Atmospheric pressure plasma jet has the ability to generate a kind of remote plasma in open air, also referred to as plasma plume, which is abundant with active species. The plasma jet has extensive application prospects in various fields, such as material synthesis, surface modification, biomedicine, and environmental protection, etc. The morphology of plasma plume is related with the distribution of active species, which is important for the investigation of plasma jet.Up to now, only a few plume morphologies have been observed in experiment. Aim to this status, an argon plasma jet is utilized to investigate the plume morphology in this paper. With varying voltage parameters including amplitude, frequency and biased value, several morphologies are observed for the first time, such as a diffuse cone, a corona and a central filament(DAF), a bead string, and a hollow structure. These results obviously enrich the plume morphology. Through comparing their waveforms for the different morphologies, it is found that plasma plume with the other three morphologies(except the DAF)presents one emission pulse per voltage cycle. Moreover, the emission pulse appears at the negative half cycle,corresponding to a negative discharge for the diffuse cone and the bead string. However, the hollow plume presents the emission pulse at the positive half cycle(a positive discharge). As a comparison, there are two emission pulses for the DAF plume, one of which is a negative discharge and the other corresponds to a positive discharge. In addition, spatial and temporal evolutions are conducted by an emICCD for the different plumes. Results reveal that the negative discharge corresponds to a negative streamer, while the positive discharge is a positive streamer. The different morphologies observed by naked eyes are temporally integrated results of the negative streamer and the positive streamer. These results are of great significance for the profound investigation of plume morphology and further study of streamer dynamics.
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