Ion and electron heating characteristics of magnetic reconnection in high field flux tube merging experiments

We present ion and electron heating characteristics of magnetic reconnection in high field flux tube (spheromak/tokamak) merging experiments in MAST, ST40 and TS-6 which drive flare-like impulsive/explosive energy release up to keV regime in laboratory plasma. By excluding collision/radiation loss with the high temperature plasma condition, several new features of reconnection heating have been explored such as the formation of fine structure whose scale size is smaller than ion skin depth. Electrons are heated around X-point typically by current sheet dissipation but forms peaked/steep profile under the influence of high guide field. Ions are mostly heated by outflow dissipation and hot spots appear around diffusion region as well as downstream. Guide field affects the structure around diffusion region where Hall effect contributes to form tilted ion temperature profile but the order of bulk ion heating is not affected by guide field. In flux tube merging configuration, those hot spots formed by outflow heating appear on the thick layer of closed flux surfaces of toroidal plasma, perpendicular heat transport is strongly suppressed when high guide field is applied and finally forms field aligned hollow structure which is globally connected to upstream region (while electron temperature keeps the peaked structure around the X-point). Both different temperatures equilibrate in the time scale of ion-electron energy relaxation time and both ion and electron temperature form triple peak distribution after merging during the energy equilibration process. The application of high power reconnection heating is adopted in the new high field spherical tokamak experiments on TS-6 and ST40, which successfully made first plasma in 2018 and expected to achieve new records in 2019 based on the extension of $\Delta T_i \propto B_{rec}^2$ scaling. The latest reports from those projects will be presented in the conference. [1] Y. Ono, H. Tanabe, et al., Phys. Rev. Lett. (2011) [2] H. Tanabe, T. Yamada, et al., Phys. Rev. Lett. 115, 215004 (2015) [3] Y. Ono, H. Tanabe et al., Phys. Plasmas 22, 055708 (2015) [4] H. Tanabe, T. Yamada, et al., Phys. Plasmas 24, 056108 (2017) [5] H. Tanabe, H. Tanaka, et al., Nucl. Fusion, to be published (2019)