Observation of structures and striations in atmospheric pressure helium surface glow discharge

Abstract

Summary form only given. The uniformity and stability of the surface glow discharge produced at atmospheric pressure is a basic requirement to use it for microwave absorption and drag reduction on moving objects. The experimental study has been performed to map the instability regime of the discharge with the variation of discharge operating parameters such as applied voltage and operating pressure. The experimental observation of glow discharge instabilities in the form of plasma spots and striations is reported. The uniform surface glow discharge was produced in helium gas at atmospheric pressure. The discharge panel was a double-sided circuit board (12 cm times 12 cm) whose fully cladded side was used as ground and the other side with multiple strips etched was stressed with high voltage (~2 kV) of audio frequency (~10 kHz) signal. The spatio-temporal discharge events were recorded using CCD camera with 25 frames per second. On reducing the applied voltage to 1.4 kV, the discharge becomes unstable and displays localized structures like stationary plasma spots. Simultaneously other phenomenon like spot-pair formation and low frequency fluctuation of few spots were also observed. Further increase in the applied voltage presents fluctuation of the spots and they interact with other spots to merge. At sufficiently high voltage, all the spots coalesce and the discharge covers the entire electrode area that indicates a uniform glow discharge. On reducing the operating pressure (~600 torr) with applied voltage 1.1 kV, the small plasma spots grow in size and arrange themselves to form a regular discharge pattern. The individual plasma spot fluctuates about their mean position. Upon further reducing the pressure to ~400 torr, the spot length increases and finally they mix with other spots. This is attributed to the plasma diffusion at low pressure operation. After few minutes of the discharge operation at atmospheric pressure, the moving striations similar to low frequency (few Hz) acoustic fluctuations were observed at applied voltage ~2-3 kV. Increase in applied voltage increases the propagation speed. The coherent oscillation excited at low voltage becomes turbulent at higher voltages ~3-3.4 kV. Further increase of applied voltage diminishes the acoustic striations and uniformity of the discharge was achieved at ~3.8 kV. Upon examining the upper surface of discharge panel, the etching of base material was found adjacent to powered metallic strips. The characterization of these experimental observations is presented. Theoretical arguments were used to understand the destabilization mechanism of the discharge