Streamer Filaments Research Articles

Experimental study of transient surface charging during dielectric barrier discharges in air gap in needle-to-plane geometry

Motivated by a deeper understanding of plasma–surface interactions, this study presents experimental investigations into the transient surface charging process during dielectric barrier discharges (DBDs) in an air gap in a needle-to-plane geometry based on a combination of the Pockels method and a custom-designed ultrafast multi-frame imaging system. We realized three-frame observations of transient surface charge distributions, with a remarkable temporal resolution of 3 ns, during positive primary discharges and negative reverse discharges when applying a positive square-wave pulse. During the positive primary discharges at the rising voltage front, following the circular expansion of the streamer over the surface, multiple streamer filaments bifurcate simultaneously from the center, resulting in a branched positive surface charge distribution. Gradient surface charge densities are observed along the channel with higher charge densities at the head, which gradually evolve into a uniform distribution along the channel as the streamers approach stagnation. No lateral expansion of positive charges is observed across the channel under the present condition. In the case of negative reverse discharges occurring at the falling edge of the voltage pulse, the neutralization of residual positive surface charges and the accumulation of negative surface charges occur simultaneously in the central region. The deposited negative surface charges exhibit a progressively expanding circular distribution characterized by increasing charge density and radius. The propagation dynamics of surface streamers and the fields induced by surface charges are investigated and discussed based on the spatio-temporal surface charge measurements. Further study suggests that the surface streamer is not driven by the over-accumulation of surface charges, but rather by the space charge field above the dielectric. The presented quantitative measurements can be used for detailed validation of DBD simulations and offer deeper insights into plasma–surface interactions.

Production of runaway electrons and x-rays during streamer inception phase

Streamers play a key role in the formation and propagation of lightning channels. In nature streamers rarely appear alone. Their ensemble behavior is very complex and challenging to describe. For instance, the intricate dynamics within the streamer zone of negative lightning leaders give rise to space stems, which help advance the stepped-leader. Another example is how the increasing morphological complexity of sprites can lead to higher sprite current and greater energy deposition in the mesosphere. Insights into the complex dynamics of a streamer corona can be obtained from laboratory experiments that allow us to control the conditions of streamer formation. Based on simultaneous nanosecond-temporal-resolution photography, and measurements of voltage, current, and x-ray emissions, we report the characteristics of negative laboratory streamers in 88 kPa of atmosphere. The streamers are produced at peak voltages of 62.2 ± 3.8 kV in a point-to-plane discharge gap of 6 cm. While all discharges were driven to the same peak voltage, the discharges occurred at different stages of the relatively slow voltage rise (177 ns), allowing us to study discharge properties as a function of onset voltage. The onset voltage ranged between 24 and 67 kV, but x-ray emissions were observed to only occur above 53 kV, with x-ray burst energies scaling quadratically with voltage. The average delay between the current pulse and x-ray emission was found to be 3.5 ± 0.5 ns, indicating that runaway electrons are produced during the streamer inception phase or no later than the transition stage, when the inception cloud is breaking into streamer filaments. During this short time span, runaway electrons can traverse the gap, hit the ground plate and produce bremsstrahlung x-ray photons. However, streamers themselves cannot traverse more than 3.5 mm across the gap, which supports the idea that runaway electron production is not associated to streamer connection to the ground electrode.

Study on Townsend first ionization coefficient in a streamer filament of the pulsed electric discharge in water

Streamers in water usually consist of several weakly ionized gaseous filaments. It is important to understand the ionization and excitation processes occurring in a streamer filament. In this paper, we studied the Townsend first ionization coefficient α in a streamer filament. The emission images of streamers in water were acquired by using an ultra-high speed camera system. Based on the emission images, it was found that the luminance of a streamer filament decreased exponentially as the distance increased from the anode tip. The Townsend first ionization coefficient in the streamer filament in water was then estimated by the emission profile method. It was found that the Townsend first ionization coefficient in a streamer filament was 5.3 cm−1 on average and did not depend on the water conductivity or the applied voltage of interest in the present paper. The Townsend first ionization coefficient estimated by the emission profile method is in agreement with that deduced from the electron density profile of the streamer filament in water, indicating that the emission profile method for measuring the Townsend first ionization coefficient can be applicable to the streamers in water. We also estimated the reduced Townsend first ionization coefficient α/N in a streamer filament in water and found that the obtained value for α/N was four orders of magnitude lower than those obtained from the glow discharge in low pressure water vapor, but at least ten orders of magnitude greater than those predicted by the empirical formulas.

