The Rectification of Radio Signals by a Thermionic Tube Containing Alkali Metal Vapor

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Abstract
The cathode of a thermionic triode tube is surrounded by a region of minimum potential caused by electron space-charge. If the cathode is of thoriated tungsten, and the tube contains caesium vapor, a few positive ions are formed by ionization of caesium atoms striking the hot cathode. Some of these ions accumulate around the potential minimum, raise the potential at that point, and allow a larger electron current to flow to the anode. The ions have a natural frequency of oscillation about the potential minimum, which is usually several hundred kilocycles. If an alternating voltage is applied to the tube, of a frequency agreeing with this natural frequency, the ions are set in oscillation about the potential minimum. This oscillation of the ions makes the potential minimum more negative, and decreases the anode current. In addition the amplitude of oscillation may build up by a resonance effect to such a large value that the ions are able to discharge to one of the neighboring conductors, leading to a still greater decrease in anode current. This kind of rectification is much greater for small alternating voltages than that resulting from curvature of the triode characteristics.

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