Fig. 3. Plot of lowest C for lasing vs laser length L. As AN cc C it follows that the threshold value for C is proportional to ML. Computer runs were performed for successively lower values of C for L = 400,100, and 25 cm, and the threshold values of C estimated. The values are compared with experimental results in Fig. 3. The absolute values of the latter were difficult to estimate as the pumping light was focused on a cylinder, but both theory and experiment show C ^L~'. Thus C can be reduced by increasing L. VII. EFFICIENCY OF THE LASER Estimates of the efficiency can be obtained from solutions of the rate equations. For a laser of length L, collecting width w and of 1 cm absorption depth, the total solar power into the laser is 0.14CLiv W. The power output is 0.5cnhvrw. If the transmission factor r is 0.05, the efficiency is 17 = 3.93 x 10~10n/CL, where n (the photon density) is in cm-3, and L is in cm. Computer runs at various values of L, C, and gas pressures were made and the maximum n obtained to give 17 (Fig. 4). For L > 5 m, 17 is roughly independent of L and C, and at 5 torr, 17 = 5 x 10~4. At these low pressures -q p and it can be seen that higher pressures increase 17, provided overheating can be avoided.
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