Short Vacuum Gap Research Articles

Behaviors and Mechanism of Electrical Breakdown in Short Vacuum Gaps Under Pulsed DC Voltage

A topology consisting of a mechanical bypass switch and a power semiconductor switch (IGBT and IEGT) in parallel is widely used in high-voltage flexible dc transmission converter and static var generator (SVG). The mechanical switch will close quickly when the power semiconductor switch fails, which will cut off the failure device to protect the whole converter valve. Therefore, the mechanical contacts of the bypass switch need to close fast (within 3 ms) without contact bounce, which could only be achieved if the gap between the two contacts is extremely short (approximately 1 mm). Vacuum interrupter (VI) is most suitable for this application due to its advantages, such as compact size, high safety, environmental friendliness, long service life, and hardly oxidized contacts. In this article, a vacuum test rig is built, and the electrical breakdown behaviors of the short vacuum gaps under continuous and pulsed dc voltages are studied. It is confirmed that breakdown voltage under pulsed dc voltage is much lower than that under continuous dc voltage owing to the free and weakly bound microparticles on the contact surfaces. Finally, a new model describing the breakdown mechanism is proposed to explain the behaviors revealed in the experimental test.

Investigation of the mechanism of switching a short vacuum gap using an auxiliary spark discharce

The ionization of a rarefied gas in a short gap by a stream of short-wave radiation and fast electrons from the plasma of an auxiliary spark discharge along the dielectric surface is detected, sufficient to initiate an arc discharge in the gap. The density of the ionizing flow of particles and radiation determines the delay time of the switching gap in relation to the start of the auxiliary discharge. With an increase in the initial concentration of free electrons in the main discharge gap, the transition time to an arc discharge is reduced, due to the development of instabilities. Keywords: plasma, arc discharge, spark discharge, ionization, instability.

Double short vacuum gaps in series under lightning impulse voltage: Estimation of breakdown voltage

The objective of this paper is to propose a method for quantitatively estimating the breakdown voltage of double short vacuum gaps in series under the lightning impulse voltage. The study found a quantitative relationship between the breakdown voltage and the maximum electric field strength at the cathode. This relationship is validated by previous experimental works when the total gap distance is less than 20 mm.

Spatial Structure and Dynamics of a Pulsed Arc Discharge in Vacuum

The process of switching a short vacuum gap using an auxiliary discharge over the surface of a dielectric is studied by the high-speed recording of images of the plasma in the optical spectral range. Based on the analysis of the obtained experimental data, it is suggested that the cathode spot and cathode flame in the ultraviolet spectral range play a significant in the formation of current channel in the discharge.

REGISTRATION OF OPTICAL IMAGES OF A VACUUM ARC DISCHARGE WITH AN ELECTRON-OPTICAL CONVERTER WITH NANOSECOND TIME RESOLUTION

Abstract—The process of switching a short vacuum gap by an auxiliary discharge over the dielectric surface was studied via recording of images of discharge plasma radiating in the optical spectral range. A two-channel electro-optical (EO) image-recording system based on an EO converter (EOC) was used. Each channel contained a unit for generating an EOC-photocathode gating pulse, a transposing objective lens, and an EOC, which was joined to a digital CCD (charge-coupled device) camera for reading information from the EOC screen. Based on the analysis of the obtained experimental data, it was suggested that the radiation of a cathode spot and a cathode flame in the UV range play a significant role in the formation of a current channel in the discharge.

High-Speed Recording of Vacuum Arc Discharge Images in the Optical Range

Switching of a short vacuum gap with an auxiliary discharge over the surface of a dielectric has been studied via high-speed recording of images of discharge plasma radiating in the optical spectral range. Based on the analysis of experimental data, we surmise that the cathode spot and cathode flame radiation play an important role in the formation of the current channel in the discharge.

Properties of Intermediate-Frequency Vacuum Arc Under Axial Magnetic Field

This paper presents an experimental study of the intermediate-frequency (400-800 Hz) vacuum arc under axial magnetic field (AMF). Arc-voltage noise at the first current semi-wave is inconspicuous even if the current is not interrupted successfully. This fact is attributed to the application of a sufficiently strong AMF provided by the current flowing through a short vacuum gap. The influence of the current frequency on peak arc voltage and the rate of rise of the arc voltage are investigated. Interrupting ability of a vacuum interrupter (VI) with cup-type AMF contacts made from CuCr50 is examined. It is found that the upper limit of current interruption is inversely proportional for frequencies in the range from 400 to 800 Hz. Phase shift of AMF is studied by the finite-element method based on a simplified symmetric model. The results show that the phase-shift angle increases with the current frequency and influences intermediate-frequency current interruption of VI with AMF contacts.

