Studying the neutral gas flow in detached W7-X discharges using EMC3-EIRENE

During operation of W7-X a key point is to pump the neutrals out of the experiment. Therefore, maximizing the neutral pressure in the subdivertor is important. The present neutral particles are generated due to plasma-wall interaction at the divertor targets. In ref. [1] it is shown for attached conditions that only a small part of the neutral particle flux is going through the pumping gap. Most particles recycle back into the plasma. On top, a part of the particle flux going through the pumping gap leaks back into the divertor chamber decreasing the subdivertor neutral pressure. While increasing the plasma density and going in that way from attachment to detachment, experiments during the campaign of 2018 of ref. [2] show that the subdivertor pressure builds up, stagnates, and drops again. The compression ratio decreases during this transition. A dedicated study of the physics behind detachment in W7-X was performed in ref. [3]. Starting from these findings, this contribution focuses on examining the dynamics of the pressure evolution to understand what it depends on and how it scales. In a first step, the neutral fluxes at the pumping gap are studied. In all simulations, special attention is put on ensuring a small numerical error on the simulated neutral quantities.

[1] T. Kremeyer et al., Nucl. Fusion 62 (2022) 036023

[2] O. Schmitz et al. Nucl. Fusion 61 (2021) 016026

[3] Y. Feng et al., Nucl Fusion 61(2021) 086012