Description
Divertor detachment and related strategies for management of the intensive heat flux on the tokamak plasma facing components is tightly connected to the transport in Scrape-off Layer (SOL). An important contributor to the SOL transport are plasma drifts, cross-field components induced by the geometry of the magnetic field, plasma pressure gradient, or electric field from potential perturbations. They affect the plasma both locally and globally, such as a local deformation of radial heat flux profiles at the divertor targets and a global asymmetry in poloidal distribution.
The analytical description of the drift transport is well understood, however, their mathematical form presents a numerical challenge for mean-fluid plasma codes. As a result, they are often omitted to improve stability and reduce computational costs. We utilize the SOLPS-ITER [1, 2] code, a 2D mean-fluid plasma solver with kinetic treatments of neutrals, to model drift-driven transport in the SOL plasma of the COMPASS tokamak, combatting the numerical challenges with the help of practical findings from [3] and [4].
COMPASS tokamak features a lower magnetic field (Bt = 1.4 T) and is therefore well-suited for this research, as dictated by a typical 1/Bt scaling of the drift velocities. A small PFR region of COMPASS offers relevancy to advanced localized radiator regimes such as the X-point radiator or compact radiative divertor [5]. We demonstrate the local effects of drift-driven transport by comparing SOLPS-ITER simulations in favourable and unfavourable Bt orientations. An improved match of experimental profiles at the divertor target is observed when drift effects are included. A special focus is given to a nitrogen impurity seeding scenario, where the drift-driven transport causes a significant poloidal redistribution of the impurity species in the unfavourable Bt configuration, strongly affecting the radiation pattern.