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Both atmospheric pressure dielectric barrier discharge (AP-DBD) deposition and direct fluorination are effective methods to modify the surface electrical properties of polymer insulation. To investigate the surface charge decay properties of Al 2 O 3 -filled epoxy resin (ER) prepared by AP-DBD and direct fluorination treatments, before and after drying, samples were studied under various treatment parameters. The experimental results show that charge decay rates for AP-DBD and fluorinated samples are more than 90% after 5 days in ambient air. The corresponding bulk and surface resistivity is reduced when the charge decay rate increases. After a longer time, the surface charge on the fluorinated samples decays faster than those on the AP-DBD samples before drying. The hydrophilicity of the fluorinated samples significantly affects the surface charge decay after drying. After 100 days of storage, the decay rates for the fluorinated samples decrease with increased storage time (less than 9% after 100 days) while for AP-DBD treated samples the decay rates exceed 24%. The surface resistivity of the AP-DBD samples is always lower than that of the fluorinated ones. Therefore, for a long storage time, AP-DBD treatment may provide a better solution to the surface charge decay of ER than direct fluorination treatment, after drying. [ABSTRACT FROM AUTHOR]

The surface charge accumulation on polymers can reduce the insulation performance of the materials and even cause surface flashover. The characteristics of surface charge accumulation and dissipation for epoxy resin (EP) exposed to various nanosecond pulse voltage parameters (i.e. pulse rise time, pulse width and voltage amplitude) are investigated. The results show that the fractional decay of surface charge increases with pulse rise time and decreases with pulse width and voltage amplitude. However, abnormal dissipation phenomenon appears when the pulse voltage reaches 9 kV. Analysis suggests that the abnormal dissipation phenomenon can be attributed to surface charge transport primarily along the surface. Furthermore, the surface charge fractional decay is less than 30% for a dissipation time of 30 min under different nanosecond pulse voltage parameters. In addition, the method of atmospheric pressure plasma jet (APPJ) is utilized to deposit SiCxHyOz thin film on the surface of epoxy resin (EP) samples. It is found that the fractional decay of surface charge for treated EP samples is promoted approximately by an average of 90%. The trap density distribution indicates that the shallow traps that are formed by APPJ treatment result in a decrease in the trap energy level. [ABSTRACT FROM AUTHOR]

This work presents experimental results on different discharge modes for microsecond-pulse gliding discharges and their transition, and then analyzes the effect of the gap distance and flow rate on the discharge modes. The current amplitude, energy consumption per pulse and voltage range are used to reflect some discharge characteristics of discharge modes. The results show that there are three discharge modes for microsecond-pulse gliding discharges, i.e. the corona, diffuse and spark. Among the three modes, the values of the amplitude of the discharge current and the energy consumption per pulse for corona mode are lowest, then the diffuse mode and spark mode accordingly. When the gap spacing is smaller than 5 mm, there is no diffuse mode, and the corona discharges directly transit to spark discharges with the increase of the voltage. When the gap spacing is or more than 5 mm, the corona-diffuse-spark mode transition with the increase of the voltage is observed. The voltage range for diffuse discharge increases with the gap spacing. Moreover, when the flow rate increases, the ignition voltage for spark discharges increases, but the ignition voltage for diffuse discharges slightly changes, resulting in the voltage range for diffuse discharges increases. Therefore, diffuse mode is more likely to be obtained in a large gap with a relatively high flow rate. [ABSTRACT FROM AUTHOR]

