High-speed dispersed photographing of an open-air argon plasma plume by a grating-ICCD camera system

In this paper, an open-air argon plasma plume is generated at atmospheric pressure by a two-electrode jet device with sub-microsecond voltage pulses at a repetitive frequency of 1 kHz. Optical emission spectroscopy measurements showed that spectral irradiance from OH and N-2 bands, and Ar lines, characterized the spectrum of the open-air argon plasma plume. The rotational temperature estimation of UV OH band spectra indicated the gas temperature of the plasma plume to be as low as room temperature. A novel diagnostic method, based on two dispersion gratings and an ICCD camera, was designed for investigating the time-and space-resolved propagation behaviour of the excited radicals in the plasma plume. Based on the dispersion feature of gratings, a series of dispersed plasma optical emission volumes, which were formed by irradiance from different excited radiation emitters (excited species) in the plasma plume, were captured in the form of high-speed images by the ICCD camera. From the sequence of dispersed emission images, it is possible to observe the time-and space-resolved behaviour of different excited species in the plasma, and meanwhile, to understand the propagation dynamics of the open-air argon plasma plume. It is found that the OH bands' emission volume exhibited a propagation behaviour distinct from that of N-2 and Ar emission volumes. The OH emissions decayed immediately as soon as the plasma travelled out from the nozzle, but were able to last for a longer duration time inside the nozzle than both N-2 and Ar emissions. The N-2 bands' emission volumes propagated to a far distance and formed the whole length of the argon plasma plume in the surrounding air. The Ar emissions decayed rapidly for the plasma inside and outside the nozzle due to the adverse effect of impurities, in particular the large concentration of diffused air in the open space. These distinct types of dynamic behaviour of the dispersed plasma emission volumes are attributed to the different generation and quenching mechanisms of their corresponding excited species and they shed light on the clear propagation dynamics of the argon plasma plume in open air.