Impact of Yacora evaluated molecular effective rate coefficients on detached SOLPS-ITER simulations

Abstract

Presently, SOLPS-ITER does not replicate the levels of plasma-molecular interactions seen during deep detachment on the Super-X divertors of MAST-Upgrade and TCV. At low temperatures ( $T_{\mathrm{e}} \lt 1\,\mathrm{eV}$ ) relevant to deep detachment, the AMJUEL reaction database contains discrepancies for detachment relevant molecular processes. In order to correct this the vibrationally resolved ground state Yacora model for H _2 (Yacora $H_{2}(X^{1},v)$ ) has been used to calculate the vibrational distribution and thus from the cross-sections of the Yacora $H_{2}(X^{1},v)$ model effective rate coefficients for these molecular processes. Reduced and extended implementations of Yacora $H_{2}(X^{1},v)$ were constructed to form EIRENE-style databases for use in SOLPS-ITER. The reduced implementation utilises the same processes as EIRENE, but with updated cross-sectional data, whilst the extended implementation adds additional processes. The improved cross-sectional data lowers the threshold for molecular charge exchange (MCX) compared to AMJUEL, whilst the inclusion of the additional processes boosts the vibrational excitation in the molecular ground state providing the additional excitation energy required for MCX to be significant at low temperatures ( $T_{\mathrm{e}} \lt 5\,\mathrm{eV}$ ). When applied to a SOLPS-ITER isolated divertor leg setup, both implementations produce greater levels of MCX that result in elevated molecular ion densities ( $D_{2}^{+}$ ) than the default rate setup. The higher levels of $D_{2}^{+}$ contribute to detachment via increased levels of Molecular Activated Recombination and Molecular Activated Dissociation. These plasma-molecular interactions elevate the neutral population. During detachment this has a strong impact on power and momentum losses, as well as elevated levels of Balmer α line emission and an earlier onset of the ion target flux rollover. These effects are strongest when the extended database is used.

Keywords

• exhaust
• plasma chemistry
• detachment
• alternative divertors
• tokamak
• scrape off layer