Comparison of a collisional-radiative fluid model of H2 in UEDGE to the kinetic neutral code EIRENE

Abstract

• H 2 density agreement of the UEDGE fluid model and EIRENE within 20% is achievable. • Vibrational H 2 populations become significant for T e < 6 eV at n e = 1 x 10 18 m −3 . • Atom-molecule equipartition becomes significant in UEDGE at n e = 1 x 10 19 m −3 . A fluid collisional-radiative model for H 2 has been implemented in the edge-fluid code UEDGE and compared to the kinetic neutral code EIRENE on a simple, 2D, orthogonal domain with a constant, static plasma distribution. The novel CRUMPET Python tool was used to implement dissociation and energy rate coefficients that consider molecular-assisted processes, binding energy, and radiation due to molecular processes into the UEDGE fluid molecular model. The agreement between the fluid and kinetic molecular models was found to be within 20% when corresponding rates were used in UEDGE and EIRENE for a domain with absorbing boundaries. When wall recycling was considered, EIRENE predicted up to a factor of 2.2 higher molecular densities than UEDGE at T < 5 eV. The difference is due to the absence of radial gradients driving diffusive wall fluxes and, thus, recycling in UEDGE and molecular self-scattering in EIRENE, and is likely dependent on plasma profiles and domain geometry. Comparison of the molecular energy sources in EIRENE and UEDGE suggest the constant elastic scattering rate coefficient used in UEDGE needs to be updated to a temperature-dependent coefficient and that atom-molecule equipartition should be considered in the EIRENE model for background plasma density in excess of 1 × 10 19 m - 3 . Finally, collisional-radiative CRUMPET simulations indicate that the vibrational molecular populations become comparable to the ground-state molecular population when the plasma temperature decrease below 6 eV and, thus, require time-dependent evaluation.