Prebreakdown Currents Research Articles

Effect of anode gas liberation on prebreakdown currents in vacuum

The effect on prebreakdown currents in vacuum of different gases introduced in the vessel is compared with the influence of gas desorbed from the anode surface. The experiments show that gases such as CO and N/sub 2/ reduce the pre-exponential factor in the Fowler-Nordheim equation. With anodic gas desorbed there may be a feedback to the emission current resulting in an apparent change of the slope of the Fowler-Nordheim plots. This can cause large apparent field enhancement factors beta , thus explaining unrealistically high values for metallic field emitters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Impulse breakdown voltage and prebreakdown currents of prestressed circuit breaker vacuum tubes

The electric strength of factory-made vacuum tubes was investigated by application of impulse voltages with various rise times between 0.08 and about 700 mu s. The investigations were performed with a contact distance of 3 mm and with the rated contact distance of 14 mm. To get a defined initial state of the contact surface, the vacuum tubes are prestressed first by breakdown conditioning with impulse voltages at the rated contact distance. For 3-mm contact distance, the breakdown voltages are nearly constant over the whole range of rise times, while for 14 mm the breakdown voltages increase for rise times shorter than about 0.5 mu s. No-load operations cause a larger spread and lower impulse breakdown voltages. There is nearly no influence on the values of the maximum breakdown voltages, but these maximum voltages occur more seldom. On the average, emission currents are higher than in the conditioned state. High-current closing operation with subsequent no-load opening of the contacts reduces the breakdown voltage and increases the emission current due to sharp microprotrusions on the contact surface. The shape of the V-t curves is the same as in the conditioned state of the contacts. >

Area effect on electric breakdown of copper and stainless steel electrodes in vacuum

The breakdown field strength E/sub b/(= alpha Vb/d) of gaps made with pure copper and stainless steel depends on the effective area S/sub eff/. It can be expressed by the single experimental equation E/sub b/=K/sub 1/S/sub eff//sup -n/, where K/sub 1/ is a constant, n is about 0.24, S/sub eff/ is the electrode area subjected to more than 90% of maximum field strength, alpha is the shape factor, d is the gap length, and V/sub b/ is the breakdown voltage. The field intensification factor beta /sub l/ obtained through the measurement of prebreakdown current can be expressed by a single experimental equation beta /sub l/=K/sub 2/S/sub eff/n, where K/sub 2/ is a constant. This means that the area of effect of E/sub b/ is represented by the area effect of beta /sub l/ and that the vacuum breakdown may be initiated by field emission.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Breakdown of large vacuum gaps under lightning impulse stress

The breakdown of centimeter gaps in high vacuum is investigated using lightning impulse voltage 1.2/50 mu s. Prebreakdown currents were measured as a function of gap spacing for different electrode materials. The prebreakdown current delay time is in the range of 10-60 mu s. Two different breakdown mechanisms characterized by the breakdown delay time were observed. Short-delay breakdown is caused by explosive electron emission; long-delay breakdown is apparently initiated by an anode response and microparticle-induced effects. The range of prebreakdown current peak values depends significantly on electrode material and increase in the order aluminum, stainless steel, and copper. The electric strength increases in the order copper, stainless steel, and aluminum, and indicates a relationship between crest value of prebreakdown current and electric strength. The prebreakdown current delay time depends statistically on gap spacing and prebreakdown current crest value. The influence of AC breakdown and low pressure argon conditioning on electric strength was measured as a function of gap spacing.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Measurement of time to flashover along dielectric surfaces in transformer oil using electro-optic coupling method

Time-to-breakdown of flashover on a solid dielectric surface in transformer oil with a point-to-plane gap configuration is studied using a microsecond rectangular high-voltage pulse generator. The time was directly measured from prebreakdown current. These prebreakdown currents were measured with a photo-optical current-measuring technique. Time-to-breakdown of the negative point was greatly decreased by the presence of solid dielectrics, more so than that of a positive point. Positive streamers on a dielectric surface were found generally to develop faster than negative ones. Without a back electrode, the magnitude of the pulse voltage has less influence on the positive streamer development than on the negative one. No effects of dielectric material or their thickness on streamer development were observed. Based on these observations, a hypothesis that the initiation condition of the streamer determines it subsequent propagation is proposed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Studies of the creepage discharge on the surface of liquids

