Effects of electron-beam confinement on klystron efficiency

This paper describes experiments which were devised to examine large-signal performance in klystrons focused by magnetic fields of finite magnitude. The experimental results are related to the theoretical analyses of Brewer, who treated small-signal bunching for magnetic fields of various magnitudes, and of Webber, who treated large-signal bunching for an infinite magnetic field intensity. The experiments agree with these theories where they apply, and additionally, the Webber theory continues to describe the transition from small to large signal for finite magnitude magnetic fields, when one uses the appropriate reduced plasma frequency in the bunching parameter. The relation of output cavity RF driving current, output cavity voltage, and magnetic field to conversion efficiency is also examined. It is found that maximum output power is obtained by maximizing driving current, but that with driving currents being equal, higher efficiency may be obtained in the beams which are magnetically more tightly confined; it appears that the higher magnetic field allows the output voltage to rise higher without defocusing the beam in the output cavity.