five times higher, and at 3 Torr there is still an inverted population. All these results are obtained assuming the wall temperature Tw is 300 K. At Tw = 400 K, the curves for 0.001, 0.01, and 0.1 Torr become identical to those in Fig. 4, but the 1-Torr curves show maximum N falling from 3.5 x 1012 to 3.5 x 1011 cm-3. At Tw = 700 K, the 0.001 and 0.01 Torr curves are still unchanged, but the 0.1-Torr curve now drops to half the above values, and the 1-Torr curves show no inverted population. It seems that if the wall temperature is not kept low, then lower gas pressures will be required. Reduction of N at high values of b for all p is due to the He deexciting the A00i level, yet the heat conduction does not increase with the pressure. For both A/l = 1 and 6, the highest N values are obtained for Tw = 300 to 400 K, C = 100, at p — 0.1 Torr with a — IO 2 and 10-4 < b < 10-2, and are about 1013 cm-3. An inversion population of this value would result in very high gain, making practical solar-pumped systems superradiant. Hence, if necessary, p, a, and b could be changed from their values at optimum N to improve the efficiency.
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