Prebreakdown process of a negative nanosecond pulsed discharge in supercritical carbon dioxide

The prebreakdown process in a 100 µm gap in supercritical carbon dioxide was investigated under negative nanosecond pulsed voltage. A bushlike negative streamer was observed by the shadowgraph method. Although currents flowed in the middle of the rising edge, a back discharge current suddenly appeared at the falling edge when a peak voltage exceeding 28 kV was applied. Electron multiplication was estimated to be approximately 108 by integrated calculation of the back discharge current. These results suggest that a negative streamer grows by steps and the inside of the bush streamer is considered to consist of a high-pressure gas.