Re-illumination and pause behavior of streamer filament of streamer discharge in water

The streamer discharge in water is a main object for studying the basic physical and chemical processes of an electric discharge in water. In this paper, the re-illuminations and the pause behaviors of a streamer filament of the microsecond pulsed streamer discharge in water with different conductivities (20 to 800 µS/cm) are studied by adopting a needle-plate electrode system and an ultra-high-speed camera system. The cause why the re-illumination of the streamer filament is difficult to detect by using the camera system as the water conductivity gets higher is discussed. It is found that the re-illumination alternately occurs among different streamer filaments, and two modes of the re-illumination are identified: one is that the whole filament quenches and then illuminates again; the other is that only the tip part of the filament quenches and then resumes the illumination. The appearance rate of the re-illumination of the streamer filament decreases rapidly as the water conductivity increases, and then drops to zero as the water conductivity exceeds 540 µS/cm. Within a water conductivity range of 20–800 µS/cm, the appearance rate of segmentation in shockwave pattern of the streamer filament is larger than 65%, indicating that the pause is a common behavior of the streamer filament. By measuring the difference in radius between two adjacent shockwave segments, the average pause period of the streamer filament is estimated at 157 ns, which is almost not affected by the water conductivity. As the water conductivity increases, the light intensity of the streamer filament increases rapidly. The light intensity of the streamer filament cannot decay to the noise level of the camera system during the pause period of the streamer filament as the water conductivity exceeds 350 µS/cm. The streamer filament produces glow on the light emission image, which makes it difficult to distinguish the “extinction and illumination” process by the camera system.

Analysis of light emission and Schlieren from short gap high voltage streamers representing lightning impulses

Light emissions and Schlieren structures were simultaneously observed from streamers produced by tens of kilovolts 1.2/50 μs impulses, representing the high voltage component of lightning, applied across a 4 cm air gap between a variety of electrode geometries and a ground plane in an unconfined environment. The results demonstrated that the light emissions and Schlieren structures coincide along the same streamer filaments but on different timescales; the light existing only during the microsecond timeframe impulse whereas the Schlieren continued to develop into the millisecond timeframe, moving towards the centre of the air gap whilst diffusing into the surrounding air within 100 ms. If an electrical breakdown did occur, the Schlieren structures outside the arc remained visible. Streamer formation theory for high voltage impulses is subsequently refined to include the observed Schlieren mechanism.

Initial pressure of the shock front launched by a streamer discharge in water

A streamer discharge in water launches a chain of isolated shock fronts as the streamer filament propagates through the water. The initial pressure of the shock front is a good estimation of the initial pressure of the streamer filament. We observed the temporal evolution of the shock front launched by a microsecond pulsed positive streamer discharge in water and investigated the initial pressure of the shock front. We discovered a method for determining the radius of the shock front measured from different discharge pulses in an accurate time sequence and obtained the temporal evolution of the shock front radius. Based on the measured temporal evolution of the shock front radius, we obtained that the initial pressure of the shock front is ∼0.6 GPa. We found that applied voltage and water conductivity do not affect the initial pressure of the shock front within the range studied.

Converging Luminosity in Column‐Sprite Filaments

AbstractSprites are electrical discharges in the mesosphere powered by the quasi‐electrostatic field following a cloud‐to‐ground lightning stroke. They are luminous structures with tendrils formed by a space charge wave with high electric fields that ionizes the atmosphere, excites optical emissions and leaves a trail of enhanced densities of electrons, and negative and positive ions. The duration of the luminous streamers in sprites is usually ∼1–10 ms. In this study, we present observations of sprite streamer filaments where the upper and lower ends continue to glow for more than 80 ms while the two ends slowly converge and fade. Simultaneous magnetic observations suggest that the field in the mesosphere is maintained at a high value by the long‐lasting continuing current of the causative stroke. We propose that the observation is a signature of currents in the filaments fed by electrons detached from negative ions at the lower origin.

