Item — High Voltage Strings Criticality — Critical to program success Present SPS concepts require buss bar voltages of 20,000 to 40,000 at the dc-RF converter. As many at 28,000 to 56,000 cells in series are required to produce these voltages. Present Capability: Existing solar arrays have produced only as much as several hundred volts output. No data exists on the effects, problems, or requirements or very high voltage strings. Potential problem areas are corona discharges, reliability, and shorted cells. Required Capability: High voltage strings of cells must put out 20,000 to 40,000 volts at high reliability in a GEO environment. Maintenance-free life of 30 years is required. Development Plan: The first step is to analyze the problems of high voltage solar cell strings. Then, experimental strings should be ground tested. After Shuttle and Space Station become available, space tests are required to verify performance for SPS applications. Item — Spectral Filters Criticality — Greatly enhance probability of program success Spectral filters can reduce cell operating temperatures by eliminating portions of the spectrum that are not converted to electrical power but only heat the cell. Lowering the cell temperature improves its operating efficiency. Present Capability: Existing spectral filters are tuned to the spectral response of silicon cells. GaAlAs spectral response is sufficiently different from silicon that existing silicon filters give poor performance. Required Capabilities: Spectral filters tuned to GaAlAs characteristics are required. They must be low in weight and cost and be able to operate at temperatures in excess of 125°C for SPS application. Development Plan: Candidate materials must be identified and screened for suitability. Promising materials should be subjected to ground tests and in-orbit tests. Low cost, high volume methods of application must be developed. Item — Ga Production Criticality — Critical to program success Each 5-Gw SPS requires in excess of 10° kg of Ga for production of the solar blankets. In addition, the rectenna requires Ga for the diodes that convert the microwave energy to de power.
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