Leonard B. Loeb

The Mechanism of Spark Discharge in Air at Atmospheric Pressure. I

The Townsend equation for the passage of a spark is analyzed and its inadequacy for explaining sparks in air at near atmospheric pressures is demonstrated. The mechanisms active in air at higher pressures, viz., the electron avalanche and its tip field, photo‐ionization in the gas and positive streamer formation are presented. A quantitative criterion for streamer formation is applied to give a quantitative theory for spark breakdown in air at atmospheric pressures. The theory gives quantitative agreement with experiment and predicts departures from Paschens law. At values of the product, pressure times gap length less than 200 mm×cm in air, the new mechanism is unimportant. The theory is applied to longer sparks at atmospheric pressures and the effect of the decrease in density of photo‐ionization in longer avalanches considered. This leads to a modification of the Meek mechanism by which the electron avalanche slows down while a retrograde positive streamer moves at high speed to the cathode and then a...

Recent Developments in Analysis of the Mechanisms of Positive and Negative Coronas in Air

The nature of the detailed mechanism of the negative Trichel corona pulses in air at atmospheric pressures is reanalyzed on the basis of recent data of English giving the time duration of the pulse and details of its structure. The onset of corona from a negative point is shown to consist of pulses of about 4 μsec. duration with a rise time of about 1 μsec. It is shown to initiate according to the classical Townsend equation, building up a positive space charge of increasing intensity at the point surface, and an electronic and beyond this, an ionic space charge further out in the gap. The intense positive space charge leads to an auto‐accelerative ionizing mechanism with Morton‐Johnson ionization near the point. The magnitude of these space charge fields account for the sputtering previously observed. On the basis of the field distributions which can be deduced, the course of the growth of the visual and electrical phenomena can be traced to the point where the space charge distortion by negative ion for...

Impulse Streamer Branching from Lichtenberg Figure Studies

The Lichtenberg figure technique for the study of the streamer mechanism in the spark transition in positive point to negative plane geometry has been extended to longer gaps. In this extension the phenomena were investigated over a wide range of variables, some of the results of which, including the nature of the figures, the potential of the streamer tips, and properties of negative streamers, are being published elsewhere. This study is confined to the observation of the growth and attenuation of branching in relation to the appearance of the commonly observed solitary spark channel. Featured in longer gaps is the very extensive branching before midgap with decline as the cathode is approached. The luminous primary‐secondary streamer sequence observed by the two‐photomultiplier technique is an instrumentally conditioned phenomenon resulting from branching. There is no such sequence otherwise indicated by these observations. The same instrumental effect deriving from the branching accounts for discrepan...

Mechanism of Cataphoretic Segregation in Inert Gas Glow Discharges

The collection of a minority gas species at the cathode in a glow discharge, termed cataphoresis, appears not to have been completely successfully explained. Observation indicates that in all cases, the minority gas segregated at the cathode, must be capable of being very effectively ionized by the vehicular gas. The recent discovery of the effective creation of ion complexes such as NeHe+ and NeA+ in pressure dependent reactions above some mm of Hg pressure by Oskam, the independent direct observation of these ions and the production of ions such as HeH+, NeH+, and AH+ with 1% H2 in the inert gas by Weimer, using the effusion mass spectrometer of Pahl, render a basis for the extension of the Druyvesteyn theory developed for metallic ions in inert gas to the general process, aid in the explanation of the Hg‐Xe anomaly observed by Kenty, and indicate certain needed corrections which are acting to make the process more efficient.

The Choice of Suitable Gap Forms for the Study of Corona Breakdown and the Field Along the Axis of a Hemispherically Capped Cylindrical Point‐to‐Plane Gap

The various corona gap forms suitable for the convenient investigation of corona phenomena of both signs in the laboratory that are capable of yielding quantitative data involving the Townsend integral for the thresholds of the manifold phenomena are discussed. For laboratory study the most convenient form of gap is the hemispherically capped cylindrical point‐to‐plane system. Analysis of the potential fall along the axis in such gaps by means of electrolytic model studies indicates that the essential parameter is L/r, the ratio of gap length, L, from point surface to plane relative to the point radius, r. The model study potentials are, however, incapable of giving accurate values of the field strength. These fields must be obtained by computation. Laboratory studies of corona indicate the desirability of relatively large values of L/r. Practical considerations based on studies at atmospheric pressure indicate point radii r of 0.025 and 0.05 cm with a ratio L/r=160 to be those giving the greatest flexibi...

