Influence of Bias Magnetic Field Configuration on Equilibrium of Field-Reversed Configuration Plasma Sustained by Rotating Magnetic Field

We have measured detailed axial profiles of electron density, floating potential, and axial magnetic field in the field-reversed configuration (FRC) sustained by the rotating magnetic field. To study the influence on the equilibrium of two kinds of bias magnetic field configuration - straight (pure solenoidal) and mirror -, experiments have been carried out in the FRC Injection Experiment apparatus. The case of mirror configuration has longer quasi-steady state compared with the case of straight configuration. The steeper density gradient outside the separatrix is generated by the uniform gradient of magnetic field due to the mirror configuration. The axial parallel diffusion is suppressed due to the steeper density gradient outside the separatrix. Therefore, the mirror bias magnetic field configuration works to improve the plasma confinement.