The Experimental Study of Time Resolved Inductively Coupled Plasma for Fast Control of High-Power Millimeter-wave

Abstract

The suppression of neo-classical tearing modes (NTMs) which is one of plasma instability is a critical issue to prevent the disruption of H-mode plasma in fusion plasma tokamak. To operate steady-state drive, we should understand not only physical phenomena of tearing modes but also optimized conditions of electron cyclotron current drive (ECCD) to suppress time resolved NTMs in range of a few kHz repetition. We propose an external switching system which can be settled in existing transmission lines and gyrotrons. The idea of switching system simply comes from interactions between millimeter-wave and glow discharged cold plasma. The cut-off and propagation of millimeter-wave can be determined by the plasma switching. A helical type inductively coupled plasma chamber is designed for high transmission of Gaussian beam (linear polarized Efield having Gaussian profiled) and generation of high-density bulked plasma. For proof-of-concept study, we conducted cold test using vector network analyzer (low power millimeter-wave, < 1mW, continuous waves) and successfully demonstrated proto-type test of millimeter-wave switching having 2 kHz repetition. For hot test, a gyrotron at UNIST (95 GHz, few kW of power) is used as a high-power millimeter-wave source. Although the gyrotron pulse length (20 us) is not enough to measure switching results (plasma switching time is 200 us), we observed wave absorption in plasma and increase of plasma density simultaneously. This result will help to understand mechanism of millimeter wave heating in inductively coupled plasma. Furthermore, this study will also contribute to understand plasma instabilities in fusion plasma.