fabrication capabilities severely limits the possible materials choices. While there have been some improvements in existing alloys, the introduction of a new alloy type, e.g., columbium-based B-66, would be more likely to resolve the problems in this area. Reflective surfaces in space degrade from the meteoroid flux encountered and from proton irradiation. The SPS reflective surfaces are usually assumed to be metalized plastic film (probably aluminized Kapton). The meteoroid problem is not considered to be significant (less than 5 percent degradation from this source over 30 years). Tests to date on degradation due to proton irradiation are quite inconclusive but potentially very severe. The only reported results (Ref. A12) indicate that an irradiation dose rate 900 times that expected at geosynchronous orbit caused reflectivity to degrade from 0.92 to 0.59 in 78 hours (900 x 78 hours = 8 years). Work is continuing in this area. 3. Processes The operational details of many of the processes required in successful implementation of an SPS are as yet unavailable, and in some cases are not even defined. Two areas that have been the subject of some study are microwave transmission of power and conversion of heat to electricity by the thermionic process. Raytheon and JPL, in particular, have dealt with the problem of microwave power transmission; Section IV summarizes these efforts. The thermionic conversion process is theoretically desirable since it is essentially a passive process utilizing large numbers of identical thermionic diodes and, therefore, has a good reliability and producibility potential. The main problem is the relatively low efficiency of these devices, coupled with the high temperature differential required between the diode emitter and collector. At an emitter tenperature of 2780°F (1800K) and a collector temperature of 1160°F (900K), which is nearly ideal and difficult to realize in space, thermionic converter efficiency is currently only about 15 percent. This figure is projected to reach 36 percent by 1995 but consi durable efforts will have to be expended to reach this goal.
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