Transient behavior of nonlinear thermal effects in an intense light beam in a uniform gas flow

The thermal self-effect in the nonsteady subsonic, transonic, supersonic, and hypersonic regimes is analyzed using simultaneous numerical solutions of the parabolic equation for narrow beams and the linearized hydrodynamic equations. The intensity profile of the light and the profile of the leading term in the perturbation of the gas density are determined for various cross sections of the beams at various times. The nature of the evolution of the intensity peak and the shift of the centroid of the intensity distribution are examined. It is found that the intensity peaks and the deviation of the beam in the transient regimes can exceed the corresponding steady-state values. The effect of the finite time required for the light source to reach its steady output level is estimated and the sensitivity of the thermal self-effect to the similarity parameters is determined. A linearized analytical solution is found for the transient hypersonic regime.