Vacuum Breakdown Research Articles

Mechanism of impulse voltage breakdown in high voltage vacuum interrupters with long contact gap

The objective of this paper is to propose a breakdown mechanism of lightning impulse voltage in high voltage vacuum interrupters (VIs) with long contact gap by complementing the Cranberg "clump" hypothesis with an impulse voltage and a non-uniform field. The Cranberg "clump" hypothesis assumed that the vacuum breakdown was due to the loosely adhering microscopic particles under a DC voltage and a uniform field. The microscopic particles might be torn from an electrode surface at a critical voltage. We considered the microscopic particles located at the area with the maximum electric field had the highest possibility to be accelerated. Therefore, the transit process of the micro-particles to the opposing electrodes was under a non-uniform field distribution driven by an impulse voltage. The rise rate of the impulse voltage applied was also considered. As a result, it was found that the breakdown voltage of a vacuum gap depends upon a 0.76 power of the gap length for the plate-to-plate contacts. The diameter of the contacts was 60 mm. The contact radius of curvature was 6 mm. In the same conditions, our previous experimental results showed that the impulse breakdown voltage was in a range of 0.70 to 0.79 power of the contact gap. So the experimental results supported the Cranberg hypothesis in its extension to the impulse breakdown under non-uniform field in high-voltage VIs. Thereafter, this proposed breakdown mechanism is a complement to the Cranberg "clump" hypothesis in terms of impulse voltage breakdown under a non-uniform field in high voltage VIs.

Features of spherical particle dynamics near a plane electrode of vacuum gap in an increasing electric field

The paper reports on numerical simulation of dynamics of a spherical particle escaped from one of the electrodes of a vacuum gap at an increasing electric field. It is shown that at a sufficiently high rate of rise of the electric field strength, the particle can be held for a time at the electrode by electrostatic force. During this time, the particle makes small jumps which involve spikes of local field strength (up to 3 - 4 GV/m) and field emission current in the space between the particle and the electrode. The possibility of activation of emission sites and initiation of vacuum breakdown on particle detachment from the electrode is discussed.

Spatiotemporal distribution of hydrogenous electrode vacuum arc discharge plasma

Vacuum arc ion sources have been used in a wide range including vacuum coating, materials surface modification, switching devices and other fields. At present, researches on the vacuum arc ion source are mainly targeting the pure metal or alloy electrodes, but the researches on the hydrogenous electrode are rarely reported. The axial and radial distributions of the hydrogenous electrode vacuum arc discharge plasma are studied by using high speed four-frame camera equipped with the narrow-band filters of strong lines of H I and Ti I. It is found that the anode is intensively luminous in the vacuum breakdown. The observation from the camera with the filter reveals that the vacuum breakdown is ignited mainly by the anode hydrogen desorption. The vacuum arc is sustained by plasmas emitted from either the cathode spots or the electrode inner wall. The cone-shaped cathode spot located at the cathode-insulator-vacuum junction is the main plasma source. It is found that the distribution of hydrogen atoms is much more homogeneous than that of titanium atoms, which may be due to the earlier beginning of hydrogen desorption, the bigger emitting area and faster diffusion speed of hydrogen atom.

Dependence of Vacuum Electrical Breakdown Field and Field Enhancement Factor on the Number of Apertures Drilled in Small Electrodes

Dark current based on field-emission current is considered as a factor causing the vacuum electrical breakdown between the multi-aperture acceleration grids in the JT-60 negative ion source. In this paper, we focused on field enhancement factor which is a key parameter of field-emission from the electrode. Vacuum breakdown tests for small sized electrodes simulating the multi-aperture acceleration grids of the negative ion source was conducted. We obtained field enhancement factor and breakdown field for multi-aperture electrodes, and we investigated the dependence of each parameter on the number of apertures. The results revealed that the increase of the average field enhancement factor after the end of conditioning due to the increase of the number of apertures decreases dielectric strength of the multi-aperture electrode.

On the Design of ITER Snubbers

High current accelerators are required being implemented in Tokamak fusion facility for heating and current driver purpose, where high voltage vacuum equipment is used. Due to that vacuum breakdown frequently occurs, snubber is often used as functional passive protection device to be inserted between the ion source and its acceleration power supply for lifetime extension with arc quenching where stray capacitance between its floating HV pole and ground plays the key role in designing the system. Due to the transformer effects of the core snubber, reactive voltage could be instantly generated by inserting time varying impedance in the main circuit to balance the suddenly disappeared resistance of the ion source, which limits the HV fault current. Based on the stray capacitance, timevarying resistor method and flux matching method are respectively developed for designing the snubbers. The two methods are integrated together by the transformer theory in the paper. By historically reviewing the requirements of ITER 1 MV snubbers with the test verified new theory, it was found that varying impedance method with new core material could give more compact and reliable design for ITER-1 MV snubbers.

