Microwave plasma torches driven by surface waves

The axial structure of air and nitrogen–argon plasma torches driven by surface waves at atmospheric pressure is studied by means of optical emission spectroscopy and radiophysics. The rotational temperature of the Q₁-branch of the OH (A²⁺Σ⁺, v' = 0 → X ²Π, v'' = 0) rotational band in the range 306–315 nm has been determined as a measure of the gas temperature. This temperature is around 3500 K along the main part of the air plasma torch and sharply drops to about 1000 K in the post-discharge zone. In the N₂–Ar mixture, the Doppler temperatures (6000–8000 K) of the H_α and H_β lines are higher than the OH rotational temperature (∼4000 K). 'Hot' excited O(3p⁵P) oxygen atoms with temperature of approximately 20000 K have been detected in the air plasma torch.