The sustainer system has an emitter that emits high energy electrons that pass from a region of high vacuum through a membrane and through the first sustainer electrode into the discharge region (the region between the sustainer electrodes in Fig. 4) (5). As the lasant gas flows through the discharge region the electrons ionize the gas molecules, thereby creating a nonzero electron density and an equal ion density throughout the region. The sustainer electrodes apply an electric field across the discharge region. This field accelerates the newly-created electrons. The energy of these electrons is transferred by collisions to the lasant molecules, thereby raising the molecules to the upper energy level of the lasing transition. The electric field is at right angles to both the gas flow and the laser beam. The two primary types of electric discharge laser are the CO2 EDL and the CO EDL. Figure 4 shows a closed-cycle, subsonic, CO2 EDL; such EDLs operate at a lasant temperature of 300-600 K (6). The CO2 laser can be made to lase on just one vibration-rotation level to vibration-rotation level transition, thereby providing a high power output that is extremely coherent and monochromatic (7). The primary CO2 output lines are at 10.6 /rm. A number of CO2 lasers, each lasing on the same vibration-rotation transition, can have their output beams “phase-locked.” After phase-locking, the collective beam is coherent (all the constituent beams are in phase) as well as monochromatic. Phase-locking is important because a large- diameter laser beam consisting of a number of parallel, smaller-diameter beams will undergo much less spreading if the constituent beams are phase-locked. CO2 EDLs have an efficiency of 10%^23%, with efficiency defined as the output beam power divided by the total input power required to operate the laser. The CO EDL is similar to the CO2 system in many respects. However, the CO EDL operates with the lasing CO at <100 K, which is considerably below the 300-600 K temperature of the lasant in a CO2 EDL. The low CO EDL temperature is required for efficient operation and is achieved by supersonic flow. The CO EDL is 239^30% efficient. Its output beam consists of a band of lines at 4.8-5.3 gm. If the beams are phase-locked on one line, they will not generally be locked on other lines. Therefore, with so many lines in the band, several output beams cannot be phase- locked together to produce a single, powerful, coherent beam. The advantages and disadvantages of the various types of lasers are listed in Table 2. 3.2 Direct Solar Pumped Laser Figure 5 shows a direct solar pumped laser. Light from the sun is reflected by the solar concentrator into a chamber containing the laser cavity. In the cavity, the reflected solar photons are absorbed by the lasant molecules, which become excited in the absorption process, to the upper laser state. Actually, this optical pumping process occurs only for those photons with frequencies in a narrow pumping band specific to the lasant. Rather (8) has considered the lasant trifluoromethyl iodide (CF3I) for use in a direct SPL. The pumping band for this lasant capture only 2.6% of the energy in the full solar spectrum; the remaining 97.4% cannot be captured and is wasted. The narrowness of the lasant pumping band has two implications: (a) the area of the solar concentrator must be quite large compared to other solar laser schemes having the same laser output power and (b) a very large, and thus heavy, radiator is needed to radiate away the heat resulting from the wasted photons. As to (a), Taussig et al. (9) estimate that the solar concentrator for a CF3I direct SPL would have to be at least an order of magnitude larger than for other solar laser
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