Repetitive Nanosecond Pulses Research Articles

0616 ナノ秒パルスプラズマアクチュエータを用いた翼上の流れ制御のための数値モデル

The significant effects of dielectric barrier discharge (DBD) driven by repetitive nanosecond (NS) pulse voltage on lift enhancement on stalled airfoils have been demonstrated. The essential effect of NS plasma is considered as thermal effect. The detailed effect of the thermal excitation on the vortex structures is, however, not fully understood. In the present work, a simple momentum model for incompressible Unsteady Reynolds-Averaged Navier-Stokes simulation was proposed, in which the effect of plasma was implemented as a wall velocity boundary condition on the airfoil. Although this simplified simulation does not take account of thermalization process and shock dynamics, the results showed discrete vortices generated by rolling-up shear layer over the leading edge. It qualitatively consisted with the trend of time-averaged pressure distribution obtained by the corresponding experiment.

Tesla变压器型重复频率纳秒脉冲源的研制

Tesla变压器型重复频率纳秒脉冲源的研制

Large-Scale Nonthermal Plasma Generated by Repetitive Nanosecond Pulses and Barrier-Free Wire electrodes in Atmospheric Pressure Air

This paper illustrates an observation of largescale cold plasma generated by repetitive nanosecond pulses and barrier-free wire electrodes in atmospheric air. The length of plasma tape achieved 110.0 cm and can be further lengthened. The applied repetitive nanosecond pulses are with ~30-ns rise time, negative ~70-kV peak, and ~750-ns width at the 500-Hz repetition frequency. The discharge images indicate that the large-scale homogeneous nonthermal plasma in open air can be stably formed with appropriate conditions.

Coaxial Diffuse Discharges Driven by Repetitive Nanosecond Pulses at Different Air Pressures

Images of diffuse discharges with a coaxial configuration in a 5-cm gap under different conditions are presented to illustrate the characteristics of large-area diffuse discharges. One ns-pulse generator with a rise time of 70 ns and a duration of 100 ns was used. Results show that the diffuse discharge is composed of plasma channels that start from bright spots on the inner electrode and expand in the direction to the outer electrode. The number of the bright spots increases with the decrease of gas pressure. Furthermore, comparing with the high repetition rate, the plasma channels are more likely to overlap with each other at low repetition rate.

Effect of Pulse Polarity on Nanosecond Surface Dielectric Barrier Discharge

This paper presents the optical emission characteristics of a surface dielectric barrier discharge sustained by repetitive nanosecond pulses with different polarities in the subsonic airflows. The plasma images presented here show that the positive polarity pulsed discharge is stable in the airflows, appearing as a brush-type discharge. While the negative polarity pulsed discharge plasma becomes a filamentary pattern that is evidently spatial inhomogeneous in quiescent air, it behaves unstable in airflows.

Schlieren Imaging of Shock-Wave Formation Induced by Ultrafast Heating of a Nanosecond Repetitively Pulsed Discharge in Air

We present schlieren images of the hydrodynamic expansion following a nanosecond repetitively pulsed discharge in atmospheric pressure air at room temperature. The discharge is created by voltage pulses of amplitude 7.5 kV, duration 10 ns, applied at a frequency of 1 kHz between two pin electrodes. The electrical energy of each pulse is of the order of 1 mJ. We recorded phase-locked schlieren images starting from a few nanoseconds after the discharge. The time-resolved images show the expansion of the heated gas channel from 50 ns and the shock-wave propagation starting at 0.5 μs.

Temporal Evolution of the Glow and Spark Regimes of Nanosecond Repetitively Pulsed Discharges in Water Vapor

Nanosecond repetitively pulsed discharges are applied to pure water vapor at 450 K and 1 atm. The plasma behavior in the glow and spark regimes is presented. The glow regime presents a streamer-like propagation, whereas the spark regime shows an homogeneous excitation of the gap.

Flow separation control over a Gö 387 airfoil by nanosecond pulse-periodic discharge

Airfoil flow separation control using plasma actuator driven by repetitive nanosecond pulse voltage was experimentally investigated. The pressure distribution on an airfoil surface was measured by means of a liquid manometer, which is free from electromagnetic interference of the plasma. By integrating the pressure distribution, the lift coefficient was computed and the effects of the input voltage amplitude and repetitive frequency were evaluated. The results show two different manners of the lift increment depending on the angle of attack. At the pre-stall and stall angle, the flow is steady and the lift increment does not depend on the frequency. A strong hysteresis effect is also observed, i.e., once the lift increases due to the plasma actuation, it is still increased even after the actuation stops. At the post-stall angle, the flow is unsteady and the lift increment becomes significant with actuation at frequencies related to inherent flow instabilities for the shear layer and wake, which are determined by the spectrum analysis based on hot-wire measurements. At the preference frequency ranges, there is a certain voltage amplitude depending on the angle of attack, at which the extended plasma layer results in an optimum increase of lift.

