The breakup of methane under ITER divertor hydrogen plasma conditions for carbon chemical erosion analysis with CH spectroscopy

Methane (CH4) was injected into the high density (ne ∼ 1020 m-3), low temperature (Te ∼ 1 eV) hydrogen plasma in Pilot-PSI to determine the CH A-X photon efficiency in this unexplored plasma regime. The effects of particle transport and particle reflection on the emission of directly excited CH under these plasma conditions were assessed with the 3D Monte Carlo code ERO. The simulations of the inverse photon efficiency showed a difference of ∼20% between full hydrocarbon sticking or no sticking (reflection). In addition it predicts that particle transport may lead to more than a factor of 10 increase. The measured inverse photon efficiency is however constant at 100 ± 30 for 0.1 < Te < 1.0 eV. The constancy is consistent with dissociative recombination of CH4+, CH3+ and CH2+ to produce excited CH instead of direct excitation. These results form a framework for in situ carbon erosion measurements in future fusion reactors such as ITER.