Prebreakdown Electron Emission Research Articles

Instabilities of prebreakdown currents in vacuum II: the nature of the emission sites

Prebreakdown electron emission between extended copper electrodes is experimentally studied with a time resolution <100 ns. When taking the current-voltage characteristics sufficiently quickly it is found that after a voltage increment the emission can relax to a lower level with time constants <1 ms. As a consequence, Fowler-Nordheim plots become flat, yielding values of the microscopic field enhancement that are too high. During the deposition of a metal-vapour plasma that originates from neighbouring arc gaps, a steady growth of the emission is observed, which leading to a considerable reduction of the voltage, down to 50% of the initial value. The results are discussed in terms of metallic-field electron emission from atomic structures that are subjected to surface migration under the influence of the electric field. The migration processes can be activated by mechanical shocks. This explains the shock effects found in the preceding paper in this issue.

Instabilities of prebreakdown currents in vacuum I: late breakdowns

Prebreakdown electron emission between extended metal electrodes is experimentally studied with time resolution of 100 ns. Switching to higher emission levels is observed and could be provoked by mechanical shocks. This is found to be analogous to spontaneous switching effects. Elementary time constants of current jumps are 1-3 µs while the whole switching process lasts up to 40 ms, thus ruling out impact of macroparticles. The breakdown voltage is found to be correlated with the emission level. Therefore, switching on electron emission can explain breakdowns with delay times >1 ms. Still longer times are found at the deposition of metal vapour plasma originating from neighbouring arc gaps. In some cases, transient increases of the emission occur that last several seconds. Therefore, in vacuum interrupters late breakdowns after current zero can be caused by mechanical shocks without particle impact and also by deposition of residual vapour at the cathode.

Relaxation of UV photon-stimulated field electron emission from particulate-based emission regimes

A newly-built photo/field electron emission microscope has been used to study the dynamic processes associated with the field-induced electron emission from particulate-based microstructures on the surface of high-voltage vacuum-insulated broad-area electrodes. It is demonstrated that the emission can be dramatically enhanced by temporarily illuminating the emitting microstructure with a UV photon beam. More importantly, the decay time of these photon-stimulated field-emitted electrons can be as long as a few minutes, suggesting the involvement of a relaxation process associated with the emitting microstructure. The findings are explained using an existing field-induced hot electron emission model, which has been proposed to explain the prebreakdown electron emission phenomenon observed with high-voltage vacuum gaps.

The dynamic, simultaneous imaging of both photon-and field-induced electron emission from “electron pin-holes” in planar vacuum-insulated HV electrodes

Details are given of a new photo-field electron emission microscopy technique that has been developed for the dynamic imaging of the prebreakdown electron emission processes that occurs on the surface of vacuum-insulated high-voltage electrodes. This new instrument can operate in: (a) a field-electron emission mode, (b) a photo-electron emission mode, and (c) the combined field- and photo-electron emission mode. Typical results obtained from the early application have confirmed the earlier finding that the prebreakdown electrons are emitted from isolated particulate surface structures on the surface of an electrode. These findings have also revealed that there are two types of associated emission process, i.e. a “surface” and an “edge” process. This successful early application has demonstrated the potential of this technique for the fundamental study of the physical origin of the prebreakdown electron emission from planar HV electrodes.

Electrical and spatial correlations between direct current pre-breakdown electron emission characteristics and subsequent breakdown events

An experimental study has established that correlations exist between the pre-breakdown electron emission characteristics and the associated breakdown events that occur in a high-voltage vacuum gap formed by OFHC Cu electrodes. A tin-oxide-coated 'transparent anode' imaging technique was employed to display the spatial distribution of pre-breakdown emission sites and the locations of subsequent direct current breakdown events. By using a real-time video recording technique, it has been possible to demonstrate that a correlation exists between the spatial location of a pre-breakdown emission site and a subsequent direct current breakdown event, and to investigate the time evolution of the two processes. The pre-breakdown electrical characteristics of emission sites, namely their switch-on fields and B values, are also correlated with the threshold field for direct current breakdown. A new breakdown-initiating mechanism is considered, and subsequently used to explain the findings that emerged from this study.

