Bai-Peng Song

Aging characterization of high temperature vulcanized silicone rubber housing material used for outdoor insulation

During the field service of composite insulators, high temperature vulcanized (HTV) silicone rubber (SIR) material used for insulators sheath and sheds is gradually aging, and thus how to effectively evaluate its aging state has become an inevitable issue. In this paper, artificially corona-aged and naturally site-aged SIR materials are employed as samples, and their properties such as hydrophobicity, leakage current, trap density/energy level, surface microstructure and chemical composition are investigated for comparative study. With the increasing of corona aging intensity, the contact angle of SIR samples drops gradually and recovers more slowly. The hydrophobicity of site-aged insulators also declines, from HC1 for 2-year service to HC5 for 15-year service. The leakage current of corona-aged and site-aged samples both increase with aging, which indicates that aging induces the increase of surface conductivity of SIR material. The peak trap density of corona-aged and site-aged samples increases with the aging duration or service duration remarkably. The changes of these properties are attributed to the changes of micro-structures and compositions in the surface layer of SIR. The scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) analysis results reflect that a hardened inorganic silica-like (SiO2) layer with many polar chemical groups and distributed micro-pores is formed on the surface of SIR material after corona aging. Because of this silica-like layer, the hydrophobicity decreases while surface conductivity and trap density of SIR material increases. Besides the traditional properties like hydrophobicity and surface conductivity, the trap density is expected to be a novel parameter for effective evaluation of aging state of HTV silicone rubber material.

Measurements of Secondary Electron Emission From Dielectric Window Materials

Dielectric window is an important component of high-power microwave (HPM) devices. However, surface breakdown easily occurs at the vacuum/dielectric interface when HPM pass through the dielectric window. This greatly limits transmission of HPM and makes the window a bottle neck of HPM technology development. Secondary electron emission (SEE) from dielectric window plays an important role in its surface breakdown. This paper studies the total SEE (including true secondary electrons and backscattered electrons) coefficients of several inorganic and organic dielectric materials including polytetrafluoroethylene, polyethylene, alumina ceramic, and machinable ceramic. The measurements are implemented by using pulsed electron beam impacting the materials with energies from 200 eV to 5 keV. In addition, surface desorbed gas property is studied with the quadrupole mass spectrometer. The performances of different materials are evaluated. The obtained results are useful for the selection of HPM window materials.

Luminescence evolution from alumina ceramic surface before flashover under direct and alternating current voltage in vacuum

The luminescence evolution phenomena from alumina ceramic surface in vacuum under high voltage of direct and alternating current are reported, with the voltage covering a large range from far below to close to the flashover voltage. Its time resolved and spatial distributed behaviors are examined by a photon counting system and an electron-multiplying charge-coupled device (EMCCD) together with a digital camera, respectively. The luminescence before flashover exhibits two stages as voltage increasing, i.e., under a relative low voltage (Stage A), the luminescence is ascribed to radiative recombination of hetero-charges injected into the sample surface layer by Schottky effect; under a higher voltage (Stage B), a stable secondary electron emission process, resulting from the Fowler-Nordheim emission at the cathode triple junction (CTJ), is responsible for the luminescence. Spectrum analysis implies that inner secondary electrons within the surface layer of alumina generated during the SSEE process also participate in the luminescence of Stage B. A comprehensive interpretation of the flashover process is formulated, which might promote a better understanding of flashover issue in vacuum.

Evolution From Cathode-Initiated to Anode-Initiated Flashover in Vacuum

This paper presents experimentally flashover evolution initiated from cathode to anode via an intensified charge-coupled device camera under the condition of adjusting electric field distribution. These results suggest that a flashover process across insulator surface in vacuum is greatly affected by the local electric field at both cathode and anode triple junctions. Flashover can be initiated at either cathode or anode. This paper might be helpful to clarify the dispute about how the flashover is actually initiated.

Secondary electron emission measurements of dielectric window materials

Dielectric window is an important component of high power microwave (HPM) generation device, however, breakdown easily occurs on vacuum/dielectric interface when HPM passes through dielectric window. Surface breakdown limits the generation and transmission of HPM, and becomes the bottle neck of HPM technology development. Secondary electron emission (SEE) plays an important role of dielectric window breakdown, so that the paper studies SEE coefficients of several dielectric window materials including polytetrafluoroethylene (PTFE), polyethylene (PE) and polymethylmethacrylate (PMMA) in vacuum. The measurements are carried out by single pulsed electron beams impacting the materials with energies of 0.6keV up to 5keV. First, we can get the primary current from the single pulse impacting aluminum. Then the secondary currents emitted from the samples are measured by the collector, which is applied to positive bias voltage (+23V) for ensuring the secondary electrons are mostly captured. The SEE coefficients δ increases with pulsed electron beam energies, then reaches the maximum, and finally decreases as beam energy continues growing. The result is that SEE coefficients δ of PTFE is the lowest, PE at the highest.

