operate at 125°C at an efficiency of 18.2%, with sunlight concentrated by thin membranes of aluminized Kapton forming the sides of 3 to 5 large "V"-shaped troughs, with the solar blankets at the bottoms of the troughs. With the walls of the troughs inclined at 60° from the floor, the concentration ratio is 2, so that 2 only 30.6 km of active solar cell area is necessary. The total inclined area of 2 the reflectors is 61.2 km . The reflectors and solar cells together intercept 61.2 2 km of sunlight and have a total mass of 7,651 metric tons, a major saving in mass compared to the silicon option. Because of the concentration of sunlight on the solar cells, the operating temperature is sufficiently high to provide continuous annealing of lattice defects produced by the radiation environment at geosynchronous orbit. Power Collection and Distribution. The electrical output of the solar arrays is collected together, transported across the rotary joint, and distributed to the microwave transmitting elements across the face of the transmitter array. The solar cells and their interconnecting tabs form the major portion of the power collection system, with strings of cells connected in series to provide 38.7 to 45 kilovolts (DC) to three main power buses of sheet aluminum 1 mm thick. For the silicon option, strings of solar panels about 1 m in width and up to about 5 km in length are ganged in parallel to produce 228 independently switched power sectors, each providing about 2000 amperes of current. In the gallium option, each of the 36 bays is switched separately, providing 6085 to 6638 amperes at 45.5 kilovolts. On the microwave transmitter antenna, most of the power is distributed directly (except for switching) to two of the electron collector plates in each of the klystrons. The remaining power elements of the klystrons are provided with regulated power and voltage. Power conditioning and conversion. Conversion from DC power to microwave power is done by approximately 100,000 to 140,000 klystron tubes, each converting about 85% of the input DC power (50 to 70 kW per tube) into microwave power. The remaining power is dissipated by heat pipes transferring waste heat to passive radiator fins or to the aluminium faceplate of the transmitter subarrays (see below). To keep the klystron filaments heated during eclipses and other power outages, batteries capable of storing up to 12 MW-hours of electrical energy are installed on the transmitter array.
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