The influence of laser-induced transient surface heating on the prebreakdown electron emission characteristics of HV electrodes

A high resolution electron spectrometer facility, incorporating a transparent anode imaging technique and a scanning laser probe, has been employed to study the effects of laser-induced transient surface heating on the prebreakdown emission process occurring on vacuum-insulated high-voltage electrodes. From studies of how I–V characteristics and site distributions vary with laser beam energy flux in a range of 10 7–10 9 W cm −2, a threshold condition has been identified for the onset of significant behavioural changes. Both bulk and transient surface heating techniques were subsequently used to investigate the mechanisms responsible for the threshold condition. Results showing I–V characteristics, site distributions and spectral characteristics are presented.

The influence of surface oxidation on prebreakdown electron emission processes

Electron spectroscopy and optical imaging techniques have been used to study changes in the current-voltage (I-V) characteristics of individual emission sites that result from the growth of 50- to 500-nm-thick layers of surface oxide. It has been established that while there is only a small effect on the I-V characteristic both the spectral shift and half-width exhibit a greatly enhanced field dependence. The fundamental implications of the results are very significant, since they can be discussed in terms of the most recently proposed nonmetallic models of the emission mechanism.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The effect of gases on the emission currents from metallic micropoints in ultra-high vacuum

The effect of various gases on prebreakdown electron emission in vacuum is studied from metallic micropoints and with gas compositions that are typical of ultra-high-vacuum systems. It was found that the presence of the gas changes preferentially the pre-exponential factor of the Fowler-Nordheim characteristics, which is related to the size of the emitting surface. The observed phenomena indicate that the combined action of several gases may create a dynamical system of adsorption and desorption leading to characteristic effects (instabilities, deviation from linearity of the Fowler-Nordheim plots, unrealistic values of the field enhancement factor) which are often ascribed to nonmetallic functions. Thus, metallic micropoints which occur in extended cathodes after sparking or arcing are possible candidates as emission sites. Since minute breakdowns are not always detectable, this statement may also hold for situations where the electrodes are thought to be virgin.

A new perspective on the origin of prebreakdown electron emission processes

An account is presented of two probe techniques that have been used to locate and study the physical properties of individual electron emission sites on broad-area high-voltage electrodes. Details are given of the typical microgeometry and elemental composition of the particulate structures responsible for the emission, and the electron spectral and projection image characteristics of the emission process. These findings are discussed in terms of a recently proposed field-induced hot-electron emission (FIHEE) mechanism that involves a composite metal-insulator emission regime. Finally, the technological implications of these recent developments are considered.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Suppression of prebreakdown current by a cathode coating of CaF2

Prebreakdown electron emission from polished stainless steel and from this surface covered with a 0.2–0.4 μm coating of CaF2 is measured at applied fields from 0.2 to 1.0 MV/cm. The CaF2 coating suppresses field emisssion by up to six orders of magnitude. In addition, the CaF2 coating increases the standoff voltage by ∼50%.

Field-Induced Electron Emission from Artificially Produced Carbon Sites on Broad-Area Copper and Niobium Electrodes

It has been experimentally demonstrated that micrometersized particles of graphite deposited on the surface of copper and niobium electrodes can promote the field emission of electrons in the range of 5-20 MV ·m-1. From measurements of the current-voltage characteristic, electron spectrum, and emission image of such sites, it has been concluded that electrons arc emitted by a mechanism similar to that operating at the naturally occurring sites responsible for prebreakdown electron emission. A hot-electron-based metal-insulator-metal (MIM) model is considered.

Switching and other nonlinear phenomena associated with prebreakdown electron emission currents

An anode probe-hole technique, in conjunction with an electron spectrometer, has been used to study the short-term temporal changes in the emission characteristics of microscopically localised field emission sites on broad-area high voltage electrodes. The observed effects, e.g. the threshold switch-on of sites, and emission mode switching, have been discussed in terms of a new field-induced hot-electron emission model which is based on the concept of dielectric switching through the formation of microscopic conducting channels.

