Experimental investigation of plasma pile-up and ejection using TS-4 plasma merging device

Plasma density pile-up and following plasma ejection were observed for the first time by radial electron density profile measurement using an 8-channel $\rm{CO_2}$ laser interferometer during the impulsive magnetic reconnection in the TS--4 spherical tokamak merging experiment. Two merging spherical tokamaks under the high guide field were merged together in the axial direction and their reconnection was as slow as the steady Sweet--Parker model under the small compressional force of the external coils. Under the strong compressional force, we observed multi-cycles of impulsive fast reconnection which is composed of the plasma pile-up inside the current sheet and its ejection from the X-point repetitively. The density pile-up in the current sheet and the plasma ejection inward accelerate the reconnection inflow about 2 times faster than the steady case. We modified the Sweet--Parker model to include those two effects [1], and found that its new theoretical inflow velocity agrees well with the measured velocity. \\[4pt] [1] Y.Ono, \textit{et al}., Phys. Plasma, vol.18, 111213(2011).