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).