NSTX plasma start-up using transient coaxial helicity injection

Future toroidal magnetic confinement fusion plasma devices such as the Component Test Facility (CTF) require non-inductive toroidal current drive. A new method of non-inductive startup, referred to as transient coaxial helicity injection (Transient CHI), has been developed on the Helicity Injected Torus (HIT-II) experiment and the National Spherical Torus Experiment (NSTX). In this method, plasma current is produced by discharging a capacitor bank between coaxial electrodes in the presence of toroidal and poloidal magnetic fields chosen such that the plasma rapidly expands into the chamber. When the injected current is rapidly decreased, magnetic reconnection occurs near the injection electrodes with the toroidal plasma current forming closed flux surfaces. In NSTX, transient CHI has demonstrated closed-flux current generation of up to 160 kA, without the use of a central solenoid. Detailed experimental measurements made on NSTX include fast time-scale visible imaging of the entire process.