Visible-Light Imaging of the Super-Alfvénic/Sonic Collisional Merging Process of Field-Reversed Configurations with a Contrast Medium-Mixed Plasmoid

The collisional merging formation process of field-reversed configurations (FRCs) was observed by using a recently developed spectroscopy system. The shape and boundary of the two plasmoids during the merging process were visualized by the “contrast medium” mixing. One of the two initial plasmoids captured a small amount of contrast medium (e.g., helium) diffused in a part of the vacuum vessel during the translation process. The contrast medium-mixed plasmoid merged with the other plasmoid which is without a contrast medium. The contrast-enhanced shape and internal structure of two plasmoids were observed using the spectroscopic systems with the line spectrum of the contrast medium ion. The emission distribution of the contrast medium ions inside the plasmoid was verified by visible-light computed tomography, which revealed that the contrast medium ions were captured only inside one plasmoid. In addition, the emission distribution resembled the density profile in the rigid rotor model, which is a pressure equilibrium model of FRCs. After the collision, the contrast medium ions diffused into the other side. This suggests the beginning of the merging with the plasmoid on the other side. The optically evaluated timescale of the merging was comparable with that observed by the internal magnetic measurements.