7.2.2 Laser Laser technology for power beaming is a good possibility. Benefits of laser beaming compared to microwave beaming are smaller receiving site, radiation outside the beam is negligible and side lobes do not interfere with other electromagnetic radiation. On the other hand, lasers have quite low system efficiency and higher losses on atmosphere. The efficiency is also dependent on the atmospheric situation. Higher power density on the beam can reduce the atmospheric attenuation by boring a hole through light clouds. In this section we discuss about different methods to create a laser power beaming system. First subsection covers the conversion of electric or solar energy to a laser beam. The next subsection covers the propagation of a laser beam through atmosphere. This is followed with a subsection where we discuss about receiving of a laser beam. These subsections are a kind of introduction to laser power beaming. In the next subsection we present some applications using laser technology. Conversion A laser consists of a laser tube containing the lasing medium, or lasant, a system for pumping the lasant with energy and a system for removing waste heat. As a laser emits monochromatic, coherent light, a laser beam propagates for great distances with very little beam spreading. For power transmission purposes die laser has to be scalable to increase the output power. The state of art in laser technology has concentrated on infrared lasers in the band of 2-11 pm. The state of art technology in photovoltaic cells requires much shorter wavelengths for power generation. Because of this some studies and experiments have been done in visual light wavelengths but these lasers are typically quite inefficient and not scalable. Main problems in scaling are gas purification and cooling. Gas purification is required to maintain the efficiency of the laser. Below we describe several laser technologies. Electric Discharge Laser Electric Discharge Laser (EDL) uses an electric field to maintain discharge and accelerate electrons to high speeds. This energy is transferred by collisions to lasant molecules. Collision transfers molecules to higher energy levels. Two primary EDL types are CO2 and CO. The CO2 EDL operates at lasant temperature of 300-600 K and its primary output line is 10.6 gm with efficiencies of 10-23%. A number of CO2 EDLs can have their output beams ‘phase-locked' so the collective beam is coherent and monochromatic. The CO EDL operates at a lasant temperature <100 K. Its output beam consists of lines at 4.8-5.3 pm with efficiency of 23-30%. This laser type can not be ‘phase-locked' since it has many separate lines in the band. This laser type is widely tested and it's operation is well understood. Because of a ‘phaselocking' capability of a CO2 laser it is quite good alternative for power beaming purposes. This laser type has however low reliability and it's lasant has easily a breakdown. [Walbridge, 1980] Solar Pumped Lasers Solar energy pumped laser is a good choice for space laser power system. Due to direct use of solar energy, no conversion from solar to electric energy is needed for beamed power. The system itself requires some electricity for controlling purposes. The temperature of the lasant has to be kept at an operating level. The cooling of the lasant requires an efficient cooling system that can be achieved with an active system. An active cooling system needs much energy, that can be either electric or mechanical. In both cases some secondary power converter is needed. Direct solar pumped lasers can use only few narrow bands of total solar energy as shown in Figure 7.33. To avoid excess heating of laser system, unused bands of solar energy have to be filtered or reflected out to reduce the heating on the lasant. The amount of useful solar energy in the total spectrum is at most 2.5%. If the efficiency of the laser is 20%, the overall efficiency from solar energy to laser beam would be 0.5%. This means that pumping energy has to be concentrated over a large area. Using solar energy value of 1.3 kW/m2 the minimum required collection area A,, for output power Po is the following:
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