Electron Acceleration at the X-point of Magnetic Reconnection in Tokamak Merging Experiments

The generation of high-energy electrons during magnetic reconnection under strong guide field in tokamak merging experiments was captured using our multi-energy soft X-ray imaging system. We observed that the spatial distribution of soft X-ray emission from the energetic electrons varies significantly as the transient reconnection process evolves. This investigation is crucial for understanding the influence of energetic electrons on the primary goal of efficient/high-power ion heating in ST start-up scenarios. Our experiment revealed the 2D evolution of the soft X-ray profile. Initially, oblique emission structure extending from the X-point downstream was observed, suggesting an electron outflow that could modify the quadrupole electrostatic potential. This emission subsequently localized into two distinct regions: near the X-point and downstream. The emission near the X-point exhibited progressive spectral hardening occurred concurrently with a decrease in electron density and an increase in the toroidal reconnection electric field. These results suggest that electrons are directly accelerated along the magnetic field lines by the reconnection electric field, and that the lower electron density and stronger electric field provide conditions for more efficient acceleration to higher energies. Experiments with varying guide field ratios revealed an enhancement of high-energy emission under stronger guide field, which supports the proposed electron acceleration mechanism.