Abstract
The ion kinetic effect on the relaxation of merging spheromaks to a field-reversed configuration (FRC) was studied experimentally using varied S* (the ratio of the minor radius to the ion skin depth) from 0.5 to 10. The merging spheromaks relax to an FRC (λp ˜ 0) or to another spheromak (λp ˜ λTaylor), depending on whether their initial poloidal eigen value λp was smaller or larger than the threshold value λ0. The varied skin depths of several ion species in the up-scaled experiment 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.
