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Identification of kink instability in 3D helical flux ropes at VEST

Authors :
Jong Yoon Park
June Young Kim
Seongcheol Kim
E. C. Jung
Y. S. Hwang
Source :
Physics of Plasmas. 29:052112
Publication Year :
2022
Publisher :
AIP Publishing, 2022.

Abstract

Local helicity injection (LHI) is a non-inductive startup and current drive method via Taylor relaxation for the spherical torus. In achieving Taylor relaxation, it has been suggested that kink instability in 3D helical flux ropes plays an important role. However, the role and occurrence of kink instability during LHI have yet to be validated. Experimentally, determining the kink mode in a flux rope relies on measuring internal information using a probe. However, for LHI, the 3D geometry complicates this measurement process. Here, we propose a new approach for determining the kink modes of 3D helical flux ropes without any internal probe measurements. It is confirmed by this approach that flux ropes exhibit two different kink modes. With increasing plasma current in the flux ropes, a transition from the coherent internal kink mode to the external kink mode is observed. Kink mode properties such as rotating frequency calculated from the kink theory agree well with the magnetic signature driven by the kink mode. During the LHI experiment in the versatile experiment spherical torus, three distinguishable phases are confirmed by the approach, consistent with NIMROD simulation. Before driving the toroidal plasma current, the external kink mode is observed for 3D helical flux ropes. As the toroidal plasma current increases, the external kink mode disappears while generating broadband internal modes instead of coherent internal kink of flux ropes. Decoupling between the toroidal plasma and flux rope results in both decay of toroidal plasma current and re-appearance of the external kink mode in the flux ropes.

Subjects

Subjects :
Condensed Matter Physics

Details

ISSN :
10897674 and 1070664X
Volume :
29
Database :
OpenAIRE
Journal :
Physics of Plasmas
Accession number :
edsair.doi...........6c8d39a756c7cdf0e4a1909bd1291ac6
Full Text :
https://doi.org/10.1063/5.0087831