Speaker
Description
Until lately, codes such as SOLEDGE3X [1] or SOLPS [2] have been routinely used without including drift effects, although past studies have shown their significance [3,4]. Thanks to the continuous development and improvements in the numerical treatment, the inclusion of drifts has now become more common and proves to be important in detachment studies [5,6]. In this contribution, we evaluate their impact on WEST dissipative divertor regimes using the SOLEDGE3X code.
Recent modelling efforts [7] show that the inclusion of drifts is necessary to obtain an agreement between simulation and experiment in WEST in attached conditions, where the poloidal ExB flux directed from the HFS to the LFS leads to a strong reduction of the outer divertor temperature that would otherwise be strongly overestimated. Based on these simulations, here we continue to explore the role of drifts in the access to detachment. It is observed, that when a nitrogen seeding is introduced, the seeding rate required to obtain a detached divertor on the HFS is significantly lowered by the drift effects. Due to an accumulation of nitrogen on the HFS, driven by the drifts, the detachment becomes strongly asymmetric, as commonly observed, with the outer target detaching less easily [8].
In the course of these studies, it has been identified that, when the drifts are taken into account, the detachment onset and XPR behaviour are sensitive to radial transport assumptions. An accurate resolution of radial transport coefficients in the pedestal, supported by reflectometry measurements, is essential to reproduce the experimental-like detachment. The radial transport around the X-point influences the feasibility of achieving a simultaneous detachment at both targets within the XPR stability limits. The results also suggest that the XPR formation and stability might depend on the transport regime, with reduced transport in the pedestal having a favourable effect on the XPR stability and control. With constant transport coefficients, typically assumed in simulations of WEST L-mode plasmas [8], the radiation and the density fronts cross the separatrix inwards of the X-point and drifts drive the cold high-density region into an unstable MARFE before a radiation collapse occurs and before the outer target detaches. Reduced transport in the pedestal slows down the propagation of the cold high-density region, directing it towards the X-point. A negative potential well develops, similar as in [9], modifying the distribution of the ExB flux around the X-point, allowing a subsequent detachment of the outer target.