and absorption are functions of the cavity wall area and aperture size. The cylinder has a small entrance hole drilled in the base facing the secondary mirror to accept and trap the incident solar energy within the cavity. (Note that the cylindrical radiator should have an opening at one side to allow the laser tube to be exposed to the blackbody radiation.) In this configuration, a GaAs-Si double layer cell is chosen. Solar cells forming a cylindrical surface, concentric to the blackbody converter, receive blackbody radiation. The idea is to form multi-junction cells, consisting of layers of materials with different bandgaps connected in series, with the material of the largest bandgap facing the incident light. Such cells can, in principle, make more efficient use of the solar spectrum. The theoretical efficiencies greater than 50%, in cells with three or more layers have been reported [4]. Cell thicknesses are usually designed to produce the optimum performance. In this approach, a thin GaAs layer (a few micrometers thick) is deposited on a 50-/«n thick Si substrate as described in Ref. [12]. An efficiency of approximately 17% and a specific power of approximately 1200 W/kg was reported from such a double-layer cell with no concentration. To obtain higher efficiencies in the GaAs layer, however, intermediate protective layers between the GaAs and Si cells and a silver mirror on the back side of the Si must be assembled very carefully. In addition, the use of anti-reflection coatings and textured surfaces minimize reflection of light from the cell surface. For efficient operation, the cell must be kept in the temperature range 250-350 K. (A typical gas laser medium should be kept at the same temperature range for a continuous lasing action.) Care must be taken to insulate the top surface contacts of the cell to prevent melting near the high temperature cavity.
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