Electric discharge lasers require electric power to drive a high-voltage discharge that pumps the laser medium to an excited discharge state and to circulate the lasant through a cooling loop to remove waste heat. For this type of system, a solar array may be employed to produce the power. This type of system is extremely inefficient, resulting in a large solar array and large radiators. The result is a system mass and cost that is not competitive with microwave power transmission systems. Direct solar-pumped lasers also are inefficient because of the narrow lasant spectral band and the broad spectral characteristics of solar energy. For this reason, an indirect solar-pumped approach is used to achieve more compatible spectral characteristics. Solar energy is focused by reflectors into a cavity collector (Figure 5). A temperature is achieved in this cavity that releases thermal radiation in the spectral region that excites the lasant. Efficiencies of this system are considerably improved. The final laser system, the free-electron laser, is shown in Figure 6. In this concept, an electron beam is formed (using a klystron as the electron source, which is accelerated in an RF accelerating cavity) that produces laser frequency energy upon passing through a magnetic field that causes lateral electron movement. The beam is directed to mirror assemblies on each end of the satellite that form a laser beam which is directed to a receiving station on the earth. The solar array provides the energy that powers the system. The system on the ground for conversion of laser to electrical energy uses optical diodes that are analogous to the microwave rectenna. Conversion efficiencies are similar to the rectenna system. This system appears to provide the highest efficiency and lowest mass of all laser systems studied. Figure 7 compares the specific masses of the laser concepts and the reference silicon solar array concept that uses klystrons for dc/RF microwave conversion. Current estimates made by the Boeing Company indicate that the lowest mass laser concept (free-electronic laser) is about twice the specific mass of the reference concept. Additional effort remains to be accomplished to evaluate and compare these concepts. Even lower mass and cost solid-state antennas need to be developed because of the importance of antenna mass on the cost of these concepts. Device development also must proceed to ensure that the requirements can be met. Magnetron concepts appear to have the best combination of characteristics, but development is needed to determine whether predicted lifetime and efficiency goals can be obtained. Because of the obvious advantages of multibandgap solar arrays in improving system
RkJQdWJsaXNoZXIy MTU5NjU0Mg==