Temporal evolution of micro-plasma in a low-current, repetitive vacuum spark soft X-ray source

Temporal evolution of micro-plasma and soft X-ray emission in a short vacuum gap, operated with a repetitive low-current (/spl sim/100 A) discharge was investigated. The device consists of two resonant circuits having different resonant frequencies for energy storage. Micro-plasma of diameter less than 1 mm is formed during spark phase with a very high-frequency (VHF) voltage, and electrodes are heated by successive high-frequency (HF) current. Thermo-field electrons are accelerated by an electric field in a cathode sheath that is formed after arc extinction. Soft X-rays were emitted mainly in the period between arc extinction and subsequent breakdown.

Soft x-ray generation after arc extinction of a short vacuum gap operated with low-current, repetitive discharge

Soft x-ray emission from a short vacuum gap, operated with a repetitive low-current arc with a period of ∼2 µs and peak current of 200 A, was investigated. Electrons are accelerated by an electric field in the cathode sheath that is formed after arc extinction, and soft x-rays were emitted in the period between arc extinction and subsequent breakdown. The bulk of the radiation was detected in the 1–3 keV soft x-ray range when a tungsten anode was employed. The dimension of the point-like x-ray source in the radial direction was less than 170 µm. The current density at the time of x-ray emission was ∼107 A m−2.

A fiber–optic interferometer for in situ measurements of plasma number density in pulsed-power applications

A fiber–optic-based interferometer has been designed, constructed, and applied to plasma measurements in pulsed-power systems. The beam from a 1310 nm solid-state diode laser is coupled into a single-mode fiber and split into two beams, one of which was passed through an acousto-optic modulator to frequency shift the light. Both beams travel through approximately 30 m of fiber, with the probe volume consisting of a short air or vacuum gap in the probe beam fiber where the light is collimated and collected by lenses. The beams are then recombined on a photodiode, producing a time-varying sinusoidal intensity signal that is phase modulated with the presence of a plasma in the probe volume. This configuration allows for remote measurements of plasma electron number density, and is robust with respect to vibration in the plasma source and electromagnetic interference. Tests indicate that phase measurement accuracies of ±0.045 rad corresponding to number density accuracies of ±1.2×1019 m−2 at 1310 nm are achievable with this device. A description of this interferometer, including refinements needed to achieve these accuracies, is presented along with the results of tests performed on a coaxial plasma gun.

Reignition development in short vacuum gaps after interruption of high-frequency currents

Very high and steep overvoltages can be produced during the reignitions of switching devices after interrupting high-frequency currents. The mechanism of such reignitions is studied in this paper experimentally by considering the light intensity emitted from the vacuum gap during a reignition. The emitted light intensity is recorded by a streak-camera. The recorded light intensities show that an emission center building at the cathode surface precedes the breakdown of the vacuum gap. This result corresponds to the simulation results of the breakdown development of short vacuum gaps presented in earlier works.

Reignitions in short vacuum gaps after interruption of high-frequency currents caused by ion bombardment

The internal energy increase of the cathode material during the post arc period as a result of the high electrical field at the cathode surface and the highly energetic ions existing in the vacuum gap is studied in this paper by solving the conservation equations of mass, momentum, and energy together, with the equation of state of electrode material in the cathode region. Solving the heat conduction equation in the cathode region, the temperature distribution of the cathode material is determined. The results for copper indicate that a breakdown can occur at the cathode surface some 100 ns after current zero depending on the form of the cathode surface inhomogeneities, the steepness of the transient recovery voltage, and the initial plasma distribution in the vacuum gap. These results are in accordance with experimental measurements.

Influence of electrode surface microstructures on the state of shortvacuum gaps after interruption of high-frequency currents

In this paper the influences of electrode surface microstructures onthe electrical field near the new cathode surface and on the distribution ofion density and velocity in the space-charge region after interruption ofhigh-frequency currents in short vacuum gaps have been studied. To investigatethese effects, the equations of conservation of mass and momentum are solvedtwo dimensionally in the space-charge region. The results indicate aconsiderable increase in ion density at the points with high electricalfields, which results in an enhancement of the electrical field at thesepoints compared with one-dimensional calculations. Considering the calculatedelectrical field near the cathode surface and the power density deposited intothe cathode surface by ions, the initiation of a breakdown during the post arcperiod under the mechanism of explosive electron emission may be possible.