Elongation and branching of stem channels produced by positive streamers in long air gaps

The elongation and branching of long positive spark discharges in the laboratory and in lightning have been attributed to the formation of thermalized channels inside a diffuse, glow-like streamer section at the leader head. It is experimentally shown here that the structured morphology of streamers produce low-density stem channels that elongate and branch well before a new leader channel section is formed. These non-thermalized stems are also shown to develop ahead of a developing leader channel. These findings are based on high-speed photography and Schlieren imaging used to visualize both the morphology of streamer filaments and stem channels. Numerical analysis is also performed to estimate the axial temperature and density of the stem channels. A stem-driven mechanism for the propagation and branching of positive long air gap discharges is proposed and discussed based on the presence of not-yet thermalized, low density channels formed by streamer ensembles at the leader head.

Study on the residual gaseous filament of microsecond pulsed positive streamer discharge in water

The residual gaseous filament soon after a streamer discharge in water contains a wealth of information closely related to the streamer propagation. We have investigated the residual gaseous filament of microsecond pulsed positive streamer discharge in water by using a shadowgraph imaging technique. Based on the obtained shadow images, we measured the average gas density, the average radius, and the average length of the residual gaseous filament. We found that the average gas density has no significant dependence on the applied voltage, but decreases from 883 to 610 kg/m3 as the water conductivity increases from 20 to 476 μS/cm. The amount of gas produced in a unit length of the streamer filament and the energy consumed for producing a unit mass of gas in the streamer filament also have no dependence on the applied voltage, but increase as the water conductivity increases. In our experiments, the energy consumed for producing a unit mass of gas in the streamer filament is far smaller than the latent heat of vaporization of liquid water, indicating that the gas produced in the streamer filament during the discharge is unlikely produced through a heating process.

Gap, shadows, spirals, and streamers: SPHERE observations of binary-disk interactions in GG Tauri A

Context.A large portion of stars is found to be part of binary or higher-order multiple systems. The ubiquity of planets found around single stars raises the question of whether and how planets in binary systems form. Protoplanetary disks are the birthplaces of planets, and characterizing them is crucial in order to understand the planet formation process.Aims.Our goal is to characterize the morphology of the GG Tau A disk, one of the largest and most massive circumbinary disks. We also aim to trace evidence for binary-disk interactions.Methods.We obtained observations in polarized scattered light of GG Tau A using the SPHERE/IRDIS instrument in theH-band filter. We analyzed the observed disk morphology and substructures. We ran 2D hydrodynamical models to simulate the evolution of the circumbinary ring over the lifetime of the disk.Results.The disk and also the cavity and the inner region are highly structured, with several shadowed regions, spiral structures, and streamer-like filaments. Some of these are detected here for the first time. The streamer-like filaments appear to connect the outer ring with the northern arc. Their azimuthal spacing suggests that they may be generated through periodic perturbations by the binary, which tear off material from the inner edge of the outer disk once during each orbit. By comparing observations to hydrodynamical simulations, we find that the main features, in particular, the gap size, but also the spiral and streamer filaments, can be qualitatively explained by the gravitational interactions of a binary with a semimajor axis of ~35 au on an orbit coplanar with the circumbinary ring.

A Method for Measuring the Propagation Speed of Streamer Discharge in Water Using its Shockwave Pattern

As a streamer discharge develops in the water, it releases spherical shockwaves from the head of a streamer filament, leaving a chain of discrete shockwaves around the filament. The position of the center of each shock wave is a good indicator of the position of the filament head at a certain moment. The velocity of shockwaves released from the filament head decays to the acoustic velocity (1450 m/s) in water after spreading for ~0.5 mm. Based on these characteristics of shockwaves, we developed a method for measuring the propagation speed of a streamer discharges in water, enabling to investigate the difference in propagation behavior of many filaments generated in a single discharge pulse. Based on this method, we investigated the difference in propagation behavior of many filaments generated in a single discharge pulse at water conductivity of $20~\mu \text{S}$ /cm and 26 kV. We found that the filaments generated in the same discharge pulse have significantly different propagation speeds, and confirmed that the propagation speed of a filament will significantly slow down after a longtime pause.

