Repetitive Nanosecond Research Articles

Characterization of a surface dielectric barrier discharge plasma sustained by repetitive nanosecond pulses

The paper discusses experimental characterization of a surface dielectric barrier discharge plasma sustained by repetitive, high-voltage, nanosecond duration pulses. The discharge pulse energy is controlled primarily by the pulse peak voltage and scales approximately linearly with the length of the electrodes. Images of the plasma generated during the discharge pulse, taken by a nanosecond gate ICCD camera, show that the plasma remains fairly uniform in the initial phase of the discharge and becomes filamentary at a later stage. The temperature rise in the discharge, operated in both continuous mode and in burst mode, is inferred from UV/visible emission spectra. Phase-locked schlieren images are used to measure the speed of the compression waves generated by the nanosecond pulse discharge and the density gradient in the wave. The density gradient is inferred from the schlieren images using absolute calibration by a pair of wedged windows. The results demonstrate that discharge filaments generate compression waves with higher amplitude and higher speed compared with waves produced in a diffuse discharge. The density gradient in the compression waves is compared with numerical modeling of propagating compression waves produced by short-pulse localized heating, and shows satisfactory agreement between the model and the experimental results.

Experimental and modeling analysis of fast ionization wave discharge propagation in a rectangular geometry

Fast ionization wave (FIW), nanosecond pulse discharge propagation in nitrogen and helium in a rectangular geometry channel/waveguide is studied experimentally using calibrated capacitive probe measurements. The repetitive nanosecond pulse discharge in the channel was generated using a custom designed pulsed plasma generator (peak voltage 10–40 kV, pulse duration 30–100 ns, and voltage rise time ∼1 kV/ns), generating a sequence of alternating polarity high-voltage pulses at a pulse repetition rate of 20 Hz. Both negative polarity and positive polarity ionization waves have been studied. Ionization wave speed, as well as time-resolved potential distributions and axial electric field distributions in the propagating discharge are inferred from the capacitive probe data. ICCD images show that at the present conditions the FIW discharge in helium is diffuse and volume-filling, while in nitrogen the discharge propagates along the walls of the channel. FIW discharge propagation has been analyzed numerically using quasi-one-dimensional and two-dimensional kinetic models in a hydrodynamic (drift-diffusion), local ionization approximation. The wave speed and the electric field distribution in the wave front predicted by the model are in good agreement with the experimental results. A self-similar analytic solution of the fast ionization wave propagation equations has also been obtained. The analytic model of the FIW discharge predicts key ionization wave parameters, such as wave speed, peak electric field in the front, potential difference across the wave, and electron density as functions of the waveform on the high voltage electrode, in good agreement with the numerical calculations and the experimental results.

A compact repetitive nanosecond pulsed power generator based on transmission line transformer

In this paper, a compact repetitive nanosecond pulsed power generator based on transmission line transformer (TLT) is developed for non-thermal plasma generation. The high voltage capacitor, the spark gap switch and the input side of the TLT are built into a compact structure to obtain a fast high-power pulse. The properly designed magnetic cores placed around the coaxial cables of the TLT have been chosen to obtain a higher voltage gain. The layout of TLT is optimized to make the generator more compact. Experiment test shows that voltage pulses of 30 kV with rise time of about 25 ns, pulse duration of about 100 ns, pulse repetition rate of 500 pps have been achieved.

Mach 5 bow shock control by a nanosecond pulse surface dielectric barrier discharge

Bow shock perturbations in a Mach 5 air flow, produced by low-temperature, nanosecond pulse, and surface dielectric barrier discharge (DBD), are detected by phase-locked schlieren imaging. A diffuse nanosecond pulse discharge is generated in a DBD plasma actuator on a surface of a cylinder model placed in air flow in a small scale blow-down supersonic wind tunnel. Discharge energy coupled to the actuator is 7.3–7.8 mJ/pulse. Plasma temperature inferred from nitrogen emission spectra is a few tens of degrees higher than flow stagnation temperature, T = 340 ± 30 K. Phase-locked Schlieren images are used to detect compression waves generated by individual nanosecond discharge pulses near the actuator surface. The compression wave propagates upstream toward the baseline bow shock standing in front of the cylinder model. Interaction of the compression wave and the bow shock causes its displacement in the upstream direction, increasing shock stand-off distance by up to 25%. The compression wave speed behind the bow shock and the perturbed bow shock velocity are inferred from the Schlieren images. The effect of compression waves generated by nanosecond discharge pulses on shock stand-off distance is demonstrated in a single-pulse regime (at pulse repetition rates of a few hundred Hz) and in a quasi-continuous mode (using a two-pulse sequence at a pulse repetition rate of 100 kHz). The results demonstrate feasibility of hypersonic flow control by low-temperature, repetitive nanosecond pulse discharges.