Exact self-similar solutions in Born–Infeld theory

We present a new class of exact self-similar solutions possessing cylindrical or spherical symmetry in Born-Infeld theory. A cylindrically symmetric solution describes the propagation of a cylindrical electromagnetic disturbance in a constant background magnetic field in Born-Infeld electrodynamics. We show that this solution corresponds to vacuum breakdown and the subsequent propagation of an electron-positron avalanche. The proposed method of finding exact analytical solutions can be generalized to the model of a spherically symmetric scalar Born-Infeld field in the ($n+1$)-dimensional Minkowski space-time. As an example, the case $n=3$ is discussed.

Ecton Mechanism of the Cathode Spot Phenomena in a Vacuum Arc

In this paper, we review the state of the art in studying the physical processes that occur in the cathode spots of vacuum arcs. The now available experimental data are interpreted in the context of the ecton mechanism of the operation of vacuum arc cathode spots. Central in this mechanism is the explosive electron emission, a phenomenon discovered by the author and his co-workers in the mid-1960s while studying high-voltage pulsed vacuum breakdown. In the light of the ecton mechanism, the cathode spot of a vacuum arc consists of individual cells which are explosive emission sites each emitting a portion of electrons termed an ecton. The cathode spot processes are cyclic in nature due to the finiteness of the ecton lifetime. It is shown that an arc is self-sustained due to the explosive emission processes initiated on the interaction of the cathode plasma either with nonmetal inclusions present in the cathode surface (first-type spots) or with liquid metal jets ejected from the zone of an active cathode spot (second-type spots). Attention is focused on the physical processes occurring during the operation of a cathode spot cell. A statistical model of a vacuum arc is used to interpret the effect of the spontaneous extinction of an arc. It is shown that an increase in the arc current is accompanied by a slight increase in the number of simultaneously operating ectons; therefore, as observed in the experiments, the parameters of a vacuum do not greatly depend on the current up to the kiloampere level.

Energetic high-voltage breakdowns in vacuum over a large gap for ITER neutral beam accelerator

CEA has undertaken tests to study the resilience of copper electrodes in vacuum against energetic high-voltage breakdowns using external capacitors to provide the energy. Earlier tests succeeded in dissipating a maximum of 150J in a 30mm gap, limited by the equivalent series resistance (ESR) in the external capacitors. Using new ones with an ESR that is a factor of 10 lower it was unsuccessfully tried to produce breakdowns at 200kV over the 85mm gap, despite the use of a UV flash lamp and a “field enhancement ring” (FER) that locally increased the electric field on the cathode by 50%. Consequently, the breakdowns had to be produced by raising the voltage to 300–350kV while maintaining the gap at 85mm. During these tests, single breakdowns dissipated up to 1140J in the 85mm vacuum gap. Inspection of the electrodes revealed that substantial amounts of copper appear have been evaporated from the anode and deposited on to the cathode. Also electrode deconditioning occurred.

真空中の放電現象—真空ギャップの絶縁破壊現象—

Suppressing an electrical breakdown of vacuum gap is one of principal factors to achieve higher performance and reliability of advanced facilities, such as particle accelerators, space crafts, vacuum interrupters, etc. Electrical breakdown mechanism of a vacuum gap is quite different from that of a gap in gases, since the electron-gas molecule collision process in vacuum is inconceivable. It is regarded that the electrical breakdown in vacuum is initiated by particles generated on the electrode surface. This article refers to breakdown mechanisms of vacuum gaps on the basis of particle generation on the electrode surface. Firstly, difference of the breakdown mechanism in gas and vacuum is explained. Secondly the outline of field electron emission based breakdown mechanism, namely anode initiated breakdown and cathode initiated breakdown, is given. Then the microparticle based mechanism, so called clump theory, is explained. In addition several experiments to improve hold-off voltage of vacuum gap are described.

