Microwave plasma optical emission spectroscopy combined with atmospheric pressure DC discharges

Atmospheric pressure microwave (MW) plasmas present considerable interest for various industrial or environmental applications. The main advantage of MW plasmas is electrode-less operation, availability of cheap microwave sources at 2.45 GHz, good microwave to plasma energy coupling and no need of vacuum devices if operated at atmospheric pressure. MW torches are sometimes used to preheat gases to high temperatures needed for other applications or for studying other discharge plasmas in preheated gases. We present spectroscopic investigations of atmospheric pressure afterglow plasmas generated by MW torch in nitrogen and air. We employ optical diagnostics: emission spectroscopy to characterise the plasma jet. In the first part of this work we analyzed the emission spectra of MW discharge generated in pure nitrogen and in nitrogen with small oxygen admixture. We also measured the plasma temperatures and analyzed spatial characteristics of MW discharge at various powers and gas flow rates. In the second part we examined MW plasma generated in air at atmospheric pressure. It showed out that the emission or rather the absence of molecular spectra at low MW powers is the limitation factor for performing the spectroscopic characteristics and direct temperature measurements of the generated plasma. This is why we developed and tested a new method for temperature measurements of near-thermal equilibrium air plasmas at atmospheric pressure. We demonstrated that this so-called corona probe method (where the corona discharge is placed into the MW plasma) can be generally applied to determine the temperature of the near-equilibrium plasmas and contrary to the thermocouple it is not limited to temperatures below 1000 grad C or affected by radiative losses and the problems of interaction with the microwave plasma and electromagnetic fields. Since there are many applications where large volume plasmas are needed and there is a good chance of expanding the volume of DC discharges by combining them with MW plasma as some previous works suggested, in the last part of our work the basic properties and behaviour of combined MW and glow (or MW and spark) discharges were examined. Spectroscopic study of these combined discharges, especially the transient spark in the gas preheated by MW plasma, is also important for the plasma assisted combustion applications. (author)