Impact of impurity radiation distribution on detachment performance and implications for STEP divertor design
Alternative Divertor Configurations (ADCs) often make use of high total flux expansion (fR) and connection length (L∥) to improve the access to and the controllability of detachment, a regime vitally important for reactor-class tokamaks such as the Spherical Tokamak for Energy Production (STEP). However, there has been little effort in assessing the benefit of fR and L∥ in the presence of impurity radiation. In this work, we relax the infinitely-thin radiation assumption of the Detachment Location Sensitivity (DLS) model and introduce DLS-Extended, a new reduced model able to estimate detachment access and front location sensitivity based on upstream conditions, the magnetic geometry and seeded impurity radiation. The prediction of radiation distribution predictions were verified against SOLPS-ITER simulations of an initial STEP design with a good match, with both the inner and outer legs featuring spatially broad radiation. DLS-Extended predicts that wide radiation weakens the detachment access benefit of fR and strengthens the benefit of L∥, while the detachment stability of the inner and outer legs is increased and decreased, respectively. This is due to a number of novel radiaton-driven effects and has implications for the optimum strike point position in reactor scale devices. These findings highlight the need for more experimental studies in the presence of broad-radiating impurities. DLS-Extended is distributed under the LGPL-3 open source license and is publicly available on GitHub.