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Dependence of MHD Burst Events and Velocity Distribution Deformation on Magnetic Field Direction in LHD Plasma

Yuichi KAWACHI¹⁾, Tatsuya KOBAYASHI^2,3,4), Mikirou YOSHINUMA²⁾, Katsumi IDA²⁾

1)

Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

2)

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

3)

The Graduate University for Advanced Studies, SOKENDAI, Toki 509-5292, Japan

4)

Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan

(Received 26 August 2025 / Accepted 22 September 2025 / Published 26 November 2025)

Abstract

We conducted energetic particle-induced MHD burst excitation experiments on the Large Helical Device (LHD) to investigate how these events—and the associated deformations of the ion velocity distribution—depend on the direction of the magnetic field. Wavelet cross-power spectrum analysis revealed that the toroidal propagation direction of the MHD bursts reverses with the magnetic field. Fast charge exchange recombination spectroscopy (fast-CXRS) was used to measure the temporal evolution of the carbon ion velocity distribution function. A bipolar signature in velocity space, as well as the resonant velocity, varied with the magnetic field direction. A strong correlation was observed between the chirping frequency and the corresponding resonant velocity, and the phase velocity was found to closely match the resonant velocity, including the sign of the propagation direction.

Copyright (c) 2025 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

charge exchange recombination spectroscopy, energetic particle, velocity distribution function

DOI: 10.1585/pfr.20.1202053

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