Search

Recently, ensemble learning paradigm has shown great potential to achieve better prediction performance in the hyperspectral image classification. However, in the traditional methods, each classifier independently searches for the optimal spectral feature subspace in series and some important features are searched repeatedly, which leads to high computing redundancy and low effective utilization of features. In this article, an evolutionary multitask ensemble learning model (EMT_EL) for hyperspectral image classification is designed. First, the model formulates the spectral feature subspaces generation into a multitask optimization problem to concurrently search for optimal feature subspaces for multiple classifiers, which would be capable to select more informative and representative feature subspaces effectively. Second, seeking the optimal feature subspace for one base classifier can assist in the optima-seeking process for some other base classifiers via sharing the useful features, which can accelerate converge toward the direction of the optimal feature subspace, avoid trapping in local optimal subspace and improve searching capability. Third, randomization-enhanced genetic operators are designed for effective and reasonable feature selection, which can facilitate the exchange of information and improve the joint searching efficiency of the feature subspace. Eventually, the quality of generated spectral feature subspaces for each base classifier is improved and the feature sharing can parse HSI data by knowing which spectral features are important. Experimental results demonstrate that the proposed method can generate the appropriate feature subspace for each base classifier, thus it has outstanding classification performance on the different hyperspectral datasets.

The trap parameter and its distribution could significantly affect the surface flashover of insulation mateirials in vacuum. To further understand the influence of traps on flashover, the surface trap parameters were regulated by surface modification. Firstly the atmospheric pressure plasma jets (APPJ) were used to treat the fluorinated ethylene propylene (FEP) film to control trap density, and electron beam irradiation was used to change surface trap energy level of polymer. Then effect of different processing conditions on the insulation properties of material surface was studied. The trap parameters were tested and calculated by means of isothermal surface potential decay (ISPD), and space charge distribution in FEP film body was also measured by means of pulsed electro-acoustic (PEA). At last surface flashover experiment under DC and pulsed voltages in vacuum was performed. The results show that the trap density of FEP surface increases while the trap energy level becomes shallow after APPJ processing. The space charge has a positive correlation with the trap density. The trap level and density increases after electron beam irradiation with different treatment time. The increase of trap level and trap density could improve the vacuum surface flashover voltage to a certain degree by surface modification.

At present, the one-step conversion of greenhouse gas into value-added liquid chemicals with a considerable selectivity is one of the biggest technical challenges to us 1 . Integrating non-thermal plasma (NTP) technology with heterogeneous catalyst is a promising way to tackle this challenge 2 , mainly owning to its strong synergy effect and high efficiency in overcoming the thermodynamic limitation of reactions. In this paper, NTP driven by nanosecond pulse power source, has been utilized for direct CO 2 /CH 4 conversion in a dielectric barrier discharge reactor. This work mainly focuses on revealing the relations between the valence states, surface compositions and defect structures of varied catalytic models on conversion performance and liquid oxygenates distribution.

For the high voltage vacuum interrupter and GIS (Gas insulated switchgear), surface flashover at the dielectric/vacuum and dielectric/air interfaces result in the insulation failure sometimes, which restricts the development of high voltage electrical equipment. The dynamic performances of surface charges on the dielectric are associated with surface flashover closely. Film deposition on the dielectric surface is regarded as a promising mothed to improve surface insulating performance of dielectric material. In this paper, a TiN film deposition method of using atmospheric-pressure plasma jet (APPJ) is proposed. Before and after the plasma modification with TiN film, the electrical properties of dielectric surface are discussed, respectively. The DC flashover voltage of sample with TiN film is increased obviously.

In order to satisfy the demand of industry development, the improvement of the surface insulation performance of polystyrene (PS) is urgent. It is believed that the accumulation of charge on the material can induce the distortion of local electric field, even cause surface flashover. Therefore, in this paper, in order to reduce the accumulation of surface charge, inorganic nano-films are deposited on the surface of PS by plasma modification technology. The plasma excitation source is AC high voltage generator with a frequency of 20 kHz. The PS samples are treated with tetraethoxysilane (TEOS) precursor assisted by plasma for 10 minutes. The working gas is argon and the pressure is sub-atmospheric (2 kPa). Before and after the plasma modification, surface charge accumulation and dissipation characteristics are measured. The results show that the decay rate of surface charge is increased obviously for modified PS. The Fourier Transform Infrared Spectrum (FTIR) indicates that new groups of Si-O-Si, Si-OH are formed on the sample surface. The trap energy level induced by the above groups is lower, namely, the shallow traps are beneficial to the dissipation of surface charge. Thus, the surface flashover withstand voltage is improved significantly.

Propagation of ionization waves (IW s) in long glass tube has been investigated in the last decade as an efficient method for understanding gas breakdown progress. However, the interaction between IWs itself still not fully understood and therefore this paper presents the experimental investigation of IW s propagation and its interaction in a long glass tube under two nanosecond pulse sources with opposite polarity. The interaction between the positive and negative IWs is studied by an intensified charge coupled device (ICCD), and the mechanism of the discharge propagation is analyzed. The experimental results show that the discharge can be divided into three stages: 1. generation of positive polar ionizing waves with velocity of 0.33 cm/ns; 2. generation of negative IW and its interaction with positive ones; 3. the decay of the discharge with the luminance attenuation. Furthermore, the luminance of the discharge channel and the speed of the IW s are enhanced because of the conducted channel behind the positive IW and the encounter of negative and positive IWs. Besides, With the change of the direction of the air flow, the initial time, the speed or the luminance intensity of IW s almost remain unchanged, which indicates the main drive of the IW s is the electric filed rather than air flow.