On the inactivation of Bacillus subtilis spores by surface streamer discharge in humid air caused by reactive species

The inactivation of Bacillus subtilis (ATCC 6633) spores deposited on a filter membrane was studied by using low-temperature plasma produced via surface dielectric barrier discharge. Spore samples were carefully prepared to avoid the formation of cell aggregates, and their inactivation was induced by multiple surface streamer discharge driven in a coplanar dielectric barrier discharge electrode geometry by an amplitude-modulated AC high voltage waveforms in humid air at atmospheric pressure. At a discharge duty cycle of 0.4, the surface dielectric barrier discharge is characterised by an average total power of 1.7 W (power density 1.5 W cm−2 and energy density ∼0.3 Wh l−1) and a low gas temperature of the plasma filaments of about 320 K. The spores were exposed by placing a sample holder at a fixed distance of 3 mm from the electrode surface covered by plasma filaments. Particular attention was paid to identifying sporicidal agents employed in the process of inactivation. Since treated samples did not come into direct contact with the streamer filaments and excessive heating was excluded thanks to the low energy density, our results indicate that the spores were inactivated mainly by reactive oxygen and nitrogen species such as O3, H2O2 and NO2–. Discharge-induced damage of the spore structure was evidenced via the detection of dipicolinic acid and leaking of intracellular components. We therefore conclude that B. subtilis spores were inactivated chemically, probably due to failure of the coat structure or membrane of the spore.

The Characteristics of Dielectric Barrier Discharge Plasma Under the Effect of Parallel Magnetic Field

The dielectric barrier discharge plasma (DBDP) has been produced by the dielectric barrier discharge (DBD) reactor that has been established to operate at atmospheric pressure. DBD reactor consists of two parallel metal electrodes, one is a high-voltage (HV) electrode with a circular iron plate shape and the other is a rectangular steel plate, and the ground electrode with a dielectric material covered one of them. The ac power supply has been used to provide the DBD circuit with ac voltage up to 15kV peak value, 50 Hz. The dielectric material used, which covered one reactor electrode, is a glass slab. The gap distance of the discharge is fixed at 3 mm. Argon (Ar) and air gases have been used in the experiment at 20SCFH (standard cubic foot per hour) flow rate, and a sinusoidal voltage is applied to the electrodes to generate a homogenous DBDP. The parameters of the discharge such as discharge current, discharge voltage, consumed power of discharge, streamer filament, and emitted plasma light have been studied. A magnetic field, with the direction parallel to the electric field, is applied to the ac and DBD, using Ar and helium (He) gases. The effect of the parallel magnetic field for generating the DBDP has been investigated. It indicates that the discharge current in the antiparallel magnetic field is lower than that without the magnetic field. Also, the DBDP in the magnetic field is more luminous than that without the magnetic field. It further indicates that the area of Lissajous figure of the DBD in the presence of antiparallel magnetic field is little reduced than that without the magnetic field. It could be concluded that the effect of the magnetic field influenced DBDP progress.

Evolution of N(4S) atoms produced under nitrogen streamer conditions: time-resolved TALIF study at reduced pressures

The evolution of N(4S) species produced by filamentary streamer discharge was investigated by two-photon absorption laser-induced fluorescence technique. A triggered single streamer filament was periodically produced in pure nitrogen at pressures of 20, 50, 100 and 200 Torr, and N(4S) species were monitored in the center of the discharge gap during the streamer formation, decay and afterglow. Under all of the investigated pressures, the population maxima of nitrogen atoms were observed in a microsecond timescale; i.e. well after the extinction of the streamer event. The concentration of nitrogen atoms increased with pressure. The maximum N(4S) concentration measured at 20 Torr was about 1013 cm−3, while it exceeded 1015 cm−3 at 200 Torr. After reaching maxima, atomic densities exhibit hyperbolic decay, which was followed up to the post-discharge time of 0.5 ms. Our results show that only a small part of N(4S) is produced during the streamer transit or within 200 ns after the transit (20%–25% increase with respect to the background density), while most of the nitrogen N(4S) atoms are produced during the streamer channel decay in timescales from units to several tens of microseconds.