Simulation on the dynamic charge behavior of vacuum flashover developing across insulator involving outgassing

A 2D simulation based on particle-in-cell and Monte Carlo collision algorithm is implemented to investigate the accumulation and dissipation of surface charges on an insulator during flashover with outgassing in vacuum. A layer of positive charges is formed on the insulator after the secondary electrons emission (SEE) reaches saturation. With the build-up of local pressure resulting from gas desorption, the incident energy of electrons is affected by electron-neutral collisions and field distortion, remarkably decreasing the charge density on the insulator. Gas desorption ionization initiates near the anode, culminating, and then abates, followed by a steady and gradual augmentation as the negatively charged surface spreads towards the cathode and halts the SEE nearby. The initiation of flashover development is discussed in detail, and a subdivision of flashover development is proposed, including an anode-initiated desorption ionization avalanche, establishment of a plasma sheath, and plasma expansion. The transform from saturation to explosion of space charges and dissipation of the surface charge are revealed, which can be explained by the competition between multipactor electrons and ionized electrons.A 2D simulation based on particle-in-cell and Monte Carlo collision algorithm is implemented to investigate the accumulation and dissipation of surface charges on an insulator during flashover with outgassing in vacuum. A layer of positive charges is formed on the insulator after the secondary electrons emission (SEE) reaches saturation. With the build-up of local pressure resulting from gas desorption, the incident energy of electrons is affected by electron-neutral collisions and field distortion, remarkably decreasing the charge density on the insulator. Gas desorption ionization initiates near the anode, culminating, and then abates, followed by a steady and gradual augmentation as the negatively charged surface spreads towards the cathode and halts the SEE nearby. The initiation of flashover development is discussed in detail, and a subdivision of flashover development is proposed, including an anode-initiated desorption ionization avalanche, establishment of a plasma sheath, and plasma expansion. Th...

Investigation of grooved surface suppressing multipactor across HPM dielectric window

Microwave breakdown phenomenon across dielectric window at the vacuum/dielectric interface limits the transmission of HPM and becomes a bottleneck of HPM technology development. Its avoidance or suppression is a major concern among researchers on HPM sources. Currently, the most effective approach is geometrical modification with the goal of altering the trajectories of electrons. In this paper, periodic rectangular grooves perpendicular to microwave electric field, are applied to improve the threshold of breakdown. To clarify the mechanism, the SEEA simulation model based on dynamic analysis and particle-in-cell (PIC) is built in this paper. The grooves change the electrons trajectories and eventually resonant multipactor condition may no longer be satisfied. The suppression effect is influenced by the grooves and each given width relates to an effective power range for suppression. Dielectric breakdown experiments under HPM of S-band at 2.86 GHz are conducted with the grooved dielectric window. The results of the experiment are consistent with theoretical analysis and simulation.

Investigation of multipactor-induced surface plasma discharge and temporal mode transition

Multipactor over a dielectric in vacuum inclines to engender interfacial gas desorption or evaporation, precipitating surface flashover and insulator failure. However, no consensus has been achieved regarding the exact mechanism during final breakdown stage, an expatiation of which therefore serves as our major motivation for this letter. By implementing the particle-in-cell simulation code, we investigate the microscopic evolution of the discharge development process and confirm the major component escalating the explosive space charge accumulation. The obtained current waveform validates the balance of charged particles between electrodes, corroborated by experimental results. A theoretical discharge model is then constructed to elucidate the physical reasoning of the previous phenomenon. Two distinct discharge modes are defined correspondingly, and the transition therein is found to be induced by rapid plasma density build-up.Multipactor over a dielectric in vacuum inclines to engender interfacial gas desorption or evaporation, precipitating surface flashover and insulator failure. However, no consensus has been achieved regarding the exact mechanism during final breakdown stage, an expatiation of which therefore serves as our major motivation for this letter. By implementing the particle-in-cell simulation code, we investigate the microscopic evolution of the discharge development process and confirm the major component escalating the explosive space charge accumulation. The obtained current waveform validates the balance of charged particles between electrodes, corroborated by experimental results. A theoretical discharge model is then constructed to elucidate the physical reasoning of the previous phenomenon. Two distinct discharge modes are defined correspondingly, and the transition therein is found to be induced by rapid plasma density build-up.

Flashover strength improvement and multipactor suppression in vacuum using surface charge pre-conditioning on insulator

Surface charging commonly appears on dielectrics in vacuum in the presence of electron bombardment, seriously aggravating the superficial withstand strength of assorted devices. Nonetheless, a pre-conditioning technique is introduced in this paper capitalizing on surface charges to play an opposite role, enhancing flashover strength and suppressing the multipactor which is frequently found over vacuum insulator. A theoretical study is first performed, incorporated with particle-in-cell simulation to show the critical condition for a single-surface multipactor to initiate. Therewith, it is proven that a negative charge accretion in cathode adjacency can prevent the multipactor from commencing with efficiency. Subsequently, an analytical model is constructed to expatiate multipactor expansion with pre-set surface charges getting involved, illustrating an upper bound of its propagation velocity, influenced by pre-conditioning. Corresponding experiments are also conducted to corroborate previous conclusions, ...

Flashover time delay characteristics of grooved insulators under nanosecond pulse voltage in vacuum

For dielectric-vacuum compound insulation systems, the discharges usually occur across the insulator surface with an onset voltage much lower than the breakdown voltage for vacuum gap of the same length, which is the main cause leading to insulation failure. It has always been a hot topic to improve the surface insulation strength in vacuum. In this paper, the relationship between the flashover time delay and electric field is studied under nanosecond impulse voltage. Experiment results suggest that the flashover time delay was proportional to N-th power of electric field for grooved insulators with different parameters. For grooved insulator, the flashover channel prefer to develop along the grooves rather than pass over the groove wall. The blocking effect of grooves to the electron multiplication in cathode-anode direction leads to the increased time delay. A narrow band of electric field with respect to time delay is formed over the flat samples. The breakdown electric field of a grooved insulator was increased by more than 100% than that of a flat one. The flashover electric field tends to be saturated when the groove depth is more than 2 mm for both PTFE and PMMA, which is correlated with the trajectory of electrons travelling across insulator surface. Grooves near electrode is more effective to increase surface insulation strength than grooves in middle area, which can be of great help to guide the structure optimization in engineering application.