required radiator area. The net result, however, is a reduction in the total system specific mass and an improvement in system efficiency. CO2 Laser Evaluation. The atmospheric transmission efficiency of any line within the “standard” 00°l—»10°0, 10.4-/zm band is very poor. To alleviate this situation, various transitions within the 00°l—»02°0, 9.4-^i.m band have been suggested (8). If low-abundance isotopic species of CO2 are employed as the lasant, a shift in output wavelength occurs and atmospheric absorption features due to natural CO2 can be avoided. After consideration of the various isotopic combinations, the specie 12CISO2 was determined to be most appropriate. To affect maximum atmospheric transmission efficiency, laser operation on midrotational R-branch lines of the 00°l—»02°0 band is necessary. Under these conditions, a laser system efficiency, rj,., of 14.1% was calculated for the thermodynamic cycle with dual heat exchangers. CO Laser Evaluation. The lasing spectra of CO lasers show a characteristic multiline output whose distribution is a function of the gas-kinetic temperature. In large-scale devices, the low gas-kinetic temperature of the lasing medium is achieved by a supersonic expansion. This results in lasing on low v transitions and improved atmospheric transmission. The spectral output above —5.4 gm is strongly absorbed by the atmosphere (9). The atmospheric transmission efficiency is a sensitive function of the multiline distribution, and any calculation of atmospheric transmission of CO laser radiation must be performed by weighting the line transmittance by the fractional laser power in each line and then summing over all the lines. Since a large fraction of the molecular absorption is attributable to the 6.3-/zm water band, seasonal variations are pronounced. Due to a significant improvement in atmospheric transmission of the lower v transitions, much effort has been expended in developing laser devices which maximize their power output on such transitions (10). Even at very low temperature many of the CO laser lines do not have satisfactory transmission characteristics. Rice (11) has developed a technique for redistributing the output line spectra of CO EDLs. An intracavity water vapor cell spoils the gain on those lines which are highly absorbed by water, the rotational populations are redistributed, and positive gain occurs only on those lines unaffected by the absorption. For the present study, the experimental spectrum shown in Figure 3 was used to calculate the transmission efficiency for a line-selected CO EDL. Above each line in Figure 3 is the fractional power residing in that line and a relative measure of the atmospheric transmission efficiency (the horizontal propagation distance at sea level for a decrease in intensity by a factor of e using the Midlatitude Winter model). The use of a line selection cell produces a significant decrease in the laser output power. This effect will be less dramatic on larger devices having longer gain lengths, although detailed calculations of the discharge efficiency of a large CO laser with line selection were not performed for the purposes of this study. In closed-cycle flow, the compressor power required to circulate the gas is a function of the flow Mach number. Thus, operation at the low gas-kinetic temperatures necessary for efficient atmospheric transmission requires a substantial fraction of the total power available just to operate the flow cycle. The dependence of total laser system efficiency, tql, on the discharge efficiency for two thermodynamic cycles is shown in Figure 4. The shaded region denotes the range of expected performance. To achieve the necessary low static temperature in the discharge cavity, a supersonic expansion of Mach number 3 to 4 is required based on the anticipated plenum stagnation temperature of approximately 360 °K. Discharge efficiencies of 0.35 to 0.50 appear possible (12), although the best reported (13) performance is 0.39.
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