arXiv subjects

U. Ebert

Publications and source records attributed to U. Ebert.

At least 19 recordsLinked to original sources

arXiv2023

Double-pulse streamer simulations for varying interpulse times in air

In this paper, we study how streamer discharges are influenced by a previous voltage pulse using an axisymmetric fluid model. We simulate double-pulse positive streamers in N2-O2 mixtures containing 20% and 10% O2 at 1 bar. By varying the time between the pulses between 5 ns and 10 microseconds, we observe three regimes during the second pulse: streamer continuation, inhibited growth and streamer repetition. In the streamer continuation regime, a new streamer emerges from the tip of the previous one. In the inhibited regime, the previous channel is partially re-ionized, but there is considerably less field enhancement and almost no light emission. Finally, for the longest interpulse times, a new streamer forms that is similar to the first one. The remaining electron densities at which we observe streamer continuation agree with earlier experimental work. We introduce an estimate which relates streamer continuation to the dielectric relaxation time, the background field and the pulse duration. Furthermore, we show that for interpulse times above 100 ns several electron detachment reactions significantly slow down the decay of the electron density.

physics.plasm-ph↗

arXiv2021

Neutral dissociation of methane by electron impact and a complete and consistent cross section set

We present cross sections for the neutral dissociation of methane, in a large part obtained through analytical approximations. With these cross sections the work of Song $\textit{et al.}$ [J. Phys. Chem. Ref. Data, $\textbf{44}$, 023101, (2015)] can be extended which results in a complete and consistent set of cross sections for the collision of electrons with up to 100 eV energy with methane molecules. Notably, the resulting cross section set does not require any data fitting to produce bulk swarm parameters that match with experiments. Therefore consistency can be considered an inherent trait of the set, since swarm parameters are used exclusively for validation of the cross sections. Neutral dissociation of methane is essential to include (1) because it is a crucial electron energy sink in methane plasma, and (2) because it largely contributes to the production of hydrogen radicals that can be vital for plasma-chemical processes. Finally, we compare the production rates of hydrogen species for a swarm-fitted data set with ours. The two consistent cross section sets predict different production rates, with differences of $45\%$ (at $100$ Td) and $125\%$ (at $50$ Td) for production of H$_2$ and a similar trend for production of H. With this comparison we underline that the swarm-fitting procedure, used to ensure consistency of the electron swarm parameters, can possibly deteriorate the accuracy with which chemical production rates are estimated. This is of particular importance for applications with an emphasis on plasma-chemical activation of the gas.

physics.plasm-ph↗

arXiv2020

Radio emission from negative lightning leader steps reveals inner meter-scale structure

We use the Low Frequency ARray (LOFAR) to probe the dynamics of the stepping process of negatively-charged plasma channels (negative leaders) in a lightning discharge. We observe that at each step of a leader, multiple pulses of VHF (30~--~80 MHz) radiation are emitted in short-duration bursts ($<10\ \mu$s). This is evidence for streamer formation during corona flashes that occur with each leader step, which has not been observed before in natural lightning and it could help explain X-ray emission from lightning leaders, as X-rays from laboratory leaders tend to be associated with corona flashes. Surprisingly we find that the stepping length is very similar to what was observed near the ground, however with a stepping time that is considerably larger, which as yet is not understood. These results will help to improve lightning propagation models, and eventually lightning protection models.

physics.ao-ph↗

arXiv2020

On the emergence mechanism of carrot sprites

We investigate the launch of negative upward streamers from sprite glows. This phenomenon is readily observed in high-speed observations of sprites and underlies the classification of sprites into carrot or column types. First, we describe how an attachment instability leads to a sharply defined region in the upper part of the streamer channel. This region has an enhanced electric field, low conductivity and strongly emits in the first positive system of molecular nitrogen. We identify it as the sprite glow. We then show how, in the most common configuration of a carrot sprite, several upward streamers emerge close to the lower boundary of the glow, where negative charge gets trapped and the lateral electric field is high enough. These streamers cut off the current flowing towards the glow and lead to the optical deactivation of the glow above. Finally, we discuss how our results naturally explain angel sprites.

physics.ao-ph↗

arXiv2017

Characteristics of a novel nanosecond DBD microplasma reactor for flow applications

