Observation of striations in RF hollow electrode discharge in argon

In this paper, the characteristics of striations observed in a 13.56 MHz radio frequency (RF) hollow electrode discharge in argon are presented. The RF hollow electrode discharge is a method for generating capacitively coupled plasmas but with a hollow electrode, which combines the effects of both high-frequency and the hollow cathode effect on the ionization. The formation and features of striations were investigated experimentally at different RF powers, gas pressures, and hollow electrode parameters. Besides, a dielectric tube was inserted between the electrodes to confine the discharge channel in some experiments. The results show that the striation spatial period ΔS decreases as the ambient gas pressure increases, while it increases as the RF power or the depth of the hollow electrode increases. However, ΔS remains invariable in different electrode gaps. It is also found that the relationship between ΔS and the electric field obeys the classical Novak's law. The striations observed in the RF hollow electrode discharge are deemed to be similar to those in the positive column of direct current (DC) glow discharges. Due to the DC self-bias voltage, there is a weak field along the plasma channel in the RF hollow electrode discharge. The electron density and the reduced electric field are stratified when the striations appear. The nonlocal effect of electron kinetics is considered to be the mechanism governing the striations in the RF hollow electrode discharge at lower pressures. At higher pressures, the local theory may interpret the formation of striations.