Coaxial helicity injection current drive in the Helicity Injection Tokamak (HIT)

A steady-state tokamak fusion reactor requires current drive. Presently studied tokamak current-drive experiments (neutral beam, electron cyclotron, and lower hybrid) drive tail particles and have a power efficiency approximately five times that of ohmic. However, at reactor conditions their efficiency relative to ohmic decreases to approximately 10-3. Helicity injection current drive utilizes plasma relaxation processes to drive current carried by the bulk population, allowing efficiency to remain near ohmic for reactor conditions. Magnetic helicity, K, is a measure of toroidal and poloidal flux linkage, proportional to their product. Thus tokamak helicity is proportional to the plasma current Ip, which decays on resistive time scales. The Prototype Helicity Injected Tokamak (Proto-HIT) experiment (R = 0.35 m, a = 0.25 m, annular thin-walled flux conserver) formed and sustained 30 kA of plasma current solely by coaxial helicity injection. The helicity injector behavior agrees with a force balance model. Surface conditioning by titanium gettering increases the toroidal current and the injector impedance, in agreement with helicity balance