α, γ, and normal, abnormal glow discharge modes in radio-frequency capacitively coupled discharges at atmospheric pressure

Discharge modes, α and γ, of a radio-frequency helium capacitively coupled discharge at atmospheric pressure were investigated with the discharge gap distance between electrodes varied from 1 to 5 mm. As similarly observed in other experiments, the α and γ mode and the α-γ mode transition were observed with large drops in the voltage (310-179 V) and the phase angle between the voltage and current (54°-18°), and a contraction of the plasma volume (8.5-0.17 cm3, at 3 mm gap distance). The discharge voltage where the α-γ mode transition occurred versus the gap distance showed a similar behavior with the Paschen curve for a gas breakdown. Depending on the gap distance, normal and abnormal glow regimes were observed in the α mode. At 1 and 2 mm, the α mode remained in the abnormal glow discharge until the α-γ mode transition occurred as the discharge current increases. At 3 mm, however, the α mode was excited as a normal glow discharge with a constant current density (17 mA cm2) but it became an abnormal glow discharge as the current increased. At 4 mm, the α mode was sustained as a normal glow discharge, then the transition to the γ mode occurred. Using a simple resistor-capacitor circuit model and a α sheath breakdown model, the discharge modes and the mode transition properties were studied.