Measured signatures of low energy, physical sputtering in the line shape of neutral carbon emission

The most important mechanisms for introducing carbon into the DIII-D divertors [J.L. Luxon, Nucl. Fusion 42 (2002) 614] are physical and chemical sputtering. Previous investigations have indicated that operating conditions where one or the other of these is dominant can be distinguished by using CD and C2 emissions to infer C I influxes from dissociation of hydrocarbons and comparing to measured C I influxes. The present work extends these results through detailed analysis of the C I spectral line shapes. In general, it is found that the profiles are actually asymmetric and have shifted peaks. These features are interpreted as originating from a combination of an anisotropic velocity distribution from physical sputtering (the Thompson model) and an isotropic distribution from molecular dissociation. The present study utilitzes pure helium plasmas to benchmark C I spectral profiles arising from physical sputtering alone.