Striations in Atmospheric Pressure AC Driven Helium Glow Discharge*

Plasma stratification has been studied for more than a century. Despite the many studies reported on this topic,  a complete understanding of the phenomena is still missing. In this work, striations at atmospheric pressure AC driven helium glow discharge in is studied for a pin-to -plate electrode geometry with an electrode spacing of 8 mm. The striations are observed by several bright and dark regions in the positive column. The AC power supply was operated at 25 kHz frequency with the pin electrode powered and the plate electrode grounded. Voltage current characteristics were determined, and the striation patterns were correlated to a range of current densities from ~0.3 to 1.6 A cm^-2. Visualization of the discharge indicate that the striations appear not during the entire voltage rise and decay but during the second half of the voltage rise and first of the voltage decay. It is also found that the striation intensity also changes temporally and has the maximum intensity when a distinct and high intensity negative glow is formed near the electrode surface.  The voltage current measurements clearly indicate the existence of a hysteresis regime pertaining to the onset and ending of the striated behavior of the positive column.  Dependence of the striation on input power and electrode geometry are also investigated. High speed imaging revealed distinct behaviors of theses striations during the positive and negative cycle of the alternating waveform. The measurements indicate that the striations travel in the velocity range of 200 – 700 m/s.