Figure 4-36. Comparison of Tube Types i. e. , at least 5 percent interception and typically 50 percent efficiency (Reference 3, Appendix F). For low-power applications, such as satellite traveling-wave tubes, periodic focusing regains its value, but for the present application further development should concentrate on the electromagnetically focused klystron. For minimum weight per kilowatt of useful power, the tube power must be a maximum. The available cathode current sets an upper limit of about 175 kW output; however, the capacity of a heat-pipe cooling system requires the most urgent study. The quantities and distribution of heat are derived here, and the next stage should be to develop a thermal model of the output cavity, with heat pipes as an integral part of the design. A toroidal shape should yield the highest circuit efficiency. It may be possible to use thin cavity walls of dispersion- hardened copper, lined outside the cavity with copper mesh as a wick for a heatpipe evaporator. The radiator area required to cool the tube body is too large for conduction alone; a panel of heat pipes is necessary to distribute the heat and maintain a nearly uniform surface temperature.
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