SECTION 4 DC-RF CONVERSION 4. 0 INTRODUCTION The primary candidates for dc to rf conversion are the amplitron and klystron. They are each separately examined for the MPTS application in Sections 4.1 and 4.2 respectively. System level considerations for them both are examined in Section 4.3 while conclusions and recommendations for both are given in Section 4.4. 4. 1 AMPLITRON The baseline design is a 5 kW, radiation-cooled amplitron operating at 2450 MHz at 85 percent efficiency. Characteristics at other operating frequencies and power levels are also determined. The amplitron is a backward wave, crossed field amplifier (CFA). Reference 1 provides a survey of CFA concepts and state of the art. This discussion covers only the special aspects of the MPTS application. A basic consideration in the design of the continuous wave (cw) amplitron for the satellite power station system is high reliability and long life. Cathode life is a major limitation for most electron tubes. In the cw amplitron, the long life requirement is met by designing it with a pure metal, secondary emitting platinum cathode. Platinum makes a nearly ideal cathode for amplitrons. It is a noble metal with very low vapor pressure and no inherent wearout mechanism. It has demonstrated good life under very high power operating conditions. The rf circuit of the amplitron consists of only 17 sections which are relatively large and rugged. The structure is solidly supported by the shell of the tube and is connected by a short thermal path to the pyrolytic graphite radiator. Heat associated with losses in the amplitron is radiated into space through the cooling fins. The amplitron is particularly desirable for space applications because of its high efficiency. Efficiencies between 80 and 90 percent have been obtained at S-band. Lightweight magnetic circuits can be used to obtain these high efficiencies by means of samarium-cobalt magnet material.
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