The nature of propagation of an intense laser beam in a plasma depends on the power and width of the beam and Ω, the ratio of plasma frequency and wave frequency. In this paper, for a given value of Ω (<1) three regimes have been obtained in the beam power-beam width plane, characterizing the nature of propagation as follows: (i) steady divergence (the beam keeps on diverging as it propagates in the plasma); (ii) oscillatory divergence (as the beam propagates in the plasma, the beamwidth oscillates between the original beamwidth and a maximum value); and (iii) self-focusing (as the beam propagates in the plasma the beam width oscillates between the original beam width and a minimum value). Ponderomotive force, collisions, and relativistic dependence of mass on quiver velocity have been considered to be the mechanisms of nonlinearity in the effective dielectric constant.

The growth of a ring ripple, riding on an intense Gaussian laser beam, through plasma has been studied. The amplitude ratio p of the ripple and the beam and the dimensionless width ρ1 of the ripple are chosen as significant coordinates. It is observed that the positive quadrant of the p, ρ1 space can be divided in three distinct regions corresponding to steady divergence, oscillatory divergence and oscillatory convergence of the ripple. The variation of ripple width with distance of propagation has been obtained for typical points in the three regions. Collisions, ponderomotive force, and relativistic dependence of mass on quiver velocity have been considered as the mechanisms, which introduce nonlinearity.

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