Long-time partial discharge (PD) is regarded as one of the main reasons for the insulation failure of high-voltage cables. In this work, we report on the application of atmospheric pressure plasma to deposit siloxane film on copper to avoid PDs. The dielectric barrier discharge (DBD) plasma is driven by AC power supply, with tetraethoxysilane (TEOS), argon, and oxygen mixture as the source gas. The effect of oxygen gas flow rate on the thin film surface morphology, chemical composition, and electrical properties is studied systematically. Our results show that the stability of the plasma deteriorated, when the oxygen flow rate exceeded 10 sccm. The addition of oxygen in the source gas induced a high oxidation level of deposited thin film. The chemical composition of thin film was in the form of SiOx ( x = 1.9) with 10 sccm oxygen compared to SiOx ( x = 1.2), when deposited in the absence of oxygen. Additionally, the optimal thin film with good stability was obtained with the surface resistivity 1.1 × 1011 Ω, when deposited in the absence of oxygen. The corresponding relative permittivity of the deposited thin film is 2.9. The simulation results demonstrated that the electric field distortion was weaken after the film deposition, which reduced the probability of PD to occur. [ABSTRACT FROM AUTHOR]

In this paper, an atmospheric-pressure dielectric barrier discharge is used to modify the surface of the epoxy material and enhance the dissipation of surface charge to reduce the accumulation of surface charge. In the experiments, atmospheric-pressure air dielectric barrier discharge is driven by a microsecond pulse generator. Surface properties of epoxy before and after the plasma treatment are characterized by water contact angle, surface potential, and surface/volume conductivity measurements. Atomic force microscope and X-ray photoelectron spectroscopy are used to investigate the changes of the morphology and the chemical composition of the epoxy surface. Experimental results indicate that the surface of epoxy is etched by the plasma and the increase of the surface roughness enhances the surface insulation ability. The O radicals in plasma and the carbonyl groups formed on the surface make the surface charge trap shallower, change the epoxy surface composition then increase the surface conductivity and accelerate surface charge dissipation. When the epoxy is treated for an appropriate time, the epoxy surface insulation performance will be enhanced obviously and the surface charge dissipation will be accelerated. [ABSTRACT FROM PUBLISHER]

Hydrophilic modification of polymethyl methacrylate (PMMA) surface is performed by atmospheric pressure plasma jet (APPJ) in Ar gas for improving the PMMA surface flashover performance in vacuum. In the experiments, APPJ is driven by a microsecond-duration pulsed generator, which has voltages of 0-30 kV, a rise time of 300 ns and a full width at half maximum of 2μs. Characteristics of the APPJ are analyzed according to its voltage and current waveform, discharge image and optical emission spectrum. Furthermore, surface properties of the PMMA surface before and after the treatment are characterized by water contact angle measurements and morphology observations. Results show that the main species of the plasma jet are composed of N2, Ar, OH, and O, among which such polar groups as OH and O enhance the hydrophilic property of the PMMA surface. The water contact angle decreases from 68° to a minimum value (16°) after the treatment. In addition, all the surface flashover voltages in vacuum for the PMMA samples treated by APPJ are higher than those for the untreated PMMA samples. [ABSTRACT FROM AUTHOR]

This paper describes experimental studies of the dependence of the X-ray intensity on the anode material in nanosecond high-voltage discharges. The discharges were generated by two nanosecond-pulse generators in atmospheric air with a highly inhomogeneous electric field by a tube-plate gap. The output pulse of the first generator (repetitive pulse generator) has a rise time of about 15 ns and a full width at half maximum of 30-40 ns. The output of the second generator (single pulse generator) has a rise time of about 0.3 ns and a full width at half maximum of 1 ns. The electrical characteristics and the X-ray emission of nanosecond-pulse discharge in atmospheric air are studied by the measurement of voltage-current waveforms, discharge images, X-ray count and dose. Our experimental results showed that the anode material rarely affects electrical characteristics, but it can significantly affect the X-ray density. Comparing the density of X-rays, it was shown that the highest x-rays density occurred in the diffuse discharge in repetitive pulse mode, then the spark discharge with a small air gap, and then the corona discharge with a large air gap, in which the X-ray density was the lowest. Therefore, it could be confirmed that the bremsstrahlung at the anode contributes to the X-ray emission from nanosecond-pulse discharges. [ABSTRACT FROM AUTHOR]

