Transport and Stability Analysis of Low qa Discharges

Abstract

An one‐dimensional stability transport code (PROSIM) [1] has been used to analyze the accessibility of low qa discharges obtained in the SINP tokamak. In order to evolve the transport properties, the code solves the electron temperature evolution equation and the poloidal magnetic field diffusion equation. To incorporate the effects of various MHD instabilities which are likely to have substantial effect on the evolution of the discharge, equations governing the MHD mixing and tearing modes have been solved at each time step. Since reverse shear profiles have been observed in the rising phases of low qa discharges, the stability calculations have been carried out for plasmas having multiple resonant surfaces. Experimental observations [2] indicate the presence of large positive spikes just before the ultra low qa (ULQ) regime is entered. In this paper we present detailed analysis of plasma evolution near the positive spike in the loop voltage. We have tried to relate our results to the complicated questio...