Electron density measurements by Thomson scattering and Stark broadening in nanosecond pulsed CH4/Ar plasma

Two independent approaches for electron density (ne) measurements, i.e. Thomson scattering and Stark broadening, are compared against each other in a nanosecond pulsed CH4/Ar plasma across a range of conditions (60 – 500 mbar and 0.5 – 2.0 % CH4). Reasonable agreement is found between both techniques up to 0.8 µs after the plasma pulse, when coherent and incoherent scattering features start to simultaneously appear in the Thomson scattering spectrum. After 2.6 µs, only incoherent scattering remains, resulting in a more accurate estimation of ne compared to Stark broadening, as the latter suffers from a low signal-to-noise ratio. The maximum ne is proportional to the total gas pressure and varies from 7 × 10^22 m^-3 at 60 mbar to 1.4 × 10^23 m^-3 at 500 mbar, whereas CH4 admixing was found to lower the ne. The electron temperature (Te), estimated from Thomson scattering spectra, rises from ~ 18000 K to 29600 K (1.55 to 2.55 eV) with increasing gas pressure, whilst being mostly independent of the CH4 admixing ratio. This comparison of two methods provides guidance to the application of Thomson scattering and Stark broadening in nanosecond pulsed plasmas, highlighting the ranges of experimental conditions and time windows where accurate estimations of ne and Te are feasible.