Paschen testing of ITER prototype cryogenic axial breaks

Axial insulation breaks are needed in forced cooled cryogenic high voltage devices for the separation of the high voltage area from the grounded pipe system. The ITER cryogenic axial breaks will be surrounded by good vacuum in case of normal operation but also under vacuum breakdown conditions sufficient dielectric strength is required for a reliable fast discharge of the coil system. A Paschen tight design of the ITER prototype breaks enables high voltage operation independent on the outer vacuum or gas conditions. Consecutively two pretested ITER prototype breaks were integrated in the insulation system of a Paschen test unit and high voltage tested. Two different ways to perform the Paschen testing were used for both breaks. The preparation of the breaks and the test setup are described and the test results are given.

Field Emission From a MOSFET-Controlled ZnO-Nanowire Cold Cathode

Field emission from a MOSFET-controlled ZnO nanowire (NW) cathode is reported. The emission current was modulated in four orders of magnitude (from 0.2 nA to 1.15 μA ) by tuning the MOSFET gate voltage from 0.8385 to 1.5255 V. The modulation of the emission current showed an exponential relationship with the MOSFET gate voltage. The MOSFET control can dramatically reduce the emission current fluctuation, i.e., from 61.4% to 3.2%, at a current level of ~ 650 nA. The related mechanisms responsible for the improvement on field-emission current stability, the correlation between the vacuum breakdown of the NWs, and the failure of the MOSFET were discussed. The technique combines cold-cathode nanomaterials with mature solid-state technology to produce a reliable cathode in an on-chip form, which shows promising potential for modern vacuum micro-/nanosized electronic device applications.

Investigation of breakdown voltage and electrical breakdown time delay in air-filled tube in presence of combined gas and vacuum breakdown mechanism

This paper presents the experimental data of breakdown voltage and electrical breakdown time delay for air-filled tube at p = 0.7 mbar pressure. The breakdown voltage was measured for product p·d (d is inter-electrode gap) from 0.005 to 0.2 mbar cm, i.e. when breakdown was a consequence of gas and vacuum mechanisms combined. It was shown that the static breakdown voltage, which was determined from experimental data of the dynamic breakdown voltage, retains approximately constant value. The shape of the memory curve, which displays the mean value of electrical breakdown time delay t¯d vs. relaxation time τ, is very similar to that previously obtained in the case of breakdown induced by Townsend's mechanism alone. UV and gamma radiation lead to the decrease of t¯d values for values τ > 30 ms and this decrease is larger in the presence of UV radiation. UV and gamma radiation shorten the relaxation time needed for memory curve to reach saturation. On the basis of exponential and Gaussian distributions of experimental data, the contribution of the statistical time delay and the formative time to total electrical breakdown time delay for different values of relaxation time is considered.

Boundary area between gas and vacuum breakdown mechanism

This paper considers the transition from the gas breakdown mechanism to the vacuum breakdown mechanism. Gas breakdown mechanisms at the points of Paschen minimum and on the left from it, as well as vacuum breakdown mechanisms and their dependence on experimental parameters have been analysed. In accordance with this analysis, an experiment with the following variable parameters has been conceived and carried out: type of gas (i.e. residual gas); gas pressure; inter-electrode gap; electrode material; electrode surface topography; type of voltage load; and position in the insulation system of an additional source of ion-electron pairs i.e. of the α-radiation Bragg peak. By a statistical analysis of experimentally obtained statistical samples of the random variable breakdown voltage (dc and impulse), it has been established that the transition from gas to vacuum breakdown mechanism takes place in a relatively broad region, in which the gas breakdown mechanism of an anomalous Paschen type and the avalanche mechanism of vacuum breakdown are present simultaneously.

Attenuation of electron gun generated surges in an electron beam evaporation system using ferrite beads

Vacuum breakdown is a very common problem during the operation of any electron gun, more so for e-guns used for vacuum evaporation. This not only spoils the coating or welding job but also causes damage to the electronics of the power system. This paper discusses an approach to attenuate the breakdown current near the gun itself, so that the surge does not propagate to cause any detrimental effect on the power source and thereby on the job. In this paper, ferrite beads on both the feed lines have been used to suppress the high frequency content and reduce the amplitude of the breakdown signal. An induction coil sensor has been designed to monitor the breakdown signal. The high frequency surge in the MHz range could be attenuated to a lower range of KHz by use of ferrite bead. A detailed quantitative analysis of the breakdown signal has been presented in this paper.

