N. D'Angelo

Device for Generating a Low Temperature, Highly Ionized Cesium Plasma

A device for generating a linear, highly ionized, low temperature cesium plasma is described. The plasma is generated by having the output of cesium atomic beam ovens impinge on hot tungsten plates placed at both ends of a cylindrical vacuum chamber. The walls of the chamber are cooled so that neutral cesium condenses on them. The theory of the device, designated as the Q machine, is presented and some experimental results given. The maximum density achieved was 2×1012/cm3, with an estimated fractional ionization of 99%, and a confining field of 5900 gauss.

Excitation of Electrostatic Plasma Oscillations near the Ion Cyclotron Frequency

Oscillations near the ion cyclotron frequency have been excited in thermal cesium and potassium plasmas by drawing current in a filament along the axis of the plasma column. The oscillations appear to be electrostatic waves propagating radially from the filament. The waves are present if the electron drift velocity exceeds about 10 times the ion thermal velocity, in agreement with the prediction of M. N. Rosenbluth. The measured phase velocity is also in agreement with the phase velocity calculated from the fluid equations.

Electrostatic Oscillations near the Ion Cyclotron Frequency

Measurements of ion wave instabilities in plasma in which the electron drift velocity is a few% of the electron thermal speed and ion and electron temperatures are about equal are reported. Experiments were performed in Cs and K plasmas 3 cm in diameter and about 60 cm long produced by surface ionization of atoms on a hot tungsten plate. If sufficient positive voltage is applied to the collector to raise the electron drift velocity to about 10 times the ion thermal velocity, oscillations appear in the collector current. Results agree with theory. (L.N.N.)

Propagation and Damping of Ion Waves in a Plasma with Negative Ions

The ion waves split up into two modes of propagation; if the fraction of negative ions is large, the collisionless damping of the faster mode is weak.

Investigation of the Kelvin‐Helmholtz Instability in a Cesium Plasma

Experiments have been performed in a thermally ionized cesium plasma, in which a relative drift between ions in adjacent layers appears to excite the Kelvin‐Helmholtz instability. The relative drifts required are of the order of ci, the ion thermal velocity, or somewhat smaller. The frequency of the oscillations is generally in the range 1–4 kc/sec. A study has been made of the type of waves excited, by means of amplitude and phase measurements.

Low-Frequency Oscillations in a Potassium Plasma

Low‐frequency oscillations (10–30 kc/sec) have been observed in thermal potassium plasmas, produced by surface ionization of potassium atoms on a hot tungsten plate. The oscillations are associated with the presence of an ion sheath near the tungsten plate and seem to be reasonably well described on the basis of a simple picture of ion waves propagating across the magnetic lines of force, in the presence of a density gradient.

Diffusion and Recombination of a Highly Ionized Cold Plasma in a Magnetic Field

Measurements of the diffusion coefficients of cold, highly ionized cesium and potassium plasmas across a magnetic field are described. The field was varied between 3000 and 9000 gauss, the ion density between 1010 cm−3 and 1012 cm−3. No current was passed through the plasma. The results support the view that diffusion proceeds according to the predictions of the classical theory (D⊥ ∼ 1/B2). No agreement is possible with Bohms diffusion theory. Also, some support is given to the theory of recombination through electron‐electron‐ion collisions and radiative cascading between excited states.

LOW-FREQUENCY OSCILLATIONS IN CESIUM THERMIONIC CONVERTERS

Oscillations with frequencies in the 100 kc range were observed in the operation of cesium converters for cathodes at 1350 to 1450 deg K. The frequencies correspond roughly to those of ion-plasma oscillations and a positive ion sheath may be present near the cathode. (L.N.N.)

``Turbulence'' of a Low‐β Plasma

The fluctuation spectra of Cs and K plasmas have been measured in the Q‐3 device. They appear to be in agreement with a recent prediction by Chen.