George A. Farrall

Experimental Study of Arc Stability. I

The stability of short arcs in gases and in metal vapors is closely related to the phenomena associated with the cathode spot. Use is made of improved techniques to study this phenomenon for a considerable range of electrode materials. The distribution of arc life for a given average current is shown to follow the survival law. These data demonstrate that a small percentage of arcs of a given current on very clean metal surfaces may have lifetimes that are extremely short or very long compared with the average life. The relation between the average life of an arc and its average current is shown generally to consist of two sharply defined sections each of the form lnt=A lnI+B. The distribution of lifetimes and the relation between average life and average current are shown for Hg, Cd, Zn, Cu2Sb, Bi, Bi‐Cu, Cu‐In, Ag, Cu3Sn, Al, Be, Cu, Cu‐W‐Th, Cu‐MoC, Mo, and W. It is shown that high vapor pressure materials tend to produce more stable arcs than those having low vapor pressure.

Low voltage firing characteristics of a triggered vacuum gap

The triggered vacuum gap is a normally nonconducting device in which a high-current metal-vapor arc can be established by a suitable pulse of current to a triggering electrode. While this gap is well suited to switching applications at high voltage, it has properties which make it useful at low voltage as well. The operation of the triggered vacuum gap has, therefore, been studied in the range 100 to 1000 volts. It was found that, although the gap could be triggered with currents as low as 0.02 amperes, consistent triggering with firing delays less than a microsecond required trigger pulses of 10 amperes or more. Little or no dependence of firing time on main gap voltage was observed. Below a few hundred volts, however, the probability of establishing a stable main discharge with a short duration trigger pulse falls off rapidly with decreasing gap voltage. The polarity of the main gap voltage and of the trigger pulse strongly influenced the firing characteristics of the gap in the range studied. These effects are discussed in detail.

Vacuum arcs and switching

This paper is a review of vacuum-arc phenomena which are related to switching devices. Despite the device overtones, the approach adopted for this paper is fundamental. Topics discussed include the drawn arc, the triggered arc, the power input to the cathode spot, cathode-spot division, arc stability, substructure of the cathode spot, dielectric recovery processes, and breakdown between electrodes subjected to repeated arcing. Frequent reference to the recent literature is made. Due to the somewhat specialized nature of certain parts of the discussion, introductory and appended sections of the paper present supplementary remarks on the concept of plasma and electrical conduction in gases, electron emission processes, and electrical contact phenomena.

Polarity Effect in Vacuum Breakdown Electrode Conditioning

We wished to determine if an electrode polarity effect existed in vacuum breakdown conditioning. To investigate this conditioning effect, we used a three‐electrode experimental tube constructed in such a manner that one electrode (1) could be made to oppose either of the other two (2 or 3) without destroying the vacuum. High‐current arcs were drawn between both possible electrode pairs to bring them to a reproducible state of apparent deconditioning. These pairs (1 vs 2 and 1 vs 3) were then, in turn, conditioned by a series of high‐voltage pulses at a gap of 2.3 mm. The following conclusions were drawn: (1) within experimental error, all conditioning took place at the pulse cathode, (2) when the arc and pulse polarities were similar, the subsequent pulse breakdown voltage was lower than when the polarities were opposed, and (3) conditioning was not due to the removal of a gas layer from the electrode, but, rather, to removal of cathode deformations. Predictions concerning the polarity effect in vacuum br...

Numerical Analysis of Field Emission and Thermally Enhanced Emission from Broad‐Area Electrodes in Vacuum

Calculations have been made of the collective field emission and thermally enhanced emission from a multiplicity of emitters in order to simulate the properties of large‐area electrodes in vacuum subjected to high voltages. The individual emitters were assumed to be noninteracting. In one set of calculations, the individual emitters comprising the groups were characterized by randomly selected values of β, work function, and projection height lying between the limits of 50 and 200, 4.4 to 5.0 eV, and 0.5×10−4 and 10−3 cm respectively. In other computations values of work function and projection height were fixed at 4.5 eV and 5×10−4 cm respectively and values of β determined from an assumed normal distribution with a central value of 100. It is the intent of this work to show the behavior of these collections of emitters in detail. Although the collective emission from these groups yielded an apparently straight‐line Fowler‐Nordheim plot, computed values of effective β and effective emitting area varied n...

Decay of residual plasma in a vacuum gap after forced extinction of a 250-ampere arc

The decaying plasma present within an experimental vacuum switch following the forced extinction of a 250-ampere arc across silver electrodes is studied. Ions and electrons are collected by applying a 6-volt potential across the vacuum device at various predetermined delay intervals after arc extinction. The magnitudes of the peak currents collected by this means vary from about ½ ampere a few microseconds after arc extinction to less than a milliampere 200 microseconds later. The results suggest that this current is principally due to ions having an initial density of 4 × 1012/cm3and moving with an average velocity of 2.6 × 105cm/s. The movement of these ions is nearly independent of the applied potential.

Further studies of electron emission areas on electropolished copper surfaces in vacuum

A lead transfer technique described in an earlier publication has been applied to the problem of locating electron emission areas on broad−area electrodes in vacuum. The objective of this work is to identify and analyze microscopic emission areas on electropolished copper and to determine their Fowler−Nordheim characteristics. The results suggest that insulating inclusions play an important part in the emission from such surfaces and indeed have emission characteristics which are similar to those expected for metallic emitters.

Arc Extinction Phenomena in Vacuum

Experiments have been performed to study the manner in which an arc between copper electrodes in vacuum extinguishes. In an ac circuit where current is limited to less than 1000 A peak, resistively, the extinctions are often prolonged over a period of many microseconds by successive reignitions of the arc caused by circuit‐recovery voltage. These reignitions tend to be suppressed by capacitance paralleling the discharge thus raising the current at which the arc is extingusihed. In a similar circuit where the current is limited by inductance, the extinction most often occurs at a minimum in high‐frequency oscillations of discharge current superimposed upon the 60‐Hz arc current near the end of the half‐cycle period. These oscillations occur at a frequency determined by the capacitance in parallel with the gap and the inductance of the wires used to connect the capacitors to the gap. For the inductive circuit with low values of capacitance, the oscillations are not apparent and extinction times of 10−7 to 1...

Techniques for the Study of Breakdown between Large‐Area Electrodes in Vacuum

From experiments dealing with electrical breakdown phenomena in vacuum, it is clear that at voltages below that level actually required to produce breakdown, there occurs a low‐level electron emission from highly localized regions of the cathode surface. This paper illustrates a technique for measuring the dependence of current upon voltage from one such region, utilizing variations in brightness of a phosphor screen. The technique avoids the possible confusion of a total current measurement which can result from the presence of simultaneously emitting areas or which can result from emission from regions of the electrode structure whose contribution to total current is unsuspected. This paper further demonstrates a method for the detailed location of particular emitting areas on the electrode surface. The procedure involves as a first step, the determination of the approximate location of the emitting region of interest using a phosphor screen. Following this, the electrode under study is used as a cathod...

Recovery Strength Measurements in Arcs from Atmospheric Pressure to Vacuum

Measurements are reported for the recovery of electric strength following interruption of an arc drawn in nitrogen at pressures ranging from atmospheric to 0.02 torr. These data are compared with similar results obtained for vacuum arcs. Recovery strength measurements followed a single half-cycle of sinusoidal arc current with a peak of 400 amperes. For conditions of gap length and pressure described by the Paschen curve minimum, full recovery is reached in a time less than a few micro-seconds. The voltage corresponding to full recovery is essentially determined by contacts, their separation, and ambient gas pressure.