We present a novel microplasma flow reactor using a dielectric barrier discharge (DBD) driven by repetitively nanosecond high-voltage pulses. Our DBD-based geometry can generate a non-thermal plasma discharge at atmospheric pressure and below in a regular pattern of micro-channels. This reactor can work continuously up to about 100 minutes in air, depending on pulse repetition rate and operating pressure. We here present the geometry and the main characteristics of the reactor. Pulse energies of 1.9 $\mu$J and 2.7 $\mu$J per channel at atmospheric pressure and 50 mbar, respectively, have been determined by time-resolved measurements of current and voltage. Time-resolved optical emission spectroscopy measurements have been performed to calculate the relative species concentrations and temperatures (vibrational and rotational) of the discharge. Effects of the operating pressure and the flow velocity on the discharge intensity have been investigated. In addition, the effective reduced electric field strength E/N has been obtained from the intensity ratio of vibronic emission bands of molecular nitrogen at different operating pressures. The derived effective E/N increases gradually from 500 Td to 600 Td when decreasing the pressure from one bar to 0.4 bar. Below 0.4 bar, further pressure reduction results in a significant increase in the effective E/N up to about 2000 Td at 50 mbar.

physics.plasm-ph↗

arXiv2017

Thunderstorm electric fields probed by extensive air showers through their polarized radio emission

We observe a large fraction of circular polarization in radio emission from extensive air showers recorded during thunderstorms, much higher than in the emission from air showers measured during fair-weather circumstances. We show that the circular polarization of the air showers measured during thunderstorms can be explained by the change in the direction of the transverse current as a function of altitude induced by atmospheric electric fields. Thus by using the full set of Stokes parameters for these events, we obtain a good characterization of the electric fields in thunderclouds. We also measure a large horizontal component of the electric fields in the two events that we have analysed.

astro-ph.IM↗

arXiv2016

Atmospheric electrification in dusty, reactive gases in the solar system and beyond

Detailed observations of the solar system planets reveal a wide variety of local atmospheric conditions. Astronomical observations have revealed a variety of extrasolar planets none of which resembles any of the solar system planets in full. Instead, the most massive amongst the extrasolar planets, the gas giants, appear very similar to the class of (young) Brown Dwarfs which are amongst the oldest objects in the universe. Despite of this diversity, solar system planets, extrasolar planets and Brown Dwarfs have broadly similar global temperatures between 300K and 2500K. In consequence, clouds of different chemical species form in their atmospheres. While the details of these clouds differ, the fundamental physical processes are the same. Further to this, all these objects were observed to produce radio and X-ray emission. While both kinds of radiation are well studied on Earth and to a lesser extent on the solar system planets, the occurrence of emission that potentially originate from accelerated electrons on Brown Dwarfs, extrasolar planets and protoplanetary disks is not well understood yet. This paper offers an interdisciplinary view on electrification processes and their feedback on their hosting environment in meteorology, volcanology, planetology and research on extrasolar planets and planet formation.

astro-ph.EP↗

arXiv2015

Influence of Atmospheric Electric Fields on the Radio Emission from Extensive Air Showers

The atmospheric electric fields in thunderclouds have been shown to significantly modify the intensity and polarization patterns of the radio footprint of cosmic-ray-induced extensive air showers. Simulations indicated a very non-linear dependence of the signal strength in the frequency window of 30-80 MHz on the magnitude of the atmospheric electric field. In this work we present an explanation of this dependence based on Monte-Carlo simulations, supported by arguments based on electron dynamics in air showers and expressed in terms of a simplified model. We show that by extending the frequency window to lower frequencies additional sensitivity to the atmospheric electric field is obtained.

physics.ao-ph↗

arXiv2015

Probing Atmospheric Electric Fields in Thunderstorms through Radio Emission from Cosmic-Ray-Induced Air Showers

We present measurements of radio emission from cosmic ray air showers that took place during thunderstorms. The intensity and polarization patterns of these air showers are radically different from those measured during fair-weather conditions. With the use of a simple two-layer model for the atmospheric electric field, these patterns can be well reproduced by state-of-the-art simulation codes. This in turn provides a novel way to study atmospheric electric fields.

astro-ph.HE↗

arXiv2013

Experimental study of the spatio-temporal development of meter-scale negative discharge in air

