The confined-flow tube should contain a heat pipe around the entire drift tunnel and penetrate to the tips of the cavities as shown in Figure 4-27, in order to maintain a uniform temperature. This minimizes possible mechanical stress due to the periodic changes in body heat with supply power. "Dowtherm A" (Reference 2, p. 124) or cesium are suitable liquids for the heat pipes. "Disper- sion hardened copper”, which contains zirconium oxide or aluminum oxide, has 95 percent of the electrical and thermal conductivities of pure copper but can withstand temperatures of 500°C or higher. It may be possible to use thin and light cavity walls surrounded by the copper mesh wick of the heat-pipe evaporator. Linear scaling from Branch and Mihran’s design for an 11 GHz, 5 kW tube (Reference 2, pp. 137, 141) suggests that the 43 kW tube described here (48 kW for the higher electronic efficiency) is feasible, one criterion being the maximum power that a heat pipe can remove. However, a more detailed thermal model of the output cavity will determine the ultimate power capability. If a solenoid is used, it should be wound directly on the heat pipe used to cool the drift tunnel and cavities, and should therefore be capable of operation at 300°C or higher. The outer surface of the solenoid can radiate directly; for example, a black body cylinder 60 cm long by 6 cm radius at 300°C radiates sideways 1430 watts. The PPM klystron and solenoid klystron are shown respectively in Figures 4-28(a) and 4-28(b). The size and weight of body radiators are calculated in Appendix G. A surface temperature of 250°C allows for 50°C rise between radiator and heat source, and the sizes allow for a ’’view factor” between body and collector and for radiation that is absorbed when the collector points directly at the sun. Heat pipes must line the backs of the radiators in order to reduce the thickness and weight required for effective conduction. The collector serves both to complete the circuit of cathode, beam and anode and to dissipate the unconverted power remaining in the beam after the output cavity. To minimize weight for space application it should radiate directly from its electrodes, and only the heat remaining when the tube is under saturated RF drive need be removed.
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