Disentangling dark and luminous phases of nanosecond discharges developing in liquid water

There is no clear experimental evidence of the underlying microscopic physical mechanisms of micro-discharges directly produced in liquids. In this study, we examine shadowgraph images and plasma-induced emission (PIE) to decouple simultaneously developing dark and luminous phases of micro-discharges with nanosecond durations in liquid water. We apply diagnostics with extremely high temporal (down to 30 ps) and spatial (down to 1 mu m) resolutions to capture tiny bush-like dark filaments that expand from the anode tip together with the formation of luminous tree-like structures. For the first time, we disentangle two closely coupled dark and luminous phases of the discharge events and determine their onsets accurately with respect to the driving high-voltage (HV) pulse. The dark filaments start appearing within similar to 3-4 ns after the onset of the HV pulse, and subsequently expand at a constant velocity of similar to 1 x 10(5)-2 x 10(5)m s(-1), depending on the HV amplitude and anode curvature. A systematic analysis of the PIE waveforms using the associated shadowgraph images reveals that the onset of the luminous discharge phase is delayed by similar to 600-800 ps with respect to the onset of the initial dark filament structures. Considering the constant propagation velocity of dark filaments, the luminous phase starts to develop when the extension of regions with a perturbed refractive index (i.e., perturbed density) reaches several tens of micrometres. An analysis of PIE tracks within the captured shadowgraph images confirms that luminous filaments develop only in regions affected by primary dark filaments and their attachment to the anode surface coincides with points of initial onset of the first dark filaments. Furthermore, the emission intensity produced during the luminous phase originates from the luminous filaments developing in the bulk liquid. Our study provides an important insight into the dynamics of different phases of micro-discharges in de-ionised water.