Ion Heating Characteristics of Merging/Reconnection Startup in MAST Spherical Tokamak Experiment

The high power reconnection heating has been investigated in MAST ST (spherical tokamak) merging startup experiment using a new 32-chords ion Doppler tomography diagnostics with optimized spatial resolution for the current sheet. The magnetic reconnection is observed to heat mostly ions in the downstream by the outflow and to heat electrons inside the current sheet. However, the high $T_e$ area is highly localized at the X-point probably due to the X line acceleration of electrons. Finally both of $T_i$ and $T_e$ profiles are observed to form triple-peak structures after the ion-electron energy relaxation time $\tau^E_{ie}$. The guide field does not affect ion heating under high guide field condition $B_t > 3B_{rec}$, while it increases significantly the localized electron heating at the X point. The increment of ion temperature reaches $\sim$200eV in TS-3 and over $\sim$1keV in MAST, as predicted in the $B_{rec}^2$ (reconnecting field energy) scaling of reconnection heating. This scaling suggests the promising high $ B_{rec}$ merging startup scenario for spherical tokamak.