Plasma aerodynamic actuator based on nanosecond-pulse surface dielectric barrier discharge (ns-SDBD) is an effective technology for active flow control, which can significantly reduce power consumption and improve the aerodynamic characteristics. In this paper, the recent progress of nanosecond-pulse SDBD actuator is introduced. Then, investigation is performed on characteristics of ns-SDBD with different parameters (rise time, pulse width and PRF) of SDBD actuator with two-electrode structure, which has not been widely discussed in the literature. The nanosecond generator can provide a voltage of 0 ~ 15 kV, a rise time of 0 ~ 500 ns and pulse width at peak of 0~ 100 μs, respectively. Experimental results show that plasma discharge mainly takes place during the rise time and fall time, respectively. When the rise time increases, the amplitude of the current gradually decreases and multiple discharges occur, while the ignition of the discharge is delayed by tens of nanoseconds due to high ionization rate. Moreover, the discharge becomes relatively homogenous at the rise time of 50 ns but the plasma length is slightly affected by the rise time. The maximum deposited energy can be obtained at the rise time of 50 ns, little effect appears on the deposited energy when the rise time increases from 100 to 500 ns, which is positively related to the intensity of induced compression wave and propagation velocity. The peak velocity of induced pressure wave decreases from 408 m/s to 369 m/s when the rise time increases from 50 ns to 400 ns. Furthermore, it is shown that pulse width has slightly effect on the current amplitude and discharge uniformity, but longer pulse width will produce more deposited energy and induce two pressure waves. In addition, with the increase of PRF from 250 Hz to 1250Hz, the body force increases from 0.245 mN to 2.107 mN, which is proportional to PRF. To sum up, the actuator operated with short time, long pulse width and high frequency is conductive to shack interaction and airflow control.

As the current in the gap of the switch increases, the gaseous products produced by the transferred arc may reduce the life of the switches. In the arcing stage, as the arc channel develops, different gas components will interact with each other. In this paper, combined the temperature and volume characteristics of high-current arc, the products of the chemical reaction are analyzed. The calculated arc radius is related to the peak and rising slope of the pulse current. As the radius of the arc increases, the volume of the arc channel expands, and more insulating gas is heated. Furthermore, two characteristic parameters of the arc are investigated respectively. The peak current ranges from 150 kA to 300 kA and the width of the pulse is between 1 ms to 3 ms. The content of nitrogen oxides is measured by a flue gas analyzer in the 4 L sealed cavity with 2.3 atm dry artificial proportional gas. It is observed that the concentration of NO2 remains stable around 570 ppm and the concentration of NO increases progressively with the increase of the peak current. According to the nitrogen-oxygen reaction condition and arc expansion theory, under the experimental parameters, the amount of nitrogen oxides increases with the augment of the calculated arc radius.

Surface trapping of insulation materials can participate in the secondary electron emission avalanche (SEEA) development and then affect the surface flashover characteristics in vacuum. To study the relationship between surface trapping and surface flashover, the trap parameters of several typical polymeric materials were deduced based on the isothermal relaxation current (IRC) theory. The surface charging was fulfilled by needle-to-plate corona discharge, and the surface potential was measured by the method of static capacitance probe. The experiment of vacuum surface flashover in direct current (DC) and pulsed voltage was developed, and the relationship between trap parameter and flashover voltage was analyzed. The research results indicate that the trap energy level and trap density of polymeric materials have relation to their flashover characteristics. The shallow trap parameter has more important effect on the flashover voltage, and the flashover voltage decreases with the increase of shallow trap density. The deep trap parameter mainly affects the flashover characteristics after several times of discharges, namely conditioning. The trap parameters have more important effect on the flashover characteristics in DC voltage than in pulsed voltage.

Nanosecond-pulse generator can sustain homogeneous DBD in a small gap at atmospheric pressure1. Time evolution of discharge of nanosecond-pulse DBD is essential for fundamental understanding of pulsed breakdown kinetics2. In this paper, a homogenous DBD is excited by a nanosecond-pulse generator, which has a rise time of 25 ns and a full width at half maximum of 50 ns. Time evolution of the homogenous is captured by ICCD images with a camera gate of 2 ns. In order to shows propagation of streamers in nanosecond-pulse DBD, ICCD images are taken from the front view, the side view and the top view, respectively. The results show that the breakdown appears first near the centre of the high-voltage electrode, followed by breakdown along the entire electrode within several nanoseconds. Note that the plasma emission in the centre of the gap starts decaying earlier than that in the rest of the gap. By the decay of the plasma emission, a luminous layer occurs near the dielectric surface, because electric field reduction in the gap to below breakdown level. The luminous layer lasts approximately 100 ns. After that, a weak emission in the gap is detected again, indicating the secondary breakdown. Furthermore, the side view images exhibit discharge dynamics during the nanosecond-pulse DBD in detail. The discharge develops as a surface ionization wave, originating near the edge of the high-voltage electrode, first extending down towards the dielectric plate, and subsequently propagating symmetrically on both side of the high-voltage electrode along the dielectric surface, with a nearly straight ionization front. The average speed of the surface ionization wave is approximately 0.05 mm/ns. In addition, the top-view images confirm the above propagation process, and the propagation distance of the ionization wave on the dielectric surface is about 3 mm.

