4. 1.4 CONTROLLING THE OUTPUT OF AMPLITRONS In the MPTS application, a large number of microwave generators must receive their dc power input from a common bus, and their microwave power output and electrical phases must be held closely to preset levels so that the subsections of the transmitting antenna which they feed are properly excited. It is necessary, therefore, to insert some form of regulation in each microwave generator. The output power and phase of the amplitron are determined by the applied dc voltage and by the value of the magnetic field which is imposed on the tube. Two approaches are discussed to control by magnetic means. The first concept is to use a movable pole piece which enables variations of the static magnetic field imposed on the tube. It is shown in Figure 4-1. The value of the magnetic field will be changed in this approach by changing the gap between the magnet poles. Figure 4-3 shows the current and voltage of the amplitron for various magnetic field levels. The pole gap can be controlled by a dc brushless motor driving a recirculating ball, differential screw. DC brushless motors have been proven satisfactory in space applications. Ball bearings are necessary for high vacuum use where no lubrication is available. The rotating anodes used in Raytheon’s X-ray tubes use vacuum ball bearing supports which have been life tested to 10, 000 hours at 3000 rpm without failure. In the MPTS application, the revolutions will be lower and the motor would be used infrequently, which may create stiction problems which should be investigated in the technology development program. The dc brushless motor will be used to adjust the pole gap as a vernier control only. The main force of the magnetic pull will be counterbalanced with a spring. The motor will weigh about 30 grams and will use only one watt of power. It can supply a force of 35 pounds and change the gauss level by over 20 percent in about one second. With the motor power off, the permanent magnet rotor will lock onto one set of stator poles to prevent any movement of the pole. 4.1.5 WEIGHT The majority of the weight is in three components: (a) the pyrolytic graphite heat radiators, (b) the magnetic circuit, and (c) the anode. A breakdown is given in Figure 4-2.
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