Low-voltage Arc Research Articles

On the diffusion theory of the low-voltage arc plasma

The thermo-diffusion term in the particle flux equations is considered according to the experimental data, which show a large variation of the electron temperature across the low-voltage arc plasma. For the case of an infinite plane parallel plasma and a constant electric field the electron diffusion coefficient at the cathode edge is estimated and it is compared with the classical theoretical expression.

Radial and Axial Electron Concentrations in an Oscillating Low Voltage Arc†

ABSTRACT Measurements have been made of the plasma electron concentration (n) along and perpendicular to the axis of a low pressure, low voltage oscillating discharge with cylindrical symmetry, maintained by an axially directed beam from a small cathode. The contours of n are sharply peaked radially at the axis between the cathode and oscillation front (meniscus), but are much flatter beyond the meniscus, near the anode. The radial contours between cathode and meniscus conform approximately to those predicted theoretically for axial electron generation, and those near the anode to those predicted theoretically for ion generation proportional to electron concentration. Contours near the meniscus, when not completely unobtainable on account of the oscillations, exhibit apparent radial maxima and minima associated with the beam diverging from the meniscus.

Investigations on the caesium-cadmium diode

It is found that the volume ionization in caesium diodes may be improved by the addition of cadmium. This is due to collisions of the second kind of caesium with cadmium atoms excited on the resonance level. Some electrical and spectral characteristics of the low voltage arc in caesium-cadmium vapours are given.

Electron Concentrations in the Cathode Region of Low Voltage Arc Discharges

The number distribution of fast and slow electrons in the cathode region of low voltage arc discharges has been calculated on the basis of diffusion theory. The results agree well with measurements obtained in hydrogen by a dynamic probe technique. A winging effect has been found which may be connected with the production of electron oscillations in the plasma.

Electron Distribution in Low-voltage Arcs

THE plasma properties of a low-voltage arc will depend on the way in which electrons are distributed through it. The electrons, as in other hot cathode discharges1,2, occur mainly in two groups, one consisting of almost monoenergetic electrons which have fallen through the cathode sheath and acquired random direction in a few mean free paths with little change of energy, the other much slower and near-Maxwellian. As part of a study of hydrogen thyratrons, measurements have been made of the concentrations of both groups in a similar device without control electrode, using Druyvesteyn's probe method3. We find that they can be accounted for by diffusion theory and report here briefly some results of general interest which have been obtained for the fast group. Comparable results have been obtained for the slow.

The Mechanism of Electrode Erosion in Electrical Discharges

This paper outlines the nature of the various classes of electrical discharges—the high voltage capacitative spark discharge, the normal arc discharge, and the low voltage arc that occurs at electrical contacts—and discusses the physical processes that determine the dependence of the rate of electrode erosion on the physical properties of the metal. It is concluded that the most important properties conducive to low erosion rates are high boiling point, high thermal conductivity and high density, so providing the physical reason for the low erosion rates observed for the platinum metals when used as electrodes and electrical contacts.

Experimental studies of CsCsFMo and CsCsFW plasma diode emitters

Measurements are presented which demonstrate that the electron emission of molybdenum in cesium vapor is greatly enhanced by the addition of cesium fluoride. This enhancement amounts to a factor of about 430 at the maxima of the respective Langmuir S-curves for a cesium pressure of 10 −2 mm Hg. The minimum effective work function of the CsCsFMo surface is 1·38 ± 0·05 eV. Preliminary measurements of the work function of cesium flouride coated tungsten have been made by obtaining retarding potential curves for photo electrons liberated from the cesium flouride coated tube wall by an external light source. An activation process is indicated by the fact that is is necessary to heat the tungsten wire to about 1100°K in order to obtain a high work function surface. A CsCsFW emitter has been incorporated into a plasma cell of unguarded parallel plate design. At the initial emitter-collector spacing of 0·25 in. attempts to operate the cell in the low voltage arc regime have met with little success, although some cross over into the useful power section of the current-voltage characteristics occurs for a cesium pressure of 0·1 mm Hg and emitter temperatures in the region about 1900°K. Arc currents extrapolated to zero voltage indicate that the emissionof the CsCsFW emitter is about the same as that from a CsCsFMo emitter.

Optical studies of the cesium arc

Experimental studies of the emitted optical radiation from cesium arc-mode energy converters were described. Three types of measurements were made: (1) Studies of details of spectra and line shapes. (2) Variation of radiation output with power output for the converter. (3) Studies of radiation during after-glow. These studies indicate that the excited states 6P 3 2 and 6P 1 2 (1·45 and 1·38 V respectively above the ground state) play an important role in the plasma generation mechanism. The results will be discussed in relation to the theory of the low voltage arc.

SOME CALCULATIONS OF THE CURRENTS IN THE IONIC CENTRIFUGE

The book, Magneto-Fluid Mechanics'' by Ferraro and Plumpton, gives equations for calculating the current taken by the end plates and cylinder of the ionic centrifuge for the two types of discharge (ibid, 47: 313-19(1961)). In the first type of discharge, where the end plates take one half the positive ions emitted by the low-voltage arc, the beam of particles to the cylinder is concluded to consist of both positive ions and electrons with like number densities and like velocities, the ion velocity is at 45° to the cylinder surface, and the electron energy is zero. In the second type of discharge, where the whole ion current is carried with electrons at low voltage to the cylinder, the energy of the ions must be very high if they are not to be caught by the end plates, and then low again at the cylinder.

