic energy is observed. This bursting activity is caused by the overlap of multiple modes, which results in a global reduction of the pressure. However, increasing beta and using a pressure profile obtained from the nonlinear evolution at the lower beta suppress this bursting behaviour. This result indicates that the pressure profile can be self-organized so that the LHD plasma could reach a high beta regime through a stable path.