High Voltage Breakdown In Vacuum Research Articles

On the variety of triggering mechanisms of high-voltage breakdown in vacuum

The paper is devoted to study of the mechanisms responsible for vacuum breakdown as a whole and the total voltage effect in particular. Experiments at DC and pulsed voltages were carried out. It has been shown that there is no manifestation of the total voltage effect at DC voltages up to 20 kV. The strong dependence of hold-off on anode temperature was recognized at DC voltages while pulsed hold-off turned out to be almost the same with heating the electrodes. This gives a basis to consider gas desorption as an insufficient factor in the initiation of pulsed breakdown. An attempt to enhance hold-off with electrostatic removing of loosely bound particles with assistance of electron flow from a thermionic cathode was undertaken in the work. The approach turned out to be ineffective.

Low-macroscopic-field electron emission from carbon films and other electrically nanostructured heterogeneous materials: hypotheses about emission mechanism

Thin flat dielectric films can be low-macroscopic-field (LMF) electron emitters, able to generate electrons when subject to a macroscopic electric field in the range 1–50 V μm −1. This phenomenon is a known cause of pre-breakdown currents in high-voltage vacuum breakdown, and is now the basis of a broad-area electron-source technology, using carbon-based thin films and other materials. The phenomenon occurs because the dielectric film is, or becomes, an electrically nanostructured heterogeneous (ENH) material, with quasi-filamentary conducting channels between its surfaces. These channels connect to emitting features near or on the film/vacuum surface, or act as electron emitters themselves. The film may contain conducting or semiconducting particles that assist with conductivity and/or act as emitting features. Several forms of thin-film LMF emitter exist: in each case the situation geometry ensures that sufficient field enhancement occurs at the ‘tip’ of the emitting feature for the emission process to be some form of tunnelling field electron emission (probably ‘cold’ in some cases, ‘hot’ in others). This paper explores aspects of the theory of thin-film LMF emission, starting from the need to understand the behaviour of emitters based on amorphous carbon films. A summary review, with extensive references, is given of relevant past work outside the immediate ‘carbon field emission’ context. Relevant aspects of semiconductor field emission theory are noted. Comment is made on the original experiments on diamond field emission, and on theoretical misconceptions in the carbon field emission literature. Analysis of carbon-film emitter behaviour suggests that emission must primarily be due to geometrical field enhancement, that in at least some cases arises from conducting nanostructure inside the film. In one case, published film characteristics can be used to show that sufficient field enhancement should be available. Some problems with an ‘internal field enhancement’ hypothesis are considered and disposed of. Difficulties with Latham’s theory of field-induced emission from ENH materials are pointed out: a new theory, largely qualitative at this stage, can explain longstanding problems: this assumes that dielectric films must be treated as ‘hopping conductors’ not semiconductors. Electron emission takes place via localised surface states: transition to a channel-limited current regime takes place when the surface states no longer have high enough occupation probability to screen the external field, and is accompanied by anomalous band bending at the channel tip. Mathematical theories of band bending and field emission for hopping conductors are required. Some consequences for the design of LMF emitters are noted.

Observations of work function changes in field-emitter arrays

The current-voltage ( I–V) characteristics for arrays of microfabricated field emitter tips have been measured as a function of cathode temperature in a range of 30 to 300°C in an turbomolecular pumped vacuum system with a base pressure of <10 -10 Torr. The current at a fixed voltage was found to increase with increasing temperature. The change was reversible provided the temperature was held constant at its new value for about 40 min. This implies that the current increase with temperature is due to a change in work function rather than a change in the emitting area or tip shape. The I–V characteristic at each temperature followed the Fowler-Nordheim field emission equation over five orders of magnitude in current and indicated the occurence of work function changes between 0.5 and 2.0 eV. Such changes could be due to adsorption on the tip of electropositive atoms originating at the anode, which reached temperatures of approximately 670°C due to the electron bombardment at high anode voltages. Conversely, the lower work function could be due to desorption of electronegative atoms (such as oxygen) under the action of the heat and the increased H 2 and CO pressure associated with the relatively high anode temperature. Reversible desorption has been previously observed from the field emitting whiskers associated with high voltage vacuum breakdown between large area metal electrodes.

