a. A voltage bias across the output gap produces an outward radial field at the far end of the gap, where the rf field has slowed the electrons. This may increase interception and also cause additional thermal problems. b. Reduction of space-charge forces and output velocity spread is the major effect of bias just before the output gap. However, in a low perveance beam these are already small. Further, part of the extra energy supplied by the bias will be lost as increased energy of intercepted electrons and the remainder must either be recovered at the collector or entirely converted to rf power if net improvement is to result. Even without a detailed design it is clear that this would be difficult, particularly in a tube of varying beam voltage. c. To apply a bias without loss of transmission, the drift tunnel must be severed over a distance shorter than the tube radius; with a voltage of 30 kV or more this could produce insulation problems. Collector depression is well developed(14-16) for tubes of low electronic efficiency (30 to 40 percent). If a collector can recover 70 percent of the energy in the spent klystron beam, electronic efficiencies of 85 to 90 percent are attainable. However, the many slow electrons leaving the output gap may present difficulties when the basic electronic efficiency is already 70 percent or higher, and a further feasibility study is recommended. Existing computer programs are capable of the analysis; 100 or more representative particles would give a realistic velocity spread. Ultimately the cost and weight of such a collector must be compared with the cost and weight of the power sources that are eliminated. 4. 2. 4 HEAT DISSIPATION AND BEAM COLLECTION A high-power klystron generates considerable heat through beam interception and high-frequency skin losses in the cavities. A calculation of the heat that must be removed is the first stage in the design of a thermal control system. This study shows that the confined-flow tube, with better beam transmission and higher circuit efficiency, can produce about three times as much useful rf power for the same body heating as the PPM tube.
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