A 0D parallel transport model for varying density regimes for use in 2D drift-fluid turbulence codes

In this paper, a new model of the parallel transport in 2D drift-fluid turbulence codes is formulated as a 3-point model and benchmarked against SOLPS-ITER. The model allows for the existence of parallel temperature gradients and local recycling near the target. To realize this, the volume of the flux tube is split into two zones: (1) a connection zone extending from the outer midplane to the ionization zone entrance and (2) an ionization zone covering the remaining space until the sheath edge. By assuming a linear increase of the parallel particle and conductive heat fluxes in the connection zone, and assuming an ad-hoc relation for the electron pressure evolution, expressions for the plasma fields at the entrance of the ionization zone are obtained. The evaluation of the integral balances over the ionization zone leads to an implicit system in terms of the parallel fluxes. To benchmark the model, we compare with reference data from density, current and recycling scans for a flux tube with SOLPS-ITER. From these scans, one observes that the model predicts well the parallel particle flux in the density and recycling scans. Discrepancies that arise at higher densities for both the conductive heat fluxes and the required floating upstream potential are analyzed and partly explained by the superlinear evolution of the heat fluxes in the SOLPS-ITER simulations. It is concluded that the model can be used to construct the effective volumetric sink terms in 2D drift-fluid turbulence models.