Developing plasma jet array is an attractive route to scale the plasma treatment area. Increasing the packing density of plasma jet array may lead to electrical and chemical jet-jet interactions. In this paper, the characteristics of pulsed driven argon plasma jet array composed with three individual plasma jets in 1-D configuration was studied. The effect of pulse duration time (from 50 ns to 1 ms), pulse rise time (50 ns to 1500 ns), pulse falling time (50 ns to 1500 ns) and pulse repetition frequency (1 kHz to 30 kHz) on the argon plasma jet array behavior was investigated. The electron density, vibrational temperature and rotational temperature were calculated based on optical emission. The maximum jet distance causing the jet-jet interactions was 1 cm. The argon plasma jet array presented as a continuously luminous strip in high speed photographs. The jet length decreased under higher pulse duration time and increased under higher pulse frequency. The discharge began from the outside jets followed by jets in-between after jet-jet interactions occurs. The spatial-temporal distribution of Ar emission showed that the Ar emission intensity in the middle jet was higher than outside jets, which indicated that air diffusion played an important role in jet-jet interactions.

As an effective method to optimize the ionization efficiency, [1] high voltage nanosecond pulsed discharge (NPD) has become an emerging technology for the production of nonthermal plasma at atmospheric pressure. Besides the unique advantages in the applications, the gas rapid breakdown mechanism, the discharge mode transition, and the physicochemical processes in NPD are also very significance for the fast pulse voltage. In our work, high resolution temporal-spatial spectra and images are employed to investigate the rapid breakdown mechanism and dynamical evolution process of high-voltage nanosecond pulsed dielectric barrier discharge under needle-plate electrode configuration at atmospheric air. Evolution dynamic processes in a discharge pulse are observed by one-shot ICCD images. There are three main stages in NPD are distinguished, which are the streamer breakdown from needle tip to plate electrode, the regime transition from streamer to diffuse, and the propagation of surface discharge on the plate electrode surface. The temporal-spatial distributions of the emission intensities of N 2 (C3∏ u → B3∏ g ) and N 2 + (B2∑ u +→ X2∑ g +) are investigated and the reduced electric field (E/N) can be calculated by the intensity ratio from the first negative systems of nitrogen ion and second positive systems of nitrogen molecular. It is found the spectra of N 2 + (B2∑ u +→ X2∑ g +) are mainly emitted from the region near the needle tip in the initial period of the breakdown process. The electrical field is maximum on the edge of the needle electrode at the initial time, and decreases with the increase of distance from the needle. After the volume discharge is extinguished at 35–40 ns, an obvious increase of the electrical field near the surface of the dielectric plate appears, which drives the surface discharge propagating to the periphery along the dielectric plate.

Characteristics of plasma synthetic jet (PSJ) actuators driven by repetitive pulsed discharge have drawn much attention. In this paper, the PSJ actuator is composed of a pair of tungsten electrodes and a boron nitride cavity without a cap. The cavity has a diameter of 8 mm and volume of 400 mm'. The distance between two electrodes ranges from 1 mm to 4 mm. The PSJ actuator is excited by a homemade generator CMPC-40D, output pulse parameters of which are as follows: the maximum voltage of 30 kV, the rise time of 0.5 μs, the pulse width of 8 μs, the repetition rate ranges from 0to5 kHz. Effects of pulse repetitive frequency (PRF) and air gap spacing on the discharge characteristics of a single actuator are investigated. Experimental results show that the discharge voltage increases with the increase of the gap spacing but decreases with the increase of the PRF. During the ignition time, the discharge voltage falls to zero in a few nanoseconds and the current discharge is damped sinusoidal oscillation. We observe that the energy dissipation oscillates in time, for a total discharge energy of 2 to 9.9 mJ. The energy decreases with the increase of the PRF, but it increases with the increase of the gap spacing. Furthermore, three actuators in series connection are studied. Experimental results show that synchronous discharge could be achieved for three PSJ actuators.

In an inhomogeneous electric field, there are several kinds of discharge modes existing in microsecond-pulse gliding discharges. In this paper, we investigate mode transition among different discharge modes and the effects of pulse repetition frequency (PRF) and gap distance on microsecond-pulse gliding discharge by measuring the voltage-current waveforms and discharge images. Experimental results showed that a diffuse discharge was obtained in atmospheric pressure in microsecond-pulse gliding discharge. When the applied voltage was 18.5 kV, diffuse-to-spark transition occurred when the PRF increased from 500 Hz to 2000 Hz. Furthermore, the diffuse discharge might appear when the electrode gap exceeded 5 mm. When the applied voltage was 12 kV, decreasing the gap spacing leaded to diffuse-to-spark transition. Therefore, both PRF and gap distance affect the discharge mode in microsecond-pulse gliding discharges. Diffuse-to-spark transition occurs when the PRF increases or the gap spacing decreases. It could provide reference for obtaining volume discharge at atmospheric pressure.

The technology of portable spark source, independent of surrounding water environment, needs to be developed for some special situations. The transfer efficiency from electric energy to seismic wave energy is the most important factor in this technology. The electrode filled with water was firstly designed to develop the pulsed discharge experiment. The seismic wave generated by pulsed discharge was measured by seismic wave detector. The relation of seismic wave amplitude and discharge parameter was observed. The electrode component is machined by stainless steel and other materials which have similar acoustic impedance with salt water to enhance the transfer efficiency from electric energy to acoustic energy. The piston-like electrodes were also designed to enhance the transfer efficiency from electric energy to mechanical energy, in which some components could be active up and down due to the impulsive force produced by pulsed discharge. The seismic wave in different electrode construction was measured and analyzed. The experiment result indicates that the amplitude of seismic wave is largest in piston-like electrodes, which also indirectly indicates that the energy of pulsed discharge in closed electrode filled with water is transferred to the ground surface layer mainly by mechanical shock.

