Design and conceptual modeling of a tokamak-like X-point divertor for the Helios quasi-axisymmetric stellarator

A novel divertor solution is presented for a pre-conceptual design point of the quasi-axisymmetric stellarator power plant, “Helios.” This solution, isomorphic to the poloidal divertor in a tokamak, features a toroidally-continuous n=0, m=2 double null configuration in which the X-points do not poloidally link the plasma. This results in a non-resonant divertor geometry which relies on neither low-order resonances nor a chaotic boundary region and which features parallel connection lengths similar to those in tokamaks. When reconstructed with a finite-build planar coilset, the configuration resembles that of a tokamak with resonant magnetic perturbations (“RMPs”) applied. Field line tracing and simplified analytic models are used to justify the assumption of tokamak-like Scrape-off Layer (“SOL”) transport and access to a high-recycling regime in such a configuration. Due to this, it is also speculated that high neutral compression akin to a tokamak will be achievable in such a configuration. Preliminary target geometries for this configuration are introduced. Even without globally optimized target geometries, peak heat flux remains below engineering limits for a reference case at similar radiated power fraction as achievable in existing machines ( f r a d = 90 % ) for a baseline P f u s = 958 MW scenario and only slightly exceeds engineering limits for a conservative, low wetted area case. Initial results indicating resilience of the X-point to pressure profile changes are shown. Lastly, preliminary feasibility studies of a disconnected double null configuration are explored, potentially enabling an engineering-simplified design with only lower targets.