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 120W. A series of plasma images captured in a time-resolution range of as fine as 10 mu s 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, N-2 and N-2(+) 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 x 10(13)-3.0 x 10(14) cm(-3) with the highest density at the tail of the plasma column. The upper limit of electron density ne is estimated to be 5.0 x 10(14) 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.