Speaker
Description
The Vacuum Neutral Model (VNM) has been implemented in UEDGE to account for neutral bypassing in the far scrape-off layer (SOL) [1], which lies outside the UEDGE computational mesh. In this work, the model is further extended to incorporate kinetic effects in the neutral flux leaking from the UEDGE boundary into the vacuum region. From a kinetic perspective, the dominant contribution to the outward-going neutral flux originates from ion–neutral charge-exchange interactions occurring approximately one mean free path inside the plasma. This allows the distribution of outward-going neutrals to be reconstructed self-consistently.
The inward-going neutral population consists of recycled particles from ion bombardment at material surfaces and reflected neutrals. While reflected neutrals can be modeled using TRIM-based reflection data, this capability is still under development and is therefore not included here; instead, inward-going neutrals are assumed to follow a half-Maxwellian distribution for simplicity based on the density and temperature obtained from the VNM. The reconstructed neutral distribution is then used to compute particle, momentum, and energy fluxes, which serve as boundary conditions for the neutral continuity, parallel momentum, and temperature equations.
This self-consistent treatment of the outgoing neutral flux is applied not only at the outer boundary of the VNM but also at the target plates. The impact of incorporating these kinetic boundary effects into the fluid neutral model on divertor detachment physics is investigated in this study.
[1] M. Zhao et al. “Vacuum neutral transport model in UEDGE for tokamak far scrape-off layer” 2025 Plasma Edge Theory Workshop, Leuven, Belgium