A new protection system against high voltage vacuum breakdowns

A passive, electrical fast-switch-off/on system for protecting electrodes subject to fortuitous high voltage vacuum breakdown has been developed. This system is based on the principle of oscillatory discharges in an RLC network coupled with the use of a diode. When a breakdown occurs, the current is automatically and rapidly switched off, preventing the damaging of electrodes and power supply by the electrical arc. The interruption lasts as long as required to stop the short-circuit metal vapor arc: a couple of milliseconds are generally sufficient. Then the current is automatically restored, allowing a rapid recovery of the normal electrical operating conditions. Besides the economical advantages of this circuit, one should note that its operating characteristics are very useful for electrode conditioning. The study of this protection circuit has been made for protecting some elements of the neutral beam injectors in the Tore Supra Tokamak experiment, but its field of application should be quite large.

Effects of particles on high-voltage vacuum breakdown and interelectrode current: J J Maley,J Vac Sci Technol, 11 (5), 1974, 892–895

Effects of particles on high-voltage vacuum breakdown and interelectrode current: J J Maley,J Vac Sci Technol, 11 (5), 1974, 892–895

Effects of particles on high-voltage vacuum breakdown and interelectrode current

Ten types of particles commonly found in electron tubes were tested for their effects on high-voltage breakdown and interelectrode current. The particles were applied to stainless steel electrodes and then tested for initial breakdown voltage, number of breakdowns necessary to condition the electrodes, and interelectrode current after high-voltage conditioning. It was found that insulating particles lower the breakdown voltage as much as 30% but the electrodes can be easily conditioned with little damage. Conducting particles and amorphous carbon particles lower the breakdown voltage up to 45%. In addition, these particles cause severe high-voltage conditioning problems. There is much more arcing during conditioning which results in severely damaged electrode surfaces. The arc craters are then a source of interelectrode leakage.

Plasma-induced field emission and the characteristics of high-current relativistic electron flow

The results of a comprehensive diode study conducted using a pulsed high-current electron accelerator are reported. Time-dependent analysis of right-cylindrical graphite cathodes has shown evidence of the field emission character of the cold-cathode diode. The effects of cathode whiskers or microprojections on the diode response have been observed. Within a few nanoseconds after the voltage is applied to the diode, the whiskers explode to form cathode flares. The observed diode perveance throughout the remainder of the pulse can be explained in terms of the expansion of the plasma cathode formed by the merger of many cathode flares. Cathode plasma velocities ranged from approximately 2 to 3 cm/μsec. The observed diode behavior was consistent with that predicted by previous studies of high-voltage vacuum breakdown.

Surface damage due to pre-breakdown currents in vacuum

Microphotographs are presented which shed further light on the temperature conditions prevailing at a stainless-steel anode surface during electron bombardment heating at voltages up to 70 k V. The possible relevance of the information obtained to studies of high voltage breakdown in vacuum is discussed.

Measurement of Electrode Erosion during High-Voltage Vacuum Breakdown

This paper is concerned with the measurement of the mass of electrode material released by the electrodes during a high-vacuum breakdown. The techniques used in the experiment described in this paper for quantitative measurement of material released from the electrodes are neutron activation analysis and gamma-ray spectrometry. The method permits determination of the small masses released during the discharge. The threshold energy density required for material release is 15 J/mm2. An empirical equation is given for the material erosion for copper and a curve is given that relates mass erosion versus stored energy for copper electrodes. The experiment indicates that the mechanism responsible for current transport between the electrodes in a vacuum breakdown is a highly ionized plasma formed from the released electrode material.

1452. Influence of electrode material on high-voltage vacuum breakdown: M H Zinn et al,Rep ACOM-2901; AD-664145, Jan 1968, 20 pages ( Sci Tech Aerospace Reps, 6 (9), 1313, N68-18345)

1452. Influence of electrode material on high-voltage vacuum breakdown: M H Zinn et al,Rep ACOM-2901; AD-664145, Jan 1968, 20 pages ( Sci Tech Aerospace Reps, 6 (9), 1313, N68-18345)

High voltage breakdown in vacuum: C Germain and F Rohrbach,Vacuum, 18 (7), July 1968, 371–377.

