Magnetrons can be operated at high temperatures and have exhibited overall efficiencies as high as 85% in a magnetron routinely used for generating high power for industrial microwave processing. To avoid the many complications of an active liquid or vapour cooling system, microwave generators must be passively cooled by radiators attached directly to them. This implies relatively small physical size because the mass of the passive radiator increases as the 2.5-3.0 power of the heat to be radiated at a given magnetron core temperature. The material that made it possible to build a lightweight radiator is pyrolytic graphite, which has three times the thermal conductivity of copper but only one-third of copper's density. Its application to an anode is simulated in Fig. 4. Studies to minimize the ratio of tube mass to microwave power generated by it indicate that the mass of the radiator is about 40% of the mass of the complete tube, that the microwave power output of the tube is in the 3.5 to 5 kilowatt region depending upon whether the tube is 85% or 90% efficient, and that the corresponding ratios of mass to power are 0.31 kg/kW and 0.23 kg/kW, respectively, as will be examined in more detail below. Defeating the Power-conditioning Ogre An area not addressed in depth in the SPS study was the problem of power conditioning involved in the interface of the solar array with the microwave generator.
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