M. P. Verma

Three regimes of intense laser beam propagation in plasmas

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.

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

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.

Ring formation in self-focusing of electromagnetic beams in plasmas

This article presents a paraxial theory of ring formation as an initially Gaussian beam propagates in a nonlinear plasma, characterized by significant collisional or ponderomotive nonlinearity. Regions in the axial irradiance-(beamwidth)−2 space, for which the ring formation occurs and the paraxial theory is valid, have been characterized; for typical points in these regions the dependence of the beam width parameter and the radial distribution of irradiance on the distance has been specifically investigated and discussed.

Three regimes of growth of a Gaussian ripple on a uniform plane electromagnetic wave front in a plasma

Starting with the scalar wave equation and relevant expressions for nonlinear dielectric constant of plasma, the propagation of a Gaussian ripple on a plane wave front of uniform intensity has been studied in the paraxial approximation. In the plane of the ripple width (along the y axis) and the ratio of the electric field intensities of the ripple and the main beam at r=0, z=0, (along the x axis) three distinct regions (for a given intensity of the main beam) can be identified which correspond to steady divergence, oscillatory divergence, and self-focusing of the ripple. The variation of the ripple width with distance of propagation has also been obtained for typical points in the three regions.

Charge distribution of particles in an irradiated dust cloud

This communication is a discussion on the charge distribution of the dust particles in an illuminated dust cloud in near space when the photoelectric emission is the dominant mechanism for electron generation. An analytical model has been developed on the basis of charge neutrality condition and balance of number density and energy of electrons; the approach of statistical mechanics has been followed. Computations correspond to a metallic dust cloud in near space environment, where Lyman-α spectral line radiation is the dominant one for photoelectric emission. A comparison of results from the present statistical theory of charge distribution with the uniform charge theory has been presented. As an interesting conclusion, the theory predicts negative charging of a few dust particles for a certain range of parameters leading to the formation of bigger particles on account of electrostatic attraction between oppositely charged particles.

Growth of a ring ripple on a Gaussian beam in a plasma

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.

Effect of self-focusing on third harmonic generation by a Gaussian beam in a collisional plasma

In this paper the third harmonic generation caused by the self-focusing of a Gaussian electromagnetic beam in collisional plasmas has been investigated. The wave equations for the fundamental and the third harmonic fields have been solved in the paraxial approximation. The wave frequency has been assumed to be much larger than the electron collision frequency. The generation of the third harmonic considering self-focusing has been investigated and graphically presented. It is seen that the self-focusing of the fundamental beam enhances the power of the third harmonic output indicating that the region of third harmonic generation is localized near the axis of the beam. The dependence of the third harmonic power on the distance of propagation for different values of initial fundamental power, beam width, and plasma density has also been plotted and discussed.

Relativistic guidance of laser beams in plasmas

The authors have identified three regimes of propagation of circularly polarized laser beams in plasmas, taking into account the relativistic laser plasma interaction. An appropriate expression for the nonlinear dielectric constant has been used in the analysis of laser-beam propagation in the paraxial approximation. Three regimes of propagation in a homogeneous plasma have been identified, viz. I. In this regime the beam keeps on diverging. II. In this regime the beam travels in a guided oscillatory and diverging mode. with the width lying between the original value and a maximum value. III. In this regime the beam becomes self-focused. with the width lying between the original value and a minimum value. For a given value of ω p /ω the regimes are characterized by dimensionless power and dimensional beam width. Tthe variation of beam width with distance of propagation has also been obtained for typical values of parameters in the three regimes. The variation of beam-width parameter with distance of propagation has also been studied for inhomogeneous plasma and penetration in overdense plasmas is indicated.

Phase matching for third-harmonic generation in collisional magnetoplasmas

This paper presents a derivation of the phase-matching conditions for generation of the third-harmonic (3ω1) and combination frequencies (ω1+2ω2), by two fundamental electromagnetic waves of frequencies ω1 and ω2, propagating in the extraordinary mode along a magnetic field in a collisional plasma. Expressions for the corresponding optimum distance of propagation for maximum output and the associated values of the phase functions have been derived. In the case of the third-harmonic generation phase matching occurs when the gyrofrequency of the electrons is twice the wave frequency (for all values of plasma frequency). In the case of the combination frequencies, the phase-matching condition is represented by an expression for plasma frequency ωp in terms of the wave frequencies ω1, ω2 and the gyrofrequency ωc of electrons. Furthermore, it is seen that in the (ωc∕ω1)−(ω2∕ω1) plane, phase matching occurs in a narrow region; for every point in this region, there is a corresponding value of (ωp∕ω1). The depend...

Self-focusing instability in ionospheric plasma with thermal conduction

In this communication, an expression for the growth rate of self-focusing instability in the ionospheric plasma has been derived after taking finite thermal conduction into account. The instability arises on account of the depletion of electrons from regions where the irradiance of the perturbation is large. In contrast to earlier work, an appropriate energy balance equation for electrons and ions and the proper dependence of thermal conductivity on electron temperature have been used. The dependence of the growth rate of the filamentation instability on the background irradiation, thermal conductivity, and the wave number of transverse perturbation has been investigated. The mid-latitude daytime ionospheric model of Gurevich has been used for numerical computations, corresponding to a height of 200km. The gradient of irradiance perturbations is assumed to be along the magnetic field of the Earth. The numerical results have been illustrated graphically and discussed.