This paper describes a 10kV cascade pulsed power supply for discharge applications with pulse width from 5μs to 30μs. Each cascade unit contains a main switch for generating a voltage switch and a chopping switch to reduce the fall time. A number of IGBTs are used for switching, and the driver devices adopt the design of pulse transformer. The trigger signal is generated by the H-bridge inverter circuit. The forward signal is used to drive the main switch, and the reverse signal is used to drive the chopping switch. Each unit contains a free-wheeling diode, which can bypass the pulsed current when the main switch is not turn-on; therefore, the trigger signal does not need to be consistent of the rise time. When the chopping switch is working, the trigger signal must be the same because the switches are in series. The reverse diode paralleled in the main switch and the pulse capacitor formed an active voltage balancing circuit. Since the capacitance is usually very large, the rise time of the chopping switch trigger signal does not need to be more synchronized. Simulation analysis proved above analysis by using the software of Saber.

Diffuse discharge sustained by microsecond pulses can generate volume discharge at atmospheric pressure, which has been widely studied since 2010s. Although the fast rising time limits the formation of spark stream under certain condition in microsecond-pulse discharges, the pulse repetition rate and gap spacing may also affect the discharge mode. In this paper, the optical emission spectra and discharge images are investigated to analyze the optical and illuminant characteristics of microsecond-pulse discharge at different discharge modes. The emission spectra were used to analyze the optical characteristics of microsecond-pulse discharge. The experimental results showed that the emission spectrum of diffuse discharge ranged from 300 nm to 425 nm. Typical spectral lines, such as OH, N 2 , N 2 +, are observed. The emission spectrum of spark discharge ranged from 200 nm to 1000 nm. The spectral lines included NO, OH, N 2 , N 2 + and O, among which the spectral lines at 777.1 nm had the largest intensity, indicated the existence of oxygen species in spark mode. Furthermore, the grey-scale image was used to estimate the illuminant characteristics of different discharge modes in microsecond-pulse discharges. The maximum grey-level values were 60, 180 and 255 when the discharge modes were corona, diffuse and spark, respectively.

Plasma aerodynamic excitation based on surface dielectric discharge is a effective approach for flow control, which can significantly improve the aerodynamic characteristics of the aircraft. However, the plasma excitation interaction region is limited and there are no significant effects by using typical two-electrodes sustained by nanosecond-pulse dielectric barrier discharge, therefore, it is difficult to satisfy the actual industrial applications. In this paper, experimental investigations on a three-electrode structure driven by a negative DC high voltage superimposed nanosecond pulses are carried out in order to obtain large-scale plasmas. A surface dielectric barrier discharges and a DC corona discharge in a three-electrode geometry are generated when the actuator are sustained by nanosecond pulses and negative DC high voltages. The nanosecond generator is homemade and could provide a voltage of 0 ∼40 kV and a pulse repetition frequency (PRF) and 0 ∼ 2 kHz, respectively1, 2. The effect of the amplitudes of DC voltage and nanosecond pulse voltage, and PRF on characteristics of nanosecond sliding discharge is studied. Experimental results show that the plasma discharge region extends when a negative DC high voltage is applied on the third-electrode, where a nanosecond sliding discharge occurs. There is a minimal potential difference threshold VP.D(|VP-VD|, here Vp is the voltage amplitude of nanosecond pulse, VD is the voltage of negative DC) to ignite a nanosecond sliding discharge and a maximum VP.D to sustain a stable sliding discharge. Arcs will be generated when the VP.D exceeds the maximum threshold. Furthermore, in order to maintain a stable sliding discharge, the VP-D should be kept between the two thresholds. Discharge intensity increases with the increase of both DC and pulse voltage amplitude. In addition, when VD is fixed at a relatively lower value, changing voltage amplitude of nanosecond pulse is preferable to maintain a nanosecond sliding discharge.

Because of the lack of crude oil resources all around the world, hydrogen resource has been developed for the feedstock instead of crude oil. As the cost-effective activation of methane, hydrogen production of the methane conversion has been extensively studied. In this paper, characteristics of the microsecond pulsed and nanosecond pulsed dielectric barrier discharge (DBD) 1 are experimentally investigated to obtain the effect of pulse repetition rate and flow rate of CH4 on hydrogen production using plasma cracking. The gas productions were analyzed by gas chromatograph. The experimental results showed that the main productions are H2, C2H6 C2H4 and C3H82. The conversion of methane and the yield of hydrogen are prominently affected by the pulse repetition rate and the flow rate of CH4. The conversion of methane and the yield of hydrogen respectively reached a maximum of 29.5% and 14.3% when the pulse repetition rate was 2000 Hz and the flow rate was 25 ml/min. Moreover, it was found that the selectivity of the gas productions significantly increased with the decrease of pulse repetition rate. The selectivity of hydrogen increased with the decrease of flow rate of CH4 when the pulse repetition rate was 2000 Hz. Considering the formation of carbon and C4 in the reactor, the carbon balances for all the cases were calculated to be between 79.26% and 98.82%, which achieved a maximum of 98.82% when the flow rate of CH4 was 100 ml /min and the pulse repetition rate was 1000 Hz.