X-ray and runaway electron generation in repetitive pulsed discharges in atmospheric pressure air with a point-to-plane gap

In this paper, using two repetitive nanosecond generators, x-rays were detected in atmospheric air with a highly inhomogeneous electric field by a point-to- plane gap. The rise times of the generators were about 15 and 1 ns. The x-rays were directly measured by various dosimeters and a NaI scintillator with a photomultiplier tube. X-rays were detected in the continuous mode at pulse repetition frequency up to 1 kHz and a voltage pulse rise time of ∼15 ns. It is shown that the maximum x-ray intensity is attainable at different pulse repetition frequencies depending on the voltage pulse parameters and cathode design. In atmospheric pressure air the x-ray intensity is found to increase with increasing the pulse repetition frequency up to 1 kHz. It is confirmed that the maximum x-ray intensity is attained in a diffuse discharge in a point-to-plane gap.

A compact repetitive high-voltage nanosecond pulse generator for the application of gas discharge

Uniform and stable discharge plasma requires very short duration pulses with fast rise times. A repetitive high-voltage nanosecond pulse generator for the application of gas discharge is presented in this paper. It is constructed with all solid-state components. Two-stage magnetic compression is used to generate a short duration pulse. Unlike in some reported studies, common commercial fast recovery diodes instead of a semiconductor opening switch (SOS) are used in our experiment that plays the role of SOS. The SOS-like effects of four different kinds of diodes are studied experimentally to optimize the output performance. It is found that the output pulse voltage is higher with a shorter reverse recovery time, and the rise time of pulse becomes faster when the falling time of reverse recovery current is shorter. The SOS-like effect of the diodes can be adjusted by changing the external circuit parameters. Through optimization the pulse generator can provide a pulsed voltage of 40 kV with a 40 ns duration, 10 ns rise time, and pulse repetition frequency of up to 5 kHz. Diffuse plasma can be formed in air at standard atmospheric pressure using the developed pulse generator. With a light weight and small packaging the pulse generator is suitable for gas discharge application.

Discharge characteristic of nanosecond-pulse DBD in atmospheric air using magnetic compression pulsed power generator

Dielectric barrier discharge driven by repetitive nanosecond pulses can offer highly efficient non-thermal plasma at atmospheric pressure and is widely used for plasma applications. In this paper, the discharge is generated using a compact pulsed power generator based on one-stage magnetic compression. The output pulse can be up to 30 kV with a rise time of about 40 ns and a full width at half maximum of 70 ns. The electrical characteristics of the discharge parameters are studied by the measurement of voltage and current waveforms. The effects of applied voltage amplitude, voltage polarity, pulse repetition frequency, and barrier dielectric on discharge characteristics are investigated, respectively. The experimental results show that the discharge current, discharge power and electron density increase with the increase of the applied voltage, and the pulse repetition frequency has a slight effect on the electrical parameters. Moreover, the discharge current is influenced by the dielectric barrier, but it is not varied with the polarity of applied pulses.

Cavity ignition and flameholding of ethylene–air and hydrogen–air flows by a repetitively pulsed nanosecond discharge

Repetitive nanosecond pulse plasma assisted ignition and flameholding of premixed and non-premixed ethylene–air and hydrogen–air flows are studied in a cavity flow at a pressure of 0.2 atm and flow velocities of up to 100 m/s. Ignition occurs via formation of multiple filaments in the fuel–air plasma, although air plasma remains diffuse until the fuel is added. After ignition occurs in the cavity, with ignition delay time of a few milliseconds, the plasma becomes diffuse and the flame couples out to the main flow. The use of a short cavity (length-to-depth ratio L/ D = 1) results in repetitive ignition and flame blow-off, caused by slow mixing between the main flow and the cavity. Increasing the length-to-depth ratio to L/ D = 3, as well as choking inlet air and fuel flows resulted in stable flameholding and nearly complete combustion in both premixed and non-premixed ethylene–air and hydrogen–air flows at u = 35–100 m/s. Air plasma temperature before fuel is added ranges from 70 °C to 200 °C. When the nanosecond pulse discharge is operated in repetitive burst mode, continuous ethylene–air flame is maintained only at a high duty cycle, which increases with the flow velocity. In hydrogen–air, the flame remains stable after the plasma is turned off. Nanosecond pulse discharge ignition of ethylene–air is compared with ignition by DC arc discharge of approximately the same power. DC arc discharge results in sporadic ignition and flame blow-off, much lower burned fuel fraction, and significantly lower flow velocity at which ignition can be achieved. Kinetic modeling is used to identify the reduced mechanism of plasma chemical oxidation and ignition of hydrogen, and to demonstrate the mechanism of energy release low-temperature reactions of radicals generated in the plasma (primarily O and H atoms).

