Electric potential structure formation in the divertor region: Investigating the mechanisms governing the electric potential in the divertor region of a magnetic confinement device employing a first-principles, flux-driven turbulence code

Modelling the exhaust power in future Tokamak reactors remains an open issue in fusion research, as the change in scale from ITER to power plants suggest that heat fluxes in the latter could exceed manageable material limits on the target plates. A potential solution to the exhaust problem is the development of so-called “alternative divertors configurations”, whose aim is to widen the plasma- wetted area to reduce the heat load at the targets by altering the magnetic geometry in the divertor area. This is achieved by increasing the flux expansion (X-divertor), sometimes in combination of placing the outer strike point at a larger radial location (super-X divertor), or increasing the number of strike points. Amongst this last class of solutions there are the so-called “Snowflake” divertors, which provides a secondary X-point and thus two additional magnetic field legs along which the heat flux might be dissipated. Experimental investigation of Snowflake divertors have shown a reduction of the peak heat flux at the targets compared to conventional Single Null geometries. In particular, it has been highlighted that the change in power distribution in the former cannot be explained by the modification of the magnetic geometry alone, which suggests additional cross-field transport mechanisms are at play. Numerical investigations using the Global Braginskii Solver (GBS) code have highlighted that drift-related transport may play a significant role in determining the output power profiles, and especially found an enhanced E × B-drift convective pattern around the X-point of reversed field configurations . In this thesis the GBS simulation data is used to determine which mechanisms are at play in setting the electrostatic potential (and the observed drift patterns) around the X-point, comparing it to previous models. A description of the profile along the outer divertor leg is found in terms of the density, and a scaling relation is recovered in terms of the parallel coordinate in the case of simplified density profiles.