Diffuse discharge could achieve glow-like discharge at atmospheric pressure1,2. It is important to understand the physics of the diffuse discharge due to the need for plasma surface treatment and the pure scientific merit. In this paper, characteristics of diffuse discharges in a blade knife-to-plate gap under different power sources and different discharge gap distances are investigated. Two homemade power supplies, microsecond power supply (CMPC-40D) and nanosecond power supply (MPC-50D), are used to produce diffuse discharges. In the experiments, diffuse discharges are generated in a blade knife-to-plate gap. The blade knife has a length of 20 cm, and the gap spacing ranges from 2 cm to 3 cm. Temporal evolution of atmosphere pressure diffuse discharges is conducted by ICCD (Andor-DH334T). Both positive and negative pulses under the conditions of gap spacing of 2 cm, 2.5 cm and 3 cm, power frequencies of 500 Hz, 1000 Hz and 1500 Hz are investigated. The discharge images taken by digital camera shows that the discharge is a diffuse mode in atmospheric pressure. Furthermore, discharge images taken by ICCD with an expose time of 2 ns are used to analyze the temporal evolution process of the diffuse discharge. The experimental results show that the diffuse in a large gap is more intense than that in a small gap. Moreover, the ICCD images present that the initial radiant area emerged near high voltage electrode, and it costs about 40 ns for forming a strip form plasma channel. The plasma channel moves forward and arrives at the negative electrode at 60 ns. It is calculated from the ICCD images that the velocity of the plasma channel is about 3×105 m/s.

Combined with plasma enhanced chemical vapor deposition (PECVD), atmospheric pressure plasma jet (APPJ) can be used for depositing film on polymer surface due to its flexibility and high stability. In this paper, the APPJ is excited by a microsecond-pulse generator, and Hexamethyldisiloxane (HMDSO), working as the reaction precursor, is injected into the APPJ discharge region by the carrier gas (Ar) to deposit on the surface of epoxy resin. Surface morphology and chemical composition of the epoxy resin surface before and after the deposition are measured by water contact angle measurement, scanning electron microscope (SEM) and fourier transform infrared spectroscope (FTIR), respectively. The decay characteristic of surface charge is analyzed by surface potential measurement system. Experimental results indicate that SiO2-like deposition area on the epoxy resin surface achieves super hydrophilicity and the maximum contact angle reaches 160°. Surface roughness increases and silicon-containing functional groups are introduced on the epoxy resin surface. In addition, surface charge decays faster after APPJ treatment than the original sample, which is beneficial to the enhancement of the flashover voltage on the epoxy resin surface.

Diffuse discharge can generate large-scale volume discharges at atmospheric pressure1, which could be used in surface modification of metal. In this paper, the feasibility of SiO2-like film deposition on copper surface by atmospheric pressure diffuse discharge is achieved. The diffuse discharge is driven by a microsecond-pulse power supply in a point-to-plate electrode configuration, and HMDSO (the precursor) is directly injected into the discharge region together with gas flow of Ar/air. The chemical composition of Cu surface before and after deposition is analyzed by fourier transform infrared spectroscopy (FTIR). Experimental results show that there is a intense peak at 1060 cm−1, which indicates SiO2-like coatings have been successfully deposited on the copper surface. Furthermore, lower HMD SO concentration can contribute to a better deposition with lower carbon content in the experiments. Moreover, surface morphology and thickness of the deposited film are also observed by atomic force microscope (AFM). The result shows that high amplitude of the applied voltage suppresses the increase of the thickness of the deposition film, but it is benefit to obtain a smooth surface. Such phenomenon can be explained by the etching of high-energy particles in plasmas. In summary, atmospheric-pressure diffuse discharge is a promising way to deposit SiO2-like film on metal surface.

Gas insulated switchgear (GIS) for power system becomes more and more compact and, thus, flashover phenomenon is more likely to occur on the surface of the support devices made of epoxy in the GIS, which is harm for the insulation and operation of the GIS device1, 2. In this paper, the flashover characteristics of the epoxy insulator under microsecond-pulse excitation are investigated. In the experiments, the epoxy insulator samples are placed in a sealed chamber filled with SF 6 or other mixed gas, and the flashover voltage and flashover time is measured. The morphology and elemental composition of the surface of the samples before and after the flashover are analyzed by atomic force microscopy (AFM), 3D digital microscope and X-ray photoelectron spectroscopy (XPS). The experimental results show that the surface morphology (roughness) is closely related to the insulation strength of the epoxy surface, and the surface elemental composition is related to the insulator's electrification level. The results will make a good reference for improving the surface flashover voltage of the epoxy insulators in the GIS device.

Electrical modelling is a very important method for studying the dielectric barrier discharge (DBD). The traditional models usually simulate the dynamical behaviors of DBD through combination of many circuit elements1 and have obtained more and more accurate results as the number of parameters increases. However the increasing parameters severely increase the complexity of the models, and as a result it is difficult to flexibly describe the dynamics of the DBD according to the actual operating situation with these models. In addition, the traditional models can not reflect the inherent mechanism of the DBD. Memristor, which is a nonlinear circuit element with both resistive and memory properties and is known as the fourth fundamental circuit element, has shown many characteristics similar to the DBD, such as the parallel hysteresis phenomenon2 and the application of one-dimensional fluid model in their physical models3. The similarities between the DBD and the memristor intrigue us to wonder whether the DBD is a memristor in nature. If this assumption is true, the electrical simulation circuit could be greatly simplified and its applications would also be pushed forward tremendously. Moreover, this theory would greatly promote studies on the mechanism of the DBD. In this paper, we demonstrate that the DBD has fingerprint characteristics of the memristor through simulations using a one-dimensional fluid model, preliminarily showing the memristive nature of the DBD.

