Effect of inner leg configuration on detachment in MAST-U

In a fusion power plant it is important to define the amount and stability of the heat flux reaching the target. This is even more important on the inner leg for spherical tokamaks, as the wetted area of the target is even more limited. As lot of attention is usually given to the study of the heat flux and its dissipation at the outer target, as that receives the largest fraction of the heat, but for increasing size of the machine a correct characterization of the heat flux on the inner target is important.

Previous studies on MAST-U shown that the inner leg detaches with a sharp transition from the target to a region near the X-point [1]. The stability of the front movement is predicted by the detachment location sensitivity (DLS) model [2,3]: if the magnetic field reduces from the X-point to the target, then the front movement should move gradually based on the balance of upstream and dissipated power, from target to X-point. Conversely, the front should move sharply. The experiment confirmed this behavior on the inner leg.

This study is the first that investigate specifically this aspect of the detachment process, by varying the magnetic configuration of the inner leg from horizontal to mostly vertical (as much as allowed). Preliminary evidence seems to suggest that the transition on the inner leg is in fact sharper with a horizontal configuration, confirming the DLS prediction.