Aspects of propagation of the creepage discharge on the surface of liquids are investigated by a high-speed Schlieren method, when an impulse voltage is applied to a point-to-plane gap arranged at the interface between the liquids and air. Distilled water, rain water, electrolytic aqueous solutions, and insulating oil, liquids with very different conductivity, were selected as the sample liquids. In the same instant of taking high-speed time-resolved Schlieren photographs, prebreakdown currents were measured using a light-emitting diode system. Through these experiments, a polarity effect in the shape and the size of the density changes, features and mechanism of propagation of the density changes, and their mean propagation velocity are discussed in conjunction with current measurements. >

Pre-Breakdown Currents Due to Filamentary Thermal Breakdown in Polyimide Films

Pre-breakdown currents in polyimide (PI) films were measured under the application of dc ramp voltage (30 V/s) and the breakdown mechanism is discussed on the basis of the results. A steep current increase was observed from about 10-3 before the breakdown and was ascribed to a thermal runway which results in a filamentary thermal breakdown path. An analytical solution of the pre-breakdown current was obtained on the basis of this filamentary thermal breakdown with a constant filamentary path; it deviated from the experimental data, suggesting a decrease in the effective radius of the filamentary path with time. A numerical calculation of the pre-breakdown current has been carried out for the filamentary thermal breakdown with a temperature distribution in the filament. The calculated pre-breakdown current agrees well with experimental results, and the effective radius of the filament also decreases as observed. From these results, we conclude that filamentary thermal breakdown occurs in PI at high temperatures.

Impulse Electrical Breakdown Fields of Short Vacuum Gaps

Prebreakdown currents for a long-duration impulse voltage were measured as a function of a wide range of gap spacing using a . air of plane-parallel copper electrodes in a vacuum of 3xlO Pa. The field-emitted component of the prebreakdown current was found to follow the Fowler-Nordheim field emission theory and the field enhancement factor at the cathode was determined from the Fowler-Nordheim plot. The B was found to increase from 15 to 230 with increasing electrode separation in the range from 0.003 to 2.5 mm. The breakdown voltage and the average field strength were also measured in the same gap range. It was found that the average field strength decreased from 700 kV/mm to 33 kV/mm with increasing electrode separation, however, the critical microscopic electric field at breakdown is determined to be (10.2±1.5) X107 V/cm in the range investigated.

Electrical Conduction in Solid Dielectrics

Electrical conduction as well as breakdown in insulating polymers has been extensively investigated for these decades by many authors in Japan. This report deals only with several recent topics because of space limitations. They are (1) per-breakdown currents, (2) interfacial phenomena, (3) effects of polymer structures and impurities and (4) behavior of ions at electrodes. The effects of collision ionization on high-field conduction and the pre-breakdown current due to the thermal breakdown are described in regard to pre-breakdown currents. Interfacial phenomena are concerned with the barrier height for electron (or hole) injection, carrier injection from the corona electrode and the behavior of carriers at polymer interfaces. The effects of chemical and physical structures of polymers and impurities upon electrical conduction have also attracted much attention recently. The electrochemical reaction and the effect of ionic space charge are concerned with behavior of ions at electrodes.

Dependence of Prebreakdown Currents in Vacuum on Anode Oxidation Conditions

A description is given of how anode surface conditions produced changes in the vacuum prebreakdown current characteristics. Surfaces were processed by Ar-ion etching or oxidation. Electrode materials used were Cu and Al. Changes in the prebreakdown current levels during high-voltage application depended upon a combination of the electrode materials and their surface-processing methods. For oxidized Cu, the levels increased with a continuous high-voltage application time; for oxidized Al, the levels decreased. A surface analysis using X-ray photo-electron spectroscopy showed that oxides on these electrodes were desorbed by electron impact. The difference in the prebreakdown characteristics between Cu and Al is discussed from the viewpoint of the evaporation properties of oxides.

