device efficiency. At 50% efficiency this ratio is unity, at 80% efficiency it is four to one and at 90% efficiency it is nine to 1. Figure 2, combines the tube efficiency (on the y-axis) with the temperature (x-axis) at which the device is being operated to form contours of radiated microwave power per unit area. The proposed one kilometer diameter transmitting antenna for the SPS system operates with a power density of about 20 kilowatts per square meter at its center, which is within the cross hatched area for an electron tube that operates at a temperature of300 degrees Celsius and 80 to 85% efficiency, but completely outside of the range of a solid state device in the other cross hatched area. In the electron tube area only the magnetron and the crossed field amplifier are capable of the high efficiency that is needed for the Solar Power Satellite. Lower power klystrons were never suggested for the SPS because of their inherently lower efficiency than the high power klystron whose efficiency can be boosted to higher values with procedures that are impractical in space. Efficiency of Crossed Field Microwave Tubes Figure 3 shows the efficiencies that have been achieved with some crossed field devices as a function of a parameter that determines the theoretical efficiency of the device to convert DC power into microwave power. The theoretical efficiency is given as the solid line. The highest conversion of DC to microwave power of 92% has been obtained with a 915 MHz magnetron that is routinely used at that conversion efficiency for industrial microwave processing. Some efficiency in the form of circuit efficiency is lost in getting the micro wave power to the useful load, but such circuit efficiency can be as high as 95%, so the overall efficiency of the 915 MHz tube can be (.95X-92) or 87.5%. Two examples of the crossed field amplifiers that have been highly successfill in broadband amplifier use for sophisticated radars also approach the theoretical curve but they are not suitable for the SPS application because of their low 10 to Figure 8-2 Contours of Density of Microwave Power Output from SPS Antenna versus Efficiency and Operating Temperature of Output Generator
RkJQdWJsaXNoZXIy MTU5NjU0Mg==