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Description
Here we report the particle acceleration and heating of magnetic reconnection under the influence of high guide field in the merging spherical tokamak formation experiments in ST40 and TS-6. In addition to the extension of ion heating scaling $\Delta T_i\propto B_{rec}^2$ in keV range, our recent experiments explored the following 3 new findings using 96CH/320CH ion Doppler tomography and Thomson scattering diagnostics: (1) formation of poloidally asymmetric global ion heating structure in TS-6 and highly localized electron heating around the X-point in ST40 via parallel electric field acceleration, (2) update of the heating scaling to $\Delta U_i\propto B_{rec}^2$ by including the contribution of electron density in collaboration with Thomson scattering measurement in ST40 from 2023, and (3) exploration of further electron heating via magnetic reconnection under the influence of high guide field in the keV range in ST40. The poloidally asymmetric ion heating structure depends on the polarity of toroidal field and the fine structure gets flipped when the guide field direction is reversed. Under the influence of high guide field, E$\times$B drift is mainly driven by in-plane/poloidal electric field $E_p$ from the quadruple potential structure, while parallel electric field $E_{\parallel}$ is mainly driven by reconnection electric field Erec (spontaneously formed toroidal electric field $E_t$ around X-point) and higher $T_i$ appears where plasma potential is positive, while high $T_e$ mainly appears around the X-point. Under the influence of toroidal effect to have higher guide field in the inboard side of outflow direction, downstream heating also forms poloidally asymmetric structure, and more heating appears in the high field side. Perpendicular heat conduction in the outflow region is strongly suppressed by high guide field $\kappa^i_\parallel / \kappa ^i_\perp \sim 2(\omega_{ci}\tau_{ii})^2 >>1$ and the field-aligned transport process leads to the formation of poloidally ring-like characteristic fine structure after merging.
[1] H. Tanabe et al., Phys. Rev. Lett 115, 215004 (2015)
[2] H. Tanabe et al., Nucl. Fusion 57 056037 (2017)
[3] H. Tanabe et al., Nucl. Fusion 59, 086041 (2019)
[4] H. Tanabe et al., Nucl. Fusion 61, 106027 (2021)
[5] H. Tanabe et al., 29th IAEA Fusion Energy Conference (FEC 2023), 1695 (2023)
[6] Y. Ono, H. Tanabe and M. Inomoto, Nucl. Fusion, accepted for publication (2024)
