A simple theory of ion noise in TWT

Summary form only given. In this paper, we present a simple theory to explain some key features of ion noise observed in experiments. It is shown that, in the event that beam scalloping due to mismatch cannot be avoided, the most effective way to suppress ion noise is adjustment of the magnetic field so that the interaction length is an integral multiple of the scalloping wavelength. The theory, which isolates the largely uncharacterized ionic processes, is able to account for the experimental observations of English and Funk, (1998, unpublished): (1) the extreme sensitivity of ion noise levels to the strength of the magnetic focussing field and cathode voltage, (2) the marginal sensitivity of ion noise to cathode heater filament voltage, and (3) the insensitivity of ion noise to RF drive level and collector voltage.