Phase resolved optical emission spectroscopy of coaxial microplasma jet operated with He and Ar

Atmospheric pressure plasma jet sources are currently in the focus of many researchers for their promising applications in medical industry (e.g. treatment of living tissues), surface modification or material etching or synthesis. Here we report on the study of excitation mechanisms of a coaxial microplasma jet with a hollow capillary as an inner electrode and a ceramic tube with metal ring as outer electrode. This microplasma jet is operated in He and Ar gas and it is investigated by means of electrical measurements, optical emission spectroscopy and space and phase resolved wavelength integrated optical spectroscopy. Measurements of a microscale atmospheric pressure plasma jet with parallel metal electrodes operated in He are shown for comparison as well. Four different modes are distinguished with He as plasma forming gas. The alpha discharge in annular space between the electrodes, observed at low applied voltages, is very similar to the discharge in the jet with parallel electrodes. As the voltage increases a gamma discharge appears, first localized at the tip of the capillary. As the voltage increases further the gamma discharge appears in the annular space as well. A hollow cathode plasma is observed at any voltage used on the symmetry axis of the jet. Only one mode of plasma operation is observed in argon gas with distinctively different behavior. We hypothesize that it is comparable to a single microdischarge of a filamentary dielectric barrier discharge.