Shock wave release behavior of a pulsed positive streamer discharge in water

We performed experimental observations to distinguish an individual shock front released by a pulsed positive streamer discharge in water. The step length between two adjacent shock wave events was measured and its dependence on water conductivity and applied voltage was investigated. We found that the step length between two adjacent shock wave events is ∼160 μm, independent of both water conductivity and applied voltage, within the range studied. Based on the bubble theory of electrical discharge in liquid, we concluded that the individual shock front is released by the electrical breakdown of a gaseous micro-segment near the head of a streamer filament in water. After launching a shock wave, the filament head pauses and waits for another gaseous micro-segment to form in front of it.

Propagation behavior of microsecond pulsed positive streamer discharge in water

The propagation behavior of microsecond pulsed positive streamer discharge in low-conductivity water was investigated via photography and shadowgraph imaging. Eight successive images were acquired from a single discharge pulse, allowing for the study of the overall streamer propagation behavior; both the emission and the shadow emission-combined images of the streamer discharge were obtained. The experimental results confirmed that the streamer at the later stage of propagation is weak in illuminance and has a propagation velocity of ∼2 km/s, which is slower than that of the secondary streamer (∼30 km/s) but as fast as that of the primary one (∼2 km/s). The streamer filaments propagated stepwise in water with two different behaviors: (1) a filament head launches a shock wave, quietly moves a short distance (or short time) ahead, and then launches the next shock wave and (2) a filament head pauses for a relatively long time while the filament illuminance decays, then restarts launching shock waves and moving ahead, and the decayed filament illuminates again. These pause and reactivation behaviors randomly alternate among the filaments, resulting in different shock-wave patterns and filament illuminance. The drop of the propagation velocity from ∼30 km/s to ∼2 km/s and the many irregular spikes in the discharge current waveform can be associated with this alternation of pause and reactivation behaviors of the filament heads.

Numerical and simulation model of the streamer inception at atmospheric pressure under the effect of a non-uniform electric field

This paper introduces a new approach of breakdown theory using a computer simulation program. Based on the concept of Minimum Stressed Volume Zone (MSVZ) a simulation model of the streamer inception in air using the needle-to-plane configuration is presented. The streamer methodology first requires accurate field computation. The non-uniform electric field distribution is conducted using Poisson's equation, which is solved based on the concept of the Finite Element Method (FEM). The advantage of using FEM in the simulation of streamer progression is the ability to simulate the potential solution for a non-uniform field gap with the external injection of charges at the beginning of each streamer step. The simulation confirms that the streamer radius is 30μm, with 108 electrons at the streamer tip, then within MSVZ the radius of the streamer filament enlarges to a value of 120μm, so, the radius of the streamer leader near stressed high voltage electrode becomes constant at 120μm. After that, when the simulated field (E) at the main leader head is supply energy with eight times of critical electric field (Ecr) value, i.e. the electric field at its tip reaches a value of E/Ecr=8 the streamer starts to branch; thus, a value of Ecr of 4.5kV/cm required for streamer initiation. The new simulation model is assessed via a comparison with other experimental and computational works.

