Self-focusing Of Laser Beam Research Articles

Effect of self-focused cosh Gaussian laser beam on the excitation of electron plasma wave and particle acceleration

AbstractThis work presents an investigation of the self-focusing of a high-power laser beam having cosh Gaussian intensity profile in a collissionless plasma under weak relativistic-ponderomotove (RP) and only relativistic regimes and its effect on the excitation of electron plasma wave (EPW), and particle acceleration process. Nonlinear differential equations have been set up for the beam width and intensity of cosh Gaussian laser beam (CGLB) and EPW using the Wentzel-Kramers-Brillouin and paraxial-ray approximations as well as fluid equations. The numerical results are presented for different values of decentered parameter ‘b’ and intensity parameter ‘a’ of CGLB. Strong self-focusing is observed in RP regime as compared with only relativistic nonlinearity. Numerical analysis shows that these parameters play crucial role on the self-focusing of the CGLB and the excitation of EPW. It is also found that the intensity/amplitude of EPW increases withbanda. Further, nonlinear coupling between the CGLB and EPW leads to the acceleration of electrons. The intensity of EPW and energy gain by electrons is significantly affected by including the ponderomotive nonlinearity. The energy of the accelerated electrons is increased by increasing the value of ‘b’. The results are presented for typical laser and plasma parameters.

Self-Focusing/Defocusing of Chirped Gaussian Laser Beam in Collisional Plasma with Linear Absorption**Supported by a financial grant from CSIR, New Delhi, India, under Project No. 03(1277)/13/EMR-II

This paper presents an investigation on the self-focusing/defocusing of chirped Gaussian laser beam in collisional plasma with linear absorption. We have derived the differential equation for the beam width parameter by using WKB and paraxial approximations and solved it numerically. The effect of chirp and other laser plasma parameters is seen on the behavior of beam width parameter with dimensionless distance of propagation. The results are discussed and presented graphically. Our simulation results show that the amplitude of oscillations decreases with the distance of propagation. Due to collisional frequency, the laser beam shows fast divergence which can be minimized by the introduction of chirp parameter. The chirp decreases the effect of defocusing and increases the ability of self-focusing of laser beam in collisional plasma.

Combined effect of relativistic and ponderomotive nonlinearity on self-focusing of Gaussian laser beam in a cold quantum plasma

AbstractIn the present paper, we have investigated self-focusing of Gaussian laser beam in relativistic ponderomotive (RP) cold quantum plasma. When de Broglie wavelength of charged particles is greater than or equal to the inter particle distance or equivalently the temperature is less than or equal to the Fermi temperature, quantum nature of the plasma constituents cannot be ignored. In this context, we have reported self-focusing on account of nonlinear dielectric contribution of RP plasma by taking into consideration the impact of quantum effects. We have setup the nonlinear differential equation for the beam-width parameter by paraxial ray and Wentzel Kramers Brillouin approximation and solved it numerically by the Runge Kutta Fourth order method. Our results show that additional self-focusing is achieved in case of RP cold quantum plasma than relativistic cold quantum plasma and classical relativistic case. The pinching effect offered by quantum plasma and the combined effect of relativistic and ponderomotive nonlinearity greatly enhances laser propagation up to 20 Rayleigh lengths.

Effect of spatial coherence on laser beam self-focusing from orbit to the ground in the atmosphere.

The effect of spatial coherence on laser beam self-focusing in the atmosphere to assist delivering powerful laser beams from orbit to the ground is studied. It is found that a fully coherent beam is more strongly compressed on the ground than a partially (spatial) coherent beam (PCB), even so, for a PCB the compressed spot size on the ground may be reduced below the diffraction limit due to self-focusing effect, and a PCB has higher threshold critical power than a fully coherent beam. Furthermore, an effective design rule for maximal compression without beam splitting of the transported PCB from orbit to the ground is presented.

