Formation of high-energy electron tail at the X-point of magnetic reconnection of two merging torus plasmas under a strong guide field

We investigated the formation mechanism of a high-energy electron tail observed near the X-point during high-guide field magnetic reconnection in torus plasma merging experiments. To enable this study, we developed a new multi-view soft X-ray (SXR) imaging diagnostic capable of measuring bremsstrahlung from high-energy electrons with both spectral and temporal resolution. The system evaluates the radiation spectrum by simultaneously detecting SXR emission in three distinct energy bands. Using this diagnostic on the TS-6 device, we captured the temporal and spectral evolution of SXR emission localized near the X-point during plasma merging. The X-point emission spectrum exhibited marked variations associated with changes in the reconnection electric field and electron density, and showed strong dependence on the guide field ratio. These results indicate that electrons are accelerated by the reconnection electric field parallel to the magnetic field. Tomographic reconstruction further provided spatially localized spectral information near the X-point, enabling direct comparison with numerical analyses using local electromagnetic fields. Test particle calculations support this interpretation by confirming that the observed emission originates from high-energy electrons and reproducing key experimental features, including the temporal evolution and guide field dependence.