Positive streamer in gases: Physical approach from low to high energies

Approach to the breakdown theory in gases based on an energy basis has been presented. The minimum stressed volume zone (MSVZ) has been used as a concept to deal with the positive streamer process in gases. These processes start at low energy values under critical electric field (Ecr) of 4.5 kV/cm. In this MSVZ, several filament streamers spread within it with a minimum streamer filament radius (MSFR) 30μm (nearly about 400 filaments). Under Ecr, an associative detachment mechanism in these filaments starts. This detachment of electrons has occurred between O2− and O2 which leading to increase the number of electrons inside the filaments. Also, the temperature increases inside the filaments due to the exothermic reaction of O− and N2. Furthermore, increases of the temperature occurred due to the collision inside these filaments. Consequently, the filament radius enlarges too within MSVZ with a value of 120μm. This will cover the entire MSVZ with 25 filaments instead of 400 which has been considered as a new breakdown index in gases (n). Therefore, a new breakdown equation has been introduced relating energy, temperature, electric field, streamer velocity, and streamer radius with index n. At low energy level, the streamer radius changes with the stressed electrode diameter, applied field and temperature changes inside the filament till it reaches nearly 7500K. However, under high energy level when the ionized species inside filaments increase, a main ionized streamer path (leader) emanates from the MSVZ toward the opposite electrode. The increased ionized species convert the medium inside this leader streamer stem to plasma state with high temperature ≅20000K. Under arc condition, the leader streamer radius nearly becomes constant at 120μm near stressed electrode and the minimum streamer velocity at Ecr reaches a value of 105m/s and n×105m/s for higher electric fields. In addition, the calculated minimum length of initiating streamer (l0) given to be nearly 30μm which is equal to its radius. The streamer starts to branch when the calculated field (E) at its head is sufficient to supply energy eight times of Ecr value.

Bubble size measurements in different acoustic cavitation structures: Filaments, clusters, and the acoustically cavitated jet

Acoustic cavitation typically forms a variety of bubble structures of generally unknown and broad size distributions. As the bubbles strongly oscillate, their (equilibrium) sizes are not directly observable. Here, a method is presented to experimentally determine the size distribution in bubble populations from high-speed imaging of the bubbles in oscillation. To this end, a spherical bubble model is applied in statistical fashion. This technique is applied to several experimentally realized bubble structures: streamer filaments, clusters, and a peculiar structure we report here on, the acoustically cavitated jet. It is generated by the sonication of a submerged jet to produce abundant cavitation at low flow velocities. Our analysis is complemented by numerical exploration of the hydrodynamic and acoustic properties of the experimental configuration in which the observed bubble structures are formed.

Evolution of N2(A3) in streamer discharges: influence of oxygen admixtures on formation of low vibrational levels

The formation of N2(A3) metastable species, produced by cathode-directed streamer discharge, was investigated using the technique of laser-induced fluorescence. A triggered single streamer filament was periodically produced in pure nitrogen (and in nitrogen with admixtures of oxygen) at total pressure of 50 Torr and metastable species were monitored during the streamer channel decay in the centre of the discharge gap.We revealed the dynamics of individual vibrational (v = 0–8) levels of N2(A3) for various oxygen admixtures (0–20%). In pure nitrogen, the observed evolution of the N2(A3) during the decaying streamer channel is evidence of initial vibrational relaxation of high vibrational levels towards the v = 2 and 3 levels, followed by a delayed increase of terminal (v = 0, 1) levels. A calibration procedure based on the rate of energy-pooling processes was used to place all detected vibronic levels in pure nitrogen on the absolute scale. Population maxima exceeding 1 × 1014 cm−3 were fixed for the v = 2 and 3 vibrational levels, while the lowest v = 0 level reaches only 3 × 1013 cm−3.Populations of v = 2–5 vibrational levels were also estimated for N2 + O2 mixtures after scaling of laser-induced fluorescence signals obtained at various oxygen admixtures. The total N2(A3) population in an air-like mixture is formed mainly by v = 3–4 vibronic levels with the population maximum of ~3 × 1013 cm−3 fixed at the shortest analyzed delay. This observation, together with the fact that we were unable to detect v = 0 and 1 levels (fluorescence signals below detection threshold), gives a strong evidence of the inhibition of Δv = 2 vibrational relaxation towards terminal v = 0 and 1 levels, causing much lower populations of the lowest v = 0–1 levels.By analyzing data obtained in pure nitrogen and in nitrogen with three different oxygen admixtures, we have estimated the quenching rate constants of N2(A3, v) + O2 and N2(A3, v) + N2 processes for v = 2–6 vibrational levels which are consistent with data from the literature. Quenching rate constants for nitrogen reflect the N2(A3, v) → N2(A3, v − 2) relaxation process, however the v = 2 rate constant shows a much larger value compared with the published data.