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.

Three regimes of intense laser beam propagation in plasmas

This paper presents an investigation of the propagation characters of Gaussian laser beam in cold collisionless plasma by considering the plasma temperature variation. The ponderomotive nonlinearity is involved and induces the nonlinear self-focusing against spatial diffraction. The second-order differential equation of dimensionless beam width parameter f(ξ) in terms of plasma temperature is established from Maxwell’s equations with paraxial approximation. The initial (P0,ρ0) plane, behaviors of f(ξ) parameter, and perturbed density n/nn0 n0 of different plasma temperatures are plotted and analyzed. The plasma temperature is divided into four regions, which leads to different propagation characters including oscillatory divergence, self-trapping, self-focusing, and steady divergence. The plasma density distribution modified by the electric field intensity and plasma temperature is discussed.

Propagation characters of Gaussian laser beams in collisionless plasma: Effect of plasma temperature

The propagation characters of Gaussian laser beam in collisionless plasma are investigated by considering the ponderomotive and relativistic nonlinearities. The second-order differential equation of dimensionless beam width parameter is solved numerically, taking into account the effect of electron temperature. The results show that the ponderomotive force does not facilitate the relativistic self-focusing in all intensity ranges. In fact, there exists a certain intensity value that, if below this value, the ponderomotive nonlinearity can contribute to the relativistic self-focusing, or obstruct it, if above. It is also indicated that there is a temperature interval in which self-focusing can occur, while the beam diverges outside of this region. In addition, the results represent the existence of a “turning point temperature” in the mentioned interval that the self-focusing has the strongest power. The value of the turning point is dependent on laser intensity in which higher intensities result in higher turning point.

Turning point temperature and competition between relativistic and ponderomotive effects in self-focusing of laser beam in plasma

In this paper the formalism of self-focusing of electromagnetic waves is extended to include nonlinear absorption by the medium. A complex eikonal has been employed, which does not need any approximation about the relative magnitudes of the real and imaginary parts of the dielectric constant or their dependence on the irradiance of the beam. The specific case of collisional plasmas has been considered as an application of the theory. It is seen that the nonlinearity in absorption tends to cancel the effect of divergence on account of diffraction. The dependence of the beam width and attenuation on distance of propagation has been illustrated for specific cases. The relevance of the investigation to radio wave propagation has also been pointed out.

Self-focusing of electromagnetic beams in collisional plasmas with nonlinear absorption

In the present work, nonlinear interaction of elliptical laser beam with collisional plasma is studied by using paraxial ray approximation. Nonlinear differential equations for the beam width parameters of semi-major axis and semi-minor axis of elliptical laser beam have been set up and solved numerically to study the variation of beam width parameters with normalized distance of propagation. Effects of variation in absorption coefficient and plasma density on the beam width parameters are also analyzed. It is observed from the analysis that extent of self-focusing of beam increases with increase/decrease in plasma density/absorption coefficient.

http://iopscience.iop.org/article/10.1088/0253-6102/65/1/78/pdf

Nonlinear Interaction of Elliptical Laser Beam with Collisional Plasma: Effect of Linear Absorption

Nonlinear penetration of an inhomogeneous laser beam in an overdense plasma

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Nonlinear penetration of an inhomogeneous laser beam in an overdense plasma