Device Design and Expected RF1 Device Design. — The Amplitron is a very desirable microwave generator for the SSPS. because of its inherent characteristics such as high efficiency, light weight, and high reliability. In this section, the Amplitron design versus its operating frequency for the SSPS concept is discussed. The frequency range considered first extended from 1 to 5 GHz. However, in addition to the microwave generation aspect, there are many other reasons for selecting a particular frequency for the SSPS, such as antenna design, the interaction of the microwave beam with the Earth's atmosphere, and noise interference. Initial studies on the type of Amplitron needed for SSPS application showed that each tube in the phased-array system would have a relatively low-power range of 5 to 10 kW. The Amplitron designed during our preliminary study is shown in Figure 57 and is the basis for the study of Amplitron design versus frequency for the SSPS program. This figure shows the essential components of the Amplitron design from the point of view of heat flow and weight. Some of the features of the SSPS Amplitron include: • A heavy outside tube envelope which will not be required in the space application; • Samarium-cobalt magnets which greatly reduce the magnet weight; and • Pyrolytic graphite which is attached to the Amplitron for radiation cooling. The basic SSPS Amplitron was designed for a power output of approximately 5 kW at a frequency of 2 GHz and the temperature drop in the vanes was limited to 50°C. A comparison of the vane temperature rise versus frequency is shown in Figure 58. At the higher frequencies, the heat path surface area is decreasing so that the vane temperature rises. The disposal of waste heat resulting from the energy conversion process will be a major problem in space. However, the high efficiency of the Amplitron will keep the waste heat at a minimum. Passive cooling of micro wave generators may be employed by attaching radiating fins made from pyrolytic graphite. This material operating in the temperature range of 250° to 400°C has a heat conductivity nearly twice that of copper. With the pyrolytic body at 325°C, the vane temperature is shown over the 1- to 5-GHz frequency range (see Figure 59). The high efficiency of the Amplitron will reduce the dissipation of wasted power in the anode circuit and thus allow this circuit to be conduction-cooled. The overall efficiency of the Amplitron can be attributed to its circuit efficiency and internal dc-to-rf conversion efficiency. The internal conversion efficiency is dependent primarily upon the value of the magnetic field utilized, while the circuit efficiency is dependent upon the I2 R losses. The percentage of the de input power dissipated in the anode circuit of the SSPS Amplitron versus frequency is shown in Figure 60. Since the circuit loss is dependent upon the skin depth, there is a variation of anode losses with frequency.
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