Ultrafast x-ray phase contrast imaging of pulsed plasma initiation in water and hydrocarbons

Plasma breakdown in liquids poses a complex multiphase environment which challenges conventional imaging techniques. During the breakdown process, emitted bright light saturates camera sensors used in optical imaging mode, which is not the case in x-ray phase contrast imaging (PCI). This work focuses on ultrafast x-ray PCI of nanosecond-pulsed plasma discharge breakdown in distilled water and heptane, for double electrode geometries. A pulsed power circuit was used (+15kV, 50mJ) in conjunction with a laser-triggered (Nd:YAG, 532nm, 30 mJ/pulse) air spark gap switch in order to reduce jitter, facilitating time-dependent diagnostics. Preliminary results show propagating streamers which connect to form a conducting channel through the liquid, which would typically be obscured by optical emission. Streamer propagation speed is estimated to be >5 km/s. A Fresnel-Kirchhoff diffraction model was constructed and used to estimate the specific gravity and density of the plasma channels. Plasma breakdown in heptane was imaged for complete and incomplete breakdown modes by varying the breakdown voltage. Ultrafast imaging results are presented, highlighting their utility for revealing nanosecond-timescale multiphase plasma environments typically obscured by strong broadband optical emission.