We study the development of a negative discharge driven by a Marx generator of about 1 MV in an air gap of 1 up to 1.5 meter, at standard temperature and pressure. We show the evolution of the discharge with nanosecond-fast photography together with the electrical characteristics. The negative discharge develops through four well-distinguished streamer bursts. The streamers have different velocities and life times in different bursts. The last burst triggers a positive inception cloud on the positive grounded electrode and a burst of positive counter-streamers emerges. The pre-discharge then bridges the gap and leaders grow from both electrodes. Finally a spark is formed. Looking closer into the pre-ionized zone near the cathode, we find isolated dots which are potential branching points. These dots act as starting points for positive streamers that move towards the high-voltage electrode. We also find such phenomena as space leaders and leader stepping in our laboratory sparks.

physics.plasm-ph↗

arXiv2013

Investigation of positive streamers by double pulse experiments, effects of repetition rate and gas

Streamer discharges are often operated in a repetitively pulsed mode and are therefore influenced by species left over from the previous discharge, especially free electrons and ions. We have investigated these effects by applying two consecutive positive high voltage pulses of 200-700 ns duration to a point plane gap in artificial air, pure nitrogen, other nitrogen-oxygen mixtures and pure argon at pressures between 67 and 533 mbar. The pulses had pulse-to-pulse intervals ({\Delta}t) between 200 ns and 40 ms. We imaged both discharges with two ICCD cameras and combined this to a compound image. We observe for values of {\Delta}t below 0.5-15 {\mu}s (at 133 mbar, depending on gas mixture) that during the second pulse the streamers continue the paths of the first-pulse streamers. We call the maximal time for which this continuation still occurs the continuation time. For N$_{2}$ - O$_{2}$ mixtures, this time has a maximum at an oxygen concentration of about 0.2%. According to our plasma chemical modelling this maximum is determined by the electron loss rate which has a minimum around this oxygen concentration. Depending on oxygen concentration the dominant recombining positive ion is N$_{4}^{+}$, O$_2^+$ or O$_{4}^{+}$ where O$_{2}^{+}$ dominates around 0.2% O$_{2}$ and recombines slowest. For increasing values of {\Delta}t we observe that after the continuation phase first no new streamers occur at all, then new streamers show up that avoid the entire pre-ionized region. Next we see new thin streamers that follow the edges of the old channels. For larger {\Delta}t (10-200 {\mu}s) the new streamers start to increase in size and move to the centre of the old channel. Finally, around millisecond timescales the new channels are completely independent of the old channels.

physics.plasm-ph↗

arXiv2013

Why isolated streamer discharges hardly exist above the breakdown field in atmospheric air

We investigate streamer formation in the troposphere, in electric fields above the breakdown threshold. With fully three-dimensional particle simulations, we study the combined effect of natural background ionization and of photoionization on the discharge morphology. In previous investigations based on deterministic fluid models without background ionization, so-called double-headed streamers emerged. But in our improved model, many electron avalanches start to grow at different locations. Eventually the avalanches collectively screen the electric field in the interior of the discharge. This happens after what we call the `ionization screening time', for which we give an analytical estimate. As this time is comparable to the streamer formation time, we conclude that isolated streamers are unlikely to exist in fields well above breakdown in atmospheric air.

physics.plasm-ph↗

arXiv2013

Streamers in air splitting into three branches

We investigate the branching of positive streamers in air and present the first systematic investigation of splitting into more than two branches. We study discharges in 100 mbar artificial air that is exposed to voltage pulses of 10 kV applied to a needle electrode 160 mm above a grounded plate. By imaging the discharge with two cameras from three angles, we establish that about every 200th branching event is a branching into three. Branching into three occurs more frequently for the relatively thicker streamers. In fact, we find that the surface of the total streamer cross-sections before and after a branching event is roughly the same.

physics.plasm-ph↗

arXiv2013

High order fluid model for streamer discharges: I. Derivation of model and transport data

Streamer discharges pose basic problems in plasma physics, as they are very transient, far from equilibrium and have high ionization density gradients; they appear in diverse areas of science and technology. The present paper focuses on the derivation of a high order fluid model for streamers. Using momentum transfer theory, the fluid equations are obtained as velocity moments of the Boltzmann equation; they are closed in the local mean energy approximation and coupled to the Poisson equation for the space charge generated electric field. The high order tensor in the energy flux equation is approximated by the product of two lower order moments to close the system. The average collision frequencies for momentum and energy transfer in elastic and inelastic collisions for electrons in molecular nitrogen are calculated from a multi term Boltzmann equation solution. We then discuss, in particular, (1) the correct implementation of transport data in streamer models; (2) the accuracy of the two term approximation for solving Boltzmann's equation in the context of streamer studies; and (3) the evaluation of the mean-energy-dependent collision rates for electrons required as an input in the high order fluid model. In the second paper in this sequence, we will discuss the solutions of the high order fluid model for streamers, based on model and input data derived in the present paper.