Interaction of a self-focused laser beam with a DT fusion target in a plasma-loaded cone-guided ICF scheme

In this paper, the interaction of a self-focused laser beam with a DT fusion target in a plasma-loaded cone-guided ICF scheme has been presented. We propose here to merge a plasma-loaded cone with the precompressed DT target in order to strongly focus the incident laser beam on the core to improve the fusion gain. The WKB approximation is used to derive a differential equation that governs the evolution of beamwidth of the incident laser beam with the distance of propagation in the plasma medium. The effects of initial plasma and laser parameters, such as initial plasma electron temperature, initial radius of the laser beam, initial laser beam intensity and plasma density, on self-focusing and defocusing of the Gaussian laser beam have been studied. Numerical results indicate that with increasing the plasma frequency (or plasma density) in the cone, the laser beam will be self-focused noticeably, while for a thinner laser beam (with small radius), it will diverge as propagate in the cone. By evaluating the energy deposition of the relativistic electron ignitors in the fuel, the importance of electron transportation in the cone-attached shell was demonstrated. Moreover, by lessening the least energy needed for ignition, the electrons coupling with the pellet enhances. Therefore, it increases the fusion efficiency. In this scheme, with employing a plasma-loaded cone, the fusion process improves without needing an ultrahigh-intensity laser beam in a conventional ICF.

Effect of self-focusing of elliptical laser beam on second harmonic generation in collisionless plasma

In the present paper, effect of self-focusing of elliptical laser beam on second harmonic generation is investigated in collisionless plasma by making use of paraxial ray approximation. There is redistribution of carriers from high field region to low field region by ponderomotive force and hence transverse density gradient gets established in plasma. This transverse density gradient in turn generates the electron plasma wave at pump wave frequency. The electron plasma wave so generated interacts with pump wave there by producing second harmonic. Effect of variation in laser beam intensity and plasma density on second harmonic yield is also analyzed.

Second harmonic generation of self-focused Cosh-Gaussian laser beam in collisional plasma

This paper presents theoretical investigation of effect of self-focusing of Cosh-Gaussian (ChG) laser beam on second harmonic generation (SHG) in collisional plasma. Due to non uniform intensity distribution along the wave front of ChG laser beam non uniform Ohmic heating of plasma electrons takes place. Non uniform heating of plasma the laser beam leads to self-focusing of the laser beam which in turn produces strong density gradients in the transverse direction. The generated density gradients excite electron plasma wave at pump frequency that interacts with the incident laser beam to produce its second harmonics. Following moment theory approach in Wentzel–Kramers–Brillouin (W.K.B) approximation second order differential equation governing the evolution of spot size of laser beam with distance of propagation has been derived. Numerical simulations have been carried out to investigate the effect of laser intensity as well as plasma parameters on self-focusing of the laser beam and also on yield of second harmonics. Results predict that ChG laser beams show smaller divergence as they propagate and, thus, lead to enhanced conversion efficiency of second harmonics.

Characteristics of Self-Focusing of a Gaussian Laser Pulse Under Lateral and Axial Plasma Density Variations

In this paper, we present a paraxial description of relativistic self-focusing of a Gaussian laser beam in a plasma channel with the density distribution as a function of radial and axial variations. We have derived the nonlinear dielectric permittivity of plasma due to ponderomotive force and relativistic effects. The effect of different conditions of the inhomogeneous plasma and the laser pulse on self-focusing and defocusing of Gaussian laser beam propagation in the plasma is studied. We find that with an increase in the value of intensity of the beam, self-focusing enhances and shifts toward lower values of normalized propagation distance. The self-focusing length also increases with an increase in channel width. When the density rapidly increases along the propagation distance, the nonlinearity increases and gives self-focusing at shorter values of $z$ .

Second harmonic generation by self-focusing of intense hollow Gaussian laser beam in collisionless plasma

The effect of self focused hollow Gaussian laser beam (HGLB) (carrying null intensity in center) on the excitation of electron plasma wave (EPW) and second harmonic generation (SHG) has been investigated in collisionless plasma, where relativistic-ponderomotive and only relativistic nonlinearities are operative. The relativistic change of electron mass and the modification of the background electron density due to ponderomotive nonlinearity lead to self-focusing of HGLB in plasma. Paraxial ray theory has been used to derive coupled equations for the self focusing of HGLB in plasma, generation of EPW, and second harmonic. These coupled equations are solved analytically and numerically to study the laser intensity in the plasma, electric field associated with the excited EPW, and the power of SHG. Second harmonic emission is generated due to nonlinear coupling between incident HGLB and EPW satisfying the proper phase matching conditions. The results show that the effect of including the ponderomotive nonlinearity is significant on the generation of EPW and second harmonic. The electric field associated with EPW and the power of SHG are found to be highly sensitive to the order of the hollow Gaussian beam.

