On the formation and control of an X-point radiator in ASDEX Upgrade: SOLPS-ITER simulations and experiments

The X-point radiator (XPR) is an attractive scenario to solve the power exhaust problem in future fusion devices. In the ASDEX Upgrade tokamak (AUG), experiments with an XPR showed a dissipated power fraction larger than 90 %, fully detached divertor targets and ELM suppression with a moderate confinement degradation [Bernert, NF, 2021]. Recently, a reduced model [Stroth, NF, 2022] was derived to explain the physical mechanisms for initiating a stable XPR. However, 2D numerical simulations are required to interpret the features not caught by the reduced model, including the spatial distribution of particle and power sources, cross-field transport and drifts in an XPR. In this work, the transport code SOLPS-ITER [Wiesen, JNM, 2015] was applied to reproduce the experimentally measured plasma conditions in AUG and to study the parameters relevant for the formation and control of an XPR.

Neutrals at the X-point play an important role in generating an XPR. In simulations with artificial baffles for neutrals, it was demonstrated that an XPR cannot be achieved without neutrals penetrating the confined region. However, once a cold and recombining XPR is created, it persists even if the gas fueling rate is reduced by an order of magnitude, which is consistent with the prediction in the reduced model and the recent experimental observations in AUG. The redistribution of plasma density and radiation caused by drifts at the XPR was studied in experiments and simulations in favourable and unfavourable field directions.

The magnetic connection length and flux expansion also show an important influence on the XPR. With the same impurity seeding rate, the simulation with a higher toroidal field showed an XPR deeper inside the confined region. In simulations for DEMO [Zohm, NF, 2013] with a snowflake divertor, an XPR regime with a radiative power fraction larger than 90 % can be achieved at low upstream density (3×10¹⁹ m^-3) and argon impurity density (∼1×10¹⁶ m^-3) at the separatrix.