Behavior of MHD instabilities of the Large Helical Device near the effective plasma boundary in the magnetic stochastic region

In the high beta experiments of the Large Helical Device (LHD), the plasma tends to expand from the last closed flux surface ( LCFS ) determined by the vacuum magnetic field. The pressure / temperature gradient in the external region is finite. The scale length of the pressure profile does not change so much even when the mean free path of electrons exceeds the connection length of the magnetic field line to the wall. There appear MHD instabilities with amplitude of 10 -4 of the toroidal magnetic field. From the mode number of the activities (m/n = 2/3, 1/2, 2/4), the location of the corresponding rational surface is outside the vacuum LCFS. The location of the mode is consistent with the fluctuation measurement, e.g., soft X-ray detector arrays. The MHD mode localized in the magnetic stochastic region is affected by the magnetic field structure estimated by the connection length to the wall using 3D equilibrium calculation.