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

Yuki TAKEMURA^1,2), Kiyomasa WATANABE¹⁾, Tokihiko TOKUZAWA¹⁾, Satoru SAKAKIBARA^1,2), Yoshiro NARUSHIMA^1,2), Masaaki OKAMOTO³⁾, Satoshi OHDACHI^1,2), Yasuhiro SUZUKI^1,2), Katsumi IDA^1,2), Mikiro YOSHINUMA^1,2), Hayato TSUCHIYA¹⁾, Ichihiro YAMADA¹⁾ and LHD Experiment Group

1)

National Institute for Fusion Science, National Institutes of Natural Sciences, Toki 509-5292, Japan

2)

SOKENDAI (the Graduate University for Advanced Studies), Toki 509-5292, Japan

3)

National Institute of Technology, Ishikawa College, Ishikawa 929-0342, Japan

(Received 26 January 2017 / Accepted 1 May 2017 / Published 12 June 2017)

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 × B rotation were experimentally investigated. The precursor rotates together with the E × B rotation at the resonant surface, and the precursor rotation slows down because of a decrease of the E × B rotation. The multi-channel fluctuation measurement of the precursor suggests that the precursor has a magnetic island, which may be related to the decrease of the E × 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.

Copyright (c) 2017 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

slowing-down mechanism, MHD instability, magnetic island, locked mode, LHD

DOI: 10.1585/pfr.12.1402028

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