Mahendra Singh Sodha

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.

Charging of dust particles in an illuminated open complex plasma system

This paper presents an investigation of the charging of dust particles in a dusty plasma, irradiated by white light in near space at satellite altitudes. In deference to the recent emphasis on the character of openness in a dusty plasma, the investigation is based on the balance of the number density and energy of electrons, ions, and neutral particles as well as the energy balance of the dust particles and the charge neutrality condition. The accretion of electrons/ions and the emission of electrons by the dust particles, the ionization of neutral particles and the recombination of electrons and ions, and binary collisions between electrons, ions, and neutral atoms are the processes considered herein; the energy exchange associated with these processes has also been considered. The formulation is applicable to dusty plasmas in space and laboratory, where the photoemission of electrons is the dominant mechanism for electron generation. As an illustration a parametric study of the charging of the dust of C...

Focusing of electromagnetic beams in collisional plasmas, with finite thermal conduction

In this paper we present a theoretical investigation of the focusing of coaxial Gaussian electromagnetic beams and of a Gaussian ripple on an electromagnetic beam of uniform irradiance in a collisional plasma (in thermal equilibrium in the absence of the beams). A self consistent solution of the electromagnetic wave equation, the energy balance equation, and Fourier’s equation of heat conduction has been obtained in the paraxial approximation. The nonuniform distribution of the electron density and thereby the dielectric function on account of the nonuniform electron temperature/density distribution causes the focusing/defocusing of the beams. The effect of thermal conduction on the temperature distribution of the electrons, on the critical curves, and the nature of focusing has been specifically studied. In common with earlier studies, three regions in the initial beam width—initial axial irradiance plane, corresponding to steady divergence, self focusing and oscillatory divergence have been characterize...

Mutual focusing/defocusing of Gaussian electromagnetic beams in collisional plasmas

In this paper the authors have considered the mutual focusing/defocusing of a number of coaxial Gaussian electromagnetic beams in a singly ionized collisional plasma (initially in thermal equilibrium) and the ionosphere (with singly charged ions). Starting from the expression of the electron temperature in terms of the irradiance of the waves, expressions for the electron density and the dielectric function in the form ∊o(z,E∙E*)−r2∊2(z,E∙E*) have been derived; the power loss by electrons to heavy particles is assumed to be much larger than that due to thermal conduction. The dominant nonlinearity considered herein is the radial redistribution of the electron density on account of the radial dependence of the electric field of the waves and consequently of the electron temperature. Using this expression for the dielectric function, the coupled wave equations corresponding to different beams have been solved in the paraxial approximation, yielding a system of coupled second-order differential equations for...

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.

Growth of embryonic dust particles in a complex plasma

A model describing the growth of embryonic dust grains on account of accretion of neutral atoms and positively charged ionic species in a complex plasma has been developed. In deference to the recent emphasis on the character of the openness in complex plasma, the investigation is based on the balance of the number density and energy of electrons, ions, and neutral particles as well as the energy balance of the dust particles and the charge neutrality condition. To discuss the kinetics of the growth of the size of dust the processes of accretion of electrons, ions, and neutral species on the dust particles, the ionization of neutral atoms, and recombination of electrons and ions, and the elastic collisions between the constituent species of the dusty plasma have been considered; the energy exchange associated with these processes has also been taken into account. The dependence of the growth of dust particles and other relevant parameters on number density of embryonic dust grains has, in particular, been explored.

Generation and accretion of electrons in complex plasmas with cylindrical particles

This paper presents an analytical model for the physical understanding of the charging of cylindrical dust particles in an open complex plasma system. Two different mechanisms, viz., thermionic emission and photoelectric emission have been considered for the electron generation from the charged cylindrical dust particles; the corresponding expressions for the rate of emission of electrons and their mean energy have been derived. A simple approach has been adopted to derive the expression for the rate of electron accretion to the dust particle. Further a new expression for the mean energy associated with the accreted electrons due to cylindrical dust particle has been derived and presented. An interesting comparison of results obtained in the case of spherical and cylindrical dust particles has also been made. Using these expressions, a formalism has been developed for the electronic processes in an illuminated dust cloud with cylindrical particles, on the basis of charge neutrality condition and number an...

Self-focusing and cross-focusing of Gaussian electromagnetic beams in fully ionized collisional magnetoplasmas

This paper presents an analysis and subsequent discussion of the phenomena of self-focusing of single electromagnetic Gaussian beams and cross-focusing of multiple coaxial beams in fully ionized magnetoplasma, taking into account the Ohmic heating of the electrons by the beams and loss of energy by electrons due to collision with the ions and electronic thermal conduction; the energy gained by ions in collision with the electrons has been equated to the energy lost on account of ionic thermal conduction. It is seen that the inclusion of the ionic thermal conduction reduces self/cross-focusing for high values of the magnetic field (νe≪Ωc) and enhances the same for low values of the magnetic field (νe≫ωc); here νe is the electron collision frequency and ωc,Ωc are the cyclotron frequencies of electrons and ions, respectively. The wave frequency is assumed to be much higher than the cyclotron frequency of the electrons. The results lead to the conclusion that considerable error occurs by neglecting ionic ther...

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.

Focusing of a dark hollow Gaussian electromagnetic beam in a magnetoplasma

This paper presents an analysis and subsequent discussion of the self focusing of a dark hollow Gaussian electromagnetic beam (HGB) in a magnetoplasma, considering ponderomotive and collisional nonlinearities. A paraxial-like approach, in which the relevant parameters are expanded in terms of radial distance from the maximum of the irradiance rather than that from the axis, has been adopted to analyze the propagation of the HGB. The nature of self focusing is highlighted through the critical curves as a plot of dimensionless radius versus power of the beam. The effect of the magnetic field and the nature of the nonlinearity on self focusing of various order HGBs has also been explored.