Initiation mechanism of negative pulsed discharge in supercritical carbon dioxide

The initiation mechanism of a negative pulsed discharge in pressurized carbon dioxide including supercritical (SC) phase was studied using pulsed Schlieren and photomultiplier techniques. The appearance of the negative streamer at its initial stage was quite different from the positive one: a bush of negative polarity and a filament of positive polarity. A clear discontinuity existed around the sub-critical phase, characterized by streamer initiation voltage versus medium condition: The streamer initiation voltage increased with the medium density in gas phase, but was mostly independent of the medium density in SC and liquid phases. Calculation of field emission current according to Fowler-Nordheim equation suggested that the streamer initiation in both SC and liquid phases was caused by field emission, while within gas phase, initial electrons might be supplied by electron detachment from CO₂^- ions.