Electron swarm properties of CCl2F2/SF6 mixtures

The ionization and attachment coefficients for CCl2F2/SF6 mixtures were determined from measurements of prebreakdown ionization currents in a steady-state Townsend discharge at a pressure p25=4 Torr and temperature of 25 °C. The curves representing the ionization coefficient (α) for reduced fields in the range from 110 to 180 V cm−1 Torr−1 exhibit a small maximum at about 15% of CCl2F2, while those representing the attachment coefficient (η) flatten out between 0% and 50% CCl2F2 and decrease at higher partial densities. The limit field obtained using the condition α=η shows two extrema, i.e., a minimum at approximately 20% and a maximum at 75% of CCl2F2, while the curves obtained from the breakdown-potential measurements available in the literature show only a maximum, at roughly the same position. Theoretical values of the transport coefficients were obtained by numerically solving the Boltzmann equation using previously determined effective sets of electron collision cross sections [J. P. Novak and M. F. Fréchette, J. Appl. Phys. 55, 107 (1984); 57, 4368 (1985)]. The good agreement between the calculated and the experimental values provides supporting evidence for the applicability of the present calculation method and collisional cross sections to analyze electron swarms in the range of fields comparable in magnitude to the limit field.

Energy distributions of carriers formed in a prebreakdown vacuum gap

The energy distributions of electrons and ions which are carriers of the pre-breakdown current, are measured for Cu and Al electrodes under a steady-state prebreakdown condition (applied voltage: DC 3 kV maximum). These profiles of energy distributions of electrons irradiating the anode are broad for both electrode materials. The spread extends across the upper half of the applied voltage, and includes two sharp peaks: the upper one due to high-energy electrons, equivalent to the applied voltage, and the lower one due to low-energy electrons (1-10 eV). The ion energy distributions measured at the cathode have peaks at high energies (corresponding to the applied voltage) and sometimes at low-energy positions (1-20 eV) as well. The implications for the carrier energy distribution and the carrier-forming mechanisms are discussed.

An analysis of field-induced hot-electron emission from metal-insulator microstructures on broad-area high-voltage electrodes

High-resolution electron energy distribution measurements of individual microscopic sites have been used to develop a detailed theoretical model of the field-induced hot-electron emission mechanism responsible for pre-breakdown currents at vacuum-gap fields of 10-30 MV m -1 . The model, which is based on the formation of conducting channels in a metal-insulator-vacuum (m. i. v.) micro-emission régime, has provided experimentally verified quantitative relations for the current-voltage characteristic, spectral shape, and the field-dependence of the spectral full width at half maximum (f. w. h. m.) and shift.

Influence of Electrode Surface Conditions on Prebreakdown Current in Vacuum

Electrode surface conditions, particularly on anode surfaces, which influence the vacuum breakdown phenomena, were investigated by using X-ray photoelectron spectroscopy (XPS) under a steady state prebreakdown condition. An experimental apparatus for the prebreakdown currents and the XPS system were built in the same vacuum chamber to enable in-situ measurements. The anode surface conditions were controlled with Ar ion etching and oxidation. Electrode material used was Cu. The anode surface conditions altered the slope of Fowler-Nordheim plots obtained from prebreakdown current-voltage characteristics. The oxidized anode gave a steep slope. Mass analysis of the prebreakdown carrier revealed that dominant ion components were H2O+ for clean anodes and O+ for oxidized anodes. The peak shift of Cu LMM Auger spectra for oxidized anodes was reduced after the prebreakdown current flow. However, O1S, Cu2p1/2 and Cu2p3/2 lines did not show the chemical shift. From these results, it was concluded that desorbed materials from the anode surface deposit on the cathode changed the field electron emission property.

飛行時間型質量分析装置の自動化

A time-of-flight mass spectrometer controlled by a micro-computer has been proposed. The operating procedure of this instrument is based on a pulse counting technique. The instrument has following operating modes : 1) qualitative mass analysis and 2) simultaneous analysis of three kinds of ions. One of these modes can be selected from the software installed in the micro-computer. The spectrometer has been applied to dc vacuum prebreakdown measurements. It is found that carriers of the prebreakdown currents are ions of residual gases in a vacuum chamber. The usefulness of this system for analyzing ions of low intensity has been demonstrated.

