Study of the effective inverse photon efficiency using optical emission spectroscopy combined with cavity ring-down spectroscopy approach

Abstract

The hydrocarbon impurities formation is inevitable due to wall erosion in a long pulse high performance scenario with carbon-based plasma facing materials in fusion devices. The standard procedure to determine the chemical erosion yield in situ is by means of inverse photon efficiency D/XB. In this work, the conversion factor between CH4 flux and photon flux of CH A → X transition (effective inverse photon efficiency PE−1) was measured directly using a cascaded arc plasma simulator with argon/methane. This study shows that the measured PE−1 is different from the calculated D/XB. We compared the photon flux measured by optical emission spectroscopy (OES) and calculated by electron impact excitation of CH(X) which was diagnosed by cavity ring-down spectroscopy (CRDS). It seems that charge exchange and dissociative recombination processes are the main channels of CH(A) production and removal which lead to the inconsistency of PE −1 and D/XB at lower temperature. Meanwhile, the fraction of excited CH(A) produced by dissociative recombination processes was investigated, and we found it increased with Te in the range from 4% to 13% at Te < 1 eV. Our work suggests that the CH spectroscopy should be reinterpreted and the conversion factor should have a new definition instead of D/XB since the electron impact excitation is not the only channel of CH(A) production. These results have an effect on evaluating the yield of chemical erosion in divertor of fusion device.