Single Streamers Research Articles

Numerical investigation of the bridging and current flow of a positive DC streamer using a 1.5D model

Abstract Single streamers, at nanosecond-timescales, can be simulated using detailed computational models with a high-dimensional representation. These models are computationally impractical for parametric explorations and simulation of longer times, that can follow many-streamer pulsations and the influence of one streamer burst on the next. This work develops a 1.5D model of a positive DC streamer for simulations beyond the electrode-gap bridging phase, and uses it to parametrically explore the impact of different terms and operational parameters. The geometry of interest is that of a tip-to-plane electrode configuration under DC voltage, and the simulation is followed for the duration of one current pulse (order 500 ns). The numerical model uses an axisymmetric boundary element method to solve for the electric field, as well as a ‘stack’ of 3 different transient solvers to improve efficiency and allow solving over longer timescales. The model is able to resolve the development of the cathode sheath during the streamer bridging phase using a kinetic flux boundary condition. It also gives qualitative agreement to current waveforms using an equivalent experimental setup. The different phases of the current pulse (streamer propagation, bridging, and current-flow phase) are discussed in detail.

On dynamics of downward-propagating sprite streamers

An approximate approach to estimating characteristics of downward-propagating single positive sprite streamers is presented. Within the framework of this approach, the dependences of streamer radius and velocity on the altitude were obtained. The influence of changes in the altitude of streamer initiation and in the electric field at this altitude on the streamer parameters is considered. Estimates of the optical emission rate are obtained.

Calculating Radio Emissions of Positive Streamer Phenomena Using 3D Simulations

AbstractWe study radio emissions from positive streamers in air using 3D simulations, from which the radiated electric field is computed by solving Jefimenko’s equations. The simulations are performed at using two photoionization methods: the Helmholtz approximation for a photon density and a Monte Carlo method using discrete photons, with the latter being the most realistic. We consider cases with single streamers, streamer branching, streamers interacting with preionization and streamer‐streamer encounters. We do not observe a strong VHF radio signal during or after branching, which is confirmed by lab experiments. This indicates that the current inside a streamer discharge evolves approximately continuously during branching. On the other hand, stochastic fluctuations in streamer propagation due to Monte Carlo photoionization lead to more radio emission being emitted at frequencies of 100 MHz and above. Another process that leads to such high‐frequency emission is the interaction of a streamer with a weakly preionized region, which can be present due to a previous discharge. In agreement with previous work, we observe the strongest and highest‐frequency emission from streamer encounters. The amount of total energy that is radiated seems to depend primarily on the background electric field, and less on the particular streamer evolution. Finally, we present approximations for the maximal current along a streamer channel and a fit formula for a streamer's current moment.

Mesoscopic ring element growth and deformation induced biofilm streamer evolution in microfluidic channels.

In a fluid environment, biofilms usually form and grow into streamers attached to solid surfaces. Existing research on single streamers studied their formation and failure modes. In the experiment on biofilm growth in a microfluidic channel, we found that rings composed of bacteria and an extracellular matrix are important elements on a mesoscopic scale. In the fluid environment, the failure of these ring elements causes damage to streamers. We simulated the growth and deformation of the ring structure in the micro-channel using multi-agent simulation and fluid-structure coupling of a porous elastic body. Based on this, we simulated the biofilm evolution involving multi-ring deformation, which provides a new length scale to study the biofilm streamer dynamics in fluid environments.

Natural sources of intense ultra-high-frequency radiation in high-voltage atmospheric discharges.

We study the sources of intense ultra-high-frequency (UHF) radiation (in the frequency range 1-6GHz) arising during the development of high-voltage atmospheric discharges. The discharges were initiated in a long discharge gap by applying an approximately 1-MV pulse with positive or negative polarity. By employing a radio registration system based on ultrawideband antennas, we managed to localize the UHF radiation sources in the discharge with centimeter accuracy and investigate their temporal and spatial correlation with the discharge structures. The vast majority of the localized sources turned out to be concentrated in the near-electrode regions. It is found that the generation mechanism of intense UHF radiation in a laboratory discharge cannot be unambiguously associated with such basic processes as the head-on collision of opposite-polarity streamers or the interaction of single streamers with the near-electrode plasma at the surface of metal electrodes. We discovered that the observed UHF emission appears basically as a precursor of the intense plasma development in a certain discharge region, whereinto a bright counterstreamer comes a bit later. The findings were confirmed by the statistical observations and results of imaging the dynamics of the discharge structures with a nanosecond temporal resolution.

