Results from the HIT-II coaxial helicity injection spherical tokamak

Summary form only given. The upgraded Helicity Injection Tokamak (HIT-II) extends coaxial helicity injection (CHI) to time scales longer than the wall resistive diffusion time. Analysis of HIT data and stability studies have developed a proposed mechanism for CHI current drive. This mechanism, consistent with these analyses, appears to be responsible for current drive inside the separatrix, and is summarized as follows: a hollow current profile plus a small (2-10 mm) edge vacuum region causes an n=1 external kink mode to go unstable at the edge; on a resistive diffusion time scale the equilibrium adjusts to this perturbation throughout the volume, producing a global helical magnetic structure locked to the electrons and not to the ions; the externally driven electrons on the open field lines cause the structure to rotate; the rotating structure, in turn, drags the electrons in the closed flux region producing current drive inside the separatrix.