福本 直之

Multiple plasmoid reconnection required for the flux closure in the transient-coaxial helicity injection (T-CHI) start-up process has been demonstrated in the Helicity Injected Spherical Torus (HIST) device. Two or three plasmoids are generated after the tearing instability of an elongated Sweet-Parker current sheet during the T-CHI. Here, we report that in the T-CHI start-up plasmas (H, D and He) with the strong toroidal (guide) field (ITF=140 kA), (i) the frequency of regular oscillations of reconnecting magnetic field decreases as the mass number increases, i.e., 250 kHz (H), 150 kHz (D) and 60 kHz (He). (ii) the oscillation propagates radially with 30 km/s (H), 20 km/s (D) and 12-18 km/s (He) from R= 0.25 m at the X-point toward the outboard side. It has been found that the propagation speed agrees with the Alfven speed. Consequently, the plasmoid reconnection could be related to the excitation of Alfven wave, leading to the ion heating in the T-CHI start-up plasmas.

The dynamo current drive with two-fluid effects has been investigated in the helicity-driven spherical torus (ST) plasmas on HIST. 2D internal magnetic field measurements of the ST configuration has verified the flux amplification and the generation of the closed flux during the coaxial helicity injection (CHI). We have found that not only MHD 〈δv×δB〉|| dynamo but also Hall 〈δj×δB〉||/en dynamo plays an important role in the helicity transport for the current sustainment. Hall-MHD induced-electric fields satisfy the parallel mean-field Ohm's law balance. We have also observed transverse propagation and damping of a low-frequency Alfven wave. The dynamo activities for the current drive may be caused by the Alfven waves.