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

Abstract

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 µs 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 (Texc) to be determined at 8000–9000 K, while the vibrational temperature (Tv), the rotational temperature (Tr) and the gas temperature (Tg) 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 ne 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 S21 of the OH A–X (0–0) band.