On the Effect of Nonequilibrium Plasma on the Minimum Ignition Energy—Part 1: Discharge Model

The mathematical model of a repetitive nanosecond pulse discharge at atmospheric-pressure conditions has been presented. The influence of initial gas temperature, chemical kinetics, and vibrational nonequilibrium on the ignition of methane-air and ethylene-air mixtures has been analyzed.

Excitation of atmospheric pressure uniform dielectric barrier discharge using repetitive unipolar nanosecond-pulse generator

Dielectric barrier discharge (DBD) excitation by unipolar high voltage pulses is a promising approach for producing non-thermal plasma at atmospheric pressure. In this study, a magnetic compression solid-state pulsed power generator was used to produce repetitive nanosecond pulses for the excitation. The DBD is created using two liquid electrodes. The electrical characteristics of the discharge voltage and current are illustrated under different experimental conditions. The nanosecond-pulse discharge current is of the order of tens of amperes. This differs from common DBD current excitated by high-voltage ac sources. Compared with the characteristics of two current pulses corresponding to two discharges for unipolar pulsed-excitation, the secondary discharge in this study is minor owing to the pulsed power and discharge configuration. Under the experimental conditions, the luminous emissions from the front and side views of the liquid electrodes show that no filament is observed and the discharge is homogeneous and diffuse in the whole discharge region. The effects of applied voltage amplitude, repetition rate, and air gap spacing on the discharge characteristic are investigated. The discharge mode does not change with the variation of the investigated parameters. A comparison of high voltage ac and nanosecond-pulse excitation is also presented. In addition, discussion of the experimental results is presented.

Flashover stressing time under repetitive nanosecond pulses in PMMA, polyamide 1010 and transformer oil

Flashover occurring under repetitive pulsed voltage perhaps may not happen at the first pulse of applied repetitive pulses. Thus, studies of flashover stressing time (FST) are important to comprehend the flashover mechanism under nanosecond-pulsed voltage. In our experiments, the pulsed power source was SPG200, based on semiconductor opening switch (SOS). Solid dielectrics employed were polymethyl methacrylate (PMMA) and polyamide 1010, and liquid dielectric was transformer oil. The dispersion of FST was statistically large at invariable or variable voltage amplitude and frequency of repetitive pulses. Furthermore, the dispersion of FST decreased, and concurrently, the odds of that flashover arising at the first pulse of repetitive pulses increased, with the increase in the voltage amplitude of repetitive pulses whose frequency was invariably retained. The flashover tended to occur at the tail of the pulse waveform, and the mean value of FST was found to gradually reduce. Moreover, the extent of reduction was prominently decreased after about 200 Hz, when the pulse frequency increased, which simultaneously restricted the variation of the voltage amplitude of the applied repetitive pulses. Thus, it was demonstrated that longer FST is the ultimate factor, which induces lower flashover field strength under repetitive nanosecond pulses.

Pure rotational CARS studies of thermal energy release and ignition in nanosecond repetitively pulsed hydrogen-air plasmas

Pure rotational CARS thermometry, complemented by UV emission measurements and ICCD imaging, is used to study kinetics of low temperature plasma assisted fuel oxidation and ignition in a repetitive nanosecond pulse discharge in hydrogen-air mixtures, with number of pulses in a 40 kHz burst varying from a few to a few hundred. Time-resolved OH emission, coupled with gated ICCD images of the plasma and the flame, demonstrate that volumetric ignition of H 2–air mixtures occurs in a spatially uniform plasma. The results are shown to agree well with predictions of a new hydrogen-air plasma chemistry model, which incorporates non-equilibrium plasma processes, H 2–air chemistry, non-empirical scaling of nanosecond pulse energy coupled to the plasma, and quasi-one-dimensional conduction heat transfer. In particular, the results demonstrate that the heating rate in low temperature hydrogen-air plasmas is much faster than in air plasmas, primarily due to energy release from exothermic reactions of fuel with O and H atoms generated in the plasma. Kinetic sensitivity analysis is used to identify dominant plasma and chemical processes of hydrogen oxidation, demonstrating that additional heat release in these reactions is a key factor in ignition kinetics. Kinetic modeling calculations demonstrate that removal of the radical generation processes by the nanosecond pulsed plasma from the model completely blocks subsequent exothermic chemical reactions, thus making ignition impossible.

