Velocity and current distributions in the spent beam of the backward-wave oscillator

Results are presented of an experimental study of the spent beam of a backward-wave oscillator. The instantaneous velocity and current of the spent beam are measured using a velocity analyzer built onto the collector of a scaled 80-mc backward-wave oscillator. The tube employs a sheet beam and interdigital line, 12 feet long. It is designed to be representative of large-space-charge tubes. The measured trajectories of the spent beam are examined to deduce the mechanism of interaction between the beam and the circuit along the whole length of the tube. It is deduced that the level of oscillation is determined by nonlinear effects in the convection current. Finally, the RF output efficiency saturation at high beam currents is found to be caused by electrons which fall back in phase from a retarding to an accelerating circuit field.