In connection with the retrofit scheme, two major guidelines were followed in this research study. Specifically, (1) the operational attitude and orbit considered is Geosynchronous Equatorial Orbit (GEO) with laser-beam pointing at typical U.S., midlatitude receptor sites, and (2) the individual independently-controllable laser transmitters are capable of being grouped at a single location in GEO and are capable of consuming, as input power, the entire power output of the baseline photoelectric power source (9.4 GW). The slant range to typical receptor sites in the U.S. from a laser in GEO at an altitude of 35,786 km, R, is taken to be 42,700 km. The zenith angle of the laser beam pointing at earth, 6, is taken to be 50°. In terms of efficiency, scalability, reliability, and atmospheric propagation, two molecular-gas electric-discharge lasers (EDL’s) have been suggested for their potential in space-to-earth power transmission systems, namely, the CO and CO2 lasers. Because of their mature state of development, laser concept selection was restricted to a choice between these two systems. Advanced laser concepts, such as direct solar pumped lasers, were excluded from consideration. The receptor site elevations employed were representative of typical locations (h = 0.5 km) and mountaintop operation (/i =3.5 km). Due to the multiplicity of independently targetable laser beams, consideration was given to both geographically separated and clustered receptor devices. CONCEPT DEFINITION Evaluation of Electric-Discharge Lasers We consider molecular-gas high-power lasers in which the excitation process is an electric discharge and the gas is circulated in a closed cycle. Gas circulation permits removal of waste heat, and closed-cycle operation minimizes the rate of gas consumption, allowing long periods of operation. In general, the laser gas mixture consists of a small amount of lasant, such as CO or CO2, added to a diluent, such as He, Ne, Ar, N2, or mixtures thereof. The total electrical power input to the EDL, P, consists of two principal terms, PPS and PCelec, the electrical power inputs required by the excitation power supply(ies) and the gas compressor or blower motor. If PL is the laser optical output power, then the laser system efficiency is The electrical power deposited into the gas, PE, is related to the electrical power input to the discharge power supplies by where -r)PS is the intrinsic power supply efficiency. Similarly, the mechanical compressor power, Pc, is related to the electrical power input to the compressor motor by where t)m is the intrinsic motor efficiency. The most often quoted efficiency in experimental laser studies is the discharge efficiency, r)d, defined as the ratio of laser output power to electrical power deposited into the gas, i.e.,
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