Characterization of Nano-Seconds Pulsed Streamer Discharges

Pulsed streamer discharges have been extensively used in many applications such as control of NO/sub X/ and SO/sub 2/ from exhaust gases, treatment of dioxins, removal of volatile organic compounds, generation of ozone, and laser excitation. An operation with a high energy efficiency is necessary for practical applications. It is very important to know the propagation mechanism of streamer discharges in order to improve the energy efficiency of pulsed discharge systems. In this paper, the emission from pulsed streamer discharges in a coaxial electrode system in air at 0.1 MPa was observed using a high-speed gated intensified charge-coupled display camera. A concentric wire-cylinder electrodes configuration was used. A positive pulsed voltage having a width of about 100 ns was applied to the central electrode. The streamer discharges were initiated at the inner electrode and terminated at the outer electrode. The propagation velocity of the streamer discharges was 1.8-3.3 mm/ns.

Recently industrial applications of ozone have increased in widely different areas including oxidation, sterilization, deodorization, bleaching and others. Especially, a demand of movable ozonizer for treatment of diesel exhaust gas has been growing up. Generally, a generation of ozone has been attained using ultra violet irradiation, electrolysis and electrical discharges such as a dielectric barrier discharge, a surface discharge, a pulsed streamer discharge and a superimposed silent and surface discharge. In the present situation, a ozonizer based on a dielectric barrier discharge has been commonly utilized in the world. However, a ozonizer based on a dielectric barrier discharge has disadvantages as a movable ozonizer since a discharge electrode, having a breakable dielectric layer and a sensitive gap separation, does not have enough resistance to vibration stress. In this study, the characteristics of ozonizer based on a pulsed streamer discharge were measured using a magnetic pulse compressor (MPC), which has a maximum output voltage of 60 kV, a maximum pulse repetition rate of 500 pulses per second and a pulse duration of about 130 ns, and discharge reactors with coaxial geometry. For the reactor of pulsed streamer discharges, an anode and a cathode electrodes are made by metallic material and a gap separation between electrodes has a little influence on a discharge condition. Therefore, this type of ozonizer has a merit to apply mobile situation. As the results, the ozonizer based on the pulsed streamer discharges has the better production yield of 150 g/kWh in the concentration range of 10 to 20 g/m

The non-thermal, chemically active plasma is one of the promising technologies for the removal of the hazardous environmental pollutants. Pulsed streamer discharges in atmospheric pressure gases have received a great deal of interest for many years. It is very important to know the propagation mechanism of streamer discharges in order to improve the energy efficiency of pulsed discharge systems. In the present work, a Blumlein generator that has a pulse width of 100 ns was used to investigate the streamer discharge propagation. The propagation images in a coaxial reactor were taken by a streak camera for both positive and negative polarities of the applied voltage. From the results, primary and secondary streamers were observed propagating from the inner wire electrode to the outer cylinder electrode, and the maximum propagation velocity of the streamer head were 0.6 - 1.2 mm/ns in the range of 43 - 93 kV of the absolute value of the applied voltage. The propagation velocity of the positive streamer was faster than that of the negative polarity. The electric field at the wire electrode for streamer onset was 12 MV/m for the positive streamer and 20 MV/m for the negative streamer.

Experimental investigation of HV short pulsed streamer discharges in dry air-fed ozonizers under various operating conditions are reported. Ozone concentration, energy input and ozone production yield (efficiency) were measured at various voltages (14 to 37 kV), pulse repetition rates (25 to 400 pulses per second, pps), flow rates (1.5 to 3.0 1/min) and different gap spacings (10 to 20 mm) at a pressure of 1.01/spl times/10/sup 5/ Pa in dry air. A spiral copper wire (1 mm in diameter) made to a cylindrical configuration (18 to 38 mm in diameter) in a concentric coaxial electrode system of various dimensions was employed. A magnetic pulse compressor provided the HV and current pulses. Higher voltage and higher repetition rates yielded higher concentrations of ozone at a fixed air flow rate. The present investigation was extended to assess the performance of this pulsed ozone generator using dry air under desired conditions of high concentration and high yield of ozone for industrial applications.

