Temperature and density characteristics of the Helicity Injected Torus-II spherical tokamak indicating closed flux sustainment using coaxial helicity injection

The electron temperature and density profiles of plasmas in the Helicity Injected Torus [HIT-II: T. R. Jarboe et al. , Phys. Plasmas 5, 1807 (1998)] experiment are measured by multipoint Thomson scattering (MPTS). The HIT-II device is a small low-aspect-ratio tokamak (major radius 0.3 m , minor radius 0.2 m , toroidal field of up to 0.5 T ), capable of inductive ohmic (OH) current drive, Coaxial Helicity Injection (CHI) current drive, or combinations of both. The temperature and density characteristics have been characterized by a ruby laser MPTS diagnostic at up to six locations within the plasma for a single diagnostic time per discharge. Observed hollow temperature profiles of CHI discharges are inconsistent with open flux only predictions for CHI and indicate a closed flux region during CHI current drive.