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The phenomenon of surface flashover in vacuum is a common problem in vacuum insulation systems. Although the theory of secondary electron emission avalanche is widely accepted, the developing process of vacuum surface flashover is not very clear and it needs to be further studied. The surface discharge process was diagnosed by spectral and mass spectrum analysis in this paper. The surface flashover was initiated by a microsecond pulse with a rise time of 500 ns and a full width at half maximum of 8 μs. Optical diagnostic methods were used to obtain the luminous properties of several polymeric materials. The time-domain optical signal was measured by a photomultiplier tube with wavelength range of 300–650 nm. The spectrograph was used to detect the optical emission from the surface discharge. The outgassing characteristics in the flashover were also measured by mass spectrometer connected to the vacuum experiment chamber. The results show that the optical signal appears before flashover and the light intensity reaches its maximum at about 80 ns after the flashover occurs. The optical emission spectrum is mainly in range of 400–800 nm, and C 2 , H and Cu spectrum lines were detected in polymeric materials. The results of mass spectrum measurement show that the outgassing of polymeric materials in surface flashover includes the desorbed gases and decomposed gases of insulation materials. It is consistent between the results of emission spectrum and mass spectrum. [ABSTRACT FROM AUTHOR]

Surface flashover characteristics of insulation materials in vacuum are key factors to determine the behavior of high voltage electrical equipment. Although the surface flashover in vacuum has been investigated for decades, the surface insulation properties of polymeric materials after repeated surface discharges have shown little attention by researchers. In order to investigate the aging characteristics of polymeric materials caused by repeated flashovers in vacuum, several typical materials were used to flashover by microsecond pulses. The flashover voltages were obtained for polytetrafluoroethylene, polymethylmethacrylate and polyamide for thousands of flashovers. The surface morphology of samples was observed after different numbers of flashovers. The trap parameter distribution was obtained by the isothermal surface potential decay method. To analyze the flashover voltage change with discharge times, the Weibull distribution was used to determine the conditioning and degradation effects in repeated discharge experiments. The results show that the morphology near the cathode triple junction and deep traps together affect the subsequent flashover voltage. The conditioning and degradation phenomena often coexist in the aging process of polymers and affect the surface insulation properties. This study could provide a reference for the type selection, design, and modification of insulation materials in vacuum. [ABSTRACT FROM AUTHOR]

Surface modification of polytetrafluoroethylene (PTFE) is performed by nitrogen ion implantation for improving the PTFE surface flashover voltage in vacuum. In this paper, nitrogen ions are produced by an electron cyclotron resonance ion source. The properties of the samples are investigated with X-ray photoelectron spectroscopy, electrometer, microhardness tester, surface potential measurement, and surface flashover system. The Stopping and Range of Ions in Matter software is used to simulate the ion implantation. Experimental results show that the surface flashover voltage of treated samples improves remarkably and increases with the injection energy. After ion implantation, the content of fluorine in the treated samples decreases, and the content of oxygen increases. The surface conductivity increases from $1.15\times 10^{{-15}}$ to 1.36 10−13 S/m, the surface potential decreases faster, and the trap density and energy increase with the implantation energy. The process of electron avalanche and the accumulation of surface charges have been suppressed, thus hindering the development of flashover and improving the surface flashover voltage. [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]

• The flashover voltage and current of G10 in LN 2 are obtained through experiments and the flashover data is analyzed by Weibull model. • The flashover process is observed by OES, and the surface evolution after G10 flashover is analyzed by optical microscope and FTIR. • The relationship between polymer flashover and trap in LN 2 is established through test of surface trap distribution and calculation of charge transport equation. The insulation failure is a bottleneck to restrict the safe operation and development of direct current (DC) superconducting power devices. The solid–liquid flashover test is an important evaluation index, but the previous researches mostly focus on engineering application, and less involve in the mechanism analysis. As a characterization method for defects existing in the dielectric, the trap has been applied to the study of flashover to occur polymer surface in liquid nitrogen (LN 2) in recent years. However, the mechanism of trap action in solid–liquid flashover is still confusing. Therefore, this paper carries out the following research. Firstly, the flashover voltage and current of glass epoxy laminate sheet (G10) in LN 2 were obtained through experiments and analysis using the Weibull model in this paper. Secondly, the flashover process of G10 was monitored. The changes of G10 surface morphology and chemical bond before and after flashover were also observed. Finally, the relationship between polymer flashover and trap in LN 2 was analyzed based on the surface trap distribution test and theoretical calculation of the charge transport equation. The results are beneficial to guide the selection and performance improvement of dielectric materials and the design of insulating structures in superconducting devices. [ABSTRACT FROM AUTHOR]

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 desorbed gases were measured in the process. By comparing the mass spectrum 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. Main gas components generated by vacuum surface flashover included H2O, N2, H2, O2, hydrocarbon fragment, CO, CO2 and sometimes high polymer fragment. C2H2 appeared in gas components when the filamentary discharge channel was formed between two electrodes. 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. [ABSTRACT FROM AUTHOR]

Microstack insulators were developed to satisfy application demands for pulsed power system high performance and miniaturization. Using a high temperature lamination method, we manufactured two types of microstack insulators from interleaved layers of brass and polyimide (PI) and from stainless steel and fluorinated ethylene propylene (FEP). The surface state and vacuum out-gassing characteristics of these microstack insulator samples were measured. Then using a nanosecond (10 ns/30 ns) pulse source with a Marx generator and a single coaxial pulse-forming line, the vacuum surface flashover characteristics of samples were tested. The results show that flashover takes place during the rising period of the voltage, and the maximum vacuum flashover field intensity for microstack insulator samples is close to 180 kV/cm. Based on the preceding work, the high frequency dielectric constants of microstack insulators were also measured using an impedance analyzer. The results show that microstack insulators have better high frequency dielectric performance than conventional insulation materials [ABSTRACT FROM AUTHOR]