Investigation of fine structure formation of guide field reconnection during merging plasma startup of spherical tokamak in TS-3U

Ion heating/transport and its fine structure formation process through magnetic reconnection have been investigated by high guide field tokamak merging experiments in TS-3 and TS-3U. In addition to the previously reported demonstration of high temperature plasma startup without centre solenoid, detailed fine structure formation process of reconnection heating has been revealed using a new 96CH/320CH ultra-high resolution 2D ion Doppler tomography diagnostics. By identifying double-axis field configuration with X-point on midplane using in-situ magnetic probe diagnostics, the detailed measurement successfully revealed that ion temperature profile forms two types of characteristic heating structure both around X-point and downstream. The former is affected by Hall effect to form tilted heating profile, while the latter is affected by transport process which forms poloidally double-ring-like structure. Achieved ion heating mostly depends on reconnecting component of magnetic field and the contribution of guide field to decrease heating efficiency tends to be saturated in high guide field regime. Under the influence of better toroidal confinement with higher guide field, the downstream ion heating is transported vertically mostly by parallel heat conduction and finally forms poloidally ring-like hollow distribution aligned with closed flux surface at the end of merging.