Electron and ion noise in microwave tubes

Abstract

Summary form only given. The measurement of noise in microwave tubes is both ancient and new. In the former category is ion noise in for instance traveling wave tubes, in the latter is electron shot noise in gyroklystrons. For most applications, the noise property of the tube is as important as the more conventional figures of merit such as power, efficiency, bandwidth, etc. In the last few years, an effort has been made to understand this noise, mostly at NRL, but also at NSWC Crane, U. Mich, and UC Berkeley. Ion noise, in various guises, has plagued microwave tubes for more than half a century. Despite this tremendous effort, very little has been documented in the scientific literature, much relevant information exists only in the lab notebooks and minds of senior engineers at the tube companies. Usually it manifests itself as a random relaxation oscillation of the microwave phase. For much of the time, it has been assumed that this noise resulted from ionization of the background gas by the beam. The ions then become trapped in potential wells of the rippled beam, and when the well fills, the ions untrap and process starts again. In fact, measured time scales of the relaxation oscillations are consistent with the time to produce this amount of ionization. However recent theoretical work has shown that this picture is only partially correct. The ions do fill the wells, but there is no tendency for relaxation oscillation. It turns out that the oscillation results from the interaction of the trapped ions in the well with the focusing of the beam. Theory and simulations give considerable insight to this process.