efficiency has been increased from 10% in 1966 to the actual 13%, although an increase of cell efficiency was a permanent design driver of past years (4). As the technology availability date is in ten years, according to the actual SPS program schedule, it seems urgent to increase the research and development effort in this field. Any improvement in cell performance would not only increase the feasibility of the SPS but would also be of immediate advantage for all future non-SPS programs. Further attention has to be given to the radiation resistance of silicon solar ceils since EOL performance determines the economy of the SPS rather than BOL efficiency. Thermal annealing of radiation induced recombination centres in the cells may be one possibility to keep the initial cell performance almost stable over the total lifetime. Recent experimental work has shown that pulsed laser beams or scanned d.c. electron beams are able to recover a significant portion of the radiation induced power loss (5). It has been shown that the degree of annealing depends primarily upon the temperature employed, and it was found that annealing is irradiation fluence dependent. Since damage from lower fluences can more easily be annealed than damage from higher fluences it is proposed to apply a periodic annealing during the mission. First results are really promising: cells have been exposed to 2x 1015l MeVe /cm2, which corresponds to approximately 10 years in geostationary orbit. As expected, the cells lost 27% of their power output. After a subsequent 2 sec annealing by means of a pulsed CO2 laser, 94% of the initial power was restored. Although thermal annealing may be a practical method of reducing particle degradation it must be noted that this process requires extremely high temperatures over a short time period. During the experiments mentioned above the cell surface temperature was increased to about 750°C while the rear side temperature was less than 100°C. Additional optimization work is necessary to make sure that the solar cell blanket design and direct annealing are compatible. In the past years there have been other approaches to make silicon solar cells radiation resistant. The most promising results were found with lithium-doped cells. This type of cell recovered nearly to its initial performance after particle bombardment was stopped. It has been assumed that lithium diffuses to the radiation induced defects and is able to neutralize them. Further studies have also shown that impurities in the silicon base material, such as oxygen, determine the properties of Li-doped cells. Cells from oxygen-lean silicon were found to recover at a much faster rate than oxygen-rich base materials. The SPS and the earlier large power missions may become drivers to reinitiate investigations on Li-doped solar cells. kV OPERATION VOLTAGE In order to be well matched with the power amplifiers of the microwave antenna, the klystrons, the SPS solar array shall be designed for an operation voltage of about 40 kV. This design shall avoid additional power conditioning equipment to step up the array voltage to the required klystron input voltage and shall further minimize the weight and the power losses of the power distribution systems. Actual arrays are designed for operation voltages of less than 100 V. For present lightweight solar arrays, the solar cells are bonded on either glass fibre or carbon fibre with reinforced capton foils with a total thickness of about 50 p.m. The solar
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