Analysis of Magnetohydrodynamic Instabilities in TJ-II Plasmas

Abstract

Magnetohydrodynamic instabilities in the TJ-II stellarator are being experimentally characterized in various plasma parameter regimes and heating scenarios. Magnetic field fluctuations data are collected, using various Mirnov coil sets distributed at different toroidal sectors of the vacuum vessel, with frequency resolution up to 1 MHz. Specific analysis is carried out with the signals from a poloidal array of 15 coils measuring poloidal magnetic field fluctuations. The appearance of low-frequency modes (some tens of kilohertz) in electron cyclotron heated plasmas depends on the rotational transform profile and plasma density. In neutral beam injection plasmas, high-frequency (150- to 300-kHz) modes have been found in plasmas with line densities in the range 0.6 × 10¹⁹ m^-3 to 3 × 10¹⁹ m^-3 and heated with on/off-axis electron cyclotron heating. They are good candidates for global Alfvén eigenmodes related to the low-order resonance n/m = 3/2.