R. J. Zollweg

Electrical conductivity of nonideal plasmas

The Spitzer formula for the electrical conductivity of fully ionized gases is modified to permit its application in the nonideal plasma region. A rigorous method of calculating electrical conductivities in partially ionized gases in both ideal and nonideal plasmas is also described. These models are based on the classical Chapman–Enskog approximation with the superposition of binary collisions without the assumption of additional electron scattering mechanisms, such as scattering by plasma oscillations. Good agreement with experiment is obtained in the nonideal region largely through a more adequate treatment of electron screening of the ionic Coulomb potential. The large electron scattering cross section for attractive ion potentials found by Mason et al. [E. A. Mason, R. J. Munn, and F. J. Smith, Phys. Fluids 10, 1827 (1967)] is found to decrease the conductivity in the nonideal region compared to the cross sections of Liboff.

Convection in vertical high‐pressure mercury arcs

Convection in wall‐stabilized high‐pressure arcs is found to be highly sensitive to the relative material properties of the discharge gases. A method has been found to experimentally determine net‐radiative‐emission coefficients that are consistent with more reliably calculated electrical and thermal conductivities. The electrical conductivity of mercury arcs obtained using the electron scattering cross section for mercury derived by Rockwood gives better agreement with experimental temperature profiles than does that obtained by use of McCutchen’s cross section. Using these material properties with a theoretical convection model, good agreement was obtained with measured arc‐temperature profiles in the midplane, with the conical appearance at the bottom of the vertical arc and with the electrical properties. For 18‐mm‐i.d. walls and mercury pressures up to 13 atm, the reduction in radiation found for high mercury pressures was found to be caused by self‐absorption and not by an increase in radial thermal...

Arc constriction in lamps containing mercury and iodine

Quantitative calculations indicate that the mechanism by which iodine causes constriction of arcs in mercury vapor lamps is the emission of molecular radiation from HgI. Calculated arc temperature profiles from the energy balance equation for various mixtures of mercury and iodine are found to agree with measured profiles when allowance is made for the emission and self‐absorption of radiation from all atomic and molecular species. Approximate emission coefficients for HgI molecular radiation and atomic iodine radiation have been determined as a function of temperature. Molecular radiation emitted from the plasma at temperatures between 2500 and 4500 °K is found to be the major cause of arc constriction for plasma pressures of the order of a few atmospheres.

Theoretical description of ac arcs in mercury and argon

The energy balance and circuit equations, together with Ohm’s law, are solved numerically to obtain temperature profiles, current, and voltage as functions of time for wall−stabilized ac arcs. Experimental measurements of current and voltage oscillograms and also temperature profiles have been made for 1.5− and 7.5−A rms arcs in mercury vapor. Derived net emission coefficients of radiation as a function of temperature for the 1.5− and 7.5−A arcs are consistent with one another; furthermore, the experimental temperature profiles for both arcs are in good agreement with our theoretical predictions. Theoretical current and voltage waveforms and central temperatures for a 20−A rms arc in argon are in good agreement with the experimental results of Detloff and Uhlenbusch. Radial convection has little influence on these results. An approximate criterion for arc extinction or reignition at current zero is given in terms of the steady−state V−I characteristic and the arc time constant. By relating the present ana...

A radiation and convection model of the vertical mercury arc containing sodium and scandium iodides

A model has been developed for a multicomponent gas discharge containing several radiative species, Hg, I, Na, Sc, Sc+, and ScI, subject to radial and axial segregation. Net emission coefficients for self‐absorbed radiation from these species are deduced individually and collectively from experiment and imprisonment theory. The vertical arc convection model has been modified to take into account the axial variation of material parameters with segregation and to calculate quartz wall temperatures. Calculated lamp parameters are found to be in reasonable agreement with experimental data.

Convection in horizontal high‐pressure mercury, mercury plus iodine, and metal halide additive arcs

A two‐dimensional (r, ϑ) numerical model has been developed to describe free convection in horizontal high‐pressure arcs consisting primarily of mercury vapor confined in quartz arc tubes. The calculated vertical location of the hot core of the arc was found to agree with experiments within 1–2 mm, both with and without an applied transverse magnetic field to force the arc downward. An energy balance calculation of the quartz wall has been used to determine the quartz wall temperature distribution. For somewhat constricted mercury plus iodine arcs, two stable arc configurations were found to exist over a range of applied transverse magnetic fields.

Electron Reflection from Cesium‐Coated Polycrystalline Metals at Low Primary Energy

The reflection of low‐energy electrons from polycrystalline metal surfaces partially coated with cesium has been investigated by the retarding potential difference technique. The technique is described and results for primary energies below 5 eV are given. Retarding potential measurements of the longitudinal component of the reflected electron energy are consistent with elastic reflection and a cosine angular distribution at low primary energy.

Oscillations and Saturation Current Measurements in Thermionic Conversion Cells

Radio‐frequency oscillations observed in cesium‐filled thermionic diodes are interpreted on the basis of a model which assumes that the cesium ions oscillate in an excess negative charge potential well outside the cathode. A simplified theoretical treatment shows that the period at the onset of oscillations is linear with cathode‐anode spacing in agreement with experiment, and relates the oscillation period to the ratio of cell current to saturation emission current. It is found that the rapid decrease of oscillation amplitude as the cell current reaches a critical value can be used to measure saturation emission currents.

Temperature‐Dependent Bismuth‐Cesium Photosurfaces

The yield of bismuth‐cesium photosurfaces is sometimes strongly temperature dependent. We have investigated this effect and found it to be dependent upon the presence of oxygen and upon the size of the aggregates making up the photosurface. The photoelectron energy spectra are also examined. It is concluded that the temperature dependence probably arises from a change in escape depth with temperature because of phonon collisions, but that the principal photoelectron energy loss may occur at oxygen impurity sites.

Thermal conductivity of cesium vapor

Abstract The thermal conductivity of cesium vapor has been measured as a function of pressure at spacings which correspond to a range from a fraction of a cesium mean free path to several mean free paths. The gas conduction was measured as the increase in power necessary to maintain a hot ribbon filament at a fixed temperature with increasing cesium pressure. A differential measurement with two filaments of different lengths was used to cancel end errors, so that a high degree of accuracy was achieved. The thermal conductivity is independent of spacing at low cesium pressures and is a constant, independent of pressure at high pressure, as expected from kinetic theory. A value of 1·7 × 10−5 W/cm-°K was obtained for this high pressure limit.