Estimating the properties of single positive air streamers from measurable parameters

We develop an axial model for single steadily propagating positive streamers in air. It uses observable parameters to estimate quantities that are difficult to measure. More specifically, for given velocity, radius, length and applied background field, our model approximates the ionization density, the maximal electric field, the channel electric field, and the width of the charge layer. These parameters determine the primary excitations of molecules and the internal currents. Our approach is to first analytically approximate electron dynamics and electric fields in different regions of a uniformly-translating streamer head, then we match the solutions on the boundaries of the different regions to model the streamer as a whole, and we use conservation laws to determine unknown quantities. We find good agreement with numerical simulations for a range of streamer lengths and background electric fields, even if they do not propagate in a steady manner. Therefore quantities that are difficult to access experimentally can be estimated from more easily measurable quantities and our approximations. The theoretical approximations also form a stepping stone towards efficient axial multi-streamer models.

Direct Connection Between Auroral Oval Streamers/Flow Channels and Equatorward Traveling Ionospheric Disturbances

We use simultaneous auroral imaging, radar flows, and total electron content (TEC) measurements over Alaska to examine whether there is a direct connection of large-scale traveling ionospheric disturbances (LSTIDs) to auroral streamers and associated flow channels having significant ground magnetic decreases. Observations from seven nights with clearly observable flow channels and/or auroral streamers were selected for analysis. Auroral observations allow identification of streamers, and TEC observations detect ionization enhancements associated with streamer electron precipitation. Radar observations allow direct detection of flow channels. The TEC observations show direct connection of streamers to TIDs propagating equatorward from the equatorward boundary of the auroral oval. The TIDs are also distinguished from the streamers to which they connect by their wave-like TEC fluctuations moving more slowly equatorward than the TEC enhancements from streamer electron precipitation. TIDs previously observed propagating equatorward from the auroral oval have been identified as LSTIDs. Thus, the TIDs here are likely LSTIDs, but we lack sufficient TEC coverage necessary to demonstrate that they are indeed large scale. Furthermore, each of our events shows TID’s connection to groups of a few streamers and flow channels over a period in the order of 15 min and a longitude range of ∼15–20°, and not to single streamers. (Groups of streamers are common during substorms. However, it is not currently known if streamers and associated flow channels typically occur in such groups.) We also find evidence that a flow channel must lead to a sufficiently large ionospheric current for it to lead to a detectable LSTID, with a few tens of nT ground magnetic field decreases not being sufficient.

Prebreakdown negative streamers in liquid nitrogen: propagation characteristics and their influence on microsecond breakdown

Further clarification of streamer-induced breakdowns in liquid nitrogen has been attracting increasing attention owing to their potential applications in superconducting systems and cryoplasmas. In this work, negative streamers in microsecond discharges were investigated using a high-speed multiple-frame imaging technique. Different external voltages, gap lengths, hydrostatic pressures, and pin tips were employed to initiate various streamers, namely bush-like streamers, single pinecone-like streamers, dendritic streamers, and filamentary streamers. Furthermore, abundant hybrid streamers involving streamer transition and combination were observed. The diversity of negative streamers may be related to the pin morphology. Generally, it was found that thinner streamers, especially filamentary streamers, could take place more frequently with higher breakdown probability under stronger filed and lower pressure. However, bush-like streamers gradually played a more dominant role upon increasing the pressure, and they were unable to cause breakdown. Moreover, several intermittent weak discharges were recorded during the growth of streamers, which might be responsible for the streamer transition and propagation. Additionally, given the evolution of streamers branches after breakdown and the influence of pressure on streamers characteristics, it was speculated that high-pressure gas could be involved inside the streamers. The possible mechanism of streamer propagation and resultant breakdown were further explored based on the analysis of streamer protrusions.

CFD-DEM modelling of biofilm streamer oscillations and their cohesive failure in fluid flow.

