L. Leopold Tracking & Communications Development Division IV-C-2-a MICROWAVE GENERATORS (1) RATIONALE The primary candidates for DC to RF conversion are the amplitron and klystron. The microwave system, consisting of microwave generators, a large planar phased array antenna, and a ground antenna/ rectifier combination must be capable of operating at high efficiencies over a 30-year lifetime with a low failure rate. Among the most important characteristics of the DC-RF converters are extremely high efficiencies and similarly high reliability punctuated by long life. In addition to the amplitron and the klystron, the traveling wavetube and solid state devices were investigated. Both the amplitron and klystron have exhibited a good record of operation in ground based systems and exhibit the best potential for the SPS. (2) OPERATING CHARACTERISTICS The amplitron possesses some characteristics, both physical and electrical, which appear very promising. Its weight for optimum efficiency of 88% is 2.27 kilograms. This includes the use of pyrolitic graphite for passive cooling of the 5 kilowatt tube. Because of the low power generated per amplifier, passive cooling will be possible. The low weight and smaller dimensions of the amplitron allow for ease of handling and mounting on the waveguides. However, it also means that many more tubes will have to be assembled, tested, transported and mounted than the higher wattage klystron. Another desirable feature is that the amplitron will use a cold platinum cathode. Since hot cathodes wear out more quickly, the cold cathode should extend the length of life of an amplitron appreciably. Raytheon estimates that a 30 year longevity of operation of the amplitron is feasible. The expected efficiency from the amplitron is somewhat higher (approximately 2% to 4%) than that to be achieved by the klystron. The power output of the amplitron is sensitive to changes in the input current. It is not markedly affected by voltage supply changes. There are many apparent advantages in the choice for the klystron. After the manufacture of a klystron, it must be baked out. This is simply done by energizing a solenoid wound around the body of the klystron. On the other hand, the amplitron requires a separate facility to bake it out prior to full operation. Since this bakeout will be done in space, the facility to complete the amplitron production will be much more complex. The klystron will operate to a higher temperature of 400°C because a solenoid coil is used to supply the magnetic fields. In the case of the amplitron, samarium-cobalt is the lightweight material to be used as magnets. It appears that there results a large drop in field intensity for a temperature change from 300°C to 350°C. Therefore, 30Q°C is the recommended upper limit for the amplitron. The phase sensitivity of the klystron is a function of tube length, and thus of gain. A 50 kW 50 dB gain klystron will be close to 2000° long. (21/2 feet long) The phase sensitivity to beam voltage will be about -25°/kV. This means that as the beam voltage goes up one kilovolt, the electrons speed up and the phase length of the tube becomes shorter by 25 degrees. It further indicates that for close phase control, the beam voltage must be regulated. Moderate changes in beam voltage may,
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