physics.plasm-ph↗

arXiv2013

High order fluid model for streamer discharges. II. Numerical solution and investigation of planar fronts

The high order fluid model developed in the preceding paper is employed here to study the propagation of negative planar streamer fronts in pure nitrogen. The model consists of the balance equations for electron density, average electron velocity, average electron energy and average electron energy flux. These balance equations have been obtained as velocity moments of Boltzmann's equation and are here coupled to the Poisson equation for the space charge electric field. Here the results of simulations with the high order model, with a PIC/MC (Particle in cell/Monte Carlo) model and with the first order fluid model based on the hydrodynamic drift-diffusion approximation are presented and compared. The comparison with the MC model clearly validates our high order fluid model, thus supporting its correct theoretical derivation and numerical implementation. The results of the first order fluid model with local field approximation, as usually used for streamer discharges, show considerable deviations. Furthermore, we study the inaccuracies of simulation results caused by an inconsistent implementation of transport data into our high order fluid model. We also demonstrate the importance of the energy flux term in the high order model by comparing with results where this term is neglected. Finally, results with an approximation for the high order tensor in the energy flux equation is found to agree well with the PIC/MC results for reduced electric fields up to 1000 Townsend, as considered in this work.

physics.plasm-ph↗

arXiv2012

Experimental study of hard X-rays emitted from meter-scale positive discharges in air

We investigate structure and evolution of long positive spark breakdown; and we study at which stage pulses of hard X-rays are emitted. Positive high-voltage pulses of standardized lightning impulse wave form of about 1 MV were applied to about 1 meter of ambient air. The discharge evolution was imaged with a resolution of tens of nanoseconds with an intensified CCD camera. LaBr3(Ce+) scintillation detectors recorded the X-rays emitted during the process. The voltage and the currents on both electrodes were measured synchronously. All measurements indicate that first a large and dense corona of positive streamers emerges from the high voltage electrode. When they approach the grounded electrode, negative counter-streamers emerge there, and the emission of hard X-rays coincides with the connection of the positive streamers with the negative counter-streamers.

physics.plasm-ph↗

arXiv2011

Probing background ionization: Positive streamers with varying pulse repetition rate and with a radioactive admixture

Positive streamers need a source of free electrons ahead of them to propagate. A streamer can supply these electrons by itself through photo-ionization, or the electrons can be present due to external background ionization. Here we investigate the effects of background ionization on streamer propagation and morphology by changing the gas composition and the repetition rate of the voltage pulses, and by adding a small amount of radioactive Krypton 85. We find that the general morphology of a positive streamer discharge in high purity nitrogen depends on background ionization: at lower background ionization levels the streamers branch more and have a more feather-like appearance. This is observed both when varying the repetition rate and when adding Krypton 85, though side branches are longer with the radioactive admixture. But velocities and minimal diameters of streamers are virtually independent of the background ionization level. In air, the inception cloud breaks up into streamers at a smaller radius when the repetition rate and therefore the background ionization level is higher. When measuring the effects of the pulse repetition rate and of the radioactive admixture on the discharge morphology, we found that our estimates of background ionization levels are consistent with these observations; this gives confidence in the estimates. Streamer channels generally do not follow the paths of previous discharge channels for repetition rates of up to 10 Hz. We estimate the effect of recombination and diffusion of ions and free electrons from the previous discharge and conclude that the old trail has largely disappeared at the moment of the next voltage pulse; therefore the next streamers indeed cannot follow the old trail.

physics.plasm-ph↗

arXiv2011

Electron density fluctuations accelerate the branching of streamer discharges in air

Branching is an essential element of streamer discharge dynamics but today it is understood only qualitatively. The variability and irregularity observed in branched streamer trees suggest that stochastic terms are relevant for the description of streamer branching. We here consider electron density fluctuations due to the discrete particle number as a source of stochasticity in positive streamers in air at standard temperature and pressure. We derive a quantitative estimate for the branching distance that agrees within a factor of 2 with experimental values. As branching without noise would occur later, if at all, we conclude that stochastic particle noise is relevant for streamer branching in air at atmospheric pressure.

physics.plasm-ph↗