Effect of Dielectric Materoals on Surface Breakdown in Vacuum and Analysis by Electron Trajectory Simulation

To study and analyze the effect of surface insulation in case of a solid insulator used in actual vacuum interrupters, prebreakdown currents, breakdown voltages and Vb-t characteristics were measured with two different dielectric materials (glass and ceramics). It has been found that the breakdown voltage with a solid insulator is lower than without it, and that this fact is closely associated with prebreakdown current. In other words, glass provides a higher surface breakdown strength than ceramics because the prebreakdown current of glass is lower than that of ceramics. Electron trajectory analysis has revealed that the above-mentioned phenomenon is caused by the presence of a critical field strength of 8.9 kV/mm at one or more points on the electrode and a prebreakdown current of the order of 10-11 A. Although the prebreakdown current for the critical field strength of 8.7 kV/mm remains too low to cause electrical breakdown, it can be predicted that the level readily increases by some million times, reaching the order of 10-4 A at the time of a breakdown. This is the result of the contribution of secondary electron avalanches on the surface of the-solid insulator.

Prebreakdown Conduction in Vacuum Gaps under Switching Impulse Excitations

The present investigation concerns prebreakdownn conduction in vacuum gaps under slow impulses. Hemispherical, cal, plane, and needle electrodes have been used in a vacuum emvironment of better than 0.1 mPa. The gap spacings were 0.5, 1, and 1.5 mm. The front times ranged from 200 μs to 5 ms. The results indicate that prebreakdown currents were smaller than the highest sensitivity of 10-7 Afor front times of about 1 ins. For larger front times, breakdown currents were-served, which increased with front time. The voltagecurrent urrent relationship appears to follow Fowler-Nordheim theory but Fowler-Nordheim plots are branched. The high magnitudes may be attributed to ion-assisted field emission based on which a model has been developed, that agrees substantially with experimental results. Applicability of non-metallictype of emission iss discussed.

Consideration on Filamentary Thermal Breakdown by Measuring Pre-Breakdown Current in Solid Dielectrics

Filamentary thermal breakdown is discussed on the basis of experimental results on the electrical conduction just prior to breakdown in polyimide (PI) and polyethylene (PE) with a dc ramp voltage applied at 90°C. In PI, the pre-breakdown current was almost proportional to the electrode area S till about 100 µs before breakdown, but a sharp current increase then appeared at smaller values of S. In PE, this sharp current increase could be detected only after about 25 ns before breakdown, using a new measuring device with higher time resolution. Assuming that this sharp current increase is due to the temperature rise of a small spot in the sample leading to filamentary thermal breakdown, the maximum radius of the filament spot was estimated at about 35 µm for PI and about 2000 Å for PE.

High-Field Conduction and Electrical Breakdown of Polyethylene at High Temperatures

The mechanism of the breakdown occurring when a dc ramp voltage is applied to low-density polyethylene (LDPE) and silane cross-linked PE at high temperatures is discussed, using experimental results on the Young's modulus Y, high-field conduction and pre-breakdown currents. The electric strength FB of PE was found to be a function of Y, irrespective of cross-linking. It was also found that, over a wide temperature range, a sharp pre-breakdown current increase could not be detected till a time before breakdown of 100 ns, the resolution limit of the measuring system. The results showed that, under the present experimental conditions, thermal breakdown is not applicable to PE over a wide temperature range and that breakdown is initiated by an electronic process which has a very short time lag to breakdown of the order of some tens of ns or less.

High-Speed Electro-Optical Measurement of Pre-Breakdown Current in Dielectric Liquids

The electrical prebreakdown currents in dielectric liquids have been investigated using photo-optical measuring techniques and ns pulses applied to a pointto-plane gap. The measured relation between breakdown time lag and gap spacing has been used to deduce the streamer propagation velocities in the fluids studied. Discharges from positive points generally were more intense and faster than those from negative points. The streamer transit times were found to be proportional to gap spacing, indicating a constant velocity. This velocity was unaffected by point radius and gap spacing for a fixed magnitude of the applied voltage, Temperature effects were determined for the case of positive points. CC14 with a positive point exhibited different current waveform shapes and very fast streamer velocities.