Effects of an externally produced static magnetic island on edge MHD modes in the Large Helical Device

Abstract

In high beta plasmas and plasmas with an edge transport barrier on the Large Helical Device (LHD), pressure-driven edge MHD modes such as m/n = 1/1, 3/4, 2/3, 3/5 and 1/2 (m, n: poloidal and toroidal mode numbers) are excited in the magnetic hill region of the plasma edge. When a sizable static magnetic island with the mode number of m/n = 1/1 was externally generated at the ι/2π = 1 surface near the plasma edge using the perturbation field coils called the local island divertor coil, the edge pressure gradient was modified in the toroidal and poloidal directions by the formation of an X-point and an O-point of the island and the achieved volume-averaged beta was appreciably reduced. This modification changed the excited mode numbers of these edge MHD modes and their radial mode structures at the inboard and outboard sides of LHD. In particular, these edge MHD fluctuations observed by soft x-ray detectors exhibited obvious enhancement on the outboard side (larger major radius) and reduction on the inboard side, regardless of the O-point/X-point position.