The Photoklystron The mass per unit area of the photoklystron area of such a configuration is estimated to be about 0.2 kg/m2. The resonant cavity/wave guide reflector area is estimated at 0.25 kg/m2. For a CR3 satellite whose combined photoklystron and radiation efficiency are 10%, and which is required to radiate 6.75 GW (5 GW rectenna output), the required photoklystron area is 16.6 km2 and the resonant cavity/wave guide reflector area is 33.3 km2, for a total of 50 km2. The resulting masses are 3.2x10s kg and 8.3x10® kg for the photoklystron and resonant cavity/wave guide reflector areas respectively. The total mass is 11.5x IO6 kg. This compares very favorably with the 13.8X106 kg and 27.6x 10(i kg for the GaAlAs Cr2 and silicon CR1 NASA/DOE reference system solar array masses (4); especially since an additional 13.5X106 kg must be added to the solar cell configuration weights for the klystron antenna array and slipring. In this photoklystron array weight estimate, no weight has been added explicitly for antennas, phase control, primary or secondary mirrors, or structure. However, an overall 50% contingency factor has been added to cover these items. It is expected that the antenna would be an integral part of the wave guide, probably periodic slots or horns, and would therefore add negligible weight. The overall satellite configuration could be similar to that proposed for the solid state sandwich system proposed by Rockwell International (5) with a primary and secondary mirror turning the solar flux through 90° and then onto the planar photoklystron configuration with rf radiating out the opposite side. See Fig. 7. This mass and area estimate naively assumes that photoklystrons can be designed which will self-oscillate at the requisite frequency and efficiency. If it should turn out that bias voltages are required, these could be provided by interspersing solar cells among the photocathode surfaces in the trough. The solar cells would thus feed nearby photoklystrons and the modular nature of the concept would be preserved. The ratio of solar cell to photocathode area would depend on how much of the energy to drive the photoklystrons had to be derived from the bias voltages. Aside from the application to the solar power satellite, the photoklystron may have other uses in space and on the earth. Communication satellites and telemetry transmitters for satellites and space probes have a need for highly reliable rf sources. A great advantage of the photoklystron is its simplicity and hence reliability. It is a rf oscillator with only passive elements. Further, we expect low cost per unit area relative to solid state energy conversion devices because the photoemitter is vapor deposited. In space applications where large areas of photoklystrons are required, it may be possible to manufacture the photoklystron in space and dispense with vacuum encapsulation. We expect the photoklystron to be relatively insensitive to degradation from charged particle radiation due to the thinness of the photoemitter. Potential ground based applications of the photoklystron include direct production of power for microwave transmission lines and use in large scale drying operations such as drying lumber, grain or tobacco. When operated in the biased mode, the output frequency is sensitive to the accelerating bias voltage so the device may be used as a voltage controlled oscillator or alternatively as a simple precision voltage measuring device. It might also be used as a transmitting light sensor for alarm systems or to decode laser or fiber optic transmissions through rf amplifiers. Acknowledgements — We acknowledge helpful discussions with Dr. William Wilson and Mr. David Cooke. This work has been supported by a grant from the Brown Foundation of Houston, Texas. The Editor wishes to thank Gordon Woodcock and Dr. Owen Garriott for their assistance in reviewing this paper.
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