Ion kinetic effect on bifurcated relaxation to a field-reversed configuration in TS-4 CT experiment

The ion kinetic effect on the bifurcated relaxation of merging spheromaks to a field-reversed configuration (FRC) was studied experimentally using varied S*, which is the ratio of the minor radius to the ion skin depth from 1 to 7. The two merging spheromaks were observed to relax to an FRC or a new spheromak depending on whether the initial poloidal eigenvalue was smaller or larger than a threshold value. The threshold initial poloidal eigenvalue for the relaxation to an FRC increased with the decreasing S* value. A decrease in S* promoted the relaxation to an FRC, annihilating the magnetic helicity, in sharp contrast with the conventional Taylor relaxation. Suppression of the low-n mode by the rotation shear of the toroidal modes is the most probable reason why the low-S* condition promotes the relaxation into an FRC.