Electrical Breakdown of Argon in Glass Cells with External Electrodes at Constant and at 60‐Cycle Alternating Potential

The electrical breakdown in borosilicate glass cells of plane parallel plate geometry with external electrodes was studied under static and 60‐cycle alternating potential using Linde spectroscopically pure grade Ar gas introduced under Alpert vacuum conditions at 7, 15, and 35 mm. Oscilloscopic studies at fast and slow sweep as well as measurement of charge, wall potential and of apparent leakage charge through the glass using a large series condenser with Keithly electrometer and recorder were made. Three differing modes of breakdown appear as potential increases. These are associated with space‐charge distribution and density. The two modes at lower potentials are Townsend discharges operating by photoelectric processes at the cathode. The third mode has streamer‐like characteristics and a rise time equal to one avalanche crossing time. All breakdowns terminate through counter potentials from charge accumulation on the glass electrodes. The fairly complete data lead to a clear understanding of the obser...

Point‐to‐Plane Corona Onsets

The effect of point material and point radius of curvature on positive and negative intermittent corona onset potentials has been studied with a point‐to‐plane gap in air at atmospheric pressure. The negative Trichel pulse onset strangely is independent of point material but does depend on point history and radius. This surprising result is shown to come from the circumstance that the Trichel pulse onset depends on current densities needed to condition the point surface and yield a higher value of the second Townsend coefficient to give the increased currents. Trichel pulse onset thus does not mark the onset of a self‐sustaining discharge. The self‐sustaining discharge initiates at lower potentials and leads to currents of a low order until the cathode spot cleans up.

The Nature and Properties of Gaseous Ions Encountered in Atmospheric Studies

Publisher Summary This chapter focuses on discussing the nature of small ions encountered in the atmosphere, differentiating between the terms, center of force, solid elastic interaction, and composite interaction. It is followed by a discussion of the small atom, and complex, cluster, intermediate, and Langevin ions. Next it discusses the consideration of the initially ionized molecules or atoms and their charge exchange, aggregation, and evolution to final forms likely to be encountered. Laboratory studies covering a larger range of variables including field strength to pressure ratios, temperatures, densities, gas types, and conditions of generation are important for prediction. Atmospheric ions that are observed here depend largely on the composition of the atmosphere, including some minority species. This chapter shows that in time, positive charges reside on species of lowest ionization potential, and negative ions reside on species of greatest stability. Both form different complex ions and hydrate with H 2 O down to 10 -4 Torr. They are stable in field strength to pressure ratios used in most atmospheric measurements, E/p o ≤ 5. The faster ions may show discrete mobilities. As aggregates of H 2 O, or chemically nucleated H 2 O, and other aerosols pick up ions, continuous or band spectra are the rule. Upper atmosphere ions may exhibit discrete mobilities.

On the appearance and mechanisms of formation of langevin-type ions and related nuclei

This paper calls attention to the existence of a stable class of nuclei which charged are termed Langevin ions and uncharged can be called Langevin nuclei. These have diameters lying around 10−6 to 10−6 cm, depending on humidity, and consist of on the order of 106 H2O molecules often formed about some chemical center. They readily lead to cloudy condensation on supersaturation. The various processes by which such nuclei are generated form the basis of this paper. The hazes produced by aggregates of such particles frequently are mistakenly called smogs though they may come from such natural causes as the evaporation of sea fogs.

The Recombination of Ions

Coincident with the study of the conductivity induced by X-rays in gases leading to the concept of gaseous conduction by ionic carriers at the Cavendish laboratory in 1896 it was observed that the conductivity in the volume of the gas declined with time after the ionizing X-rays were cut off. This decline was ascribed by J. J. Thomson [1] and his group to a neutralization of the positive ions by the negative ions in the volume of the gas and termed recombination. Assuming for simplicity that there were just one species each of negative and positive ions, that they were present in equal numbers, that at some initial time designated as zero, t = 0, there were n + = n - = n 0 ions per cm.3 distributed uniformly and isotropically in space but with different signs at random relative to each other both on a macro and a micro scale, and that they by mutual “encounters”, (unspecified), of ions of opposite sign experienced a neutralization by exchange of charge termed recombination, it was stated that: Loss of ions dn and thus of conductivity in a time dt would be proportional to n + the number density of positive ions, n - the number density of negative ions and the time interval dt. Thus it follows that: