Stable filamentary structures in atmospheric pressure microwave plasma torch

Abstract

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.

Experimental characterization of TIAGO torch discharges: surface wave discharge behavior and (post-)discharge kinetics

“…With increasing power the dart becomes longer and the plume more voluminous, as was also found in [34]. A linear increase in the dart length with applied power has also been reported for other torch-like devices at atmospheric pressure working with argon at 2.45 GHz [33,46]. In [46] Hrycak et al used powers higher than those presented in this research, while Snirer et al [33] worked with powers ranging from 10 to 300 W; so, in this research, a wide interval of powers is covered.…”

“…An increase in electron density with power was also reported in [50], where values of the same order of magnitude were shown at 915 MHz. Furthermore, a linear decrease of electron density when moving far from the nozzle has been previously reported in other microwave plasma torches, for example by Snirer et al in [33]. However, the results reported herein allow for a more complete discussion regarding the behavior of the dart as a SWD.…”

“…A linear increase in the dart length with applied power has also been reported for other torch-like devices at atmospheric pressure working with argon at 2.45 GHz [33,46]. In [46] Hrycak et al used powers higher than those presented in this research, while Snirer et al [33] worked with powers ranging from 10 to 300 W; so, in this research, a wide interval of powers is covered. The dart length values are in good agreement with those previously reported [34] whereas plume lengths are reported for the first time in this research.…”

“…Our results also fit with those shown in other microwave plasma torch-like devices. In [33] Snirer et al reported an increase in gas temperature with both power and axial distance from the nozzle-specifically, gas temperature was shown to be higher in the middle of the dart than at the gas exit, which is also observed in the TIAGO dart. Comparable values of gas temperature and an increase with applied power were also observed by Miotk et al [50] at 915 MHz.…”

“…Concerning the experimental demonstration of microwave plasma torch plasmas behaving as SWDs, unfortunately neither complete evidence for nor direct identification of this behavior is found in the literature. For example, in [32,33] a linear decrease in electron density was shown when moving far from the field applicator along the plasma discharge; however, formal and complete experimental proof of this being a SWD was not presented.…”