Equilibrium Transition Study of Merging Spheromaks to a Field-Reversed Configuration

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A novel merging formation of field-reversed configuration (FRC) has been developed by use of two merging spheromaks with equal but oppositely directed toroidal magnetic fluxes. A significant increase in ion temperature from 10eV to 200eV was documented when their magnetic reconnection annihilates their toroidal magnetic energies. The ion acceleration by reconnextion outflow and the follwing selective ion heating through ion viscosity were found to cause this new equilibrium transition from the low-beta (0.1) spheromaks to the high-beta (0.7-0.9) FRC.This high-beta relaxation annihilates the initial magnetic helicity, revealing a new high-beta relaxation different from the low-beta Taylor relaxation. The merging spheromaks were also found to relax either to an FRC or to a new spheromak, deoending on whether their initial magnetic helicity is smaller or larger than a threshold value under the fixed total magnetic energy. Experimental verification of this formation process lead us to anew scenario of FRC slow-formation, heating and flux (current) -amplification by ohmic heating coils. Th toroidal current (poloidal flux) of the produced FRC was amplified successfully by the factor 2.5 within 30musec.