Time-resolved radical species and temperature distributions in an Ar–O2–H2 mixture excited by a nanosecond pulse discharge

Time- and space-resolved temperature and absolute concentrations of OH and H are measured by UV Rayleigh scattering, Laser-Induced Fluorescence (LIF), and Two-Photon Absorption LIF (TALIF), respectively, in an Ar–O2–H2 (80:20:2) mixture at P=40torr and T0=300K. The mixture is excited by a 50-pulse burst from a repetitive nanosecond pulse discharge (NSPD) in a point-to-point geometry, operated at 100kHz pulse repetition rate and 5Hz burst repetition rate. One-dimensional radial distributions of temperature and species concentrations across the discharge filament, during and after the burst, are obtained from Rayleigh scattering and fluorescence images of the laser beam. Both temperature and species concentration profiles are found to expand with time, with peak centerline values of Tpeak≈1200K, [H]peak≈6.0⋅1015cm−3, and [OH]peak≈1.0·1015cm−3. Secondary maxima in OH distributions are detected near the periphery of the filament after a few tens of discharge pulses. Experimental results are compared with predictions of a plasma chemical kinetics model. The model reproduces trends in temporal evolution of radial distributions of temperature, as well as OH and H concentrations. Based on a rate of species production analysis, the secondary peaks in OH radial distributions are found to be caused by radial diffusion of H atoms from the central region of the discharge filament, with subsequent formation of HO2 and OH via reactions H+O2+M→HO2+M and H+HO2→OH+OH in the low-temperature peripheral regions. The results demonstrate significant potential of the present approach for quantitative, time- and spatially-resolved studies of coupled radical reaction kinetics and diffusion over a wide range of temperatures, pressures, and mixture compositions.

Experimental study on surface modification of PET films under bipolar nanosecond-pulse dielectric barrier discharge in atmospheric air

Abstract Dielectric barrier discharge (DBD) is widely used for surface modification of polymer films. In this paper, DBD characteristics under bipolar repetitive frequency nanosecond pulse in atmospheric air are studied and surface properties of polyethylene terephthalate films under homogeneous DBD and filamentary DBD modification are compared through scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS) and dielectric test equipment. It is found that the discharge is homogeneous when gap spacing d is less than 1.2 mm and filamentary when d is within the range of 3.0 mm to 5.8 mm. SEM pictures reveal that films under homogeneous DBD present a smooth surface while intensive “gully-like” etches appear on the surface of the films under filamentary DBD, which can result in local insulation defects and is disadvantageous to surface modification. It is found from the XPS analysis that a number of oxygen-containing polar groups are introduced onto the surface of the film modified by homogeneous DBD compared with the untreated one. Experimental results for dielectric properties indicate that the three parameters: relative dielectric constant ɛr, dielectric loss tangent tan δ and breakdown voltages Vb are all changed in different degree after surface modification. And possible reason for the phenomenon is discussed.

Thermal and hydrodynamic effects of nanosecond discharges in atmospheric pressure air

We present quantitative schlieren measurements and numerical analyses of the thermal and hydrodynamic effects of a nanosecond repetitively pulsed (NRP) discharge in atmospheric pressure air at 300 and 1000 K. The plasma is created by voltage pulses at an amplitude of 10 kV and a duration of 10 ns, applied at a frequency of 1–10 kHz between two pin electrodes separated by 2 or 4 mm. The electrical energy of each pulse is of the order of 1 mJ. We recorded single-shot schlieren images starting from 50 ns to 3 µs after the discharge. The time-resolved images show the shock-wave propagation and the expansion of the heated gas channel. Gas density profiles simulated in 1D cylindrical coordinates have been used to reconstruct numerical schlieren images for comparison with experimental ones. We propose an original method to determine the initial gas temperature and the fraction of energy transferred into ultrafast gas heating, using a comparison of the contrast profiles obtained from experimental and numerical schlieren images. This method is found to be much more sensitive to these parameters than the direct comparison of measured and predicted shock-wave and heated channel radii. The results show that a significant fraction of the electric energy is converted into gas heating within a few tens of ns. The values range from about 25% at a reduced electric field of 164 Td to about 75% at 270 Td, with a strong dependance on the initial gas temperature. These experiments support the fast heating processes via dissociative quenching of N2(B3 Πg, C3 Πu) by molecular oxygen.

