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A Fast Electron Transport Model for Lower Hybrid Wave Sustained Plasmas

Akira EJIRI, Hibiki YAMAZAKI, Yuichi TAKASE, Naoto TSUJII, Osamu WATANABE, Yi PENG, Kotaro IWASAKI, Yuki AOI, Yongtae KO, Kyohei MATSUZAKI, James H.P. RICE, Yuki OSAWA, Charles P. MOELLER¹⁾, Yasuo YOSHIMURA²⁾, Hiroshi KASAHARA²⁾, Kenji SAITO²⁾, Tetsuo SEKI²⁾ and Shuji KAMIO²⁾

The University of Tokyo, Kashiwa 277-8561, Japan

1)

General Atomics, San Diego, CA 92186, U.S.A.

2)

National Institute for Fusion Science, Toki 509-5292, Japan

(Received 20 October 2021 / Accepted 10 March 2022 / Published 13 May 2022)

Abstract

In the TST-2 spherical tokamak (ST), non-inductive start-up by lower-hybrid waves (200 MHz) has been studied and a plasma current of 27 kA was achieved. For a comprehensive understanding of the wave sustained plasmas, a fast electron transport model combined with an X-ray emission model is constructed. The electrons in the model show a velocity random walk induced by the wave and collisional slowing down. Simultaneously, they show diffusion in real space. Electron generation and loss at the limiters are also considered. Using the model we can calculate the powers, such as the power from the wave to electrons (i.e., deposition power), collisional bulk electron heating power, power to the limiters. In addition, plasma current, electron density, neutral density, X-ray spectrum expected by a certain measurement system are obtained. Comparison with experimental data shows that a major part of the LHW deposition power is lost by fast electrons hitting the outboard limiter, while a minor part is used to heat cold bulk electrons. The diffusion in real space is well described by the RF induced radial transport, which is often used to interpret fast ion diffusion in ICRF heating. The present work suggests that the RF induced transport of fast electrons is the dominant loss mechanism.

Copyright (c) 2022 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

rf induced transport, lower hybrid wave, spherical tokamak, fast electron

DOI: 10.1585/pfr.17.1402037

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