Review of plasmoid reconnection and ion heating in the CHI experiments on Helicity Injected Spherical Torus (HIST)

The key role of plasmoid-mediated magnetic reconnection was experimentally investigated during transient-Coaxial Helicity Injection (T-CHI) for non-inductive plasma start-up in the HIST device. The fast magnetic reconnection is required for the flux closure and ion heating at once in T-CHI discharges. Here, we will review them as follows; (a) In the ejection of plasma flow from the magnetized coaxial plasma gun during the helicity injection phase, a narrow current sheet is elongated and broken apart at some points inside it due to a tearing instability, so that three or four small-size plasmoids are born in sequence. The separation and coalescence of the multiple plasmoids are repeated through the reconnection, leading to a bigger plasmoid, namely closed flux surfaces. We observed the phenomena as magnetic field oscillations, which was newly named as the plasmoid oscillation. (b) Doppler ion temperature increased from ~10 eV up to 80 eV during the plasmoid oscillation. The ion heating driven by the regular reconnection during the repeated plasmoid merging has been for the first time identified on HIST.