Dissociative recombination and electron-impact de-excitation in CH photon emission under ITER divertor-relevant plasma conditions

TL;DR

This study investigates the mechanisms behind CH photon emission in edge plasmas relevant to ITER, highlighting the dominant role of dissociative recombination and the importance of electron-impact de-excitation in modeling emission profiles.

Contribution

It introduces a detailed modeling approach including dissociative recombination and electron-impact de-excitation, validated against experiments in a plasma generator.

Findings

Dissociative recombination dominates CH excitation at T_e < 1.5 eV.

Electron-impact de-excitation significantly reduces CH emission intensity.

Modeling including quenching aligns well with experimental data.

Abstract

For understanding carbon erosion and redeposition in nuclear fusion devices, it is important to understand the transport and chemical break-up of hydrocarbon molecules in edge plasmas, often diagnosed by emission of the CH A^2\Delta - X^2\Pi Ger\"o band around 430 nm. The CH A-level can be excited either by electron-impact or by dissociative recombination (D.R.) of hydrocarbon ions. These processes were included in the 3D Monte Carlo impurity transport code ERO. A series of methane injection experiments was performed in the high-density, low-temperature linear plasma generator Pilot-PSI, and simulated emission intensity profiles were benchmarked against these experiments. It was confirmed that excitation by D.R. dominates at T_e < 1.5 eV. The results indicate that the fraction of D.R. events that lead to a CH radical in the A-level and consequent photon emission is at least 10%. Additionally, quenching of the excited CH radicals by electron impact de-excitation was included in the modeling. This quenching is shown to be significant: depending on the electron density, it reduces the effective CH emission by a factor of 1.4 at n_e=1.3*10^20 m^-3, to 2.8 at n_e=9.3*10^20 m^-3. Its inclusion significantly improved agreement between experiment and modeling.