(Use of this formula assumes a uniformly illuminated transmission aperture. For illustrative purposes, figure 5 is not shown as such an aperture, but in reality would have negligible spacing between output facets.) Where m is the diffraction order (m = 1, 2, 3 ...), 1 is the laser wavelength (0.8 gim), L is the length of one side of the laser-transmitting square aperture (8 m), and Z is the transmission distance having a maximum of 3,100 km and a minimum of 1,800 km. Thus, the central beam spot size will be 0.3 m at 3,100 km and 0.18 m at a 1,800-km transmission distance. This spot size contains 81.5 percent of the total beam power, so the photovoltaic receiver should be larger than this to capture the power in the other lobes of the beam profile. It is interesting to note that only 0.095 kW of solar power is incident on a 0.3-m diameter area. While the SP-100 nuclear reactor is a relatively near-term development program, the laser diode array, as proposed in this paper, is an emerging technology. Laser diodes emitting several watts are readily available, but coupling many diodes to form a high power, coherent array has not been demonstrated as yet, although much progress has been made in recent years [9, 10], This is not off-the-shelf technology, but because of the high laser power per unit mass potential, this technology is being closely monitored for space power-beaming applications. IV. Rover Laser-to-Electric Power Converter The purpose of the laser-to-electric converter on the rover is to track the incident laser beam, capture it, and convert it into electricity. Although there are many concepts for converting laser light into electricity, we have chosen the photovoltaic device because of its high efficiency, low mass, simplicity of operations, and few moving parts. One characteristic of laser emission is its very narrow frequency spectrum. If the band gap energy of the photovoltaic-converter-material is made (tuned) slightly less than the laser photon energy, then high (50 percent), laser-to-electric conversion efficiency can be achieved [11]. The conversion process generates heat which must be dissipated, thus the photovoltaic device must be cooled by a circulating cooling fluid. A heat-rejection system is necessary to maintain a converter operating temperature of 320K. Figure 8 shows a cross sectional view of the photovoltaic device. Laser emission at 0.8 gm is transmitted through a protective cover glass into the GaAlAs active region which has been tuned for maximum conversion efficiency at 0.8 gm (band-gap energy of 1.55 eV). The absorption coefficients of GaAs were wavelength-shifted to match a wavelength of 0.80 gun. These absorption coefficients were then used to calculate the converter efficiencies. A computer code developed by Heinbockel [12] was modified to use these absorption coefficients as well as to allow specification of electron and hole diffusion lengths. Table III lists the device parameters used in these calculations. The metal contacts carry the electric current to the load. A diamond layer is used to electrically insulate the device from the cooling channel but also to allow good thermal conductivity between the active region and the cooling channel.
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