First Results form TS-6 and ST-40 Tokamak Merging Experiments, -Global and Local Energy Conversion of Magnetic Reconnection in Laboratory Plasmas

The high-power reconnection heating of merging tokamak plasma has been developed mainly in TS-3, TS-4 and MAST experiments. This unique method is caused by the promising scaling of ion heating energy that increases with squire of reconnecting magnetic field B_rec. We studied mechanisms for this scaling of reconnection (ion) heating up to 2.2keV mainly using TS-6 and ST-40 experiments and PIC simulations and found the following features: (i) the ion heating energy is as high as ~40-50% of poloidal magnetic energy of two merging tokamak plasmas, (ii) the ion heating energy is not affected by (guide) toroidal field B_t, in the region of B_t/ B_rec >1 under two conditions: (a) compression of current sheet to ion gyroradius and (b) full-isolation of the merging tokamak plasmas from coils and walls. The sheet compression to ion gyro radius was found to be a key condition to realize the fast reconnection as well as the high power ion heating consistent with the B_rec^2-scaling prediction. Under this condition, the ion heating energy is determined uniquely by B_rec ~ B_p not by B_t in the conventional tokamak operation region: B_t/ B_rec >1. The merging tokamak plasmas need to be fully pinched off from the PF coils (no link with coils and walls) for the purpose of minimizing loss of the hot ions heated by the reconnection/ merging. The ion heating was found to occur not only in the local downstream area of reconnection but also in the global merging tokamak area, realizing the ratio of the ion heating energy to the electron heating energy of reconnection around 4. This promising scaling realized ion temperature as high as 2.2keV and is expected to realize the burning ion temperature >10keV (under electron density n_e~1.5x10^19 [m^-3]) just by increasing B_rec over 0.6T, leading us to the high-B_rec field merging tokamak experiments: TS-6 in U. Tokyo and ST-40 in Tokamak Energy Inc.