STUDY OF THE ULTRA-LOW ASPECT RATIO TOKAMAK, ULART

We investigate experimentally and theoretically the global MHD characteristics of an ultra-low aspect ratio tokamak (ULART). Since the ULART requires a substantially smaller toroidal field current, I-tf, than conventional tokamaks, it has important reactor advantages. By fully utilizing the TS-3 merging spheromak facility with a slender center conductor, we have carried out an experimental study of the ultra-low aspect ratio tokamak with aspect ratio reaching as low as 1.05. The ULART is found to be similar to the spheromak in its strong paramagnetism and magnetic helical pitch. In this extreme limit, we investigate the transition of the spheromak (q(a) = 0, I-tf = 0) to a ULART plasma (q(a) = 5-20, I-tf < I-p). It is observed that a small current at the center conductor can significantly improve the overall stability of the formed plasmas by effectively stabilizing the tilt mode. We identify a threshold of I-tf << I-p with q(cyl)(a) << 1 for global tilt/shift modes. This initial observation is in agreement with a global MHD theory.