Empirical Line Ratios for Optical Emission Spectroscopy in low-temperature, low-density, magnetised Ar plasmas

Spectroscopic evaluation of electron temperature and density in low-temperature, low-density, magnetized plasmas can be difficult, but necessary in situations where chemical erosion and physical sputtering prevent the use of other diagnostics, such as Langmuir probes (LP). Further, in such cases, because of the low densities and temperatures, the vessel and environment involved, theoretical line ratios derived from Collisional-Radiative (CR) models may not be easily applicable. This is the case, for example, of low-temperature (< 15 eV), low-density (< 1011 cm-3), magnetised plasma used for plasma-material interaction studies where chemical erosion and physical sputtering can be significant. The aim of the present work is to define an empirical line ratio (ELR) method derived from an extensive calibration campaign with the two diagnostics, using LP measurements as a reference. The ELR method is useful to permit the use of OES independent of LP in conditions that are critical for the latter, resulting in an effective instrument for the evaluation of plasma parameters. Further, the use of two different lines of sight and the influence of the magnetic field intensity on the measurements are also discussed. The proposed ELR method is demonstrated here for pure Ar linear plasmas and is in principle applicable also to other similar cases.