Formation and sustainment of a 250 kA tokamak by coaxial helicity injection

Abstract

Summary form only given. Steady-state tokamak fusion reactors require an efficient method of current drive. 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/. Coaxial helicity injection (CHI) current drive utilizes plasma relaxation processes to drive current carried by the bulk population, allowing the reactor efficiency to remain near ohmic. 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. Magnetic helicity is proportional to the product of linked flux, which in a tokamak is the toroidal flux (generated by external coils) times the poloidal flux (generated by plasma current). Relaxation occurs on reconnection time scales where helicity is approximately constant. Helicity (plasma current) decays on resistive time scales, and is replenished by CHI. No Ohmic transformer is required, thus 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 is used to produce low-aspect-ratio tokamaks with toroidal currents reaching 250 kA and sustained over 140 kA for many resistive diffusion times.