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

The CH A-X molecular band is measured upon seeding the hydrogen plasma in the linear plasma generator Pilot-PSI [electron temperature T-e=0.1-2.5 eV and electron density n(e)=(0.5-5) X 10(20) m(-3)] with methane. Calculated inverse photon efficiencies for these conditions range from 3 up to >10(6) due to a steeply decreasing electron excitation cross section. The experiments contradict the calculations and show a constant effective inverse photon efficiency of similar to 100 for T-e < 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 n(e) > 5 X 10(20) m(-3) and 0.1 eV < T-e < 1 eV. These results form a framework for in situ carbon erosion measurements in future fusion reactors such as ITER. (C) 2009 American Institute of Physics. [doi:10.1063/1.3238295]