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 ∆-X 2 Π Gerö band around 430 nm. The CH A-level can be excited either by electronimpact 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.

Hydrocarbon injection experiments on molybdenum targets facing high-density plasmas in Pilot-PSI were simulated with the 3D Monte Carlo impurity transport and PSI code ERO. Impurity transport and calculation of redeposition profiles were decoupled by calculating carbon redistribution matrices with ERO. Redeposition was found to be strongly dependent on the electron density. The calculated average number of recycling events of hydrocarbon molecules on the surface went up from from 1.5 for n e = 5 • 10 19 m −3 to 19.2 for n e = 4 • 10 20 m −3 ; at the latter density, only 2.4% of the hydrocarbon molecules escapes the simulated plasma beam without returning to the target at least once. Agreement with experimental deposition profiles in argon was fair. The results in hydrogen point towards a strong gradient in chemical erosion yield along the target.

Methane was injected into a high density low temperature hydrogen plasma beam (n e ≤ 10 21 m-3 , T e ~ 1 eV) in Pilot-PSI to study carbon transport and provide a benchmark case for modeling. Gas injection was performed through molybdenum samples that acted as collectors of redeposits. The CH AX band emission plumes are localized near the injection hole with a typical width of ~3 mm and a decay length of ~1.5 mm. The geometry of the emission profile is not influenced by the electron temperature. The redeposition efficiency and codeposit structure depend on the surface temperature. Surface temperatures below 500 K promote growth of a-C:H films across the surface and lead to redeposition of up to 38% of injected carbon. Above 800 K the redeposition efficiency decreases strongly and carbon microparticles are found near the injection hole.

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