Tokamak startup using outboard current injection on the Pegasus Toroidal Experiment

Abstract

Localized current injection near the outboard midplane is used to form 0.1 MA plasma discharges with no induction supplied from a central solenoid in the ultra-low aspect ratio Pegasus Toroidal Experiment. The discharges are initiated by driving open-field-line currents that perturb the vacuum magnetic field such that the magnetic topology transitions to a tokamak-like configuration. The plasma is subsequently driven via helicity injection from the edge current sources and poloidal field induction. Intermittent n = 1 MHD activity is observed during periods of strong edge current drive and each event leads to a rapid inward expansion of the plasma volume and a drop in the plasma inductance. The plasmas are sufficiently turbulent such that the equilibrium approaches the lowest energy state described by Taylor relaxation theory. In agreement with that theory, the maximum I p scales with ( I TF I inj / w ) 1/2 , where I TF is the toroidal field rod current, I inj is the injected edge current and w is the radial width of the average poloidal magnetic flux in the driven open flux region.