Spectroscopic Investigation of a Microwave-Generated Atmospheric Pressure Plasma Torch

The investigated new microwave plasma torch is based on an axially symmetric resonator. Microwaves of a frequency of 2.45 GHz are resonantly fed into this cavity resulting in a sufficiently high electric field to ignite plasma without any additional igniters as well as to maintain stable plasma operation. Optical emission spectroscopy was carried out to characterize a humid air plasma. OH-bands were used to determine the gas rotational temperature T_rot while the electron temperature was estimated by a Boltzmann plot of oxygen lines. Maximum temperatures of T_rot of about 3600 K and electron temperatures of 5800 K could be measured. The electron density n_e was estimated to n_e ≈ 3 · 10²⁰m^-3 by using Saha's equation. Parametric studies in dependence of the gas flow and the supplied microwave power revealed that the maximum temperatures are independent of these parameters. However, the volume of the plasma increases with increasing microwave power and with a decrease of the gas flow. Considerations using collision frequencies, energy transfer times and power coupling provide an explanation of the observed phenomena: The optimal microwave heating is reached for electron-neutral collision frequencies νen being near to the angular frequency of the wave ω.