Reillumination of submicrosecond pulsed corona-like discharges in water

Abstract

Pronounced reilluminations can be observed for discharges that are generated in water by submicrosecond high-voltage pulses. This phenomenon can appear during the decrease of the applied voltage, depending on the pulse characteristics, especially fall times. The respective processes are expected to be crucial for optical investigations of discharge characteristics and also for applications. Accordingly, the development of individual corona-like discharges in water was investigated in a needle-to-plate geometry for their ignition by single rectangular highvoltage pulses with a duration of 100 ns. The pulse amplitudes of about 50 kV and adjusted pulse fall times of 20, 26, and 43 ns were used. Voltages, currents and light emissions were synchronized with subsequent images of individual discharge events. Two distinct stages, a propagation phase and a reillumination phase, were identified. The individual discharges expanded for all investigated fall times with the same velocity of (29 ± 2) km s −1 . No significant differences of streamer morphologies and electrical characteristics were observed for the propagation phase. As the voltage began to decrease, light emissions first went through a minimum before increasing again. The following maxima were strongly dependent on fall times. Corresponding images showed that discharge channels were reilluminated within 6 ns. However, not all of the previously observed filaments reignited at once for longer fall times and instead single channels reilluminated subsequently. The development of reillumination can be explained from the external electric field in the electrode gap during the streamer propagation, which determines the head charge distributions at the discharge channel ends. This space charges generate an internal electrical field in the filaments resulting in the reillumination during the falling edge of the applied high-voltage pulse.

Hydrogen Peroxide Production of Individual Nanosecond Pulsed Discharges Submerged in Water of Elevated Conductivity

“…Thus, energy that was stored at the heads of the filaments during the propagation process was utilized for reillumination which is why negative values have been assigned to E rel . [28] In general, propagation energies increased with propagation time. For the lowest conductivity, a maximum energy of 24 mJ was derived, which rose with conductivity to 43 mJ or 53 mJ.…”

“…The setup for this study was already described in detail previously. [28] In short, a tungsten needle (99.95 % purity, Metall Maier), sharpened to a tip radius of (35 � 1) μm, was placed at a distance of (5.0 � 0.2) mm above a titanium plate with a diameter of 46 mm (99.5 % purity, Selfan Fine + Metal GmbH), which was integrated in the bottom of a polymethylmethacrylat chamber. The chamber had a volume of 1 mm 3 and was filled with 800 μl of conductive water.…”

“…This current minimum was associated with the reillumination of discharge filaments, which was described in detail previously. [28] Accordingly, propagation energy, E prop , and reillumination energy, E rel , were defined as the energy dissipated before and after the current signal changed polarity, respectively.…”

“…[16,27] For simplification, a single substitute streamer can summarily describe the entire discharge. [10,28] The production of hydroxyl radicals can then be calculated by the integration of local •OH concentrations over the streamer volume. It can be assumed that the overall •OH-production rate, k 1 , in a streamer is constant as long as the temperature along the filaments of a streamer remains the same.…”

“…The reillumination energy, E rel , is a measure for the electrical energy that is stored in the streamer and corresponds to the charge accumulated at the streamer head. [28,35] In deionized water, up to 4 mJ were stored this way, i. e. about 17 % of E prop (cf. Figure 3(C) and (D)).…”