The improvement of the field emission properties from graphene films: Ti transition layer and annealing process

Chemical-reduced graphene oxide (rGO) films were deposited on titanium (Ti)-coated silicon substrates by a simple electrophoretic deposition. The rGO films were annealed under argon atmosphere at different temperatures. The morphology and microstructure of the rGO films before and after annealing were characterized using scanning electron microscope, X-ray diffraction and Raman spectroscope. The field emission behaviors from these rGO films were investigated. The results show that, Ti-based transition layer can improve the stability of field emission from the rGO film, and the annealing at appropriate temperature is in favor of the field emission. Particularly, the rGO film displays an unexpected vacuum breakdown phenomenon at a relatively high current density. In addition, it is found that the field emission property of the rGO film is dependent on anode-sample distance and the film exhibits lower turn on field at larger anode-sample distance.

Visual and Electrical Evidence Supporting a Two-Plasma Mechanism of Vacuum Breakdown Initiation

The energy available during vacuum breakdown between copper electrodes at high vacuum was limited using resistors in series with the vacuum gap and arresting diodes. Surviving features observed with SEM in postmortem samples were tentatively correlated with electrical signals captured during breakdown using a Rogowski coil and a high-voltage probe. The visual and electrical evidence is consistent with the qualitative model of vacuum breakdown by unipolar arc formation by Schwirzke [1, 2]. The evidence paints a picture of two plasmas of different composition and scale being created during vacuum breakdown: an initial plasma made of degassed material from the metal surface, ignites a plasma made up of the electrode material.

Measurements of Field Electron Emission Sites Distribution for Oxygen Free Copper Electrodes and Microscopic Observations of Emission Sites

Electron emission is one of causes leading to electrical breakdown in vacuum. Distributions of field emission sites on a plane electrode before and after current conditioning procedure were investigated by using an Electron Emission Microscope. After the investigation of emission sites distribution, microscopic observation on some of the emission sites were carried out by using an Electron Emission Microscope. The observation revealed that field emission occurs at the foreign particles embedded on the electrode surface. To characterize the emission sites, we observed the absorption current image and analyzed the chemical composition of the emission sites by using an auger electron spectrometer. Observation and analysis suggest that the foreign particle was a semiconducting or insulating material composed of carbon.

Electrical Breakdown in Vacuum

Electrical Breakdown in Vacuum

Analysis of Desorbed Gases from Oxygen-free Copper Stimulated by Irradiation with Single Pulsed Electron Beam

Desorbed gases from anode stimulated by electron irradiation are regarded as one of causes leading to vacuum breakdown. To understand the vacuum breakdown phenomena, it is important to analyze composition and quantity of electron stimulated desorption gases. Single pulsed electron beam irradiation method was developed for the detailed analysis of the desorbed gas characteristics. It is possible for this method to reduce the modification of surface condition due to electron beam irradiation and enables to measure the transition of gas desorption rate from the virgin surface state to a certain period. Using this method, we investigated the desorbed gases from oxygen-free copper electrode. It was found that electron irradiation caused H2 gas desorption from copper electrode and the conditioning of the gas desorption. Moreover, a large amount of H2 gas was desorbed when the first electron beam was irradiated. By using desorbed gases analysis method stimulated by irradiation with single pulsed electron beam, it is possible to observe change of gas desorption rate from virgin surface to a certain number of electron irradiated surface state. Analyzing desorbed gas from the sample in detail can evaluate the electrode material in vacuum.

Huge volume expansion and structural transformation of carbon nanotube aligned arrays during electrical breakdown in vacuum

We observed a huge volume expansion of aligned single walled carbon nanotube (SWCNT) arrays accompanied by structural transformation during electrical breakdown in vacuum. The SWCNT arrays were assembled between prefabricated palladium source and drain electrodes of 2 μm separation on a silicon/silicon dioxide substrate by dielectrophoresis. At high electrical field, the SWCNT arrays erupt into a large mushroom-like structure. Systematic studies with controlled electrical bias show that above a certain field the SWCNTs swell and transform to nanoparticles and flower-like structures with a small volume increase. Further increases in electrical bias and repeated sweeping results in their transformation into amorphous carbon as determined from scanning electron microscopy and transmission electron microscopy (TEM). Cross-sectional studies using a focused ion beam and TEM show the height of a 2–3 nm SWCNT array increased to about 1 μm with a volume increase of ∼400 times. Electron energy loss spectroscopy reveals that graphitic sp 2 networks of SWCNT are transformed predominantly to sp 3. The current–voltage measurements also show an increase in the resistance of the transformed structure.