Atmospheric-pressure non-thermal plasmas become more and more widely used in the fields of ignition and combustion. In this paper, a home-made microsecond-pulse power supply is adopted to investigate the characteristics of gliding discharges in air flow. The generator could produce repetitive pulses with an output voltage of up to 30 kV, a pulse width of ∼8 μs, and pulse repetition frequencies (PRFs) of 1 Hz ∼3000 Hz1, 2. The voltage-current waveforms and the discharge images are obtained to analyze the characteristics of microsecond-pulse gliding discharges. Furthermore, effects of the gas flow rate, the PRF, and the electrode gap on the electrical characteristics and discharge luminance are conducted. Experimental results show that there are two typical discharge modes in microsecond-pulse gliding discharges i.e. diffuse discharge and spark discharge. Moreover, the gas flow rates slightly affect the electrical characteristics of diffuse discharge but significantly affect the breakdown voltage of the spark discharge. PRF and electrode gap place a more significant role on the gliding discharges in both diffuse discharges and spark discharges.

Pulsed discharges, especially with fast rise time and high over-voltage, usually accompany by some specific pheromones in the nonuniform electric field. In this paper, the appearance of bright spots on the electrodes in air breakdown and the inversion of polarity effect in SF 6 breakdown are investigated. Experiments were carried out by using three experimental setups with a full width at half maximum (FWHM) at a matched load of 2, 30–40 and 130 ns. The experimental results show that in the case of air breakdown, bright spots on the flat electrode appear for both positive and negative polarity of the voltage pulse. At the negative polarity of the electrode with a small radius of curvature, these bright spots on the flat electrode arise when current through the gap changes its direction and pulse duration increases. At the positive polarity of the electrode with a small radius of curvature, these bright spots on the flat electrode appear due to the participation of the dynamic displacement current in the gap conductivity. In the case of SF 6 breakdown, with the FWHM of 2 ns, the inversion of polarity effect occurs at the pressures of 0.25 MPa and lower for SF 6 and SF 6 -N 2 , and with the FWHM of 30–40 ns, the inversion of polarity effect occurs at the pressure lower than 0.12 MPa for SF 6 . Such inversion of the polarity effect may be related to the ionization wave front. The ionization wave velocity is higher for positive polarity of voltage pulse than that for negative polarity of voltage pulse.

A 6 cm long helium plasma jet torch was generated under atmospheric pressure. A tungsten needle was inserted into the middle of quartz tube and worked as a high HV electrode. The current and voltage of discharge were monitored by an oscilloscope. The spectra of plasma jet were recorded by optical emission spectrum1. In order to improve the spatial resolution of the optical system, a 0.6-mm-in-diamter field diaphragm is placed in front of optical fiber and ensured a spatial resolution of 6 mm. Intensity of N 2 , He and O in axial and radial distribution were investigated. In addition, spatial distribution of rotational temperature, vibrational temperature, electron excitation temperature, and electron density were well studied 2. Results showed that He and O intensity decreased with the increase distance from plasma plume root, while N 2 intensity peaked at 3 cm. Spatial distributions of rotational temperature, vibration temperature and electron excitation temperature were similar. They decreased first and then increased with the distance.

The configuration and characterization of plasma jet are essential for their various applications. In this paper, two different configurations are used as high electrode: single tungsten electrode and tungsten electrode covered with quartz tubing (inner tubing). The high electrode is inserted into the middle of quartz tubing (outside tubing). The effect of inner quartz tubing on electrical property was investigated. Furthermore, the plasma jet was analyzed by optical emission spectroscopy (OES) by changing the pulse repetition frequency and outside tubing diameter. The experimental results showed that with quartz tubing, the initial excitation voltage was increased but the current decreased. With a higher frequency, the discharge would be more intense but with a lower voltage tolerance ability. The plasma jet plume injected to air straightly in 5 mm quartz tubing, while the plume curled up in 10 mm quartz tubing. OES observed that the intensity of reactive species in plasma jet increased when higher frequency and larger quartz tubing diameter was used.

Pulsed power can be used for the generation of low-temperature plasma, and different discharge modes, which has been widely used in the field of surface modification1, 2. In this paper, modification on the copper surface by nanosecond-pulse diffuse discharges with a tube-to-plane gap is conducted in atmospheric air. The characteristics of the diffuse discharges are investigated to determine the modification conditions. The experimental results show that there are three typical regions in the diffuse discharges, i.e. central region, diffuse region and the edge region. Different regions play different roles in the surface modification. It is found that diffuse region has the best performance in the treatment. Furthermore, the hydrophilic property and surface energy are significantly enhanced after the modification. Compared with the untreated sample, both the nitrogen and oxygen proportions for the diffuse region and the edge region significantly increase, but the corresponding carbon proportion decreases. However, nitrogen and oxygen proportion for the central region slightly changes after the modification, but the corresponding carbon proportion increases. In addition, Micro-hardness measurements show that the surface hardness increases about 25% after the treatment.

Nanosecond-pulse dielectric barrier discharge (DBD) can provide non-thermal plasmas with extremely high energy and high density for achieving complicated physical and chemical reactions in surface treatment of polymers. In this paper, the surfaces of low density polyethylene (LDPE) films are modified by atmospheric-pressure non-thermal plasmas generated by nanosecond-pulse DBD 1, 2. The DBD plasma is excited by a home-made nanosecond-pulse generator with a rising time about 70 ns, and a full width at half maximum of 200 ns. The surface water contact angle, surface free energy, surface roughness and surface chemical composition of the LDPE surfaces are measured by contact angle measurement, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) before and after treatment. Space charge distribution in LDPE before and after surface treated is estimated by pulsed electro-acoustic method (PEA). The aging characteristics are also studied by placing the treated materials in atmospheric air for a period of time. Experimental results show that as the treatment time increases, the contact angle decreases and the surface energy increases. Both the surface roughness and the oxygen-containing groups increase after the treatment. In addition, PEA measurements show that the plasma treatment can significantly suppress the space charge injection of the LDPE surface.

