Streamers In Water Research Articles

Developing characteristics of the positive streamer in water with microsecond pulse voltage

Underwater discharge is the typical method used to generate plasma in a liquid phase environment and is employed in many engineering applications. This study analyzes the formation and development process of the positive streamer in water under microsecond voltage. The effects of voltage amplitude, liquid conductivity, and the presence of bubbles on the underwater discharge characteristics are analyzed by establishing a two-dimensional finite element simulation model of a needle-plate gap. The simulation results show that the electron density of the streamer in water can reach 1023 m−3, and as the applied voltage amplitude increases, the development speed of the streamer increases and the head of the stream bifurcates. Moreover, when the conductivity of the water is high, the development speed of the streamer and the density of charged particles increase. Furthermore, the presence of bubbles significantly impacts the development of the discharge morphology, causing the channel to have multiple bifurcations.

Insight into the dynamic evolution behavior of subsonic streamers in water and their voltage polarity effect

Due to the complex interaction between liquid, gas, and plasma, the pre-breakdown process in water under quasi-static moderate electric fields, namely the development of subsonic streamers, was unclearly understood so far. In this paper, the dynamic evolution behavior of subsonic streamers and their voltage polarity effects were investigated. It was indicated that the whole streamer development process can be divided into two successive stages: bottom-up period characterized by root spherical expansion and OH (309 nm) emission line; top-down period characterized by head burst expansion and Hβ (486 nm), Hα (656 nm), and O (777 nm) emission lines. Further analysis revealed that the magnetic pinch effect on the internal plasma distribution determines the expansion mode of the streamer. The low capture energy of the solvated electron and local space charge accumulation make the positive streamer propagate faster at a low voltage level. However, the limited carrier resource and relatively divergent internal plasma distribution (weak magnetic pinch effect) hinder the propagation acceleration of the positive streamer with the applied voltage. Thus, the voltage polarity effect variation can be observed at high voltage levels. Finally, a novel framework model was proposed to depict the dynamic evolution behavior of subsonic streamers. Our results can provide a deeper insight into the electrohydrodynamics of dielectric fluid and promote the relevant industry applications.

Determination of the Pre-breakdown Electric Field near a Cathode Streamer in Water

The electrical strength of liquids under the influence of an impulse voltage is determined by the streamer initiation and propagation processes. For a streamer to grow and develop, a strong field must be present near its head. The field strength near a cathode streamer developing in water is estimated. Computer simulation of electro-optical studies of the pre-breakdown stage in water in the submicrosecond range that have already been carried out is performed, and the relevant processes are analyzed. During the simulation, all geometric, optical, and electrophysical conditions of the experiment were fully reproduced. A cathode streamer developing in a system of spherical electrodes in water is considered. The mathematical simulation was carried out using the finite element method inside the measuring cell volume. The experimental kerrogram intensity distribution field was numerically reproduced using the proposed model. The obtained 2D matrix was visualized, and the calculated kerrogram was compared with the experimental one. Calculations with a step of 2.5–5 µm allowed the fragments of interest to be analyzed in detail. Two streamer model versions were considered: a simplified one and a modified one, with the latter being closer to the real streamer kind. Computer simulation of the field near the streamer has shown the following. If the permittivity is assumed to be independent of the field strength, its value at the cathode streamer head does not exceed 1.9 MV/cm. At the same time, if the permittivity non-linearity is taken into account, the field strength estimated using both the streamer model was found to be around 2.4–3.1 MV/cm. It is shown that, despite the fact that the field strength somewhat exceeds the threshold value equal to 3 MV/cm, at which the dipole saturation state is possible, this does not lead to a noticeable change in the result. Thus, it has been confirmed that the field strength near the cathode streamer is essentially lower compared with that near the anode streamer.

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.

Temperature measurement of initiation process of microsecond pulsed discharge in water by quantitative schlieren method

In the initiation process of microsecond pulsed discharge in water, Joule heating is believed to be responsible for the formation of a low-density region. Then, electron avalanche can be easily induced in this low-density region. The aim of this paper is to study the feasibility of the quantitative schlieren method in the temperature measurement of water in the initiation process of microsecond pulsed discharge. A lens-type schlieren system was applied in the measurement of the radial temperature around the tip of the electrode. Finite element analysis was used to simulate the electric field and temperature during the discharge. Measurement results of the schlieren quantitative schlieren method are compared with the simulation results of finite element analysis in terms of temperature and energy conversion. The comparison shows that most electrical energy is consumed in Joule heating of water in the initiation process of microsecond pulsed discharge. In the region with no thermal fluid, the temperature distributions obtained by the quantitative schlieren method and finite element analysis are in good agreement. This work is helpful in the quantitative study of the role of Joule heating in the initiation of the streamer in water.

