Observation of Internal Structure of Edge MHD Modes in High Beta Plasmas on the Large Helical Device

Abstract

Edge MHD modes are excited with an appreciable amplitude by the rise of the edge pressure gradient in the Large Helical Device (LHD), where the related rational surfaces are located near the last closed flux surface (LCFS). Internal structure of these edge MHD modes in high beta plasmas was measured by using a set of soft X-ray (SX) diode arrays. The relative amplitude of SX fluctuations related to these MHD modes increases rapidly toward the LCFS. The phase relation among SX fluctuation signals obtained by the SX-diode array clearly indicates the poloidal mode structure.