Abstract
In the Large Helical Device (LHD), edge coherent MHD modes such as m/n = 2/3 or 1/2 (m, n: poloidal and toroidal mode numbers), of which the rational surface is located in the plasma edge region of the magnetic hill, are strongly enhanced after an L–H transition. In this case, an edge MHD mode localized in a region further in than the location of the enhanced MHD modes is often stabilized because of the decrease in the pressure gradient there. Enhancement of the edge MHD mode amplitude stops a substantial rise in the edge pressure gradient after the transition. In an L–H transition plasma, soft x-ray (SX) fluctuations, δIsx, related to edge MHD modes are clearly detected. The radial profiles of the amplitude and the phase differences among the SX-detector channels are sensitively dependent on the m-number. The relative amplitudes of the SX fluctuations, δIsx/Isx, which may reflect the eigenfunction of the MHD mode, rapidly increase towards the plasma edge in which the relevant rational surface exists.
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