A novel electrostatic-focusing depressed collector for linear beam tubes

In a theoretical study, various types of retarding electrostatic fields are compared with respect to their usefulness in a depressed collector. Without space charge, fields having focusing properties lead to higher collector efficiencies ηcollthan defocusing fields. For a special case, the hyperbolic field, ηcollis given as a function of the angular beam spread, beam diameter, and initial beam energy. ηcollincreases with 1) decreasing angular beam spread, 2) increasing ratio of collector size to beam diameter, 3) increasing initial beam energy, and 4) the number of collector stages. Experimentally, a 3-stage collector of the hyperbolic field type was studied in connection with a PPM-focused high-power Dual Mode TWT, having a pulse-up ratio of 8 dB. At 50-, 75,, and 100- percent depression, the tube efficiency was 26 percent in the low-power mode and 34 percent in the high-power mode, corresponding to collector efficiencies of 76 and 71 percent, respectively.