In most case, stable, relatively uniform, and large-volume plasmas in atmospheric air are desirable for various applications. Therefore, different methods are used to obtain a diffuse discharge, among which exciting power source is a key parameter. In this paper, pulsed discharges can be produced by microsecond or nanosecond pulses in open air. Two kinds of pulsed powers are applied to the HV electrode. The home-made generators can produce repetitive microsecond pulses or nanosecond pulses of up to 30 kV, respectively. The output pulse of one generator has a rise time of 300–500 ns and a full width at half maximum (FWHM) of about 6–10 μs. The output pulse of the other generator has a rise time of 70 ns and a FWHM of 100 ns. Diffuse discharges are obtained in a highly non-uniform gap in open air. Two kinds of electrode configurations are used. One type uses a commercial saw blade as high-voltage electrode, and the other type uses a series of knife blade as the HV electrode. Electrical characteristics of the diffuse discharges in open air are studied according to the voltage, current waveforms, and discharge images. Results show that diffuse discharge can be obtained with an axial distance of 30 cm and a gap spacing of 2 cm for both nanosecond pulse and microsecond pulse when the pulse repetition rate is 1 kHz. Nanosecond pulse is better for sustaining a uniform diffuse discharge than microsecond pulse excitation. In addition, the nanosecond pulse generator can provide more energy to the excitation of diffuse discharges than the microsecond pulse generator in our experiment.

With the plasma applications, kinds of high voltage (HV) pulsed generators of different principles and different parameters are developed for the increasing requirement. For the reliability and compactness of HV pulsed generators, the study on pulse transformer is required. Pulse transformer can easily achieve HV output and compress pulse width. In this paper, a wideband pulse transformer was developed with repetitive frequencies varying from 0 to 10 kHz. By calculating the magnetization characteristics and choosing the suitable core material, the volume and winding lines, turns, cross-sectional area, winding method, and insulation of pulse transformer are designed. Depending on semiconductor switches and pulse transformer, a compact repetitive frequency microsecond generator was developed, with the output voltage amplitude of up to 30 kV, repetitive frequency varying from 0 Hz to 10 kHz and pulse width of 8 μs. Finally, the generator is used for the large-area dielectric barrier discharge experiments, and discharge voltage, discharge current, and the preliminary discharge characteristics are presented.

The aim of the work was to take measurements during subnanosecond breakdowns with a time resolution no worse than 100 ps. The current through gaps filled with atmospheric pressure air was investigated during the generation of runaway electrons and X-rays. Three generators, when ensuring the formation of a runaway electrons preionized diffuse discharge (REP DD), were used. It is confirmed that during a subnanosecond voltage rise time, the amplitude of the dynamic displacement current can be higher than 4 kA. It is demonstrated that during the generation of supershort avalanches electron beams with amplitudes of 10 A and more. The conductivity in the air gap at the breakdown stage is ensured by the ionization wave, whose front propagates from the electrode of small curvature radius, and by the dynamic displacement current between the ionization wave front and the plane electrode.

In this paper, diffuse discharges sustained by nanosecond pulses are obtained in air gaps with a multi-needle-to-plane discharge electrode configuration at atmospheric pressure. A repetitive nanosecond generator with a rise time of 70ns and a full width at half maximum of 100ns is employed to drive diffuse discharges. Characteristics of the diffuse discharge were investigated by analyzing voltage-current waveforms and discharge images. Experimental results showed that a large-area diffuse discharge could be obtained with a pulse repetition frequency of up to 1 kHz. Furthermore, as the applied voltage increased, discharges transformed from corona discharge to diffuse mode, meanwhile the discharge current changed rapidly. It was also found that the discharge intensity varied according to the position of the needles; and discharge intensity in the center was weaker than that near the margin. In addition, the composition of the measured discharge current was calculated and analyzed.

Surface dielectric barrier discharges with five different grounded electrode's widths are produced in order to investigate the effect of grounded electrode's width on electrical characteristics. The electrical parameters including applied voltage, total discharge current and transported charges are measured and calculated, respectively. Results show that the total discharge current has two obvious spikes. The first current spike of the total current has a peak which results in some fluctuations of transported charges in V-Q Lissajous figures, but the peak of the first current spike hardly changes with the electrode's width. The second spike peak value increases with the electrode's width, and reaches its maximum at an optimum width. All the V-Q loops at different electrode's widths are “almond-shape”, and the transported charges and energy per pulse increase with the electrode's width. The main plasma action area increases with the electrode's width, but keeps relatively constant when the grounded electrode's width is larger than the optimum width. In addition, the grounded electrode's width has no obvious influence on the plasma light intensity.

Atmospheric pressure low-temperature plasma jet has become one of the hot research issues in plasma science and technology, and a plasma jet array is a tendency for plasma jet application. In this paper, atmospheric pressure low-temperature plasma jet array is generated by a one-dimensional device of two quartz tubes with a single ring electrode configuration, and the feed gas is helium. The discharge excitation power is a repetitive nanosecond-pulse generator. The main discharge parameters such as applied voltage, total discharge current, emission spectra were measured. Results show that nanosecond pulses can produce a stable one-dimensional plasma jet array. The produced two plasma jets fired in the same initiate voltage and show the well uniformity. When the quartz tube placed extremely closely, with the increase of gas flow, the visual length of one tube remain unchanged, which has much weaker intensity than another. Because the asymmetric electric field distribution, a great quantity of charges shift to one tube, forming the coupling phenomenon. Experimental results show that with the distance increase between two tubes, both the jet length and intensity are decreased.

