and operating data taken on accelerated life tests that were carried on for more than ten years in negative grid tubes. Final Magnetron Tube Design A cross section of the magnetron designed for the SPS is shown in Fig. 10. The drawing has been simplified to show the principal features of the tube. The design as shown contains the permanent magnets, the buckboost coil for controlling power output, the ‘voice coil' driven tuner to phase lock the output to the input at high gain levels, a carburized thoriated tungsten cathode with 60-year life expectancy, and a pyrolytic heat radiator to directly dispose of waste heat to space. The tube design weighs 1018 grams and delivers 3.2 KW of power at an efficiency of 85% or 5.0 kw at an efficiency of 90% for a mass-to-power ratio of 0.32 kg/kW and 0.20 kg/kW respectively. The various functions that make up the design have been evaluated for their mass. The contribution of each function to the total mass is shown by percentage in Table 2. The Magnetron in the Subarray In a 7.5 gigawatt transmitter there would be approximately 2 million magnetrons arranged in subarrays similar to that shown in Fig. 11. This design represents the format at the end of the DOE/NASA SPS study, before being upgraded with the necessary circuitry to use the high-gain, phased-locked magnetron in the ‘Magic T' combiner format. Details of this circuitry are given elsewhere [6].
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