Plasma and Fusion Research

Volume 15, 2402007 (2020)

Regular Articles

Observation of Parametric Decay Instability on TST-2 Lower Hybrid Wave Driven Plasma
Yongtae KO, Yuichi TAKASE, Akira EJIRI, Naoto TSUJII, Osamu WATANABE, Hibiki YAMAZAKI, Charles P. MOELLER1), Yasuo YOSHIMURA2), Kotaro IWASAKI, Yuki AOI, Kyohei MATSUZAKI, James H.P. RICE, Yi PENG and Yuki OSAWA
The University of Tokyo, Kashiwa 277-8561, Japan
1)
General Atomics, San Diego, CA 92186, USA
2)
National Institute for Fusion Science, Toki, 509-5292, Japan
(Received 29 November 2019 / Accepted 6 February 2020 / Published 13 March 2020)

Abstract

Parametric decay instabilities (PDIs) in lower hybrid (LH) frequency range were observed in TST-2 in a various parameter range. LH wave measurement has been performed using four RF magnetic probes. We have been studied experiments aiming at efficient non-inductive current drive with the outboard and the top antennas installed on the outboard side and the top side of the vacuum vessel, respectively. Since PDIs are believed to deteriorate the current drive efficiency, we investigated conditions to avoid them. Operations with various combination of antennas (outboard, top), toroidal magnetic field strength and direction (CW, CCW BT), working gas (hydrogen, deuterium) and density (normal, high) were investigated. In the outboard launch case, PDI sideband frequency δf associated with ion cyclotron quasi-modes was observed with large degree in any BT strength in both hydrogen and deuterium plasma. On the other hand, no clear such sideband δf peak was observed only in deuterium plasma with the top launch case. Among various scenarios, the top launch with CW and CCW toroidal field in deuterium plasma seems to be desirable in terms of avoiding PDIs.

Keywords

spherical tokamak, lower hybrid wave, magnetic probe, parametric decay instability

DOI: 10.1585/pfr.15.2402007

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