Parallel flows as a key component to interpret Super-X divertor experiments

TL;DR

This paper emphasizes the importance of parallel flows in understanding Super-X divertor experiments, revealing discrepancies between models and experiments, and demonstrating how flux expansion influences plasma behavior.

Contribution

It introduces a self-consistent model including parallel flows, explaining experimental discrepancies and analyzing their impact on plasma detachment and flow dynamics.

Findings

Parallel flows weaken the flux expansion effect on detachment.

Total flux expansion can promote supersonic flows at the OSP.

Experimental results on TCV align better with the new model than with the original 2PM.

Abstract

The Super-X Divertor (SXD) is an alternative divertor configuration leveraging total flux expansion at the Outer Strike Point (OSP). While the extended 2-Point Model (2PM) predicts facilitated detachment access and control in the SXD configuration, these attractive features are not always retrieved experimentally. These discrepancies are at least partially explained by the effect of parallel flows which, when self-consistently included in the 2PM, reveal the role of total flux expansion on the pressure balance and weaken the total flux expansion effect on detachment access and control, compared to the original predictions. This new model can partially explain the discrepancies between the 2PM and experiments performed on tokamak \`a configuration variable (TCV), in ohmic L-mode scenarios, which are particularly apparent when scanning the OSP major radius Rt. In core density ramps in lower Single-Null (SN) configuration, the impact of Rt on the CIII emission front movement in the divertor outer leg - used as a proxy for the plasma temperature in the divertor - is substantially weaker than 2PM predictions. Furthermore, in OSP radial sweeps in lower and upper SN configurations, in ohmic L-mode scenarios with a constant core density, the peak parallel particle flux density at the OSP is almost independent of Rt, while the 2PM predicts a linear dependence. Finally, analytical and numerical modeling of parallel flows in the divertor is presented. It is shown that an increase in total flux expansion can favour supersonic flows at the OSP. Parallel flows are also shown to be relevant by analysing SOLPS-ITER simulations of TCV.