CH spectroscopy for carbon chemical erosion analysis in high density low temperature hydrogen plasma

Abstract

The CH A−X molecular band is measured upon seeding the hydrogen plasma in the linear plasma generator Pilot-PSI [electron temperature Te=0.1–2.5 eV and electron density ne=(0.5–5)×1020 m−3] with methane. Calculated inverse photon efficiencies for these conditions range from 3 up to >106 due to a steeply decreasing electron excitation cross section. The experiments contradict the calculations and show a constant effective inverse photon efficiency of ∼100 for Te<1 eV. The discrepancy is explained as the CH A level is populated through dissociative recombination of the molecular ions formed by charge exchange. Collisional de-excitation is observed for ne>5×1020 m−3 and 0.1 eV<Te<1 eV. These results form a framework for in situ carbon erosion measurements in future fusion reactors such as ITER.