Reignition phenomena after high-frequency current interruption with short vacuum gaps

Based on a large number of measurements of high-frequency (HF) current interruptions in vacuum at small contact gaps (/spl les/600 /spl mu/m), the statistical reignition behavior of vacuum switching devices after a HF current zero is investigated. Three types of reignitions can be classified. Statistical evaluation of post-arc current measurements for different parameters at current zero and different HF current ignition processes gives information about the stress of the gap as a result of the transient recovery voltage after a HF current zero (HF-TRV) and the accumulated post-arc charge. Comparing post-arc current values at the beginning of the HF-TRV and at the moment of reignition reveals a production of charge carriers during the recovery interval. Possible reasons for the different types of reignitions are discussed.

Electric strength of vacuum gap under various voltage conditions

Although the electric strength of vacuum gap under various voltage conditions has been discussed in many paper, research on this subject is still ongoing. The first page of our investigations was examining the prebreakdown state of vacuum gap under ac voltage excitation. Field electron emission in short vacuum gap (0–2 mm) and microdischarges in long vacuum gap (over 10mm) were analyzed. Field electron emission current was measured with an unbalanced hv bridge. Next, using voltage-current characteristics analysis, the parameters of field electron emission were obtained. The microdischarges phenomenon was monitored with a multichannel pulse-height analyzer. The relation between breakdown voltage and the parameters characterizing prebreakdown phenomena was then determined. Finally, the characteristics of electric strength under ac (50 Hz), switching impulse (250/2500 μs) and lighting impulse (1.2/50 μs) voltages were obtained. The investigations were carried out with CuCr contacts in a wide range of interelectrode distance (0.5–12 mm).

Interrupting capacity of vacuum interrupters depending on the frequency of current

Results of arc reignition voltages during current interruption of frequencies from 5.9 to 60 kHz by a short vacuum gap are presented. Measured arc reignition voltages depend on current amplitude and frequency, the values of preliminary arc current at the moment of switching on the HF current, and the discharge mode in the preceding current semiwave. Threshold amplitudes of the first semiwave of currents l/sub m1l/ and I/sub m1h/ as a function of frequency are determined. I/sub m1l/ and I/sub m1h/ divide current into three ranges to which different kinds of arc reignition voltage distribution correspond. Particularly large dispersion of reignition voltages takes place in the current range from I/sub m1l/ to I/sub m1h/. The threshold current I/sub m1h/ is inversely proportional to frequency in the range from about 10 to 60 kHz, which is in agreement with the elaborated mathematical model.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The influence of circuit parameters and contact materials on the reignition of high-frequency vacuum arcs

The extinction or reignition of high-frequency (HF) transients plays an important role in the generation of voltage surges. Investigations with currents of several hundred amperes peak and current frequencies from 100 kHz up to 1 MHz were made to examine the reignition behavior of short vacuum gaps using different contact materials. The experiments were carried out in an UHV test chamber connected to an L-C test circuit with voltages in the 10-kV range. Data on the statistical distribution of reignition voltages and their dependence on the parameters as well as on different TRV conditions are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Breakdown time lags in short vacuum gaps: J D Chalmers and B D Phukan, Vacuum, 32 (3), 1982, 145–150

Breakdown time lags in short vacuum gaps: J D Chalmers and B D Phukan, Vacuum, 32 (3), 1982, 145–150

Breakdown time lags in short vacuum gaps

Measurements of breakdown time lags have been made in gaps of up to 1 mm in ultra-high vacuum. A clear discontinuity has been found in the distribution of these times demonstrating that breakdown takes place either within 30 ns of applying the voltage or after a long time delay of up to several ms. Comparison of measured and calculated data suggests that, as the applied voltage is increased, there may be a change in breakdown mechanism from one involving heating and evaporation of the emitting site at the cathode to one involving evaporation of the target site at the anode.

Photographic observations of impulse breakdown in short vacuum gaps.

Reaction of a dithiol with B(OH)3 and base in MeOH followed by reaction with xylylene dibromides proceeds selectively to give the corresponding ortho, meta or para-cyclophanes in good yield (70–86 %). The syntheses of examples of all three types of benzenophanes are discussed. The molecular structures of 3,8-dibenzo-1,6-dithiacyclodecane (8) and 2,5,8-trithia-(9)-p-benzenophane (5) have been determined by X-ray crystallography. Crystal data for 8: space group P21/c with a=8.2745(10), b=4.9330(10), c=16.5500(12) A, β=100.37 (1) °, Z=2; R=0.043 (Rw=0.045) for 949 reflections (I>2.5σ(I)). Crystal data for 5 (150K): Space group P21/c with a=10.310(1), b=7.430 (1), c=17.681(2) A, β=113.27(1) °, Z=4; R=0.065 (Rw=0.065) for 1969 reflections (I>2.5σ(I)).