Merging Formation of High-Beta Spherical Tokamak with Absolute Minimum-B in TS-6 merging experiment

The high-power reconnection heating of merging spherical tokamak (ST) plasmas has been developed by TS-3, TS-4U, UTST, TS-6, MAST and ST-40 experiments and PIC simulations by NIFS. It allows us to realize direct access to burning high-beta ST plasma often with absolute minimum-B without using any additional heating like neutral beam injection (NBI). Those merging experiments confirmed the promising scaling of ion heating energy increasing with square of reconnecting magnetic field ~ poloidal magnetic field, provided that the current sheet is compressed to the order of ion gyroradius. We found that the reconnection heating forms interesting high-beta ST plasmas with hollow ion temperature profiles. This high-beta ST often has an absolute minimum-B profile in the second stability regime for ballooning instabilities. It is also noted that the toroidal field does not affect ion heating energy as high as 40-50% of poloidal magnetic energy of merging ST plasmas when the merging ST plasmas have safety factor q>1. The toroidal magnetic field does not affect the global hollow ion temperature profile of the produced new ST plasma but peaks its electron temperature profile locally at the O-point.