Plasma and Fusion Research

Volume 15, 2402063 (2020)

Regular Articles

Parametric Decay Wave Observation in HFS X-Mode Injection in QUEST
Shinichiro KOJIMA, Hatem ELSERAFY1), Kazuaki HANADA1), Hiroshi IDEI1), Ryuya IKEZOE1), Yoshihiko NAGASHIMA1), Makoto HASEGAWA1), Takumi ONCHI1), Kengoh KURODA1), Kazuo NAKAMURA1), Takahiro MURAKAMI, Masaharu FUKUYAMA, Ryoya KATO, Ryota YONEDA2), Masayuki ONO3), Akira EJIRI4), Yuichi TAKASE4) and Sadayoshi MURAKAMI5)
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
1)
Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
2)
Department of Physics & Astronomy, University of California Los Angeles, Los Angeles, CA 90095, USA
3)
Princeton Plasma Physics Laboratory, Princeton, NJ 08540, USA
4)
Graduate School of Frontier Sciences, The University of Tokyo, Kashiwa, Tokyo 277-8561 Japan
5)
Graduate School of Engineering, Kyoto University, Kyoto 615-8540, Japan
(Received 5 December 2019 / Accepted 5 July 2020 / Published 19 August 2020)

Abstract

The parametric decay wave (PDW) caused by three-wave parametric decay process was measured in a plasma-injecting X-mode electron cyclotron wave (ECW) from the high field side (HFS) of the Q-shu University Experimental Steady-State Spherical Tokamak (QUEST). The intensity of the low-frequency PDW on the HFS X-mode injection was significantly enhanced over the O-mode ECW injection from the low field side (LFS), where the mode conversion to electron Bernstein wave (EBW) was not expected. As the comparison was executed using plasmas with the same magnetic field and injection power, the wave injection method was considered as the primary cause of the difference in the PDW excitation. The frequency range of the low-frequency PDW was consistent with that of the lower hybrid wave (LHW) range, which was expected to be excited during the mode conversion to EBW. The low-frequency PDW intensity evolved in response to the plasma density, plasma current and injection power. This observation suggests that the low-frequency PDW intensity is a reliable indicator for the efficient mode conversion to EBW.

Keywords

electron Bernstein wave, parametric decay instability, X-B mode conversion, high field side injection, QUEST

DOI: 10.1585/pfr.15.2402063

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