EFFICIENCY ENHANCEMENT OF DUAL-MODE TRAVELING WAVE TUBES AT SATURATION AND IN THE LINEAR RANGE BY USE OF SPENT-BEAM REFOCUSING AND MULTISTAGE DEPRESSED COLLECTORS.

An axisymmetric, multistage depressed collector of fixed geometric design was evaluated in conjunction with an octave-bandwidth, dual-mode traveling wave tube. For operation of the dual-mode TWT at and near saturation, the collectors increased the TWT overall efficiency by a factor of 2 one-half to 3 one-half . Collector performance was relatively constant for both the high and low TWT modes and for operation of TWT across an octave bandwidth. For operation of the TWT in the linear, low-distortion range, collector efficiencies of 90 percent and greater were obtained, leading to a five- to twelvefold increase in the TWT overall efficiency for the range of operating conditions covered and reasonably high ( greater than 25 percent) overall efficiencies well below saturation. With collectors of this efficiency it becomes practical to design dual-mode TWT's such that the low mode can represent operation well below saturation. Consequently, the required pulse-up in beam current can be reduced and this mitigates beam control and dualmode TWT circuit design problems.