Surface Treatment of Polyethylene Terephthalate Films Using DBD Excited by Repetitive Unipolar Nanosecond Pulses in Air at Atmospheric Pressure

Dielectric barrier discharge (DBD) is proved to be an effective method for surface treatment of polymers over the past 20 years and is now widely used in the fields of surface modification. In this paper, surface treatment of polyethylene terephthalate (PET) films for improving the hydrophilicity using DBD excited by unipolar nanosecond pulses is presented. Two typical discharges exhibiting homogeneous and filamentary modes are obtained under certain experimental conditions, and then, they are used to modify the surface of PET films. The microstructure and properties of the film surface before and after treatment are characterized with water contact angle measurement, scanning electron microscope, atomic force microscope, and X-ray photoelectron spectroscope. The results show that, with the treatment power density of 192 and 158 mW/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in filamentary and homogeneous modes, respectively, surface morphology and property after treatment are significantly changed, and the physical etching and introduction of oxygen-containing polar functional groups account for the decease of surface contact angle and the increase of surface roughness. Compared with the filamentary DBD treatment, the homogeneous DBD (HDBD) is more effective in surface treatment, such as lower contact angle, higher roughness, and more oxygen-containing function groups after treatment. It is attributed to the fact that the HDBD can achieve a uniform treatment and can make the physical and chemical interaction much sufficient.

Pure rotational CARS thermometry studies of low-temperature oxidation kinetics in air and ethene–air nanosecond pulse discharge plasmas

Pure rotational CARS thermometry is used to study low-temperature plasma assisted fuel oxidation kinetics in a repetitive nanosecond pulse discharge in ethene–air at stoichiometric and fuel lean conditions at 40 Torr pressure. Air and fuel–air mixtures are excited by a burst of high-voltage nanosecond pulses (peak voltage, 20 kV; pulse duration, ∼ 25 ns) at a 40 kHz pulse repetition rate and a burst repetition rate of 10 Hz. The number of pulses in the burst is varied from a few pulses to a few hundred pulses. The results are compared with the previously developed hydrocarbon–air plasma chemistry model, modified to incorporate non-empirical scaling of the nanosecond discharge pulse energy coupled to the plasma with number density, as well as one-dimensional conduction heat transfer. Experimental time-resolved temperature, determined as a function of the number of pulses in the burst, is found to agree well with the model predictions. The results demonstrate that the heating rate in fuel–air plasmas is much faster compared with air plasmas, primarily due to energy release in exothermic reactions of fuel with O atoms generated by the plasma. It is found that the initial heating rate in fuel–air plasmas is controlled by the rate of radical (primarily O atoms) generation and is nearly independent of the equivalence ratio. At long burst durations, the heating rate in lean fuel air–mixtures is significantly reduced when all fuel is oxidized.

Surface modification of polyimide films using unipolar nanosecond-pulse DBD in atmospheric air

DBD-induced surface modification is very versatile to increase the adhesion or hydrophilicity of polymer films. In this paper, the DBD is produced by repetitive unipolar nanosecond pulses with a rise time of 15 ns and a full width at half maximum of about 30 ns. The power densities of the homogeneous and filamentary DBDs during plasma treatment are 158 and 192 mW/m 2, respectively, which are significantly less than that using ac DBD processing, and the corresponding plasma dose is also mild compared to AC DBD treatment. Surface treatment of polyimide films using the homogeneous and filamentary DBDs is studied and compared. The change of chemical and physical modification of the surface before and after plasma processing has been evaluated. It can be found that both surface morphology and chemical composition are modified, and the modification includes the rise of hydrophilicity, surface oxidation and the enhancement of surface roughness. Furthermore, the homogeneous DBD is more effective for surface processing than the filamentary DBD, which can be attributed to the fact that the homogeneous DBD can modify the surface more uniformly and introduce more polar functional groups.

