Observations on ion oscillations in a cylindrical-beam tetrode under hard vacuum conditions

A study of the physical mechanism of ion oscillations was begun after the discovery of spurious sidebands in the spectrum of a DC-operated low-voltage reflex klystron which was undergoing development at this laboratory. These sidebands were observed to be as strong as 40 db below 1 mw in some tubes and were separated from the carrier by as little as 0.6 mc or as much as 15 mc, depending on which klystron was examined and what voltages were applied to it. It was observed that the frequency separation and magnitude of the sidebands were a function of the exterior cooling conditions, an observation which suggests a relation between the sidebands and the gas pressure within the tube. It has been recognized 1 that ion plasma oscillations can be a source of noise in microwave tubes; consequently it was decided to build a cylindrical tetrode structure whose DC configuration was similar to that of the klystrons and to study the details of the oscillations.