Emission of high-energy ions from low-voltage arcs. Comments on H. H. Teh's paper

Experimental agreements and disagreements are pointed out concerning the paper Pseudo-High-Energy Positive Ions in Plasma Jets,'' H. H. Teh, J. Nucl. Energy, Part C,

Emission of Relatively High-Energy Ions from Low-Voltage Arcs

Emission of Relatively High-Energy Ions from Low-Voltage Arcs

Plasma electron oscillations

After emerging from the cathode sheath of a low voltage arc at low pressure, the primary electrons excite and participate in an u.h.f. oscillation in the plasma. This attains large amplitude rather abruptly a few mm from the cathode. The present paper contains a concise description and brief commentary on discharges of this type and of their relation to some other forms of u.h.f. and lower frequency discharges. Most of the processes involved are probably markedly non-linear. The possibility of electrical turbulence is mentioned.

Production of Doubly Charged Helium Ions

A 0.6 μa beam of He++ ions has been produced and accelerated to 7.6 Mev in an electrostatic generator. The He++ ion source, contained in the high voltage electrode of the generator, consists of a second small 400 kv electrostatic generator driven by the upper belt pulley of the main machine. He+ ions obtained from a low voltage arc type ion source in the dome of the small generator pass through a region of low pressure gas to become doubly ionized before entering the main accelerating tube of the large generator.

Radiative Processes in Thermionically Controlled Discharges in Helium

Recent measurements have been made of the intensity of radiation from the low voltage arc in helium as a function of gas density, tube current and tube potential. The experimental results indicate that the radiation is the result of a primary electron process. This process has been generally assumed to be direct excitation. Such an explanation is not fully in accord with the phenomena observed. The possibility of an unrecognized process is suggested.

Construction and Performance of a Low Voltage Arc as a Source of Positive Ions

A low voltage arc of the ``capillary'' type which has given reliable performance since its installation in 1940 is described. The design stresses simplicity and reliability, but no special attempt is made to attain very low consumption of gas. Operating at an arc current of 0.75 ampere and consuming 25 cc of deuterium at N.T.P. per hour, a total beam current of 1.1 milliampere is produced with a probe potential of 3500 volts. About 50 microamperes of deuterons, magnetically selected from this beam, can be focused on a target.

Cesium Vapor Lamps*

A study of the characteristics of low voltage arc discharges in cesium vapor led to the development of a practical lamp that was used in a successful communication system using infra-red radiation. The radiated energy is confined almost entirely to the cesium resonance lines at 8521 and 8944 angstroms. Data are presented that give structural details, operating characteristics, infra-red output, and efficiencies of several lamps of different sizes. Excellent modulability is attained throughout the audiofrequency range with current modulation up to 100 percent.

The sputtering of platinum in hydrogen and deuterium

The rates of sputtering in hydrogen and deuterium have been compared using ions drawn from a low voltage arc accelerated through a known potential. The rates are equal within a few per cent.One of us (K.I.R.) is indebted to the Industrial Research Council of Eire for a maintenance grant.

A Neutron Generator for Biological Research

A neutron generator is described which has been built for biological research. It embodies a Cockcroft and Walton circuit to generate 400 kV and an ion tube fitted with a low-voltage arc for the bombardment of deuterium by deuterons. A film of deuterium paraffin-wax on copper, cooled by running tap water, is used as the target. Materials and apparatus cost £600, the shed to house it £150, and subsequent maintenance £80 per annum. Two physicists and a mechanic spent two years in the building and improvement of the generator.

The Study of the Diffraction of Protons of Medium Energy by Vapor Molecules

The diffraction of 15–20 kv protons by carbon tetrachloride vapor has been investigated with a low voltage arc source and electrostatic focusing with photographic registration. The principal difficulties encountered were with space charge dispersion of the beam near the ion source, and with neutralization of the beam as it passed through the vapor. The cross sections for neutralization and re-ionization of protons and hydrogen molecular ions in several gases and vapors were determined. The sensitivity of photographic film was found to diminish as the emulsion was outgassed but to increase fifteen-fold when it was treated with oil. When suitable precautions were taken to minimize the background scattering, diffraction patterns were obtained showing three maxima, the measurements of which are in satisfactory agreement with electron diffraction data.

The Variation with the Density of Mercury Vapor of the Intensity of the Spectral Lines of Mercury Excited by Electron Impact

The variation in the intensities of mercury lines with the abundance of mercury present was investigated experimentally both for mercury alone and for mercury vapor in a mixture with another gas. The source consisted of a normal low voltage arc and this source was used because it closely approximates the simple ideal source desired; namely, one in which electrons of definite and controlled energy are projected through a space containing mercury and possibly other atoms so that an electron may not make more than one exciting or ionizing collision in its passage through the gas. Tests indicated that the ideal conditions were closely approximated. Theoretical formulae were developed for the relationship between the intensities of the mercury lines and the density of the mercury vapor (abundance of mercury). The measured values were found to be in good agreement with those given by the formulae. The relation of this investigation to the practices of quantitative spectrochemical analysis is pointed out. An analytical curve for the determination of mercury in such mixtures as were employed can be gotten from the data and the equation of the curve can be derived from the formulae derived from the theory given.