is near optimum; the input gap should be longer in order to raise R/Q and the beam-loading conductance, and thus increase the coupling and the tube gain, g. The input and output cavities resonate at the drive frequency (in the presence of the beam), while intermediate cavities of fundamental frequency are unloaded and tuned above drive frequency to give nearly phase lead of voltage from current for high efficiency. Second-harmonic cavities are tuned slightly below . Appendix E gives the calculation of the required resonant frequency from the bunched current and the gap voltage. 4. 2. 8 TUBE LIFETIME The goal of the satellite power system is to produce near-maximum design power with minimum maintenance over 25 to 30 years. For existing tubes this is optimistic. For example, field tests of high-power klystrons by Varian Asso- ciates showed an average life of about 16, 700 hours (2 years). Of 22 klystrons reported, seven failed with unknown cause, while cathode failure was reported in eight, collector burnout in five, and output cavity arcs in two. In the vacuum of space (less than 10 Torr) an output gap of 4. 5 mm should safely withstand an rf voltage of 100 kV or more; for ground-based tubes at about 10 Torr, 60 kV/ cm is about the maximum safe field. A 200 microsecond ”pulse-off” period would extinguish occasional arcs. The space environment has the further advantage of making vacuum sealing unnecessary on both the output waveguide and the collector. The tube must be protected, however, against loss of rf drive to avoid overheating the radiating collector. Heat pipes and the cathode remain as the most likely parts to fail. Reported lifetimes for heat pipes have attained only 41,000 hours, but the technology is relatively new. A conventional "nickel matrix oxide” cathode is still the most promising, with a current density of up to 0. 24 A/cm , whereas cold field-emitting cathodes are as yet unproven on linear-beam tubes. The high vacuum has the advantage of virtually eliminating ion bombardment in the tube. 4. 2. 9 WEIGHT AND COST The weights and costs of three proposed klystrons are shown in Figures 4-33 to 4-35. Figure 4-36 summarizes the results.
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