Biofilm streamer motion under different flow conditions is important for a wide range of industries. The existing work has largely focused on experimental characterisations of these streamers, which is often time-consuming and expensive. To better understand the physics of biofilm streamer oscillation and their interactions in fluid flow, a computational fluid dynamics-discrete element methodmodel has been developed. The model was used to study the flow-induced oscillations and cohesive failure of single and multiple biofilm streamers. We have studied the effect of streamer length on the oscillation at varied flow rates. The predicted single biofilm streamer oscillations in various flow rates agreed well with experimental measurements. We have also investigated the effect of the spatial arrangement of streamers on interactions between two oscillating streamers in parallel and tandem arrangements. Furthermore, cohesive failure of streamers was studied in an accelerating fluid flow, which is important for slowing down biofilm-induced clogging.

Formation of Multiple Current Sheets in the Heliospheric Plasma Sheet

When spacecraft cross the heliospheric plasma sheet (HPS) that separates large-scale magnetic sectors of opposite directions in the solar wind, multiple rapid fluctuations in the sign of the magnetic radial component are often observed. These fluctuations indicate the change of the sign of the azimuth electric current density within the HPS. Possible mechanisms of formation of the multilayered current structure of the HPS are proposed. Within the context of a stationary magnetohydrodynamic solar wind model, we test one of the most popular hypotheses regarding an extension of multiple current sheets from the streamer belt oriented along the neutral line of the solar magnetic field into the solar wind. Properties of self-consistent distributions of the solar wind key characteristics are investigated as function of the fine structure of streamers. The results show that both single and multiple streamers can be the source of the set of a multilayered structure with alternating azimuthal currents. The significance of the results for the interpretation of solar wind data is discussed.

Investigations of positive streamers as quasi-steady structures using reduced order models

Single streamers are currently well simulated using detailed computational models. Most of these models are inhibitively complex to use for modelling many-streamer interactions in a streamer corona. This work develops reduced order models of single positive streamers in atmospheric pressure air that replicate the core macroscopic behaviour of detailed models while using a simpler physics representation. Models are developed using the 1.5D framework, with emphasis placed on solving the equations of motion in the streamer reference frame. The solution in this quasi-steady frame is shown to be a good representation of the instantaneous state of the streamer. Finally, a method of uniquely characterizing the instantaneous state of a streamer using its macroscopic parameters (velocity, radius, tip electric field and channel electric field) is developed. This characterization is interpreted graphically, with streamers treated as quasi-steady structures which evolve in time at a rate much slower than the time scale of electron transport. Previous work in the literature is shown to be well captured by this interpretation.

Probe Measurements of Parameters of Streamers of Nanosecond Frequency Crown Discharge

Investigations of the parameters of single streamers of nanosecond frequency corona discharge, creating a voluminous low-temperature plasma in extended coaxial electrode systems, are performed. Measurements of the parameters of streamers were made by an isolated probe situated on the outer grounded electrode. Streamers were generated under the action of voltage pulses with a front of 50–300 ns, duration of 100–600 ns, and amplitude up to 100 kV at the frequency of 50–1000 Hz. The pulse voltage, the total current of the corona, current per probe, and glow in the discharge gap were recorded in the experiments. It was established that, at these parameters of pulse voltage, streamers propagate at an average strength of the electric field of 4–10 kV/cm. Increasing the pulse amplitude leads to an increase in the number of streamers hitting the probe, an increase in the average charge of the head of a streamer, and, as a consequence, an increase in the total streamer current and the energy introduced into the gas. In the intervals up to 3 cm, streamer breakdown at an average field strength of 5–10 kV/cm is possible. In longer intervals, during the buildup of voltage after generation of the main pulse, RF breakdown is observed at Еav ≈ 4 kV/cm.

Broadband Processing of Conventional and Deep Tow Marine Streamer Seismic Data

We have developed WiBand, a de-ghosting and broadband processing method that can be applied to conventional streamer data acquired with single component streamers. We review two case studies of WiB...

