Self-organized luminous pattern formation observed above the anode surface of a DC glow discharge in pure He

Abstract

We generated a pin-to-plate He DC glow discharge in a chamber and experimentally observed self-organized luminous pattern formation above the anode. We varied the gas pressure (0.1–1atm), the discharge current (1–80mA), and the distance between the anode and cathode (1–12mm) to investigate the experimental conditions for the pattern formation. We also investigated the gas composition in the chamber and optical emission spectra. We found that the pattern formed farther away from the discharge axis at a higher gas pressure and a longer distance between electrodes. On the other hand, at a lower gas pressure, a higher discharge current, and a shorter distance between electrodes, orange dots were observed near the discharge axis. Under some conditions, both the pattern and the orange dots were observed simultaneously. Furthermore, by sorting the results by pd-product (gas pressure×distance between electrodes), we found that the pattern formed at a threshold value for the pd-product. When 1% of N2 or Ne was added to He, the structure of the pattern changed, but the tendency for the pattern to form at a threshold value for the pd-product was similar. When the pd-product is larger, the reduced electric field and electron energy are most likely to be lower. Considering the voltage applied to the discharge region and the optical emission spectra, the plasma is considered to be a recombining plasma. We propose that the pd-product is one of the key factors, and that considering the recombining plasma, in which ionization by HeM (metastable He) occurs under weak electric fields, is important for the pattern formation in DC glow discharges.