In this work, we examine initial phases of micro-discharges produced in deionised water by high-voltage (HV) pulses of nanosecond duration. We apply opto-electrical diagnostics with extremely high temporal (down to 30 ps) as well as spatial (down to 1 μm) resolution. Frozen interferometric and shadowgraph images show three distinct events. The first, the subcritical (no-discharge) event, is characterised by periodic perturbations of the index of refraction which depart from the anode surface and are pulled away at the speed of sound as an expanding envelope defined by the shape of the anode tip. One-dimensional hydrodynamic modelling of the subcritical phase under conditions mimicking curvatures of real anode tips reveals basic characteristics of perturbations caused by dynamic balance between the hydrostatic and electrostrictive pressures consistent with experimental observations. The second, the dark or non-luminous discharge event, is characterised by the onset of a few isolated very tiny tree-like structures growing from the anode tip. Depending on the HV amplitude, the initial structures occur with a delay of ∼2-3 ns after onset of the HV pulse and subsequently expand with average velocity of ∼1 × 10 5 -2 × 10 5 m s −1 , creating very dense bush-like structures made of thin hair-like filaments in a few nanoseconds. The third, the luminous discharge event, follows (nearly simultaneously) the dark discharge event and unveils much simpler tree-like morphology determined by the extension of non-luminous bush-like structures. Characteristic dimensions of observed events range from about 1 μm (typical diameter of non-luminous filaments) to tens of micrometres (characteristic diameters of luminous filaments). Furthermore, we address a possible role of microbubbles developing in the anode region due to the periodic HV pulses and verify that the UV-vis-NIR spectrometric signatures of the luminous phase notably change when replacing non-degassed deionised water with degassed.

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.

This contribution investigates the effects of duty cycle and mass flow of synthetic air and oxygen on the efficiency of ozone generation in multi-hollow surface dielectric barrier discharge (MSDBD). It discloses that the efficiency of ozone generation in MSDBD is significantly higher compared with standard coplanar DBD, surface DBD and volume DBDs. Ozone production yield reached 205.5 ± 29.1 g (kW h)−1 (40% duty cycle, 8 slm) and 413.91 ± 58.7 g (kW h)−1 (100% duty cycle, 8 slm) at an energy cost of 8.7 and 4.3 eV/molecule for synthetic air and oxygen, respectively. Such high ozone yields arose out of the intrinsic characteristics of MSDBD ceramics, which were efficiently cooled by the flow of the working gas. The amplitude modulation of low-frequency 5 kHz high-voltage sine waveforms facilitates controlled O3 production at a nearly constant rate of yield. Since the correct evaluation of ozone production yield requires precise determination of the discharge power, the concentration of ozone and working gas-flow, considerable attention was paid to measurements of these parameters. It is confirmed and experimentally demonstrated herein that correct determination of discharge power lies with Lissajous figure methods, while the determination of power through the direct integration of product u(t)i(t), where i(t) is measured by Pearson current probe, leads to systematically lower values of calculated power with consequent overestimation of the ozone production yield. The correct determination of discharge power is clearly the key to the proper calculation of ozone production yield and efficiency. Under the DBD discharge conditions presented herein, ozone production yield and efficiency achieved figures as high as 19.5% and 35.2% of theoretical limits recently established for air and oxygen, respectively.

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