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

Abstract

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 μm) 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 ∼3–4 ns after the onset of the HV pulse, and subsequently expand at a constant velocity of ∼1 × 105–2 × 105 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 ∼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.

Demonstration of Dynamics of Nanosecond Discharge in Liquid Water Using Four-Channel Time-Resolved ICCD Microscopy

“…We conducted the present experiments using the discharge reactor described in detail in recent works [9,10,12] and illustrated in Figure 1. The discharge reactor consists of a stainless-steel body equipped with HV and capacitive probe interfaces, optical windows, and a combined inlet/outlet for the working liquid.…”

“…The electrode system used in this study was composed of an electrochemically sharpened tungsten anode pin and grounded metallic chamber walls. We used the hyperboloid-like anode pin applied in [12] with a slightly more flattened tip, owing to progressive consumption caused by thousands of shots. The anode was stressed using positive nanosecond HV pulses transmitted through a coaxial cable.…”

“…Any dataset related to a single discharge event consists of a sequence of four ICCD images and several oscilloscopic waveforms. The recorded oscilloscopic waveforms (HV, PIE, laser pulse, MCP gate monitors) were corrected for various propagation delays, as described in detail in [12]. At the end of this procedure, each ICCD image was associated with a precise time window (with respect to the HV onset) during which the exposure of a given image was performed.…”

“…This indicates that, compared with Figure 5, The Ch1-Ch4 detectors were exposed by much lower photon fluxes; therefore, the gate width 1 ns of the MCP gates appeared to be a reasonable minimum for revealing the shining cores of the main luminous filaments with a reasonable signal-to-noise ratio. The wavelengths (λ c = 532 and 656 nm) used to acquire monochromatic images were selected based on previously reported results [7,12]. The first wavelength (532 nm) coincides well with the short wavelength edge (~500 nm) of the continuum emission registered during the first few nanoseconds [7], whereas the second wavelength (656 nm) allows the tracking of the emission intensity of the H α line superposed with the NIR continuum [12].…”

“…For example, in our recent studies, we employed time-resolved emission and shadowgraphic microscopy to reveal fundamental morphologic signatures and estimated the average propagation velocity of expanding dark and luminous filaments [7,8,12]. We established that the dark or non-luminous discharge event is initiated at a certain delay after the onset of the HV pulse through a few isolated, tiny tree-like structures emerging from the surface of the anode tip.…”