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Effects of Toroidally Distributed Divertor Biasing on Scrape-Off-Layer Plasma in the QUEST Spherical Tokamak

Kazuo TOI, Takumi ONCHI¹⁾, Kengo KURODA¹⁾, Shinichiro KOJIMA¹⁾, Hideki ZUSHI¹⁾, Makoto HASEGAWA¹⁾, Masaharu FUKUYAMA¹⁾, Shoji KAWASAKI¹⁾, Aki HIGASHIJIMA¹⁾, Tatsuya IKEZOE¹⁾, Hiroshi IDEI¹⁾, Takeshi IDO¹⁾, Kazuaki HANADA¹⁾ and QUEST Experiment Group¹⁾

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

1)

Research Institute for Applied Mechanics, Kyusyu University, Kasuga 816-8580, Japan

(Received 18 November 2020 / Accepted 7 January 2021 / Published 5 March 2021)

Abstract

A novel divertor biasing using four biasing plates that are arranged toroidally every 90° on the upper divertor plate is applied to low-density plasmas of the QUEST spherical tokamak. When some of these plates are biased in-phase by applying a sawtooth waveform voltage of 85-V amplitude and 50-Hz repetition, up to approximately 35% reduction of the particle flux to the divertor is observed during positive biasing. The input power for the flux reduction is approximately 0.2 kW for low-density tokamak plasmas produced by ∼130-kW electron cyclotron wave injection. Additionally, the signal of a plate probe placed in the low-field side of the mid-plane of the vacuum vessel indicates enhanced losses of fast electrons during positive biasing. The enhanced loss is attributed to small resonant magnetic perturbations produced by the bias-driven currents in the scrape-off layer. This novel divertor biasing is expected to provide a new experimental tool for studying divertor heat load control and fast electron confinement in a tokamak device.

Copyright (c) 2021 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords

scrape-off layer (SOL), divertor heat load, divertor biasing, radial transport in SOL, SOL current, resonant magnetic perturbation

DOI: 10.1585/pfr.16.2402024

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