Pulsed Discharge Plasmas in Contact with Water and their Applications

Large‐volume pulsed discharge plasmas have been studied to gain insight on their phenomena and to realize their applications in various areas, such as sterilization, water treatment, and cancer treatment. Since 2000, when a review paper titled ‘Streamer Discharges in Liquids and their Application’ was published, a number of related papers have been reported in the literature; these reports are summarized in this review paper. Four types of pulsed power‐generated large‐volume discharges in contact with water are presented as representative examples. The discharges are reviewed from the perspectives of the physical and chemical phenomena, as follows: the initiation of filamentary and bush‐like streamers in water; the propagation and extinction of streamer discharge plasmas; the production, behavior, and collapse of bubbles; the measurement of temperature, density, electric field, shock wave, and radicals; the wide discharge on the water surface. Finally, the applications of these large‐volume discharge plasmas are described. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC.

A Study on the Primary Mode of Pulsed Positive Streamer Discharge in Water

The primary mode of the pulsed positive streamer discharge in water has been studied by the shadowgraph imaging techniques utilizing an ultrahigh speed frame camera system. The propagation speed and the flare angle of the fan-shaped primary streamer were measured and their dependence on the water conductivity and the applied voltage were investigated. The initial propagation speed of the primary streamer is about 4–6 km/s and rapidly decreases to about 1.5 km/s. The water conductivity and the applied voltage have no significant influence on the propagation speed of the primary streamer. The flare angle of fan-shaped structure of the primary streamer ranges from 150° to 210°, and increases with increasing the applied voltage. In the voltage range 15 to 17 kV, the water conductivity has no significant influence on the flare angle of the fan-shaped structure, but as the applied voltage increases beyond 18 kV, the flare angle of the fan-shaped structure decreases with increasing the water conductivity. Based on the plasma discharge in a bubble and the fact that the radius of primary streamer is about 200–400 $\mu \text{m}$ , the primary streamer in water seems to be incepted by the positive ions at the head of the discharge filament generated in the microbubble, which penetrates into the water through spattering process and forms a gaseous trajectory in the bulk water.

Propagation and branching process of negative streamers in water

Streamer discharges in water, which are a pre-breakdown phenomenon caused by underwater discharge, have gained attention because of the development of plasma-based technologies for water purification and medical treatment. In this study, a single-shot pulsed negative high voltage of –18 to –21 kV with a rise time of 100 ns was applied to a needle electrode in ultrapure water, and streamers were continuously imaged using an optical system combining a high-speed video camera, a streak camera, and an ultra-high-speed camera with a microscope lens. These observations were synchronized with the applied voltage and the discharge current to investigate the effect of the discharge on streamer propagation. Negative streamers propagated with a velocity of 820 ± 50 m/s in the presence of pulsed currents, and 100 ± 20 m/s when pulsed currents were absent. The streak camera detected light emission when the pulsed currents appeared, and it was also observed that 1480 m/s pressure waves were generated during streamer propagation. Furthermore, we developed a simple new method of imaging weak density changes similar to those detected using the Schlieren method or Mach–Zehnder interferometer. This method simply involves inserting a pair of polarizing plates on the optical axis, so that the pressure waves can also be imaged in two-dimensional photographs. Our results indicated that the pressure waves were generated from the propagating streamer head when the pulsed currents appeared in the waveform. Analysis of temporal resolution with nano-second order clarified that the branching phenomenon occurred at different times resulting in the branching streamer propagation with different directions.

Subsonic streamers in water: initiation, propagation and morphology

In this paper, we focus on the development and morphology of underwater subsonic streamers in a pin–plane electrode configuration. The processes of initiation, propagation and branching of underwater subsonic streamers are studied in detail, and the effects of applied voltage amplitude and polarity are discussed. The results show a similar initiation process for both polarities including the formation of a low-density shadowed region, an initial bubble and streamers. Joule heating is thought to be the dominant intrinsic dynamic in the initiation process. There is a distinct difference in the propagation mode and the morphology between positive and negative subsonic streamers. Positive subsonic streamers develop in the form of a bubble cluster with a constant low speed of 20–80 m s−1, but the development mode transition from bubble cluster to secondary streamer occurs at the final stage of streamer propagation. On the other hand, negative subsonic streamers develop in the form of tree-like structures with a thinner root and propagate faster at a speed of 100–370 m s−1, and the propagation velocity increases significantly near the final breakdowns. A specific phenomenon has been observed in which the breakdown voltage of positive polarity can be higher than that of negative polarity in micro-second underwater discharge. The above results indicate that the polarity effect of subsonic streamers is quite different from that of the supersonic streamers.