重复频率纳秒脉冲介质阻挡放电对聚对苯二甲酸乙二酯表面改性

重复频率纳秒脉冲介质阻挡放电对聚对苯二甲酸乙二酯表面改性

Energy coupling to the plasma in repetitive nanosecond pulse discharges

A new analytic quasi-one-dimensional model of energy coupling to nanosecond pulse discharge plasmas in plane-to-plane geometry has been developed. The use of a one-dimensional approach is based on images of repetitively pulsed nanosecond discharge plasmas in dry air demonstrating that the plasma remains diffuse and uniform on a nanosecond time scale over a wide range of pressures. The model provides analytic expressions for the time-dependent electric field and electron density in the plasma, electric field in the sheath, sheath boundary location, and coupled pulse energy. The analytic model predictions are in very good agreement with numerical calculations. The model demonstrates that (i) the energy coupled to the plasma during an individual nanosecond discharge pulse is controlled primarily by the capacitance of the dielectric layers and by the breakdown voltage and (ii) the pulse energy coupled to the plasma during a burst of nanosecond pulses decreases as a function of the pulse number in the burst. This occurs primarily because of plasma temperature rise and resultant reduction in breakdown voltage, such that the coupled pulse energy varies approximately proportionally to the number density. Analytic expression for coupled pulse energy scaling has been incorporated into the air plasma chemistry model, validated previously by comparing with atomic oxygen number density measurements in nanosecond pulse discharges. The results of kinetic modeling using the modified air plasma chemistry model are compared with time-resolved temperature measurements in a repetitively pulsed nanosecond discharge in air, by emission spectroscopy, and purely rotational coherent anti-Stokes Raman spectroscopy showing good agreement.

Enhancement of ignition characteristics of lean premixed hydrocarbon–air mixtures by repetitive pulse discharges

A newly developed small-sized inductive energy storage (IES) circuit that uses a semiconductor switch for the turn-off action is successfully applied to an ignition system in spark ignition engines operating under lean fuel–air ratios. This IES circuit can generate repetitive nanosecond pulse discharges. Experiments are conducted by means of a spherically expanding flame configuration for C3H8–air mixtures under various conditions. Findings show the investigated ignition system improves the inflammability of lean combustible mixtures in terms of extended flammability limits, shorted ignition delay time, and extended dilution limits, compared with conventional spark ignition systems.

An Efficient, Repetitive Nanosecond Pulsed Power Generator with Ten Synchronized Spark Gap Switches

This paper describes an efficient, repetitive nanosecond pulsed power generator using a Transmission-Line-Transformer (TLT) based multiple-switch technology. Within this setup, a 10-stage TLT and ten high-pressure spark-gap switches are adopted. At the input side, ten spark-gap switches are interconnected in series via the TLT, so that all the spark-gap switches can be synchronized automatically. At the output side, all the stages of the TLT are connected in parallel, thus a low output impedance (5 Ω) is obtained, and a large output current is realized by adding the currents through all the switches. Experimental results show that 10 spark-gap switches can be synchronized within about 10 ns. The system has been successfully demonstrated at repetition rates up to 300 pps (Pulses Per Second). Pulses with a rise-time of about 11 ns, a pulse width of about 55 ns, an energy of 9-24 J per pulse, a peak power of 300-810 MW, a peak voltage of 40-77 kV, and a peak current of 6-11 kA have been achieved with an energy conversion efficiency of 93-98%.

Electrical characterization of dielectric barrier discharge driven by repetitive nanosecond pulses in atmospheric air

Dielectric barrier discharge (DBD) is an important method to produce non-thermal plasma, which has been widely used in many fields. In the paper, a repetitive nanosecond-pulse generator is used for the excitation of DBD. Output positive pulse of the generator has a rise time of about 15 ns and a full width at half maximum of 30–40 ns, and pulse repetition frequency varies from single shot to 2 kHz. The purpose of this paper is to experiment the electrical characteristics of DBD driven by repetitive nanosecond pulses. The variables affecting discharge conditions, including air gap spacing, dielectric thickness, barrier fashion, and applied pulse repetition frequency, are investigated. The relationship between electric field, discharge current, instantaneous discharge power across air gap, and estimated electron density with the length of air gap, dielectric thickness, barrier fashion, and pulse repetition frequency is obtained respectively, and the experimental results are also discussed. In addition, two typical images exhibiting homogeneous and filamentary discharges are given with different experimental conditions.