High voltage breakdown in vacuum: C Germain and F Rohrbach,Vacuum, 18 (7), July 1968, 371–377.

High voltage breakdown in vacuum (invited paper)

Recent developments in the theory and experiment of vacuum breakdown phenomena are reviewed. It is shown that, for small gaps between clean metal electrodes, satisfactory explanations may be found for the initiation and development of electrical discharges in vacuum. The theory is based on a detailed analysis of the electron field emission from the cathode and the associated current thermal interaction with either electrode. This theory fails when applied to large gaps (of the order of centimetres). Some experimental support is found for the hypothesis that breakdown initiation arises from microparticles accelerated across the gap by the electric field, but further investigation is required before the explanation of vacuum breakdown in large gaps becomes complete.

Influence of electrode material on high-voltage vacuum breakdown

Influence of electrode material on high-voltage vacuum breakdown

1239. The role of positive ions in high voltage breakdown in vacuum: H C Bourne et al,US Govt Res Develop Repts,41(4), 25 Feb 1966, Suppl p45, PB-168912, 17 pages

1239. The role of positive ions in high voltage breakdown in vacuum: H C Bourne et al,US Govt Res Develop Repts,41(4), 25 Feb 1966, Suppl p45, PB-168912, 17 pages

527. Some studies of high voltage vacuum breakdown across large gaps. Investigation of the properties of oxide-coated aluminum electrodes: F Rohrback, Report CERN-64-50, European Org Nucl Res, Geneva, Nov 1964

527. Some studies of high voltage vacuum breakdown across large gaps. Investigation of the properties of oxide-coated aluminum electrodes: F Rohrback, Report CERN-64-50, European Org Nucl Res, Geneva, Nov 1964

Role of Positive Ions in High-Voltage Breakdown in Vacuum

Measurements were made of the electron emission from targets of magnesium, aluminum, steel, copper, gold, and lead when bombarded by ions from hydrogen, helium, nitrogen, xenon, and mercury. The ions, whose energy was varied over the range from 10 to 140 kilovolts, were produced by ionization of the residual gas in the anode electrode and emerged through a small opening into an essentially uniform accelerating field. An initial rapid rise of electron emission with ion energy was followed in all cases by a slow linear increase. The emission ratio varied from 2 to 20 and was maximum for nitrogen ions on steel. The effect of extractive gradient at the bombarded metal surface was slight and no simple dependence on the nature of the ion or of the bombarded metal was observed. The significance of these measurements on the electronion interaction theory of high-voltage breakdown in vacuum is discussed.

The Initiation of Electrical Breakdown in Vacuum

The hypothesis is suggested that initiation of high voltage breakdown in vacuum is due to traversal of the high voltage gap by a clump of loosely adhering material. The implication of this hypothesis for uniform-field gaps is that the breakdown voltage is proportional to the square root of the gap length. A summary the literature is presented which supports this conclusion for a range of voltages from 20 kilovolts to 7 megavolts, and for a range of gap distance from 0.2 mm to 6 meters. Additional qualitative evidence is presented which tends to support the proposed hypothesis.

The Insulation of High Voltages in Vacuum

Breakdown studies have been made between electrodes in high vacuum at constant voltages from 50 to 700 kv. These further demonstrate the inadequacy of the field emission theory to account generally for high voltage breakdown in vacuum. Experiments are described which investigate some of the ``total voltage'' breakdown mechanisms, including positive-ion emission by electron impact, electron emission by positive-ion impact and by photons. In the d.c. case these processes contribute to a steady interchange of charged particles between cathode and anode which increases with voltage until breakdown ensues. At higher breakdown voltages the cathode gradient has diminished far below the value for field emission. Measurements of electron emission by electrons with energies up to 300 kv for tungsten, steel, aluminum, and graphite are reported. The possibilities of predicting and of improving the insulating strength of electrode gaps in high vacuum by the study of the coefficients of the electrode materials are discussed.