A comparative summary on streamers of positive corona discharges in water and atmospheric pressure gases

From an intention of summarizing present understandings of positive corona discharges in water and atmospheric pressure gases, we tried to observe streamers in those media by reproducing and complementing previously reported results under a common experimental setup. We used a point-to-plane electrode configuration with different combinations of electrode gap (7 and 19 mm length) and pulsed power sources (0.25 and 2.5 e s duration). The general features of streamers were similar and the streamer-to-spark transition was also observed in both the media. However, in the details large differences were observed due to inherent nature of the media. The measured propagation speed of streamers in water of 0.035 × 10 6 ms −1 was much smaller than the speed in gases (air, N 2 and Ar) from 0.4 to 1.1 × 10 6 ms −1 depending on species. In He the discharge looked glow-like and no streamer was observed. The other characteristics of streamers in gases, such as inception voltage, number of branches and thickness did also depend on the species. The thickness and the length of streamers in water were smaller than those in gases. From the volumetric expansion of a streamer in water after the discharge, the molecular density within the streamer medium was estimated to be rarefied from the density of water by about an order of magnitude in the active discharge phase. We derived also the electron density from the analysis of Stark broadened spectral lines of H and O atoms on the order of 10 25 m −3 at the earlier time of the streamer propagation. The analyzed background blackbody radiation, rotational temperature of OH band emission and population density of Cu atomic lines yielded a consistent temperature of the streamer medium between 7000 and 10 000 K. Using the present data with a combination of the analysis of static electric field and previously reported results, we discuss the reason for the relatively low streamer inception voltage in water as compared to the large difference in the medium density between water and gases.

Estimations of the electric field strength of nonelectrode streamers in water

The electric field strength of a nonelectrode streamer in water has been estimated using data of electro-optical measurements. The microscopic field strength on the tips of the anode-directed and cathode-directed streamers is approximately 10 MV/cm, while the macroscopic field strength of cathode-directed and anode-directed streamers near the streamer zone is approximately 300 kV/cm and 2 MV/cm, respectively.

Spatiotemporal analysis of propagation mechanism of positive primary streamer in water

Currently, further clarification of pre-breakdown phenomena in water such as propagation mechanisms of primary and secondary streamers are needed because applications of aqueous plasma to environmental and medical treatments are increasing. In this study, a series of primary streamer propagations in ultrapure water was visualized at 100-Mega frames per second (100 Mfps) in the range of 400 μm square using an ultra high-speed camera with a microscope lens when a single-shot pulsed positive voltage was applied to a needle electrode placed in a quartz cell. Every observation was synchronized with the waveforms of the applied voltage and the discharge current. The primary streamer, having many filamentary channels, started to propagate semi-spherically with a velocity of about 2 km/s when the pulsed currents occurred. Although most filamentary channels disappeared 400 ns after the beginning of the primary streamer, a few of them continued propagating with almost the same velocity (about 2 km/s) as long as the repetitive pulsed currents flowed. Shock waves were iteratively generated and streamer channels were formed while the repetitive pulsed currents were flowing. Thus, we concluded that the positive primary streamer in water propagates progressively with each repetitive pulsed current.

Streamers in Water and Along the Insulator Surface in a Wire–Cylinder Gap

Two types of streamers in water, namely, volume streamers and surface streamers, have been observed in a coaxial discharge gap where the central anode wire was surrounded by a perforated dielectric cylinder inside the discharge gap. The speed of surface streamers (~ 100 km/s), which develops along the solid-liquid interface on the outer surface of the dielectric cylinder, exceeds that of the volume streamers (~ 42 km/s) by more than a factor of two. The axial surface streamers serve as sources for numerous thin secondary radial volume streamers.

Aspects of Plasma in Water: Streamer Physics and Applications

AbstractPhysical mechanisms for streamer initiation and electrical breakdown in water are discussed. The focus is on various processes that could lead to the inception and propagation of streamers in water. Physical details associated with observed disparities in breakdown at the anode versus the cathode, differences in luminosity, and variations in streamer propagation speeds are discussed. Creation of a localized low‐density region near the site of a field enhancement is shown to be essential for streamer initiation. Finally, the use of a trigatron‐type electrically triggered switch with a triple‐point geometry is shown to be a low‐cost, compact alternative to laser (or acoustic) switching. Such controlled breakdown could be used for various applications including water purification and bio‐decontamination.magnified image

Streamers in water and other dielectric liquids

Experimental results on the inception and propagation of streamers in water generated under the application of high electric fields are reviewed. Characteristic parameters, such as breakdown voltage, polarity of the applied voltage, propagation velocities and other phenomenological features, are compared with similar phenomena in other dielectric liquids and in gases. Consequently, parameters that are expected to influence the development of streamers in water are discussed with respect to the analogous well-established models and theories for the related mechanisms in gases. Most of the data support the notion that an initial low-density nucleation site or gas-filled bubble assists the initiation of a streamer. Details of this theory are laid out explaining the observed differences in the breakdown originating from the anode versus the cathode locations. The mechanisms can also be applied to streamer propagation, although some observations cannot be satisfactorily explained.

