Investigation of atmospheric pressure RF discharge with coexisting α and γ -modes

An atmospheric pressure radio frequency discharge working at coexistence of alpha and gamma-modes was investigated using electrical and optical measurements. The voltage and current characteristics showed a phase shift which is typical for alpha mode discharge. Time resolved discharge patterns were captured at a condition favorable for the coexistence of alpha and gamma-modes to assist in analyzing the working mode. Spectral line profile of HeI line at 492.19 nm and its forbidden counterpart was recorded and analyzed to estimate the electric field distribution near the instantaneous cathode. The combined spectroscopic and imaging measurement showed the simultaneous presence of both modes in the plasma volume at certain conditions. The strong HeI line Stark broadening and consecutive analysis indicated a presence of a short plasma sheath with a strong electric field, characteristic for gamma-mode, whereas a long extending low-field region was attributed to a presence of simultaneous alpha-mode. The electric field analysis method presented here can be used in a case where a very thin plasma sheath exists which is often detected in electrical discharges sustained at atmospheric pressure.