Striations in Atmospheric Pressure AC Driven Glow Discharge in Helium/Argon mixture

In this work, striations in an atmospheric pressure AC driven glow discharge having a helium-argon feed gas mixture have been studied. A pin-to-plate electrode configuration having a separation of 8 mm was driven by a 25 KHz AC signal. In our prior work for a pure helium system the striations were identified to be dictated by the ions as the measured striation velocity matched the drift velocities of the helium ions. To confirm the role of the ions on the striation characteristics we systematically introduced trace amount of argon (0 – 1%) into the feed gas mixture and the striation behavior was characterized. Voltage-current characteristics were determined and change of striations patterns as function of the gas mixtures and input power were correlated with a range of current densities from ~0.5 to 2 A/cm². The striation pattern was found to diminish as argon concentration increased. It is found that the striations pattern changes temporally and have maximum intensity when a distinct and high intensity negative glow is formed near the electrode surface. Furthermore, unlike the pure helium case where no radial variation in the striation is observed, with argon the striations also showed a radial increase in its emission intensity when the discharge current increased rapidly. The voltage-current measurements clearly indicate the existence of a hysteresis regime, which is evaluated based on the gas mixture as well as power input. High speed imaging revealed that with increase in the argon concentration the striation velocity decreased significantly, representative of an average helium-argon ion drift velocity.