Effect of Initial-Plasmoid Density Reduction on Collisional Merging Process of Field-Reversed Configurations

A super-Alfvenic/sonic collisional merging formation of field-reversed configurations (FRCs) with low-density and high-temperature initial-FRCs was attempted on the FAT-CM device at Nihon University. To vary the density and temperature of initial-FRCs, the low-density/high-temperature (LD/HT) FRC formation technique was applied to the initial-FRC formation. The electron density of initial-FRCs formed using the LD/HT FRC formation technique was reduced to about 50% of that in the standard cases. The ion temperature was increased as the electron density decreased because the plasma pressure completely balances with the external magnetic pressure in an ideal FRC. The ion mean-free-path also increased to the equivalent value of the diameter of the initial-FRCs. Therefore, the initial-FRCs will be collision-less. These collision-less initial-FRCs were successfully translated. The observation results of the collisional merging formation process of FRC from the internal magnetic probe array and two axially arranged interferometers indicate that the performance of the FRC formed after the collision and merging declined in cases with collision-less FRCs and it depends on the kinetic energy in the collision process. (C) 2022 The Japan Society of Plasma Science and Nuclear Fusion Research