Fig. 3. Solid-state sandwich power panel. Source: Reference 12. interconversion between de power and microwave power at both terminals of the system, are being obtained. The application of new technology is projected to raise this efficiency to almost 70%. The devices which are being considered for converting de voltage to rf power at microwave frequencies in the SPS are crossfield amplifiers (Amplitrons) and linear beam devices (Klystrons). The Amplitron uses a cold platinum metal cathode operating on the principle of secondary emission to achieve a nearly infinite cathode life. With an output of 5 kW it could operate at an efficiency of 90%. The Klystron could operate at an efficiency of 80% with an output of 70 kW but will require a more complex cooling system. Microwave solid-state power transistors are being investigated as it appears feasible to combine them with solar cells in a sandwich power panel to form a resonant cavity feeding to waveguides. (See Fig. 3.) The performance of the DC-RF conversion devices which could be used with the SPS design options is shown in Table 3. Considerations of mass, costs, and efficiency at specific frequencies have led to the selection of a frequency within the industrial microwave band of 2.40 to 2.50 GHz for the SPS reference system. 4.1.1. Microwave beam transmission. The transmitting antenna for the SPS reference system is designed as a circular, planar, active phased array having a diameter of about 1 km. Micro wave power can be transferred at high efficiency when the transmitting antenna is illuminated with an amplitude distribution which is of the form (l-r2)n and when the phase front of the beam is carefully controlled at the launch point to minimize scattering losses (11, 14).
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