Observation of the Transition of Internal Magnetic Structure During the Collisional Merging Process in Field-Reversed Configurations*

The Field-Reversed Configuration (FRC) is identified as a high-beta plasmoid, ideally possessing only poloidal magnetic field. With the ion Larmor radius within the separatrix exceeding or being comparable to the plasma radius, most of the FRC region is perceived as an unmagnetized kinetic plasma. In the FAT-CM device at Nihon University, FRCs undergo collisional merging at relative velocity surpassing that the Alfvénic and ion sonic speeds. Post-collision, the initial-FRCs merge into a singular FRC within microseconds, hinting at a process analogous to the rapid magnetic reconnection observed in astrophysical plasmas. The magnetic structural transition of the collisional merging process was analyzed using a magnetic probe array installed at the central cross-section of the FAT-CM device. The observed changes in the internal magnetic structure during collision deviate from those seen after the merging. This outcome indicates that the internal structure undergoes transitions at each phase.