The plasma radiation from a pulsed streamer discharge at 0.2–2 Torr in the air has been studied. Pulsed discharges at these pressures at a distance from the electrodes consist of diffuse ionization waves and, therefore, are widely studied, including within the framework of modeling high-altitude atmospheric discharges, in particular, columnar sprites. Two setups with different excitation parameters were used to excite air at low pressures. When plasma was formed using a capacitive single-barrier discharge feeding by voltage pulses with an amplitude of up to 7 kV, diffuse red-colored ionization waves up to 120 cm long were formed. Due to the formation of an ionization wave by a discharge forming by voltage pulses with an amplitude of up to 33 kV between two electrodes, X-ray and runaway electron beam were recorded.

The earlier experimental setup for the study of a multi-streamer single pulse sliding discharge was supplemented by a photoelectric system for recording streamer radiation with a time resolution of 1.5 ns. Measurements of streamer delay and propagation velocity in Ne of atmospheric pressure at two supply voltage polarities were performed. The results showed that during each discharge pulse, regardless of its polarity, streamers of two types propagate across the gap in opposite directions. The first ones, primary streamers, start from a high-voltage electrode and move to a grounded electrode. The other type of streamers, reverse streamers, propagate in the opposite direction from a grounded electrode only after primary streamers have reached it. The experiments were carried out using voltage amplitude of 3 and 3.5 kV applied to plane-parallel electrodes on an alumina ceramic substrate with a rectangular discharge area of 9.5×11.6 cm2 in size. The total delay time range of the streamers was up to 2500–2700 ns and their motion velocity up to (3.5−5.6)×107 cm/s, depending on the discharge voltage and polarity. The dependence of delay time and velocity of the streamers' head motion on their position in the gap is presented. A qualitative explanation of the primary streamer characteristics is proposed, while the obtained experimental data are not sufficient for a similar explanation with respect to reverse streamers. The presented results are significant for studying the physical mechanisms of counterpropagating surface streamer/ionization wave dynamics, while the experimental methodology allows recording their spatiotemporal evolution in the gap.

In pre-ionized pulsed streamer discharge, regulating the seed-electron density and its spatial distribution during the pre-ionization stage provides an effective approach to control streamer development and plasma chemical activity. Using electrical measurements, optical diagnostics, and a two-dimensional axisymmetric fluid model, this study systematically analyzes the behavior of pulsed streamer discharges under seed-electron pre-ionization and quantitatively clarifies how seed-electron density and spatial distribution govern the evolution of the primary streamer (PS), the secondary streamer (SS), as well as the generation of reactive species. The results show that UV pre-ionization enhances the discharge intensity and promotes streamer development. Under the same experimental conditions, simulations indicate that the minimum seed-electron densities required to initiate the development of PS and SS are 10 10 m -3 and 10 12 m -3 , respectively. When the seed-electron density exceeds 10 10 m -3 , PS development is enhanced, leading to increased formation of N 2 (C), N 2 + and O 2 + . Once the density exceeds 10 12 m -3 , SS development is accelerated, and the generation of N and O increases markedly. The spatial distribution of seed electrons is also critical to discharge development. Placing electrons near the anode region accelerates breakdown and promotes PS development, whereas electrons near the cathode region favor SS development, intensifies the radial electric field and electron density near the cathode region, and ultimately leads to a large discharge volume. These findings clarify how seed electrons regulate pulsed streamers and provide guidance for tailoring pre-ionization strategies to optimize plasma reactivity.

