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 × 10 20 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.

Anomalous ion heating has been observed for the first time during flux/current amplification by doublepulsing coaxial helicity injection (CHI) in the helicity injected spherical torus (HIST) device. Doppler ion temperature increases significantly near the separatrix on the inboard side during the second CHI pulse, whereas electron temperature remains constant. The experimental results indicate that selective ion heating may be associated with viscous damping of poloidal flows driven by the CHI pulse.

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