Optical diagnostics of a low power - Low gas flow rates atmospheric-pressure argon plasma created by a microwave plasma torch

We employ a suite of optical techniques, namely, visual imaging, optical emission spectroscopy and cavity ringdown spectroscopy (CRDS), to characterize a low power, low gas flow rates, atmospheric-pressure argon microwave induced plasma. The plasma is created by a microwave plasma torch, which is excited by a 2.45 GHz microwave with powers ranging from 60 to 120 W. A series of plasma images captured in a time-resolution range of as fine as 10 νs shows that the converging point is actually a time-averaged visual effect and the converging point does not exist when the plasma is visualized under high time resolution, e.g. <2 ms. Simulations of the emission spectra of OH, N₂ and in the range 200-450 nm enable the plasma electronic excitation temperature (T _exc) to be determined at 8000-9000 K, while the vibrational temperature (T_v), the rotational temperature (T_r) and the gas temperature (T_g) at different locations along the axis of the plasma column are all determined to be in the range 1800-2200 K. Thermal equilibrium properties of the plasma are discussed. OH radical concentrations along the plasma column axis are measured by CRDS and the concentrations are in the range 1.6 × 10¹³-3.0 × 10¹⁴ cm ^-3 with the highest density at the tail of the plasma column. The upper limit of electron density n_e is estimated to be 5.0 × 10¹⁴ cm^-3 from the Lorentzian component of the broadened lineshape obtained by ringdown spectral scans of the rovibrational line S ₂₁ of the OH A-X (0-0) band.