Investigations of low qa discharges in the SINP tokamak

Low edge safety factor discharges including very low qa (1 < qa < 2) and ultra low qa (0 < qa < 1) have been obtained in the SINP tokamak. It has been observed that accessibility of these discharges depends crucially on the fast rate of plasma current rise. Several interesting results in terms of different time scales like Tqa, τR etc have been obtained using a set of soft wares developed at SINP. From fluctuation analysis of the external magnetic probe data it has been found that MHD instabilities m = 1, n = 1 and m = 2, n = 1 etc. play major role in the evolution of these discharges. To investigate the internal details of these discharges, an internal magnetic probe system has been developed using which current density jθ and other related parameters have been estimated. By carrying out a resistive stability analysis, evidence of the above-mentioned MHD instabilities have again been found. The physical processes lying behind the accessibility and evolution of the low qa discharges have been thoroughly investigated. (author)