Tunable diode laser absorption diagnostics for atmospheric pressure plasmas

A tunable diode laser was used as a light source for line-of-sight absorption measurements in an atmospheric pressure plasma. Spectrally-resolved argon absorption lineshapes at 8115 and 8104 Å, corresponding to the 4s3P2→4p3D3 and 4s3P1→4p3D1 transitions of argon, were recorded in a 1 kW, 27 MHz inductively-coupled argon plasma 11 mm downstream from the load coil. Lineshapes were fit to Voigt profiles, allowing inference of both Stark broadening and shift which, in turn, specifies the electron number density and the kinetic temperature. A population temperature for each of the probed levels was determined from frequency-integrated line intensities. The difference between ionization temperatures, determined from measured electron number-density values assuming Saha equilibrium between atoms and ions, and population temperatures suggests the presence of a suprathermal electron number density. The methods presented here provide important new capabilities for plasma diagnostics.