Time Evolution of Pulsed Streamer Discharge in Water

Pulsed positive streamers in water were observed in the needle-to-plate electrode configuration with a gap of 20 mm. The characteristics of streamer propagation were investigated by using an intensified charge-coupled device camera. A few branching with a channel diameter of ~250 mum was observed. The streamers in water did not reach the plate electrode at the stable operation voltage range below the transition to spark. Under the present experimental condition, the propagation velocity of the streamer head was 3 times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> m/s, which is one order of magnitude lower than that of the streamer in air.

Degradation of organic molecules by streamer discharges in water: coupled electrical and chemical measurements

This paper presents an experimental study of pre-breakdown discharges in water (usually called ‘corona’ or ‘streamer’) used to degrade organic molecules for depollution purposes. Streamers in water are generated by high voltage pulses of short duration in order to minimize energy losses by Joule heating. Measurements of the electrical energy used to develop streamers can be taken only when losses due to the electrical conduction in the test cell becomes negligible, i.e. at high voltage. Coupled to electrical measurements, the formation of hydrogen peroxide and the degradation of organic compounds such as 4-chlorophenol and 4-nitrophenol is studied. Chemical effects are correlated with the amount of electrical energy released, which depends mainly on conductivity and applied voltage. On the other hand, the chemical yield of these processes, expressed in terms of mole/joule is independent of voltage and conductivity.

Underwater streamer propagation analyzed from detailed measurements of pressure release

In this paper we describe experimental observations connected with the propagation of primary and secondary streamers in water. Using a Mach-Zehnder interferometer we determined the pressure field surrounding the streamer channel at a given instant in time with high temporal and spatial resolution. This pressure field contains information on the time evolution of the pressure pulse inside the discharge channel. The pressure history in the channel has been reconstructed by comparing the experimentally obtained fringe shifts in the interferograms with those derived from one-dimensional hydrodynamic calculations in cylindrical geometry. Assuming different trial pressure pulses, it has been possible to establish the channel pressure iteratively. A reproduction of the experimental data from secondary streamers requires short (2–3ns) pressure pulses with amplitudes of 2–3GPa. These findings are inconsistent with the assumption of bubble-initiated propagation of secondary streamers. It has also been inferred from estimates of the channel diameter that self-propagation of secondary streamers occurs at field strengths at the streamer tip of more than 2GV∕m. We can therefore conclude that field induced dissociation and ionization of molecules in the bulk liquid are the most likely mechanism for secondary streamer propagation. Rather high electrical conductivity (&amp;gt;0.2S∕m) is achieved at fields of 2GV∕m and an ionization wave is launched from the streamer tip into the liquid. To advance the streamer the electric field must be expelled from the newly generated section. This occurs with the Maxwellian relaxation time of a few nanoseconds. During this time the region of high conductivity is transformed into a plasma channel of lower density and a pressure wave is launched into the liquid. A different mechanism is suggested for primary streamer formation. Because of the low conductivity in the channels it is more likely that gas bubbles or phase instabilities are involved in this case.

Observation of electric field enhancement in a water streamer using Kerr effect

We report the observation of the Kerr effect in a streamer head in experiments with electrical breakdown of water. The propagation of the streamer in water is associated with localized enhancement of the electric field surrounding the streamer head. Polarizational measurements show that the electric field around the streamer head is estimated to be 1.7–2.2MV∕cm, which is approximately six times higher than the maximum interelectrode electric field of 0.37MV∕cm. Electrostatic simulations support the experimental data.

Streamers in water filled wire-cylinder and packed-bed Reactors

Streamers in a coaxial reactor filled with tap water have been investigated. Up to six primary plasma channels originated simultaneously from a common point at the center electrode, from where they branched out radially, driven by the electric field. Points of origin were distributed over the wire with an average separation of 5 mm. The streamers propagated at above 60 km/s velocity near the center electrode and slowed to about 30 km/s when they reached the middle of the gap. The streamers accelerated again as they approached the outer electrode. Packing the reactor with silica gel beads generated more homogeneous plasma.