There is a region of parameters where laser action is possible despite high gas and wall temperatures. Plots of N at b = 1 and 10-4 show that, at b = 1, TV is drastically reduced as Tw is increased, but when b = IO 4 there is still an inverted population even at Tw = 1000 K. (However, this result should be treated with caution as temperatures this high can introduce rotational effects which are not included.) The high temperature region occurs where the concentration of He and CO2 is small. At high temperatures He as a coolant is unnecessary; Eq. 8 converts to the resultant conductivities of the other gases as b —» 0, and with T112 dependence the conductivity is adequate. Again, as b—»0 the deexcitation of CO2 (001) by He becomes unimportant. III. EFFICIENCY AND POWER OUTPUT OF THE LASER III.l. Efficiency The overall efficiency can be subdivided into the product of several efficiencies. First, if absorption occurs over a bandwidth Xj to X2, the fraction of the solar spectrum used or “solar utilization efficiency” is t)s: for Br2. If the radiation passes through the gases x times by reflection and the absorption is small, the fraction absorbed or “absorption efficiency” r]A = xa(Br2)d is independent of C and obeys a similarity law, -qA = iqA(xpd). However, p is limited by temperature rise considerations and x is limited by the reflectivity of the reflecting wall (—99%) to about 50. If p were 0.1 Torr, a = 3 x 10"19 cm2, d = 2 cm, then pA would be 0.1. An increase in pA could be achieved at higher pressure. If the gas were cooled more effectively by fins (A/l = 6), there would still be adequate population inversion at p = 3 Torr, and with x = 15 then r)A —» 1. The fraction of absorbed quanta that ends up producing CO2 (001) is obtained from the gas kinetics (Fig. 2) and is Fkta/[(ki + k^a + £3]. This fraction we call the “kinetic efficiency,” pk. Assuming F = 0.5, kt = 6 x 10-12, k2 = 10“, and k3 = 4.7 x 10-13 cm3s-1; then r]k —> 0 as a —> 0, pk = 0.18 for a = 1, and —> 0.19 as a —» 00. The laser possesses a quantum efficiency r)Q = (Eoo! - E^/e where e is the averaged energy absorbed, = 2.7 eV, and E^ - E100 — 0.1 eV, so = 0.039. The product can be termed the “energy utilization efficiency” of the fraction of spectrum absorbed, and the “total solar efficiency” rj = PsPaPqPk- A plot of p vs a is shown in Fig. 5 for p(Br2) =0.1 Torr, d = 2 cm, and different values of x. If it is assumed that pA = 1 or complete absorption occurs, then a value of 1.3 x 10”3 (or slightly more, depending on F) is possible. It is to be understood that these efficiencies are achieved only in those regions of Fig. 4 where the gain is sufficient for steady lasing. 1IE2. Power Output of the Laser The cooling requirements suggest the laser should be a fiat plate of shallow depth of a few cm, and hence the exposed laser area A, should be large to handle high
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