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- ItemImpact of the electrode proximity on the streamer breakdown and development of pulsed dielectric barrier discharges(Bristol : IOP Publ., 2022) Wubs, J.R.; Höft, H.; Kettlitz, M.; Becker, M.M.; Weltmann, K.-D.The impact of the electrode proximity on the streamer breakdown and development of pulsed-driven dielectric barrier discharges (DBDs) in a single-filament arrangement has been investigated in a gas mixture of 0.1 vol% O2 in N2 at 0.6 bar and 1.0 bar. The gap distance was varied from 0.5 mm to 1.5 mm, and the applied voltage was adapted correspondingly to create comparable breakdown conditions in the gap. The development of the DBDs was recorded by an iCCD and a streak camera system, while fast electrical measurements provided insight into discharge characteristics such as the transferred charge and consumed energy. The results demonstrate that breakdown in a smaller gap is characterised by a slower streamer propagation but a significantly higher acceleration. It can therefore be concluded that the proximity of the cathode has a strong impact on the characteristics of the streamer breakdown. However, after the streamer has crossed the gap, the discharge structure in front of the anode was found to be the same independent of the actual gap distance.
- ItemEffective streamer discharge control by tailored nanosecond-pulsed high-voltage waveforms(Bristol : IOP Publ., 2021) Huiskamp, T.; Ton, C.; Azizi, M.; van Oorschot, J.J.; Höft, H.In this paper we present our solid-state nanosecond pulse source (the solid-state impedance-matched Marx generator) which can generate arbitrary waveforms and which can be used for pulsed discharge generation. The purpose of the development of such a generator is twofold: by being able to change the waveform at will, we aim to control the discharge generated by such pulses very precisely which can be very useful for plasma applications, but also for more fundamental studies. In the presented study, we applied the arbitrary-waveform pulse source for streamer discharge generation in a cylinder-wire-like arrangement and used the arbitrary-waveform capability to change the rise time (in our experiments we used 6.8-26.2 ns) of unipolar positive pulses with 6-10 kV amplitude and 80 ns duration. Additionally, we introduced variations of a step in the rising edge of the waveform. We performed measurements both in air and nitrogen to electrically characterize the discharge while analyzing the streamer propagation in the plasma reactor with intensified charge-coupled device imaging and measured ozone generation (in air). The results show that we can indeed control the propagation of the streamer discharge with the stepped waveform, but that the rise-time variation has little effect on the streamer propagation in our system. However, the streamer velocity and structure differs significantly comparing discharges in nitrogen and air for the same applied voltage waveform. Additionally, for some of the stepped waveforms we found a slight increase of the ozone yield for air at low overall energy densities.
- ItemUpscaling from single- to multi-filament dielectric barrier discharges in pulsed operation(Bristol : IOP Publ., 2022) Höft, H.; Becker, M.M.; Kettlitz, M.; Brandenburg, R.A study on the scalability of discharge characteristics of a single-filament dielectric barrier discharge (DBD) to a spatially one-dimensional multi-filament arrangement driven by the same high-voltage (HV) pulses was performed for a gas mixture of 0.1 vol% O2 in N2 at 1 bar. Both arrangements feature a 1 mm gap with dielectric-covered electrodes featuring two hemispherical alumina caps for the single-filament and two parallel alumina-tubes for the multi-filament arrangement. The DBDs were characterised by electrical measurements (for peak current, energy, and power) accompanied by iCCD and streak imaging to determine the filament number and the discharge development in the gas gap and on the surfaces. It was found that the electrical quantities scale with a constant factor between the single- and multi-filament arrangement, which is expected to be related to the filament number. In the multi-filament arrangement, the pulsed operation leads to filament formation in the entire gap in lateral direction within less than 2 ns. Furthermore, particular breakdown or discharge inception regimes were identified for the multi-filament DBDs. These regimes could be generated at the falling slope of asymmetrical HV pulses featuring e.g. a double-streamer propagation, which was previously reported for single-filament DBDs. Consequently, it was proven that the discharge manipulation by varying the HV pulse widths obtained for single-filament DBDs can also be applied in a one-dimensional multi-filament arrangement, i.e. an upscaling based on the knowledge for single-filament DBDs seems to be generally possible.
- ItemDouble-propagation mode in short-gap spark discharges driven by HV pulses with sub-ns rise time(Bristol : IOP Publ., 2020) Höft, H.; Becker, M.M.; Kolb, J.F.; Huiskamp, T.The object of this study is the investigation of spark discharges ignited by unipolar positive rectangular high voltage (HV) pulses with 200 ps rise time and (15 ± 2) kV amplitude with 3 ns duration full width at half maximum in synthetic air in a 1.2 mm pin-to-pin gap (tungsten electrodes) at atmospheric pressure. The discharge development was recorded by synchronised iCCD and streak camera measurements in single-shot operation, revealing a two-stage propagation mode. The discharge started with a fast initial breakdown across the entire gap (∼10 mm ns−1) during the HV slope, followed by a much slower (∼0.1 mm ns−1) propagation originating from both electrodes towards the gap centre. The combination of high-resolution diagnostics with numerical modelling indicated that the initial breakdown phase is caused by the rapid increase of electric field strength during the steep HV slope, which leads to the simultaneous fast propagation of a positive and a negative streamer.