New results from MAST are presented that focus on validating models in order to extrapolate to future devices. Measurements during start-up experiments have shown how the bulk ion temperature rise scales with the square of the reconnecting field. During the current ramp up models are not able to correctly predict the current diffusion. Experiments have been performed looking at edge and core turbulence. At the edge detailed studies have revealed how filament characteristic are responsible for determining the near and far SOL density profiles. In the core the intrinsic rotation and electron scale turbulence have been measured. The role that the fast ion gradient has on redistributing fast ions through fishbone modes has led to a redesign of the neutral beam injector on MAST Upgrade. In H-mode the turbulence at the pedestal top has been shown to be consistent with being due to electron temperature gradient modes. A reconnection process appears to occur during ELMs and the number of filaments released determines the power profile at the divertor. Resonant magnetic perturbations can mitigate ELMs provided the edge peeling response is maximised and the core kink response minimised. The mitigation of intrinsic error fields with toroidal mode number n>1 has been shown to be important for plasma performance.

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 center solenoid, the detailed fine structure formation process of reconnection heating has been revealed using new 96CH/320CH ultra-high-resolution 2D ion Doppler tomography diagnostics. By identifying the double-axis field configuration with the X-point on the midplane using in situ magnetic probe diagnostics, the detailed measurement successfully revealed that the ion temperature profile forms two types of characteristic heating structure, both around the X-point and downstream. The former is affected by the Hall effect to form a tilted heating profile, while the latter is affected by the transport process which a forms a poloidal double-ring-like structure. The achieved ion heating mostly depends on the reconnecting component of the magnetic field, and the contribution of the guide field to decrease the heating efficiency tends to be saturated in the 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 a poloidal ring-like hollow distribution aligned with the closed flux surface at the end of merging.

We present recent results from high resolution 2D imaging measurement of reconnection heating during central solenoid (CS)-free merging startup of spherical tokamak plasmas in TS-3 using an ultra-high resolution 2D ion Doppler tomography and in-situ 2D magnetic probe arrays. The new high-resolution 2D ion Doppler tomography diagnostic has successfully resolved the formation of fine structure during magnetic reconnection and it has been found that magnetic reconnection increases the ion temperature inside a current sheet as well as in the downstream region of an outflow jet. The maximum ion temperature is obtained during a current sheet ejection event and the double peak structure of ion heating becomes clearer after the end of reconnection. The maximum ion heating increases with the reconnecting magnetic field. The high temperature region in the downstream typically propagates vertically along a closed flux surface and MAST-like fine structure formation has successfully been reproduced in this laboratory experiment for the first time.

A new type of spontaneous formation of plasmoid is observed in our toroidal plasma merging experiment on TS‐6 using a newly developed high‐resolution magnetic probe array, which has axially gyro‐scale spatial resolution (ΔZ ∼ 1.1 cm). In the axially elongated new machine, the plasmoid is formed near the mid‐plane with a configuration of two X‐points in the early phase of magnetic reconnection when the initial two flux tubes (merging tokamaks) are still in their respective formation region. The plasmoid is then merged with the major upstream plasmas at the two X‐points, which finally triggers the increment of reconnection speed. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

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