On the energy density of radiation in a nonlinear medium and the stabilityof light waves

Expressions are derived for the energy density of the light field in transp arent media with a substantial nonlinearity of the refractive index. For th e media with defocusing nonlinearity, the possibility of anomalous behavior of energy density is discovered, which consists in its decrease with incre asing radiation intensity. The anomaly is shown to be associated with the i nstability of regimes of propagation of intense light waves. The threshold intensity for the absolute instability is shown to be considerably lower th an the corresponding value for the convective instability.