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The configuration consisting of a dielectric rod, surrounded by a plasma shell and a concentric outer conducting cylinder, is investigated theoretically and experimentally for its propagation characteristics. It is predicted from the theory that such a configuration will support propagation in a backward wave mode (oppositely directed phase and group velocities). These backward waves were observed in experiments using a configuration essentially identical to the above (the dielectric rod had an air space down the center used to insert a probing antenna), with the plasma provided by a cesium discharge. Agreement between these observations and calculations is excellent. A forward wave was also observed at low frequencies, and is attributed to the influence of the sheath at the plasma-conductor interface. The observed dispersion curve for this forward wave is also in good agreement with that calculated using the expected sheath size of the cesium plasma-metal interface.

A stable cesium plasma with a density of 1015 ions/cm3 and 50% ionization was produced by a Penning arc. A hot, high work function, anode is shown to be a significant factor in the achievement of this very dense highly ionized plasma. Plasma instabilities, which are normally present in the Penning arc, were found to be rotating in the j × B direction. These instabilities are damped when the arc pressure is 0.5 torr and the arc boundary is irregular. Radial and axial plasma density variations were measured. The radial density gradient decreased when the plasma became stable. The axial plasma density variation indicates that the prime source of plasma electrons is the hot cathode.

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