Trigger mechanism for the abrupt loss of energetic ions in magnetically confined plasmas

K Ida,H Tsuchiya,T Akiyama,K Itoh,T Tokuzawa,M Yoshinuma,S.-I Itoh,T Kobayashi

doi:10.1038/s41598-018-21128-z

Abstract

Interaction between a quasi-stable stationary MHD mode and a tongue-shaped deformation is observed in the toroidal plasma with energetic particle driven MHD bursts. The quasi-stable stationary 1/1 MHD mode with interchange parity appears near the resonant rational surface of q = 1 between MHD bursts. The tongue-shaped deformation rapidly appears at the non-resonant non-rational surface as a localized large plasma displacement and then collapses (tongue event). It curbs the stationary 1/1 MHD mode and then triggers the collapse of energetic particle and magnetic field reconnection. The rotating 1/1 MHD mode with tearing parity at the q = 1 resonant surface, namely, the MHD burst, is excited after the tongue event.

Highlights

Abrupt loss of plasma energy due to MHD activity has been commonly observed and various collapse events have been reported in toroidal plasmas[1]
The Large Helical Device (LHD) is a Heliotron-type device equipped with three tangential neutral beams with a beam energy of 160–180 keV and two perpendicular beams with a beam energy of 40–50 keV
When the two perpendicular beams are injected into the plasma with relatively low density of 1–2 × 1019 m−3, the MHD bursts appear to be associated with the enhancement of high frequency RF signals

Summary

Introduction

Abrupt loss of plasma energy due to MHD activity has been commonly observed and various collapse events have been reported in toroidal plasmas[1]. In order to explain the transition from fragile stability to an actual unstable state, an abrupt event which triggers the collapse is necessary. This is recognized as a trigger problem and is commonly observed in magnetized plasma in tokamak plasmas[3,4] and solar flares[5]. A tongue-shaped deformation[17,18] is observed just before the MHD bursts[19] This is considered to be a candidate for the trigger mechanism causing the MHD bursts. The causality relation between the MHD burst and the collapse of energetic ions is discussed

Methods

Results

Conclusion