Cesium and potassium plasma experiments

Abstract

We review experiments performed on cesium and potassium thermal plasmas produced by thermal ionization of neutral atoms on tungsten and tantalum surfaces. A brief description of typical cesium or potassium thermal-plasma devices is given in section 2 of the paper. Section 3 deals with actual experiments, and the material is divided as follows:a. Experiments on plasma diffusion across a magnetic field. In these experiments it is found that when no current is passed through the plasma, diffusion proceeds according to the predictions of the “classical” theory (D⊥ ≈ 1/B2). The magnitudes of the diffusion coefficients are also in agreement with the results of classical theory based on ion-electron collisions.b. Excitation and propagation of low-frequency waves. Three different experiments are described. The first deals with ion waves excited by passage of a small current along the magnetic lines over a very small part of the plasma cross section. The frequency of the oscillations is generally slightly in excess of the ion cyclotron frequency, and the waves are electrostatic ion waves propagating almost at a right angle to the magnetic lines. The second experiment deals with ion waves propagating in the azimuthal direction across the magnetic lines and the radial density gradients of the plasma column. The phase velocity of these waves has the same magnitude as the ion macroscopic velocity of the zero-order motion. The third experiment deals with excitation, propagation along the magnetic lines and damping of ion acoustic waves. The phase velocity and the damping of the waves have been measured. In particular, the wave attenuation appears to be explainable only in terms of collisionless (Landau) damping.c. Spectroscopy experiments. We give a brief description of a spectroscopic determination of the recombination coefficient in cesium.d. Beam-plasma interaction. We discuss the effect of this interaction on the amplification of a microwave signal applied as a modulation to an electron beam that passes through a 2.2-cm length of cesium plasma.Section 4 contains a few considerations about experiments that appear feasible with alkali-metal plasma devices, some of which are under way. In particular, the production of a cesium plasma in a stellarator geometry is considered.