Coaxial helicity injection current drive in a low-aspect ratio tokamak

Summary form only given. Economical steady-state tokamak reactors require both an efficient method of current drive and high beta operation. Proposed tokamak current-drive methods (neutral beam, electron cyclotron, and lower hybrid) drive tail particles and have reactor power efficiencies of approximately 10/sup -3/. The NSTX ultra-low aspect ratio tokamak (ULART, A=R/a<1.3) design shows global betas up to 44%. Coaxial helicity injection (CHI) current drive uses plasma relaxation processes to drive current carried by the bulk population, allowing the reactor efficiency to remain near ohmic. No transformer is required, thus CHI-driven tokamaks can be constructed with a low aspect ratio. The helicity injected tokamak (HIT) produces a steady-state toroidal current by driving an edge current and allowing relaxation to drive current throughout the plasma. Relaxation occurs on reconnection time scales where helicity is approximately constant. Helicity (plasma current) decays on resistive time scales, and is replenished by CHI. HIT is constructed with a low aspect ratio, R/sub 0/=0.3 m, a=0.2 m, (A=1.5), with an on axis field up to B/sub 0/=0.67 T. Diagnostics include magnetic probes, FIR interferometry, Thomson scattering, XUV photodiode arrays, and VUV spectroscopy. CHI produces low-aspect-ratio tokamaks with currents up to 250 kA and sustained over 140 kA for many resistive diffusion times.