Time and spatially resolved LIF of OH in a plasma filament in atmospheric pressure He–H₂O

Abstract

The production of OH in a nanosecond pulsed filamentary discharge generated in pin–pin geometry in a He–H2O mixture is studied by time and spatially resolved laser-induced fluorescence. Apart from the OH density the gas temperature and the electron density are also measured. Depending on the applied voltage the discharge is in a different mode. The maximum electron densities in the low- (1.3 kV) and high-density (5 kV) modes are 2 × 1021 m−3 and 7 × 1022 m−3, respectively. The gas temperature in both modes does not exceed 600 K. In the low-density mode the maximum OH density is at the centre of the discharge filament, while in the high-density mode the largest OH density is observed on the edge of the discharge. A chemical model is used to obtain an estimate of the absolute OH density. The chemical model also shows that charge exchange and dissociative recombination can explain the production of OH in the case of the high-density mode.

Time-resolved absolute OH density of a nanosecond pulsed discharge in atmospheric pressure He–H2O: absolute calibration, collisional quenching and the importance of charged species in OH production

“…The decay time of the LIF signal then needs to be measured at each time together with the total LIF signal, to be able to correct the time dependent LIF signal (and thus OH density) for this. An example is shown in figure 2, for a high-density discharge in He + 0.84% H 2 O, with similar conditions as the discharge treated in [9]. In this case, the correction for time-resolved quenching changes the time-resolved OH density significantly.…”

“…The authors have shown before that the self-destruction processes of OH are dominant in the loss of OH at the observed high OH density (see e.g. [9,39,40]). …”

“…In the current work, positive high voltage pulses (V applied = 1.5 kV) are applied to the upper electrode with a repetition frequency of 1 kHz and a pulse duration of 170 ns. This corresponds to the low-density mode, as defined in [9]. A delay generator (BNC, model 575) is synchronized with the function generator that controls the high voltage pulses, to adjust the delay of the laser and the diagnostics (intensified Charge-Coupled Device (iCCD) cameras, time digitizer) relative to the voltage pulse.…”

“…In this work the filamentary discharge is stabilized in space between two needle electrodes at a sufficiently short distance, which allows the application of diagnostics with a high resolution [9]. The discharge is investigated in a mixture of water with an atomic gas, helium.…”

“…This facilitates modelling the observed OH dynamics because of the lower amount of reaction channels. As described in [9] two discharge modes occur, a low-(applied dc voltage <3 kV) and a high-density (applied dc voltage >3 kV) plasma mode. The names lowand high-density mode are related to the strong difference in the electron density or current density.…”