Influence of bias voltage and oxygen addition on the discharge aspects of a diffuse argon plume in an atmospheric pressure plasma jet

A remote plasma, also referred to as a plasma plume (diffuse or filamentary), is normally formed downstream of an atmospheric pressure plasma jet. In this study, a diffuse plume is formed by increasing the bias voltage (U_b) applied to the downstream electrode of an argon plasma jet excited by a negatively pulsed voltage. The results indicate that the plume is filamentary when U_b is low, which transits to the diffuse plume with increasing U_b. The discharge initiated at the rising edge of the pulsed voltage is attributed to the diffuse plume, while that at the falling edge contributes to the filament in the plume. For the diffuse plume, the discharge intensity decreases with the increasing oxygen content (C_o). Fast photography reveals that the diffuse plume results from a negative streamer, which has a dark region near the nozzle with C_o = 0%. However, the dark region is absent with C_o = 0.5%. From the optical emission spectrum, the electron density, electron excitation temperature, gas temperature, and oxygen atom concentration are investigated.