Considerations in the design of a CFA for the power from space application

In the power-from-space application for microwaves, gigawatts of solar-derived energy are to be relayed efficiently and continuously from equatorial geosynchronous orbit to earth by means of a microwave beam and then converted into base-load electrical power. The complete system has been studied in considerable detail, and the following set of requirements has been established for the microwave generator: high efficiency, a capability to radiate directly into space waste heat resulting from any inefficiency, an operating frequency of 2450 MHz, extremely long life and reliability measured in terms of scores of years, a very high signal-to-noise ratio, a high ratio of power output to mass, and a low cost of manufacture. Broad bandwidth, high gain, and a vacuum-tight enclosure are not required. Using this set of requirements, the design of a CFA of the continuous-cathode, reentrant-beam type has been studied in detail. The resulting device has the following characteristics: 85-90% overall efficiency, 6.5 kW of continuous output power, 2450 MHz frequency, 7 dB gain, 3.6 1b weight including samarium-cobalt magnet, pyrolytic graphite cooling radiator, and constant power output regulation system. Long life and high reliability are obtained with the use of a pure-metal secondary emitting cathode.