The investigation of the life properties of the carburized thoriated tungsten cathode in the microwave oven magnetron made use of a magnetron that had an optical window in it so that the operating temperature of the cathode could be observed. It was observed that the magnetron wanted to operate its cathode in a temperature-limited emission mode, which of course would be the condition for the maximum life of the cathode and therefore of the tube. It was also found that this state of operation coincided with the very low noise performance of the magnetron. The Power Conditioning Interface with the Solar Photovoltaic Array. An outstanding feature of the MDA is its ability to interface directly with the solar voltaic array Figure 8-7 Phase Error Performance for MDA Phase- Locked Amplifier and Frequency-Locked Oscillator to operate the array at its maximum efficiency. The maximum efficiency of the array occurs at a specific value of voltage whereas the efficiency of the magnetron is relatively insensitive to operating voltage. Changing the voltage of operation of the magnetron to match that of the solar array is accomplished by means of the buckboost that is wound on the tube. This buckboost coil adds and subtracts from the magnetic field provided by the permanent magnets fitted on the tube. Without the use of this buckboost coil, which requires only a few Watts of power, it would be necessary to have an auxiliary power conditioning that would be very complex, and in any event, represent some inefficiency that would have to be radiated to space in the form of heat. This buckboost coil is shown schematically in Figure 4 of Appendix A, but its design has been detailed in reference [2]. Low Emission of X-rays One of the advantages of the magnetron over a linear beam tube is that the kinetic energy of all of the electrons that reach the anode is only a small fraction of the applied DC voltage to the tube whereas an appreciable fraction of the electrons in the linear beam tube have the full kinetic energy corresponding to the voltage applied to the tube which will be in the twenty to sixty kilovolt range. In the magnetron no electrons with energies in excess of 2 kilovolts will reach the anode. X-rays at this voltage level are very soft and will be easily absorbed in the copper metal that is part of the structure of the tube. Low emission of X-rays is desirable for three reasons: (1) maintenance of SPS transmitter involving the presence of people, (2) damage to solid state components that X-rays might cause, and (3) avoidance of an X-ray source in space whose flux density at the Earth’s surface would be many times that arriving from any other source in space. Ability to Withstand Bombardment by Highly Energetic Particles in Space
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