Experimental Study of Ion Kinetic Effects on Relaxation of Merging Spheromaks to an FRC

Abstract

The ion kinetic or skin depth effect on the relaxation of merging spheromaks to a field-reversed configuration (FRC) was investigated experimentally using varied S^* (the average number of the ion skin depth) from 0.5 to 10.The merging spheromaks relax either to an FRC (λ_p˜0) or to another spheromak (λ_p˜λ_Taylor), depending on whether their initial poloidal eigen value λ_pwas smaller or larger than the threshold value λ₀. The up-scaled TS-4 experiment under varied skin depths with several ion species revealed that the threshold value λ₀ for the relaxation to an FRC increased inversely with the S^* value. The decrease in S^* promoted the relaxation to an FRC, annihilating the magnetic helicity, in sharp contrast with the conventional Taylor relaxation. The low S^* relaxation to an FRC was observed to suppress the all low-n modes with the fast rotation of those modes.