Stable filamentary structures in atmospheric pressure microwave plasma torch

This paper experimentally investigates the processes governing the single- and multi-filament regimes in an atmospheric pressure microwave (MW) torch operated in argon. Optical emission spectroscopy and spectral imaging are the principal diagnostics techniques which are employed. MW power is found to be the main parameter controlling the number of filaments. The single-filament regime exhibits many properties typical for surface wave discharges, e.g. a linear decrease in electron density along the axis or the existence of a central dip in the radial/lateral emission profiles. Simple geometric quantities, such as the length or thickness of the filament(s), vary almost linearly with the input MW power, and exhibit discontinuities at successive filament splitting events. These take place at similar values of filament maximum thickness, and may be due to skin-depth limited power transfer. The presence and chemistry of a low-emission intensity plasma shell surrounding the filament(s) is also investigated. The gas temperature is estimated from the OH band and complemented by Schlieren imaging, which revealed that a much larger cone of gas is being heated by filaments than is their diameter.