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 5mm. 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 (310–179V) and the phase angle between the voltage and current (54°–18°), and a contraction of the plasma volume (8.5–0.17cm3, at 3mm 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 αmode. At 1 and 2mm, the α mode remained in the abnormal glow discharge until the α–γ mode transition occurred as the discharge current increases. At 3mm, however, the α mode was excited as a normal glow discharge with a constant current density (17mA∕cm2) but it became an abnormal glow discharge as the current increased. At 4mm, 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.

Microsize jet-type plasmas were generated in a single pin electrode structure source for two separate input frequencies of 50kHz and 13.56MHz in the ambient air. The copper pin electrode radius was 360μm, and it was placed in a Pyrex tube with a radius of 3mm for helium gas supply. Due to the input frequency difference, the generated plasmas showed distinct discharge characteristics for their plasma physical appearances, electrical properties, gas temperatures, and optical properties. Strengths and weaknesses of both plasmas were discussed for further applications.

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Controllability of small size atmospheric pressure plasma generated at low frequency in a pin to dielectric plane electrode configuration was studied. It was shown that the plasma characteristics could be controlled by geometrical and operational parameters of the experiment. Under most circumstances, continuous glow discharges were observed, but both the corona and/or the dielectric barrier discharge characteristics were observed depending on the position of the pin electrode. The plasma size and the rotational temperature were also varied by the parameters. The rotational temperature was between 300 and 490K, being low enough to treat thermally sensitive materials.