Laser beam propagation model with a hydrodynamic treatment of the transmission medium.

Pulsed laser beam propagation through the atmosphere is described by a model that contains the transient effects of air wave development. This study considers the initial stages in the propagation of a laser beam; the computer calculation follows the process during the time regime in which pressure gradients are important. Energy absorbed by the medium from the beam during a time increment is added to the existing internal energy in the form of excited molecular states. Deactivation rates are specified by pressure, temperature, and component species such as CO(2) and H(2)O. Absorption and deactivation processes change the temperature of the air and cause the buildup of the cylindrically symmetric air waves. A Richtmyer-von Neuman finite difference scheme follows the waves' development, and a pseudoviscosity term is used to smooth steep pressure gradients. During the growth of the related gradients in the index of refraction the equations for power flow in the initially diffraction-limited beam are continually changed to allow for refraction. Power densities in the focal plane are obtained as a function of time, atmospheric constitution, power, beam source aperture diameter, range, and radius from the beam axis in the focal plane.