Discharge Characteristics of SF6 in a Non-Uniform Electric Field Under Repetitive Nanosecond Pulses

The characteristics of high pressure sulphur hexafluoride (SF6) discharges in a highly non-uniform electric field under repetitive nanosecond pulses are investigated in this paper. The influencing factors on discharge process, such as gas pressure, pulse repetition frequency (PRF), and number of applied pulses, are analyzed. Experimental results show that the corona intensity weakens with the increase of gas pressure and strengthens with the increase of PRF or number of applied pulses. Spark discharge images suggest that a shorter and thicker discharge plasma channel will lead to a larger discharge current. The number of applied pulses to breakdown descends with the increase of PRF and ascends with the rise of gas pressure. The reduced electric field (E/p) decreases with the increase of PRF in all circumstances. The experimental results provide significant supplements to the dielectric characteristics of strongly electronegative gases under repetitive nanosecond pulses.

Analysis and experimental study on formation conditions of large-scale barrier-free diffuse atmospheric pressure air plasmas in repetitive pulse mode

Atmospheric air diffuse plasmas have enormous application potential in various fields of science and technology. Without dielectric barrier, generating large-scale air diffuse plasmas is always a challenging issue. This paper discusses and analyses the formation mechanism of cold homogenous plasma. It is proposed that generating stable diffuse atmospheric plasmas in open air should meet the three conditions: high transient power with low average power, excitation in low average E-field with locally high E-field region, and multiple overlapping electron avalanches. Accordingly, an experimental configuration of generating large-scale barrier-free diffuse air plasmas is designed. Based on runaway electron theory, a low duty-ratio, high voltage repetitive nanosecond pulse generator is chosen as a discharge excitation source. Using the wire-electrodes with small curvature radius, the gaps with highly non-uniform E-field are structured. Experimental results show that the volume-scaleable, barrier-free, homogeneous air non-thermal plasmas have been obtained between the gap spacing with the copper-wire electrodes. The area of air cold plasmas has been up to hundreds of square centimeters. The proposed formation conditions of large-scale barrier-free diffuse air plasmas are proved to be reasonable and feasible.

Resonant Charging Performance of Spiral Tesla Transformer Applied in Compact High-Voltage Repetitive Nanosecond Pulse Generator

The compact, small-sized device based on a pulsed transformer combined with a semiconductor switch is a feasible solution that generates the high-frequency and high-voltage pulse. Compared with other types of transformers, spiral Tesla transformer is much easier to achieve a high output-voltage ratio. By an in-depth analysis of the resonance circuit of Tesla transformer, the expressions of secondary voltage, transformation ratio, peak, and steepness of the primary current are deduced and analyzed in this paper. The development test of a small-size Tesla transformer prototype has been performed. The key electrical characteristics are further measured, estimated, and discussed. The specification parameter selection of the primary semiconductor switch must rely on the characteristics of primary current. Applied to a 6.0 kΩ resistor, the high-voltage repetitive pulse generator based on the Tesla transformer prototype can generate a series of pulses with a peak voltage of 100 kV and a rise time of 40 ns at the repetition rate of 200 Hz. Atmospheric air nonequilibrium plasma can be formed using the developed pulse generator and some typical images are presented.

A Comparative Study of Water Electrodes Versus Metal Electrodes for Excitation of Nanosecond-Pulse Homogeneous Dielectric Barrier Discharge in Open Air

Atmospheric pressure low-temperature plasmas produced by dielectric barrier discharge (DBD) provide a promising approach for civilian application of pulsed power technology. In this paper, repetitive nanosecond pulses were generated using a magnetic compression solid-state pulsed power generator, and the rise time and pulse duration of the nanosecond pulse are ~30 and 70 ns, respectively. The DBD in open air is created using two kinds of electrodes, i.e., water and metal electrodes. The electrical, luminous, and optical characteristics of the DBDs under these two electrodes are studied and compared. The experimental results show that no filaments are observed and the discharge is homogeneous when water electrodes are used. The DBD still behaves in a filamentary mode when the discharge gap is extended to 4 cm in the case of metal electrodes. The results are validated by fast images taken by an intensified charge-coupled device camera. In addition, some discussion about the experimental results is presented. Improvement of discharge uniformity is due to the effect of resistive stabilization using water electrodes.

Generating diffuse discharge via repetitive nanosecond pulses and line-line electrodes in atmospheric air

Diffuse discharge in atmospheric air can generate extremely high power density and large-scale non-thermal plasma. An achievable method of generating diffuse discharge is reported in this paper. Based on the resonance theory, a compact high-voltage repetitive nanosecond pulse generator (HRNPG) has been developed as discharge excitation source. The HRNPG mainly consists of repetitive charging circuit, Tesla transformer and sharpening switch. With the voltage lower than 1.0 kV, the primary repetitive charging circuit comprises two fast thyristors as low-voltage switches. A spiral Tesla transformer is designed to provide a peak transformation ratio of more than 100. The HRNPG prototype is capable of generating a pulse with over 100 kV peak voltage and ~30 ns rise-time at the repetition frequency of 500 Hz. Using the copper line electrodes with a diameter of 0.4 mm, the gaps with highly non-uniform electric field are structured. With the suitable gap spacing and applied pulse, the glow-like diffuse discharge has been generated in line-type and ring-type electrode pairs. Some typical images are presented.