Radial dependence of surface streamer-channel luminosity: experimental evidence of Gaussian radiative profiles in Ar and N2

Radial distributions of electronically excited species produced during surface streamer propagation were obtained by applying the Abel inverse transform to projected luminosities of single streamers. The streamers were generated in an argon and nitrogen surface coplanar dielectric barrier discharge at atmospheric pressure and their magnified microscopic images were registered with high time resolution. Selected regions of the projected luminosities were processed by the Abel inverse transform procedure based on the Hankel–Fourier method assuming cylindrical symmetry of the streamer channel. Projected as well as Abel-inverted profiles were fitted by Gaussian functions. It is shown that the projected profiles, in addition to the Abel-inverted ones, can be well approximated by the sum of two coaxial Gaussians with two different half-widths and weights. The sharper Gaussian component with higher weight characterizes the radial dependence of the primary cathode-directed streamer-channel luminosity. The second (broader) Gaussian component probably originates either from the pre-breakdown Townsend phase or from the second wave propagating towards the anode.

Saffman-Taylor streamers: Mutual finger interaction in spark formation

Bunches of streamers form the early stages of sparks and lightning but theory presently concentrates on single streamers or on coarse approximations of whole breakdown trees. Here a periodic array of interacting streamer discharges in a strong homogeneous electric field is studied in density or fluid approximation in two dimensions. If the period of the streamer array is small enough, the streamers do not branch, but approach uniform translation. When the streamers are close to the branching regime, the enhanced field at the tip of the streamer is close to 2Einfinity, where Einfinity is the homogeneous field applied between the electrodes. We discuss a moving boundary approximation to the density model. This moving boundary model turns out to be essentially the same as the one for two-fluid Hele-Shaw flows. In two dimensions, this model possesses a known analytical solution. The shape of the two-dimensional interacting streamers in uniform motion obtained from the PDE simulations is actually well fitted by the analytically known "selected Saffman-Taylor finger." This finding helps to understand streamer interactions and raises new questions on the general theory of finger selection in moving boundary problems.

Streamer propagation over a liquid/solid interface

A needle immersed in transformer oil is located just above a sheet of glass of which the opposite surface is conducting. By applying high-voltage (HV) steps, streamers which propagate radially with numerous branches are generated. The characteristics of these discharges (mean propagation velocity, final extension, and electric current and charge) are determined, and the measured charge is compared with the predictions of two simple models. A technique for guiding the discharges gives single quasi-rectilinear streamers, which allows a more accurate investigation. The mean electric current and the final charge indicate that the streamer channels are, to a first approximation, at the potential of the needle. The simple picture of a finite potential gradient along the channels is discussed with reference to the intermittent character of the discharge phenomena.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Potential distribution along single negative creeping streamer in transformer oil

A needle immersed in transformer oil is located just above a sheet of glass the opposite surface of which is conducting. By applying steps of high voltage, streamers are generated which propagate radially with numerous branches. A technique for guiding the discharges gives single quasirectilinear streamers, which allows more accurate investigation. The characteristics of these discharges (mean propagation velocity, final extension, electric current and charge) are determined and the measured charge is compared with the predictions of two simple models. This indicates that the streamer channels are not equipotential. A capacitive probe technique with associated optical fibres allows one to accurately determine the spatio-temporal variation of the potential along the streamer channels.

Waveforms of Prebreakdown Primary Streamers in a Short Positive Point-Plane Gap in Air

Current pulses of single and repetitive prebreakdown streamers have been measured in a positive point-plane gap in dry air at pressures of 3.33–26.7 kPa and at various overvoltages. The pulses of the single streamers exhibited a complex double-peaked form and, in contrast to the commonly held belief, differed significantly from those of the repetitive streamers. To explain the complex waveforms observed, a qualitative physical picture of the processes occurring during the streamer-cathode contact is outlined.

The development of single streamers started by laser light at high overvoltages in rare gases

Investigations on the temporal development of single streamers performed with an electro-optical streak camera are reported. The starting electrons being released by the focused radiation of a giant-pulse ruby laser, any influence of the electrodes can be excluded. In rare gases the propagation of streamers was observed in a homogeneous field (gap distance 11 cm), beginning from low E/p and up to higher E/p corresponding to overvoltages of several hundred per cent. The velocities of different streamer stages are measured. Below a certain E/p value they are relatively low (some 108 cm s−1) and proportional to E/p. Above this value, a considerable increase in streamer velocity up to 5 × 109 cm s−1 is observed.