Light-beam instability and breakup during thermal self-interaction in a moving medium

The linearized equations for perturbations in a one-dimensional laser beam propagating in a moving medium with thermal nonlinearity are used to demonstrate that such perturbations may grow exponentially when the beam intensity exceeds some critical value. The propagation of a Gaussian light beam with intensities much greater than the critical value is calculated numerically. The results obtained show that: (1) the characteristic crescent shape of the beam during thermal self-interaction in a moving medium is unstable; (2) the beam may break up into filamentary structures similar to certain experimentally observed structures; and (3) the number, location, and size of the filaments depend significantly on the perturbations of the beam shape.