Low Aspect Ratio Tokamak Formation and Sustainment by Coaxial Helicity Injection Current Drive

Coaxial helicity injection is used to form and sustain low aspect ratio tokamaks at currents of up to 250 kA in the Helicity Injected Tokamak experiment. Plasma currents can be sustained at an average of 225 kA for 2 ms, with on axis electron thermal energies up to 80 eV, or for longer times, 140 kA average for 7 ms, many resistive diffusion times. Spectroscopic measurements of the higher current discharges suggest burn-through of oxygen impurities. These plasmas have a rotating n = 1 distortion, appearing only on the outer, bad-curvature region. Equilibria reconstruction, fitting to experimental data, shows tokamak q profiles achieved with hollow plasma current profiles.