The absorbing surface on receiving spot should have a high absorptance and a small hemispherical emittance. A device made of a dense forrest of aligned metal whiskers have been demostrated. The diameter of these whiskers is of the order of incoming radiation and their spacing is several wavelengths. This device has an efficiency of 98% and a hemispherical emissivity of 0.26 at 550 'C over a wavelength range of 0.5-40 pm using tungsten. Another method to maximize the absorption of incoming radiation is an open sphere with a cone heading to the entrance opening of the sphere shown in Figure 7.37. The meaning of a cone is to minimize the escape of re-radiated energy. Convective losses due to air heating are minimized by purging dry air. Converters Photovoltaic cells for infrared have been researched and Mr. Beverly mentions two candidates 50% efficiency in his paper with. These are mercury-cadmium-telluride for 4-18 pm and lead- tin-telluride for 4-13 pm. Their lifetime and stability in different weather is uncertain. Another semiconductor device for infrared mentioned by Mr. Beverly is a tuned optical diode. This is an infrared analog of the microwave rectenna diode. Its power handling capability is very low and there is no experimental effectiveness determined. In paper written by Boeing Aerospace is mentioned an optical rectenna that is an entirely analogous to the microwave rectenna. It consists of microminiature 10 pm wavelength dipole antenna and rectifier elements as shown in Figure 7.36. In paper written in Boeing Aerospace it is approximated that efficient operation requires intensities of almost gigawatts/m2 to overcome forward voltage drop in the rectifying diode. This amount of intensity can be easily reached with pulsed laser beam and using efficient optics to amplify the laser beam. Another way to increase the voltage level is fabrication of antenna elements with gain. The optical concentrator could be eliminated with a gain as low as 20 db. [Beverly, 1980a] [Boeing Aerospace, 1980] The thermoelectric laser energy converter (TELEC) generates electricity by collecting electrons diffused out of the plasma. Electrons are energized by absorbing laser energy through inverse bremsstrahlung. Electrons are collected using different area and temperature for anode and cathode. Theoretical efficiency using 10.6 pm laser beam is above 42% but experimental values are much lower. Device scaling has been considered to improve the efficiency but laser window limitations may cause a problem. The cell vapor may condense on the surface of the window and reduce the energy density on plasma. [Beverly, 1980a] Figure 7.36 Optical Rectenna Configuration. [Boeing Aerospace, 1980]
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