Divertor Optimisation and Power Handling in Spherical Tokamak Reactors

A key aspect in the design of a spherical tokamak reactor is the optimisation of the plasma equilibrium, together with a compatible divertor configuration, and the corresponding poloidal field system. This is a complex multi-disciplinary problem, integrating plasma physics and engineering in order to satisfy a multitude of often conflicting requirements and constraints. The equilibrium design process employed in this work takes into account the reference plasma operating scenario, the power exhaust solution, and the engineering limits.Managing the heat exhaust proves to be one of the most challenging issues in a compact device such as the UKAEA STEP reactor. With a smaller major radius, the available target area over which the scrape-off layer heat load must be deposited is relatively small as compared to conventional aspect ratio devices. Consequently, alternative and advanced divertor concepts need to be considered, having significant implications for the whole reactor design. Here we address the inner divertor power handling challenges. With very limited inboard space, and the small radius of the inner strike point, the associated heat loads are likely to exceed the power handling capacity of standard divertors (SD). An alternative divertor configuration approaching an X-divertor (XD), created by inducing a secondary X-point near the inner strike point, is compared with an SD configuration optimised for the maximal connection length and maximal poloidal flux expansion. The inner X-divertor, simultaneously achieving strong poloidal flux expansion, increased connection length and higher divertor volume, proved to be advantageous in reducing target heat loads and favouring detachment. Amongst a number of viable exhaust solutions considered, the inner-X divertor is indeed emerging as a promising candidate.