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.

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The multichannel ion Doppler spectroscopic (M-IDS) system using a fast camera as detectors has been newly developed to measure the spatial profiles of impurity spectral line emissions from plasmas. The M-IDS system developed with eight viewing chords is characterized by the simplest optical systems, a fast sampling frequency, high number of shooting, and a low crosstalk of 3.5% on each chord. In order to verify the usefulness and reliability of the M-IDS system, we have applied it for the start-up plasma produced in the helicity injected spherical torus device. Resultantly, we have successfully measured the radial profiles of Doppler ion temperature T i.D and Doppler ion velocity v i.D of the plasma with a good Gaussian fitting. However, we have found that the sampling frequency is upper limited by 25 kHz because the signal detected with a faster frequency from the CIII impurity line emission is too weak to be analyzed, and thus the sensitivity of the total system needs to be improved. C⃝ 2017 Wiley Periodicals, Inc. Electron Comm Jpn, 100(2): 68-75, 2017; Published online in Wiley Online Library (wileyonlinelibrary.com).