Experimental Study on Slowing-Down Mechanism of Locked-Mode-Like Instability in LHD*)

Abstract

In order to clarify the mechanism responsible for slowing down the precursor of an instability in Large Helical Device (LHD), whose behavior is similar to the locked mode instability in tokamaks, the spatial structure of the precursor of the locked-mode-like instability, and the relationship between the rotation of the precursor and the E x B rotation were experimentally investigated. The precursor rotates together with the E x B rotation at the resonant surface, and the precursor rotation slows down because of a decrease of the E x B rotation. The multichannel fluctuation measurement of the precursor suggests that the precursor has a magnetic island, which may be related to the decrease of the E x B rotation. In addition, the reason for the appearance of the precursor with a magnetic island is discussed. The precursor appears when a magnetic island grows initially without rotation but then shrinks and begins to rotate.