Experimental studies of plasmoid reconnection by using CHI start-up plasma on HIST

Plasmoid reconnection for the flux closure has been for the first time demonstrated by transient-coaxial helicity injection (T-CHI) in the Helicity Injected Spherical Torus (HIST) device. The intensive measurement of internal magnetic structures indicates that 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), (i) regular oscillations of reconnecting magnetic field, electron density and ion flow observed in the T-CHI process indicates repetitive generation of small-size plasmoids due to the magnetic reconnection, (ii) one of reconnecting plasmoids grows up a large-size plasmoid, results in the formation of doublet-type ST configurations. Consequently, the plasmoid reconnection could be the leading mechanism for the formation of multiple X-point, i.e., the fast flux closure in the T-CHI discharge.