High Power Reconnection Heating and Acceleration in UTokyo Tokamak Merging Experiments

High power reconnection heating/ acceleration experiments for direct access to the burning plasmas have been developed in TS-3, TS-4, TS-6, MAST and ST-40 merging tokamak experiments. We studied mechanisms for ion and electron heating/ acceleration of two merging tokamak plasmas by means of two imaging diagnostics: the stereo-type soft X-ray imaging for high-energy electrons and the ion Doppler imaging for ion temperature/ velocity. In the early and middle reconnection phases, we found (i) high energy (non-thermal) electrons in the downstream of reconnection which increases with reconnecting magnetic field B_rec, (ii) high energy (non-thermal) electrons peaked at around X-point which increases with guide (toroidal) field B_t. The reconnection electric field is considered to accelerate electrons significantly only along the toroidal X-line. In the middle and late reconnection phases, we observed (iii) huge downstream heating of ions whose energy is as high as ~40-50% of the reconnecting (poloidal) magnetic energy. The ion heating energy is not or weekly affected by guide (toroidal) field B_t, in the tokamak operation regime: B_t/B_rec>2, because the multi-blob-formation, merging, and ejection cause intermittent ion heating especially in the high B_t regime, weakening the B_t dependence of ion heating. Compression of current sheet thickness to ion gyroradius is found to be a key for realizing the fast reconnection as well as the high power ion heating consistent with the B_rec²-scaling of ion heating energy. Under this condition, the ion heating energy is determined only by B_rec~B_p (poloidal magnetic field), and not or weakly by B_t. This promising B_rec²-scaling of ion heating energy allows us to realize the burning plasma with ion temperature T_i>10keV (under electron density n_e~1.5x10¹⁹ [m^-3]) by increasing B_rec of merging tokamak plasmas over 0.6T.