Fig. 7. A sketch showing a possible photoklystron SPS array configuration. The photoklystrons generate a rf wave which travels along the wave guide and emerges from the horn. The inset in the upper left hand comer shows the total satellite. The secondary mirror and photoklystron array rotate to follow the earth. VII. PHOTOKLYSTRON APPLICATIONS In the SPS application of the photoklystron, the entire solar array becomes the rf radiating surface. Instead of a cluster of high power klystrons we invision a large array of low power photoklystrons. This system lends itself to modular construction with radiating units being added as needed. This could reduce the high initial capitol cost for a given system. Moreover, the lower radiation energy density would reduce the hazard for astroworkers and make possible the addition of new modules without shutdown of the entire system. To be competitive with a solar cell/klystron system (including bus bar and slipring), efficiencies for the photoklystron of about 12% will have to be demonstrated. Based on our present estimate of about 10% and the fact that reflex klystron efficiencies higher than this have been achieved, it appears possible to reach the 12% figure. In order to obtain a mass per unit area estimate of an SPS configured with photoklystrons, we have developed a hypothetical design using resonant cavities and solar reflectors to concentrate sunlight on the photocathode surfaces. In this design, each photoklystron excites the center of a resonant cavity which forms a wave guide with adjacent resonant cavities. A traveling wave then moves down a line of adjacent photoklystrons. Since the resonant cavity/wave guide occupies more cross-sectional area than the photocathode surface, solar reflectors placed sunward of the resonant cavity/wave guide concentrate sunlight onto the photocathode surface. A concentration ratio of 3 is used. The resonant cavity/wave guide walls are made of aluminized 1/2 mil Kapton as are the solar reflectors. The photocathode consists of photoemissive material vapor deposited on a 1/2 mil Kapton (or similar uv transparent material) substrate. Wire grids are used at the resonant cavity gap. The reflector electrode is again an aluminized Kapton sheet. The resonant cavity/wave guide may be formed by separating the two sheets of aluminized Kapton by dielectric spacers or honeycomb material and drawing the sheets together into contact at the edge. Figure 7 is a sketch of this design.
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