Speaker
Description
In a fusion power plant it is paramount to control the heat flux reaching the targets. Most of the exhaust heat flux is directed to the outer leg, that hence is often the main focus, but for increasing machine size a correct characterization of the heat flux on the inner target is important, even more for spherical tokamaks as the inner target wetted area is more limited.
Previous studies on MAST-U have shown that the inner leg detaches with a sharp movement from the target to a region near the X-point, while it detaches gradually on the outer leg [1]. The detachment onset and front movement sensitivity is predicted by the detachment location sensitivity (DLS) model [2,3]: if the magnetic field reduces towards the strike point (outer leg) the front movement should be gradual from target to X-point. Conversely (inner leg), the front should move sharply. The experiments confirmed these behaviors.
The aim of this study is to investigate the impact of changing the inner leg configuration on the detachment process. We varied the inner leg orientation from horizontal to ~60 degrees while maintaining the same core shape, and made it almost vertical allowing for more significant changes (negative triangularity). In these experiments the infrared video bolometer (IRVB), that can reconstruct the total emissivity profile in the divertor with high spatial resolution [4], was the prime diagnostic. Preliminary evidence suggests that even an inner leg close to vertical causes a sharp transition from target to X-point and detachment starts at lower upstream density for a more horizontal inner leg, both in agreement with the DLS model. It also appears that the radiation at the target elongates when the DLS profile becomes locally marginally stable, and that the X-point radiation elongates towards the inner target for increasing poloidal length of the inner leg, pointing to a dominant role of neutral dynamic for the location of the cold radiating regions. Experiments in lower single null rather than balanced double null have been performed to obtain a higher and more reliable heat flux on the inner leg, with results currently being analyzed.
We also decreased the density from an already detached plasma, observing the reattachment process. Reattachment seems to be also characterized by a sharp movement of the front, similar to detachment. Further analysis and experiments will provide deeper insight into the physics of inner leg detachment and the factors governing its stability.