X-rays and gamma-ray hardening of the solar panels for SPS will require major advances in the state-of-the-art. Minority carrier lifetimes decrease with accumulated dose, lowering the solar cell short-circuit current and open-circuit voltage output capability. Solar cells can be hardened to some degree against ionizing radiation using gold or lithium doping. Selective doping can reduce both transient and permanent damage radiation susceptibility. (Given the size of the power satellites, however, doping with gold is impractical.) The silicon option for the power satellite includes annealing lasers which could be used to restore solar cell capabilities degraded by neutrons, x-rays, and gamma rays, provided the system as a whole had survived intact. It is unclear whether the self-annealing feature for the gallium option would allow recovery in a reasonable time under similar circumstances. Often the most effective and easiest way to harden electronic systems against x-ray and gamma radiation is at the system functional level. By providing ample design margins and overlap between various subsystem/circuit requirements and predicted responses, the engineer automatically allows for some degradation of the subsystems without loss of the system function. Also, by requiring large noise margins, the spurious responses of the system to gamma-ray pulses may be reduced. Another system approach is to design so that the system function can accommodate a few microseconds of upset, that is, the system can "hiccough-and-recover.” For example, a radar might lose information from a few of its pulses but the next scan would fill in the loss; or a communications receiver would put out a large ’'static’’-type noise burst, but would recover. (This approach would seem essential for the pilot beam and phase control system for the microwave power transmission system of the SPS.) It is more difficult to design digital logic circuits to hiccough and recover from lost bits, but data checks and redundancies can provide some nuclear hardness. It is generally impractical to shield against neutrons. The design approach is to use devices which are specifically constructed to be insensitive to neutron displacement effects. Again, due to the enormous size of the devices used and the power handling requirements, solving the neutron degradation problem will require the development of new techniques and significant advances in the state-of-the- art.
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