Prebreakdown Electron Emission

Recent evidence suggests that the prebreakdown currents that flow between vacuum-insulated high voltage electrodes frequently originate from a non-metallic emission mechanism which is thought to be assocaited with some form of insulating/ semiconducting surface oxide or impurity concentration. This paper develops this theme by giving a brief review of the growing body of experimental evidence that can be cited in support of the above contention, a summary of the recent findings of on-going experimental investigations into this new emission mechanism, a discussion of possible new physical models and, finally, an assessment of our present understanding of prebreakdown electron emission and the technological implications.

The origin of prebreakdown electron emission from vacuum-insulated high voltage electrodes: R V Latham, Vacuum, 32 (3), 1982, 137–140

The origin of prebreakdown electron emission from vacuum-insulated high voltage electrodes: R V Latham, Vacuum, 32 (3), 1982, 137–140

The origin of prebreakdown electron emission from vacuum-insulated high voltage electrodes

The first part of this paper is devoted to a brief review of the recent experimental evidence that has established the ‘non-metallic’ origin of pre-breakdown electron emission currents. This is followed by a discussion of two new models of the emission mechanism, both of which are based on a composite microstructure consisting of a semiconducting/insulating microinclusion in intimate contact with a metallic substrate electrode. Using a band diagram interpretation, it has been possible to give a new quantitative significance to the linear Fowler-Nordheim current-voltage characteristic that is typical of a high voltage gap.

The Effect of Glass Debris on Electron Emission and Electrical Breakdown of Vacuum Interrupters

Pre-breakdown electron emission and breakdown voltages have been studied for three experimental vacuum interrupters, two of which contained numerous glass particles the largest of which were typically 50 μm in their longest dimension. The contaminated interrupter differed from the uncontaminated interrupter in the following ways. 1) Continuously recorded Fowler-Nordheim plots had lower slopes and showed several stepwise changes in emission as the applied high voltage was varied. 2) 60 Hz breakdown voltages were lower and occurred randomly for sustained ac voltage at constant peak amplitude. 3) Electron emission current with dc high voltage applied was markedly sensitive to mechanical shock. Changes in emission occurred at statistically varying time intervals which may be several tens of milliseconds from the imposed shock. We conclude that the presence of glass particles within the interrupter strongly alters the emission properties of highly stressed internal surfaces and significantly degrades dielectric capability.

720. Compression piston gauge: G I Polukhin and V A Ryzhov, Izmerit Tekh, No 8, 1971, 89–90 ( in Russian)

Details are given of a new photo-field electron emission microscopy technique that has been developed for the dynamic imaging of the prebreakdown electron emission processes that occurs on the surface of vacuum-insulated high-voltage electrodes. This new instrument can operate in: (a) a field-electron emission mode, (b) a photo-electron emission mode, and (c) the combined field- and photo-electron emission mode. Typical results obtained from the early application have confirmed the earlier finding that the prebreakdown electrons are emitted from isolated particulate surface structures on the surface of an electrode. These findings have also revealed that there are two types of associated emission process, i.e. a “surface” and an “edge” process. This successful early application has demonstrated the potential of this technique for the fundamental study of the physical origin of the prebreakdown electron emission from planar HV electrodes.

An Apparatus for Investigating 50 c/s Voltage Pre-breakdown Electron Emission in Vacuum Insulation at Low Temperature(124°K)†

ABSTRACT An apparatus is described in which voltages up to about 100 kv a.e. can be applied to coaxial electrodes with a 5 mm radial gap, in a vacuum of 2 × 10-5 torr. Electron emission is monitored in terms of the resultant x-radiation and by superimposing the ratemeter output on a CRO display of the 50 c/s waveform, it is possible to distinguish between emission from the two electrodes. Conditioning of the aluminium electrodes is carried out by raising the voltage at constant x-radiation level, and occurs more readily at room temperature than with the electrodes cooled (inner electrode 124°K, outer electrode 200°k). The electron omission is not changed by cooling the electrodes. Anodizing one of the electrodes suppresses the electron emission from this electrode.