Characterization of a low temperature atmospheric-pressure argon microwave induced plasma using visual imaging, OES, and CRDS combined

We employ a suite of optical techniques, visual imaging, optical emission spectroscopy (OES), and cavity ringdown spectroscopy (CRDS), to characterize a low power, low gas flow rates, atmospheric-pressure argon microwave induced plasma (MIP). A series of plasma images captured in a time resolution range of 10 mus-0.05 s shows that the converging point is a visual effect of integrating the image of a rapidly rotating conical helix. The plasma electronic excitation temperature T_exc, vibrational temperature T_v, rotational temperature T_r, and gas temperature T_g at different locations along the axis of the plasma column are determined from the simulations of the emission spectra of OH, N₂ and N₂ ⁺ in the range of 200-450 nm. Thermal equilibrium properties of the plasma are discussed. OH radical concentrations along the plasma column axis are measured by CRDS and the effects of plasma gas compositions on the generation of OH radicals are investigated. An upper limit of electron density ne is estimated from the Lorentzian component of the broadened lineshape obtained by ringdown spectral scans of the rovibrational line S₂₁ of the OH A-X (0-0) band.