Global stability study of the ultra low aspect ratio tokamak, ULART

By introducing a slender current carrying conductor through the geometric center axis of the Tokyo University TS-3 device, the authors have generated ultra low aspect ratio tokamak (ULART) configurations with aspect ratio as low as 1.05. In this extreme limit they study the transition of the spheromak (q{sub edge} = 0, I{sub tf} = 0) to a ULART plasma (q{sub edge} = 2--20). The authors investigate the global MHD characteristics of ULART by comparing theoretical results with the experimental data obtained. A small current in the center conductor (compared with the plasma current) is found to significantly improve the overall MHD stability characteristics of the formed plasmas by effectively stabilizing the global tilt/shift mode. Theoretical calculations of the threshold toroidal field current required for stability and the growth rates of the tilt/shift modes agree well with the TS-3 data.