Optical cavity-based measurements of electron density in a non-equilibrium atmospheric-pressure plasma

Electron density estimates in dielectric barrier discharge plasmas (DBD) have large uncertainties. They can be calculated from an electric field model if the ion densities and charge states are known. They can be measured using Thomson scattering if the plasma is in thermodynamic equilibrium and if the path-integrated electron densities are large enough. Other methods infer an electric field based on shifts or broadening of ion spectral lines. In this work, we present an interferometric method for measuring the electron density in DBD plasma bullets. It uses a laser locked to a high-finesse optical cavity to measure sub-milliradian phase shifts. We report a peak electron density of (6.0±1.2)×1020 m−3 in our Ar DBD plasma. Our method demonstrates a direct and minimally invasive diagnostic for measuring electron densities in low-density atmospheric-pressure plasmas.