Recent literature (2000−2008) on the functionalization methods leading to soluble carbon nanotubes is reviewed. Carbon nanotubes (CNTs) can be solubilized by a series of methods, including their functionalization by the aryl diazonium process, use of elemental metals, simple inorganics, acids, esters, aldehydes, amines, aromatics, macrocycles, thiols, biomolecules, polymers, and using such techniques as pulsed streamer discharge, microwave treatment, cryogenic crushing, or γ-irradiation. In a difference with the functionalization methods applied in the past decade of the 20th century (frequent use of strong acidic media as a first step and further attachment of various organic moieties), current techniques involve a much wider number of functionalizing agents paying main attention to macrocycles, biomolecules, and polymers, as well as using such special techniques as use of radionuclides, irradiation, microwave treatment, or pulsed streamer discharge. Reactivity, physicochemical properties, and main applications of soluble CNTs are analyzed.

The formation and propagation of pseudo-filamentary dielectric barrier discharge in atmospheric air are investigated through a 2D fluid model. The discharge development can be divided into three stages: the volume streamer stage, the surface streamer stage, and the reverse discharge stage. The simulations show that the streamer head becomes wider and the electron density of the volume streamer head increases six times when the volume streamer interacts with the dielectric, and the volume streamer transforms into the surface streamer after the interaction. Compared with volume streamers, surface streamers have a smaller radius, a higher electric field, and a higher electron density. Furthermore, the parameters that may influence the discharge characteristics are also studied. It is found that a larger dielectric permittivity, a thinner dielectric, or a shorter voltage rise time leads to earlier inception of volume streamers, faster propagation of surface streamers, and higher current density. It is observed that the velocity of the surface streamer increases first, and then, decreases with the accumulated charges on the surface.

Axisymmetric two-dimensional (2D) simulation is often used in case of streamer discharge in atmospheric-pressure air. To validate the 2D simulation of streamer discharge, the simulation has to be compared with the measurement results. However, there is difficulty in performing the comparison because the streamer is not a 2D phenomenon but a three-dimensional phenomenon. In this study, a pseudo-2D axisymmetric single-filament pulsed positive streamer discharge was generated in atmospheric-pressure air and compared with our 2D simulation. The propagation, emission intensity, shape, and cathode current of the streamer were compared. The ozone density was also measured using laser spectroscopy and compared with the simulation because our simulation intends to simulate the dynamics of reactive species. The measurements and simulations exhibited some agreements and disagreements. The measurement also showed a cathode fall and other cathode phenomena, which were not considered in the simulation.

Electronegative gas components and gas pressure significantly change residual charge dynamics, which are critical for pulse-periodic streamer discharge behaviors. Evolutions of repetitively pulsed positive streamer discharge and the streamer-to-spark transitions were investigated at high pressures and compared between typical weak (O2) and strong (SF6) electronegative gas mixtures. Pulse-sequence resolved electrical and optical diagnostics were implemented to capture discharge evolutions in long pulse trains. We observe that streamer inception and propagation under subsequent pulses in N2 and N2–O2 mixtures are similar, including the earlier inception of the primary streamer and the accelerated propagation of the secondary streamer. The repetitively pulsed breakdown is extended to the low pulse repetition frequency region with the addition of O2. Discharge evolutions are unexpectedly different in N2–SF6 mixtures. Subsequent discharge channels prefer to propagate around the periphery of the inception cloud region with large radial deviations. Another difference is the precursor channel identified besides multiple streamer channels. Effects of electronegative gas on streamer evolutions under positive repetitive pulses have been qualitatively analyzed. Collisional electron detachment and photo-ionization are crucial in N2–O2 mixtures. With the presence of strong electronegative gas (SF6), the leader formation is probably induced by the earlier corona inception and longer voltage stressing period under following pulses in a pulse train, which are caused by the strong electronegativity of SF6 and the small ion mobility.

