Abstract

The cryogenic cw CO laser excited by e-beam sustained discharge has the laser efficiency (30-40)% at the hundred kilowatt level [1]. For a compact laser device of this class a laser gas mixture with densities in the range of 0.5 of the normal one is needed. Then any inhomogeneities in gas density exert strong influence on laser beam quality. Experimental rough estimates [1] gave the laser beam divergence about 0.01 rad at the power 85 kW when using the unstable resonator. A probable origin of a such strong divergence is a spatial distribution of refraction index associated with nonuniform energy release into the gas. A spatial spectrum of index nonuniformities has in general two components: one associated with sizes of discharge volume, and second is due to the small scale turbulence. Setting apart the last problem a two-dimensional semi-analytical model was formulated to describe the effects of nonuniform gas heating in the discharge. The first problem is the large number of physical variables involved: vibrational populations on about 100 levels; charged particle concentrations: gasdynamic variables; laser radiation intensities for about ten vibrational transitions. The second problem is that all these quantities are coupled each to other, and their variations should be described self-consistently. Despite the high productivity of the modem computers, integrating of a full set of equations in 2-D space is still not easy. We have employed the analytical approach to the vibrational kinetics developed earlier: for steady state conditions by Gordiets et al [2], for vibrational evolution in time by Zhdanok et al [3]. Analytical theory allows us to replace 100 vibrational kinetic equations by 3 non-linear equations. Special tests demonstrated a good accuracy of the approximate method (deviation from the numerical calculations using the full kinetic model is as a rule within 10%).

© 1996 IEEE