Plasma and Fusion Research

Volume 16, 2402048 (2021)

Regular Articles

Initial Results from High-Field-Side Transient CHI Start-Up on QUEST
Kengoh KURODA, Roger RAMAN1), Makoto HASEGAWA, Takumi ONCHI, Brian A. NELSON1), John ROGERS1), Osamu MITARAI2), Kazuaki HANADA, Masayuki ONO3), Thomas JARBOE1), Masayoshi NAGATA4), Hiroshi IDEI, Takeshi IDO, Ryuya IKEZOE, Shoji KAWASAKI, Takahiro NAGATA, Aki HIGASHIJIMA, Shun SHIMABUKURO, Ichiro NIIYA, Shinichiro KOJIMA, Akihiro KIDANI, Takahiro MURAKAMI, Kazuo NAKAMURA, Yuichi TAKASE5) and Sadayoshi MURAKAMI6)
Kyushu University, Fukuoka 816-8580, Japan
1)
University of Washington, Seattle, WA, USA
2)
Institute for Advanced Fusion and Physics Education, Kumamoto 861-5525, Japan
3)
Princeton Plasma Physics Laboratory, Princeton, NJ, USA
4)
University of Hyogo, Himeji 671-2280, Japan
5)
University of Tokyo, Kashiwa 277-8561, Japan
6)
Kyoto University, Kyoto 615-8540, Japan
(Received 16 November 2020 / Accepted 15 February 2021 / Published 9 April 2021)

Abstract

Transient coaxial helicity injection (t-CHI) current start-up using a new design simple electrode configuration has been implemented on the QUEST. Discharges injected from the low field side (LFS) and from the high field side (HFS) were examined. Compared to the LFS injection case, the HFS injection has the advantages of providing access to a higher toroidal field and better controlling the location of the injector flux footprint location. Although the present PF coils on QUEST are not well positioned to form the injector flux on the HFS injector region and there has been a frequent occurrence of the spurious arcs, known as absorber arcs, HFS injection has shown flux evolution in a shape that is suitable for the formation of closed flux surfaces. The discharges were improved by installing an in-vessel-coil and adding a new cylindrical electrode to the existing CHI electrode. The results show that the new cylindrical electrode allowed the flux to evolve stably while allowing both the inner and the outer injector flux footprint to remain in the vicinity of the cylindrical electrode. This configuration which inherently generates a narrow injector flux footprint width resulted in discharges that strongly suggested the persistence of the CHI generated plasma after the injector current was reduced to zero. These studies have informed us of the need to improve the CHI gas injection system so that the absorber arcs could be better controlled in the HFS injection configuration.

Keywords

CHI, non-inductive current drive, spherical tokamak, magnetic reconnection, ECH

DOI: 10.1585/pfr.16.2402048

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