A Regularly Swelling Hollow Plume Generated in an Atmospheric Pressure Argon Plasma Jet Excited by a Positively Biased Sinusoidal Voltage

Far different from common plasma plumes, a regularly swelling hollow plume is observed in an argon plasma jet. The results indicate that the regularly swelling plume appears only at a low-frequency voltage (lower than 1 kHz) that is positively biased. Waveforms of applied voltage and light emissions reveal that there is a barrier discharge inside the nozzle, which initiates once every half voltage cycle. Outside the nozzle, it initiates once every positive half voltage cycle and never at the negative half cycle. By fast photography, it can be found that the positive discharge stems from the rod end of the powered electrode and bends down to hug the tube surface, which finally propagates to the outside as a cathode-directed streamer due to the charge overflow effect. The bifurcating streamer propagates along the periphery of the argon flow, which results in a hollow structure of the plasma plume. Due to abundant active species provided by the positive discharges inside the nozzle, the diameter of the plasma channel is comparatively larger in the swelling regions than in other regions, which induces the swells of the plume. Furthermore, the negative barrier discharge is diffuse, whose influence on the swells can be neglected. Finally, the contribution of active species generated upstream of the swells is verified by investigating the distance between two adjacent swells as a function of gas flow rate and driving frequency.