Residual charges residing at gas volume and trapped at the solid dielectric surface are crucial memory effect agents for evolutions of repetitively pulsed surface streamer discharges. The low-photon-energy laser irradiation can intentionally release and then exploit volume and surface residual charges to regulate discharge characteristics, enabling more flexibilities than conventional voltage waveform modulation methods. However, important parameters and regulation mechanisms of the laser irradiation on evolutions of repetitively pulsed surface streamers are still ambiguous. Evolutions of repetitively pulsed surface streamers in atmospheric pressure air were investigated after low-photon-energy pulsed visible (532 nm) laser irradiations. The effects of the laser irradiation modes and pulse repetition frequency (PRF) dielectric material on streamer inception and propagation features were determined by pulse-sequence and temporally resolved diagnostics. The laser irradiation and increased PRF have similar effects on the evolution of streamer light emission. PRF had little effect on the streamer velocity. When the relative permittivity of the dielectric is smaller, the number of streamer branches increases, which is dominated by the photoionization. Interestingly, the morphology and propagation velocity of the streamer vary greatly depending on the laser irradiation mode. Streamer characteristics exhibit noticeable changes only when the laser perpendicularly irradiates the solid surface rather than the laser beam is tangent to (slightly above) the surface. The effect of laser-induced surface trapped electron desorption on streamer characteristics is analyzed, focusing on the distribution morphology and density of desorption electrons.

In this study, spatially resolved measurements of the emission intensity of OH (A2Σ↦X2Π, 0-0) and the vibrational temperature of N2 (C) have been performed during a positive pulsed streamer discharge with a wire-plate electrode configuration at atmospheric pressure. The effects of pulse peak voltage, pulse repetition rate and the added O2 flow rate on the spatial distributions of the emission intensity of OH (A2Σ↦X2Π, 0-0) and the vibrational temperatures of N2 (C) perpendicular to the wire in the direction towards the plate (in the radial direction) are investigated. It has been found that the emission intensity of OH (A2Σ↦X2Π, 0-0) increases with increasing pulse peak voltage and pulse repetition rate and decreases with increasing the distance from the wire electrode. When the different oxygen flows are added in N2 and H2O mixture gas, the emission intensity of OH (A2Σ↦X2Π, 0-0) decreases with increasing the flow rate of oxygen. The vibrational temperature of N2 (C) is nearly independent of pulsed peak voltage and pulsed repetition rate, but increases with increasing the added O2 flow rate and keeps almost constant in the radial direction under the present experimental conditions. This measurement plays a crucial role in understanding the discharge characters of pulsed streamer discharge and establishing the molecule reaction dynamics model of pulsed streamer discharge.

Pulsed power technology is gradually forming a development trend of civil-military integration, which puts forward more requirements for pulsed power generators. This paper takes magnetic switches (MSs) as the starting point and reviews recent advancements in pulse generators based on MSs. First, the working mechanism of the MS "rapid inductance drop after magnetic core saturation" is analyzed. Second, the basic uses of MSs are introduced with specific examples, namely, magnetic compression unit, saturated pulse transformer, and magnetic delay switches. Then, the typical topologies of pulse generators based on MSs are discussed, including transmission line, Marx, Fitch, linear transformer driver, and semiconductor opening switch pumping circuits. These circuits' technical characteristics and parameter levels are highlighted. Finally, the existing problems and future development trends of MS-based solid-state pulse generators are discussed.

Influence of streamer-to-glow transition in NOx removal from a simulated diesel exhaust gas using pulse streamer discharge (PSD) reactor is described in this paper. A co-axial cylinder type discharge chamber was used as the PSD reactor which was driven by inductive energy storage (IES) pulse power generator with semiconductor opening switch (SOS). The pulsed power generator supplies both polarity of 30 kV pulse with 300 pps repetition rate and 25 ns pulse width. The co-axial cylinder plasma reactor consists of 1 mm diameter tungsten wire and 19 mm i.d. copper tube with 30 cm length. NO removal from the simulated gas (N2:O2=9:1, Initial NO concentration=200 ppm) increased with input energy into plasma reactor. The energy efficiency for NO removal was obtained to be 25 g/kWh at 30 % removal in gas flow rate of 2 L/min. However, the energy efficiency decreased to 4.6 g/kWh with increasing capacitance of the primary capacitor from several hundreds pF to several nF. This decrease is caused with streamer-to-glow transition.