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Merging startup of high beta spherical tokamak by using outer poloidal field coils

Author: Michiaki Inomoto
Requested Type: Consider for Invited
Submitted: 2014-05-28 19:07:07

Co-authors: T.G. Watanabe, K. Yamasaki, A. Wang, T. Ushiki, X. Guo, H. Ishikawa, H. Nakamata, N. Kawakami, T. Sugawara, K. Matsuyama, Y. Ono

Contact Info:
GSFS, The University of Tokyo
5-1-5 Kashiwanoha
Kashiwa, Chiba   277-856
Japan

Abstract Text:
There is no enough room for a center solenoid (CS) coil in spherical tokamak (ST) device because of its tight aspect ratio. Development of CS-less start-up methods are strongly required to achieve ST-type fusion reactor and many unique methods such as EC/EB, LH waves, helicity injection, and plasma merging have been proposed. Recently, merging start-up has been attempted in MAST using internal poloidal field (PF) coils and showed rapid increase in both ion and electron temperatures by magnetic reconnection[1], however, electron heating mechanism by magnetic reconnection with strong toroidal field (i.e. guide field) perpendicular to the reconnecting field is still unclear and need to be investigated to optimize merging start-up of high beta ST. Theoretical and numerical works revealed that the presence of guide field greatly changes the collisionless reconnection process in a qualitative way. One of the essential changes is that electrons are efficiently accelerated by the reconnection electric field to achieve high kinetic energy.

In the UTST device, two initial ST plasmas are formed by PF coils located outside of the vacuum vessel and their merging provides ion and electron heatings by magnetic reconnection. During the non-steady reconnection process in the UTST with toroidal guide field 20 times higher than the reconnection field, a sharp ring-shape visible light emission was found near the X-point. Toroidally accelerated electrons by reconnection electric field are supposed to ionize/excite carbon impurities in the middle phase of reconnection. Soft X-ray (SXR) burst from the X-point region was also observed simultaneously with the enhancement of the reconnection electric field. The SXR intensity showed almost linear increasing trend with the guide field and reconnection electric field, suggesting that the SXR emission was induced by parallel accelerated energetic electrons.

[1] Y. Ono, et al., Plasma Phys. Control. Fus., 54, 124039 (2012).

Characterization: 1.2

Comments:
I'd like to submit my abstract for US-Japan CT Workshop as well as EPR2014. I'm sorry but I can attend only on Aug 7 and 8.

Workshop on Exploratory Topics in Plasma and Fusion Research (EPR) and US-Japan Compact Torus (CT) Workshop
August 5-8, 2014
Madison, Wisconsin

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