Characteristics of plasma current start-up by transient coaxial helicity injection on HIST

Transient coaxial helicity injection (T-CHI) was successfully demonstrated in the Helicity Injected Spherical Torus (HIST) device for non-inductive plasma current start-up. The spherical torus (ST) requires no central solenoid coil. The characteristics of the T-CHI start-up discharges on the HIST were investigated in detail. The toroidal plasma current reached 60 - 80 kA, and the current density profile was significantly modified by variations in the bias poloidal coil flux. The Doppler ion temperature and electron temperatures reached 10 - 15 eV, and the electron density of 1 × 10²⁰ m^-3, depending on the bias flux. Internal magnetic field measurements using a two-dimensional magnetic probe array verified that closed flux surfaces formed after fast magnetic reconnection during the current rising phase. The amount of closed flux was quantitatively related to the bias flux. We also present the first experimental measurements of helicity balance during the T-CHI process. These results validate the capability of T-CHI for higher current generation in the ST. (author)