Gliding discharges can generate nonthermal plasma at atmospheric pressure. Therefore, they are widely used in plasma-assisted ignition and combustion. In this paper, the gliding discharge sustained by a resonant ac power supply was obtained with a pin-to-pin electrode geometry, and the characteristics of such discharge were investigated through its voltage–current and discharge images. Inceptive, nonstable, and stable stages involved in this gliding discharge were studied. Experimental results showed that the discharge was first ignited by a spark with a current peak of tens of amperes and soon turned into a stable stage, where the discharge respectively behaved in repetitive spark mode with a current peak of several amperes in a small gap and glowlike mode with a current peak of several microamperes in a relatively large gap spacing. Under a certain situation, a nonstable discharge was observed, which changed from a spark discharge to a glowlike discharge after some time. In addition, both the discharge stability and the breakdown voltage were influenced by the air flow. The discharge became unstable when the air flow exceeded 3 L/min. [ABSTRACT FROM PUBLISHER]

Atmospheric-pressure gas discharge driven by high voltage pulses with fast rise-time and short duration has attracted significant attention for various plasma applications. In this paper, discharges were generated in a highly non-uniform electric field by point-plane gaps in open air by four repetitive nanosecond-pulse generators with repetition rate up to 1 kHz. The rise time of generators was 25 (generator #1), 15 (generator #2), 3 (generator #3), and 0.2 ns (generator #4) and a full width at half maximum was 40, 30-40, 5, and 1 ns, respectively. The experimental results show that there were typical discharge fashions, i.e., corona, diffuse, spark, or arc modes. The variables affecting the discharge characteristics, including the gap spacing and applied pulse parameters, were investigated. Especially, the diffuse discharges were investigated and discussed. With generator #1 at voltage 70-120 kV, characteristics of measured x-rays on the discharge modes were studied, and it indicates that counts of x-rays in a diffuse discharge are up to a peak value under the experimental conditions. With amplitude of voltage pulses in incident wave up to 18 (generator #3) and 12.5 kV (generator #4), runaway electron beam in low pressure helium, nitrogen, and air in a pulse-periodic mode of discharge with repetition rate up to 1 kHz was obtained. Electron beam was registered behind a thin foil in a pressure range from several to tens of Torr. X-ray radiation was obtained in a wide range of pressures, as well as at atmospheric pressure of helium, nitrogen, and air. Voltage pulses of positive and negative polarities were used. Generation of runaway electrons with pulses of positive polarity appeared because of reflected voltage pulses of reverse polarity. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• Using APPJ surface modification, a TiC x O y N z film can be deposited on PS. • The surface withstanding voltage of modified samples are improved by 50%. • The maximum SEY of sample with TiC x O y N z film is approximately decreased to 1.5. Surface flashover at the solid dielectric/vacuum interface is a fundamental physical phenomenon, which restricts the development of pulsed power system and high voltage electrical equipment. The initiation and developing processes of surface flashover at the solid dielectric are associated with electrode-dielectric interface, dielectric surface and other external factors. Especially, the surface charges transport properties that related with surface flashover developing process are determined by the physicochemical of the dielectric surface. Therefore, the surface insulating performance can be enhanced by deposited with appropriate functional films. TiN film has a low secondary electron yield (SEY), which can help to suppress the secondary electron multipactor effect and suppress the surface flashover. In this paper, a preparation method of film contained TiC x O y N z using atmospheric pressure plasma jet (APPJ) is proposed. The experimental devices include AC power supply, plasma jet chamber and gas/precursor system. The AC power supply has a power of 1000 VA and frequency of 20 kHz. The precursor tetraethyl titanate (TTEO) is pumped into plasma jet chamber by argon gas and the working gas is nitrogen. The experimental results show that the maximum SEY of sample with TiC x O y N z film is approximately decreased to 1.5. The flashover voltage of sample with TiC x O y N z film is increased obviously. The morphology of the dielectric surface indicates that the rough surface is introduced after modification. Also, the physical processes of film deposition are investigated by the emission spectroscopic analysis. [ABSTRACT FROM AUTHOR]