Striation characteristics in atmospheric pressure AC-driven glow discharge operating in monoatomic gas

Striation in an atmospheric pressure AC-driven helium glow discharge operating at 25 kHz is experimentally characterized. Striation behavior is characterized by the visualization of discharges and voltage–current measurements over a range of plasma power. Voltage–current characteristics reveal that striations form in the “normal” glow regime of operation, with a higher number of strata at low current, diminishing as discharge current increases. The relative differences and similarities in the striation patterns during the positive and negative cycles are identified. High-speed imaging shows the striations to be traveling in time—“moving striations.” The spatial luminosity of the striations is tracked over time, enabling a reliable determination of average striation velocities. The measured striation velocity is compared with estimated He+ and He2+ drift velocities and is found to be in good agreement in the positive column region of the discharge. Experiments were conducted with trace amounts of argon (0%–2%) to explore the effects of ions on striation patterns. The presence of a trace amount of argon was found to constrict the discharge, increase the gas temperature, and suppress the formation of striation. Dominant recombination and ionization pathways in the positive column are analyzed to identify the underlying reason for the formation of striations in pure helium and their absence in a helium–argon mixture.