A Tesla-type repetitive nanosecond pulse generator for solid dielectric breakdown research

A Tesla-type repetitive nanosecond pulse generator including a pair of electrode and a matched absorption resistor is established for the application of solid dielectric breakdown research. As major components, a built-in Tesla transformer and a gas-gap switch are designed to boost and shape the output pulse, respectively; the electrode is to form the anticipated electric field; the resistor is parallel to the electrode to absorb the reflected energy from the test sample. The parameters of the generator are a pulse width of 10 ns, a rise and fall time of 3 ns, and a maximum amplitude of 300 kV. By modifying the primary circuit of the Tesla transformer, the generator can produce both positive and negative pulses at a repetition rate of 1-50 Hz. In addition, a real-time measurement and control system is established based on the solid dielectric breakdown requirements for this generator. With this system, experiments on test samples made of common insulation materials in pulsed power systems are conducted. The preliminary experimental results show that the constructed generator is capable to research the solid dielectric breakdown phenomenon on a nanosecond time scale.

Effect of Nanosecond Repetitively Pulsed Discharges on the Dynamics of a Swirl-Stabilized Lean Premixed Flame

The effects of nanosecond repetitively pulsed (NRP) plasma discharges on the dynamics of a swirl-stabilized lean premixed flame are experimentally investigated. Voltage pulses of 8 kV in amplitude and 10 ns in duration are applied at a repetition rate of 30 kHz. The average electric power deposited by the plasma is limited to 40 W, corresponding to less than 1% of the thermal power of 4 kW released by the flame. The investigation is carried out with a dedicated experimental setup that allows for studies of the flame dynamics with applied plasma discharges. A loudspeaker is used to acoustically perturb the flame and the discharges are generated between a central pin electrode and the rim of the injection tube. The velocity and CH* chemiluminescence signals are used to determine the flame transfer function, assuming that plasma discharges do not affect the correlation between the CH* emission and heat release rate fluctuations. Phase-locked images of the CH* emission show a strong influence of the NRP discharges on the flame response to acoustic perturbations, thus opening interesting perspectives for combustion control. An interpretation of the modifications observed in the transfer function of the flame is proposed by taking into account the thermal and chemical effects of the discharges. It is then demonstrated that by applying NRP discharges at unstable conditions, the oscillation amplitudes can be reduced by an order of magnitude, thus effectively stabilizing the system.

Spacer flashover characteristics in SF6 under repetitive nanosecond pulses

Very fast transient overvoltages (VFTO) may result in insulation failures inside or outside gas insulated switchgears (GIS), and most of them are caused by surface flashovers along insulating spacers according to operational experiences. For a good understanding of the discharge process produced by high voltage pulses with a fast rising time, an experimental study on spacer flashover under repetitive nanosecond pulses in high pressure sulphur hexafluoride (SF6) is presented. A solid-state pulse generator is used to generate nanosecond pulses with a rise time of 15 ns and a full width at half maximum of 30~40 ns. The pulse repetition frequency (PRF) varies from single shot to 1 kHz. Brass parallel-plane electrodes and cylinder epoxy resin spacer are used to represent a configuration of uniform electric field and insulator in GIS, respectively. In this experiment, the parameters of amplitude of discharge current, flashover time lag and number of applied pulses to flashover are investigated. And their relationships with gas pressure and PRF are analyzed.

Experimental study on conduction current of positive nanosecond-pulse diffuse discharge at atmospheric pressure

Nanosecond pulsed discharges have various discharge modes, such as corona, diffuse discharge, spark or arc. A dense diffuse discharge is particularly desirable for various applications at atmospheric pressure. In this paper, a magnetic-compression pulse generator was used to produce repetitive positive nanosecond pulses for excitation of a diffuse discharge. The output pulse of the generator had a rise time of about 25 ns and a full width at half maximum of 40 ns. Electrical characteristics of the diffuse discharge were studied by measuring its voltage and current waveforms, as well as images of the discharge. The conduction current was calculated by the measured voltage and current, which was a true discharge current. The experimental results show that a stable diffuse discharge could be obtained at atmospheric pressure, and the conduction current was unipolar and had similar amplitude of several amperes under our experimental condition, which has the similar amplitude with the displacement current. Furthermore, the air gap spacing and pulse repetition frequency (PRF) affected the intensity and mode transition of the diffuse discharge. The conduction current increased with the PRF but decreased with the air gap spacing. Therefore, the diffuse discharge was likely available under some conditions of proper air gap, high PRF with positive pulse.