The identification of the dominant ionic species in the cesium plasma diode has been established experimentally as the atomic ion, Cs+, instead of the molecular ion, Cs2+. Values of the atomic ion density, Ni, have been determined spectrographically by measurements of the radiative recombination spectrum leading to the 6P state. Electron densities, Ne, were measured independently by the Stark broadening of selected cesium lines. Within experimental uncertainty for a broad range of diode operating conditions, Ni=Ne. Thus, not only are Cs+ ions directly observed, but any important concentrations of Cs2+ would violate plasma neutrality. The experimental results are consistent with the existence of local thermodynamic equilibrium.

Identification of the Ionic Species in a Cesium Plasma Diode

Relations between optically-thick full-widths and optically-thin full-widths are given for Lorentzian and Gaussian line shapes. The Lorentz form is seen to predominate for most situations due to the strength of the wings of a Lorentz shaped line. The excessively large widths often measured for alkali resonance lines, as shown by these relations, are due to optically thick observations.

Widths of optically thick lines

A dc probe was placed behind a small slit in the collector electrode in each of several cesium‐vapor thermionic diodes. These systems were used to study the velocity distribution of the small fraction of the emitted electrons which passed through the collector slit. This technique overcomes several of the inherent disadvantages in the use of the dc Langmuir probe in cesium vapor. The error due to momentum defocusing in the collector slit is calculated with the presence of a plasma in the emitter—collector interspace. The probe log‐current vs voltage curves could be accurately fitted to a Maxwell—Boltzmann velocity distribution over several cycles in log‐current. In this manner, effective temperatures were defined as a measure of the randomness of the velocity component normal to the collector surface. No fully randomized, high‐temperature electron group was detected in the unignited mode of these diodes. The effective electron temperature in the unignited mode became increasingly larger than the emitter temperature as the latter temperature was increased, but the departure was always less than 25%. The effective electron temperature increased rapidly with diode current as the ignited mode was entered and rose to be 470% higher than the emitter temperature.

Measurement of electron temperature in low-pressure cesium thermionic energy converters

Electron excitation temperatures have been measured in a low‐pressure (0.05‐Torr Cs, 2‐Torr total pressure) argon‐cesium discharge that uses a heated cathode (900–1100 K). The excitation temperature determinations are based upon a model that allows calculation of cesium excited state densities for low electron density (<1011 cm−3). The model assumes that the dominant creation processes for excited states are electron impact excitation from the ground state and radiative cascade from higher levels, while destruction is by spontaneous emission. Maxwellian electron energy distributions were used and the plasmas were considered to be optically thin. The model indicates that cascade contributions to the production of excited states can be as high as 50% for some cesium levels. Predicted emission spectra with cascade contributions to spontaneous emission intensities agree well with measured spectra except for radiation trapped transitions from low nP states to the ground state. Excitation temperatures are deter...

The effect of radiative cascade on electron excitation temperature measurements

Excitation temperatures and Saha temperatures are determined for an r.f. excited cesium discharge in the electron density range from 2·5x109−3 x 1011/cm3. The consequence of self-absorption of both resonance and nonresonance states in measuring temperatures is discussed. It is shown that cesium ion excitation is inversely related to ion density and that this excitation cannot be described by a “ temperature”. Addition of helium over the pressure range 0·1–10 torr has no effect on ionization or cesium excitation other than to allow observation of Cs spectra at a lower pressure than is possible in pure cesium. A discussion of the utility of these observations in describing the non-equilibrium state of the discharge is included.

A spectroscopic study of low density cesium and cesium-helium discharges

Experimental cesium line shapes

A tantalum oven that produces quiescent isothermal cesium plasmas at precisely known temperatures and pressures is described. The apparatus consists of a closed, cesium vapor filled tube heated by radiation within a resistance heated vacuum furnace. Plasma temperatures up to 2300°K and cesium vapor pressures up to 10 Torr are obtainable. Temperatures are measured by calibrated unsheathed W/W‐26%Re thermocouples. Cesium vapor pressures are known through careful control of the cesium condensation conditions. Designed especially for spectroscopic studies, the oven features two sapphire windows mounted on a line‐of‐sight axis through the plasma. Details of construction and operation are described. Performance data showing temperature profiles to 1% over a 10 cm length are given.

C. Summers

Papers

Experimental cesium line shapes

Isothermal Plasma Oven