Abstract
A merging formation of field-reversed configuration (FRC) has been studied for the first time without central conductor, using two merging spheromaks with opposing toroidal magnetic field. Without the n=1 stabilization effect of the central conductor, the merging process was maintained stably probably due to (1) plasma flow shear generated by the counterhelicity reconnection and (2) line-tying effect of the magnetic field lines. This relaxation from the force-free (≈0.05-0.1) spheromaks to the high-beta (≈0.7-1) FRC is attributed to efficient (≈80%) conversion of the toroidal magnetic energy of the former to the ion thermal energy of the latter. This energy conversion was caused by a direct ion heating and acceleration effect of reconnection under the toroidally symmetric condition. This ion heating energy was found to increase inversely with the field amplitude parallel to the X-point line. Further plasma acceleration by high-field compression was observed to increase the reconnection speed, revealing the new fast reconnection mechanism called the current sheet ejection.
