Investigation of the density shoulder formation by using self-consistent simulations of plasma turbulence and neutral kinetic dynamics

Simulations of plasma turbulence in a lower single-null magnetic configurations are presented. The plasma dynamics is modelled by the drift-reduced two-fluid Braginskii equations that are coupled with a kinetic model for a single neutral species that considers ionization, charge-exchange, recombination and elastic collisions. The effect of increased core fuelling on the plasma scrape-off layer (SOL) density and temperature profile is investigated. The increase in core fuelling leads to an increase of the e-folding length in the near SOL and, in the far SOL, to an increase of the plasma density. These results are in agreement with experimental measurements, and in particular with the observations of the formation of a density shoulder in high-fuelling scenarios. The physical mechanisms underlying the increase of the far SOL density are analysed comparing parallel and perpendicular fluxes in the SOL and considering also simulations with similar parameters but without neutrals. Despite the increase of the blob size, the increase of the far SOL density observed at high-fuelling rates is found to be mainly caused by the strong decrease of parallel transport, due to the cooling of electrons resulting from ionization events.