Dielectric barrier discharge (DBD) is an important method to produce non-thermal plasma in atmospheric air, which is widely used in a variety of industrial fields. In this paper, the discharge is generated in atmospheric air using a magnetic compression solid-state pulsed power generator. 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 characteristics are studied by the measurement of voltage and current waveforms, discharge images, and optical emission spectroscopy. The experimental results show that when the air gap is less than 3 mm, no filaments are observed and the DBD is diffuse in the whole discharge regime. The rotational and vibrational temperatures of the DBD are calculated according to the emission band of the nitrogen second positive system near 380.5 nm. The relationship between the emission intensity and the rotational temperature with the applied voltage and pulse repetition frequency are investigated and discussed. The experimental results show that the emission intensity is sensitive to the applied voltage and pulse repetition frequency; however the rotational and vibrational temperatures indicates insensitivity to the applied voltage and pulse repetition frequency.

Pulsed discharge is a promising approach for producing non-thermal plasma at atmospheric pressure. Pulsed power generators vary widely in performance and should be chosen according to the application requirements. In this paper, a repetitive microsecond-pulse generator was constructed using a cascading of power modules, where all solid-state switches were considered as the key units. The generator was able to produce repetitive pulses with a peak voltage of 2 kV, a pulse duration of a few microseconds, and a pulse repetition frequency of 5 kHz at a 1000Ω resistive load. Output pulse voltage could be adjusted by varying ac input voltage. Both the pulse width and frequency could be controlled by changing the driving signal of the IGBT. In addition, this generator was successfully used in the plasma application.

Cold atmospheric-pressure plasma is rich in active particles, with good physical and chemical properties. It has a wide range of applications in areas such as surface modification of materials, and biomedical and environmental treatment. A dielectric barrier discharge is a traditional way to generate cold plasma, but its confined discharge gap limits some applications. The plasma jet is a method to produce plasma beyond the plasma generation region, and has a relatively simple electrode structure. It can be applied in a narrow space and on complex surfaces. In this paper, a single needle electrode is used to generate a plasma jet that is driven by unipolar repetitive nanosecond pulses with rise time of 25 ns and a full width at half maximum of 40 ns. The characteristics under different experimental conditions are observed. On the basis of previous work, film surfaces of polyethylene terephthalate and polymethylmethacrylate are modified, and hydrophilic and hydrophobic results are achieved correspondingly.

The relation of desorbed gases and surface discharge in vacuum is a complex problem without reliable theory. The vacuum experimental platform of direct current and nanosecond pulse source was built, and the quadrupole mass spectrometer connected to vacuum cavity was used to analysis the out-gassing characteristics of insulation materials. Then the vacuum surface discharge experiments were done and the out-gassing characteristics were measured in the process. By comparing the mass spectrograms before discharges with ones after discharges, we obtained the out-gassing composition of several insulation materials. Meanwhile we analyzed the probable sources of different gases. The results show that the high polymer surface may decompose and the dissolved gases may be extracted besides gas desorption in discharges. The discharge pattern has direct effect on their out-gassing characteristics. We'll thoroughly research their relation in subsequent work.

Nanosecond pulsed discharge has various discharge forms, especially, dense diffuse discharge is closely related to maximum x-ray radiation and runaway electron beams. In the paper, a two-stage magnetic compression solid-state pulse generator was used to produce repetitive nanosecond pulses for excitation of nanosecond-pulse diffuse discharge. Output pulse of the generator had a rise time of about 25 ns and a full width at half maximum of 40 ns. Electrical characteristic of diffuse discharge was studied by measuring its voltage-current waveforms and discharge image. In addition, the conduction current was calculated by subtracting the displacement current from the measured current. The experimental results showed that a large-scale and stable diffuse discharge could be obtained at atmospheric pressure. Furthermore, it indicates a good agreement between the measured current without discharge in single shot mode and the calculated displacement currents in all cases during the experiment. The calculated conduction current was unipolar and had amplitude of several amperes or less.

Surface dielectric barrier discharges excited by high nanosecond voltage pulses are a promising approach for producing plasma required by airflow control. Charge transport is an important parameter of discharge characteristics, and the discharge energy can be obtained using an electric charge - gap voltage (Q-V) Lissajous figure. Based on a repetitive nanosecond-pulse generator, the characteristics of Q-V Lissajous figures in surface discharge in atmospheric air were investigated in this paper. Results showed that typical Q-V Lissajous figure was like an ellipse and the characteristic of the Lissajous figure varied with the discharge parameters. Both transport charge and energy in single pulse are in proportion to applied voltage amplitude. The energy obtained by Lissajous figure is always lower than by V-I-t integration, and the average power was able to reach hundred milli-watts while the instantaneous power was able to reach tens of kilo-watts. The applied voltage played a more important role in the uniformity of surface plasmas.

Achieving high target recognition accuracy is a pursuing and challenging issue for hyperspectral data analysis. The complicated imaging environment and noise interference lead to heterogeneous spectra within the homogeneous object, which makes the current spectral target recognition methods be lack of robustness. In this paper, a robust spectral target recognition method is proposed based on the combined spectral signatures, in which two key techniques are concerned. One is support vector data description (SVDD), which can tolerate the spectral variations of different pixels in the same object. Another is an effective spectral signature combined of spectral reflectance and spectral derivative, which can be robust to data characteristics with different spectral-amplitude variation. The proposed method outperforms the classical methods with only spectral reflective information in term of target recognition accuracy and robustness.