/spl alpha//spl gamma/ and normal, abnormal glow discharge modes in radio-frequency capacitively-coupled discharges at atmospheric pressure

Summary form only given. Discharge modes, alpha and gamma, of a radio-frequency helium capacitively-coupled discharge at atmospheric pressure were investigated with the discharge gap distance between electrodes varied from 1 mm to 5 mm. As similarly observed in other experiments, alpha- and gamma-mode and the alpha-gamma mode transition were observed with large drops in the voltage (310 V to 179 V) and the phase angle between the voltage and current (54deg to 18deg), and a contraction of the plasma volume (8.5 cm³ to 0.17 cm³, at 3 mm gap distance). The discharge voltage at which the alpha-gamma 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 alpha-mode. At 1 and 2 mm, the alpha-mode was remained in the abnormal glow discharge until the alpha-gamma mode transition occurred as the discharge current increases. At 3 mm, however, the alpha 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 alpha-mode was sustained as a normal glow discharge, then the transition to the gamma-mode occurred. The effect of the gap distance was resulted from the different system impedance. Using a simple resistor-capacitor circuit model and the alpha-sheath breakdown model, the discharge modes and the mode transition properties were studied