Influence of a Toroidal Field on Plasma Confined in a Toroidal Octupole

Abstract

A toroidal magnetic field Bθ of up to 360 G was added to the Wisconsin toroidal octupole producing a stellarator-like field with a large rotational transform and large shear. The influence of this Bθ on plasma injection, transport, density distribution, lifetime, and electric-field fluctuations was studied. Injection and transport were not inhibited by Bθ and proceeded in much the same way as for Bθ = 0. The density distribution after filling was unchanged and remained peaked slightly off the separatrix toward the hoops. The lifetime was smaller with Eθ, but was still consistent with expected losses to the hoop supports. In the originally V″ unstable region outside the last stable field line, the amplitude of the electric field fluctuations decreased when Bθ was added, but in the interior of the plasma, the fluctuations increased monotonically with Bθ. In the plasma interior the maximum diffusion coefficient derived from the fluctuations increased from 10−4 DBohm to 10−2 DBohm as Bθ was increased from zero to 140 G, but the observed lifetime, limited by hoop supports, was always ∼10 τBohm. Large potential deviations which were affected by magnetic field perturbations occurred in the region of zero average shear. Large external-field perturbations were added with no effect on the lifetime. Fluctuations associated with stable inverted density gradients were reduced or eliminated by Bθ.