Computer studies of noise generation in crossed-field amplifiers

Results from several different types of computer models of cross-field amplifiers have contributed to an understanding of some of the physical mechanisms responsible for noise generation in these tubes. These mechanisms include the build-up of charge densities by the (secondary emitter) cathode in excess of that required to supply the anode current and the effect of space charge on the production of fluctuations in the current carried through an individual spoke to the anode. In comparisons with measured S/N (signal-to-noise) ratios, computed results have, in some cases, done surprisingly well, reproducing certain known trends (showing increasing S/N ratio with increasing drive power, for example) and even producing decent quantitative agreement with measured RF output fluctuation levels. The general level of agreement is far from perfect, however, and certain further model improvements and tests are necessary before final judgments can be offered as to the applicability of simulation codes to the calculation of actual noise powers above a floor level determined by the nature of the simulation algorithms and the computer on which they are run.<>