Effect of Decay Index on Elongation Ratio in Non-Circular Tokamak TNT-A

Abstract

In a non-circular tokamak (TNT-A), the relation between elongation ratio k and decay index n(=−(r⁄B_z)·(∂B_z⁄∂r)) is investigated, varying the curvature of the shaping field. Both active and passive control have been done in the region of −0.9<n<−0.2. By Thomson scattering, magnetic probes and flux loops measurements, a D-shaped, stable plasma of T_e0∼180 eV, n_e0∼10¹³ cm⁻³, q_a>2 (the MHD oscillation of m=2 mode appears) with elongation ratio of up to 1.5 is obtained. The numerical curve of plasma equilibrium in (k, n) space fits well with the measurements. The effects of the iron core and shell on the decay index are important in a non-circular tokamak. The iron core makes the decay index lower, and the shell makes the field curvature smaller. The calculation and measurements show that the decay index is mainly affected by the iron core.