Recent progress of magnetic reconnection research in the MAST spherical tokamak

In the last three years, magnetic reconnection research in the MAST spherical tokamak achieved major progress by use of new 32 chord ion Doppler tomography, 130 channel YAG- and 300 channel Ruby-Thomson scattering diagnostics. In addition to the significant plasma heating up to $\sim$1keV [1], detailed full temperature profile measurements including the diffusion region have been achieved for the first time. 2D imaging measurements of $T_i$ and $T_e$ profiles have revealed that magnetic reconnection mostly heats ions globally in the downstream region of outflow jet and electrons locally at the X-point [2]. The higher toroidal field in MAST ($B_t>0.3$T) strongly inhibits cross-field thermal transport scaling as $1/B_t^2$ and the characteristic peaked $T_e$ profile at the X point is sustained on a millisecond time scale. In contrast, ions are mostly heated in the downstream region of outflow acceleration inside the current sheet width ($c/\omega_{pi}\sim$ 0.1m) and around the stagnation point formed by reconnected flux mostly by viscosity dissipation and shock-like compressional damping of the outflow jet. Toroidal confinement also contributes to the characteristic $T_i$ profile, forming a ring structure aligned with the closed flux surface. There is an effective confinement of the downstream thermal energy due to a thick layer of reconnected flux. The characteristic structure is sustained for longer than an ion-electron energy relaxation time (\tau^E_{ei}\sim4-11$ms) and the energy exchange between ions and electrons contributes to the bulk electron heating in the downstream region. The toroidal guide field mostly contributes to the formation of a localized electron heating structure at the X-point but not to bulk ion heating downstream. [1] Y. Ono {\it et al}., Phys. Plasmas {\bf 22}, 055708 (2015). [2] H. Tanabe {\it et al}., Phys. Rev. Lett. {\bf 115}, 215004 (2015).