Second harmonic generation by self focused q-Gaussian laser beam in preformed collisional parabolic plasma channel

Abstract This paper presents an investigation of self-focusing of a q-Gaussian laser beam and its effect on second generation in a preformed collisional parabolic plasma channel. In the presence of a q-Gaussian laser beam redistribution of carriers takes place on account of nonuniform heating produced due to collisional nonlinearity, as a result of which, a transverse density gradient is established in the channel which in turn generates plasma wave at pump frequency. Generated plasma wave interacts with the incident laser beam and generate second harmonics of the incident laser beam. Moment theory has been used to derive differential equation governing the evolution of spot size of laser beam propagating through the channel. The differential equation so obtained has been solved numerically. The effect of the intensity of laser beam, deviation of intensity distribution of laser beam along its wavefront from Gaussian distribution, plasma density and depth of channel on beam width of laser beam as well as second harmonic yield has been investigated.

Strong terahertz field generation by relativistic self-focusing of hollow Gaussian laser beam in magnetoplasma

AbstractThe present paper proposes a model for the generation of Terahertz (THz) radiation by self-focused hollow Gaussian beam (HGB) in collisionless magnetized rippled density plasma. At high intensities, the change in the electron mass occurs due to relativistic effect, introducing a nonlinearity in the plasma leading to the self-focusing of the HGB. The nonlinear interaction of this highly intense self-focused HGB with the electron plasma wave in the rippled density plasma, satisfying proper phase matching conditions, results in the resonant excitation of THz radiations at the beat frequency. We have studied the dependence of generated THz radiations on the order of the HGB as well as on the static background magnetic field. The results show that the intensity of the generated radiations is highly sensitive to both of these parameters. For the current scheme the power of the generated THz waves comes out to be of the order of Gigawatts.

Second-harmonic generation by relativistic self-focusing of cosh-Gaussian laser beam in underdense plasma

AbstractThis paper presents theoretical investigation of effect of relativistic self-focusing of cosh-Gaussian (ChG) laser beam on second-harmonic generation in an underdense plasma. Steep transverse density gradients are produced in the plasma by the electron plasma wave excited by relativistic self-focusing of ChG laser beam. The generated plasma wave interacts with the pump beam to produce its second harmonics. Following Jeffrey Wentzel Kramers Brillouin (J.W.K.B) approximation and moment theory the differential equation governing the evolution of spot size of laser beam with distance of propagation has been derived. The differential equation so obtained has been solved numerically by the Runge–Kutta method to investigate the effect of decentered parameter, intensity of laser beam as well as density of plasma on self-focusing of the ChG laser beam, and generation of its second harmonics. It has been observed that the peak intensity of the laser beam shifts in the transverse direction by changing the decentered parameter and a noticeable change is observed on focusing of the laser beam as well as on conversion efficiency of second harmonics.

Self-Focusing of Quadruple Gaussian Laser Beam in an Inhomogenous Magnetized Plasma with Ponderomotive Non-Linearity: Effect of Linear Absorption

The propagation of quadruple Gaussian laser beam in a plasma characterized by axial inhomogeneity and nonlinearity due to ponderomotive force in the paraxial ray approximation is investigated. An appropriate expression for the nonlinear dielectric constant has been developed in the presence of external magnetic field, with linear absorption and due to saturation effects for arbitrary large intensity. The effects of different types of plasma axial inhomogeneities on self-focusing of laser beam have been studied with the typical laser and plasma parameters. Self-focusing of quadruple Gaussian laser beam in the presence of externally applied magnetic field and saturating parameter is found significantly improved in the case of extraordinary mode. Our results reveal that initially converging beam shows oscillatory convergence whereas initially diverging beam shows oscillatory divergence. The beam is more focussed at lower intensity in both cases viz. extraordinary and ordinary mode.

Terahertz radiation by self-focused amplitude-modulated Gaussian laser beam in magnetized ripple density plasma

AbstractThis paper presents a theoretical model for efficient terahertz (THz) radiation by self-focused amplitude-modulated laser beam in preformed ripple density plasma. The density of plasma is modified due to ponderomotive nonlinearity which arises because of the nonuniform spatial profile of the laser beam in magnetized plasma and leads to the self-focusing of the laser beam. The rate of self-focusing depends on the intensity of the amplitude-modulated beam as well as on the externally applied magnetic field strength. The electron also experiences time-dependent ponderomotive force by the laser beam at modulated frequency. A nonlinear current at THz frequency arises on account of the coupling between the ripple density plasma and nonlinear oscillatory velocity of the electrons. The yield of the generated THz radiation enhances with enhancement in self-focusing of the laser beam and applied magnetic field.

Propagation of circularly polarized quadruple Gaussian laser beam in magnetoplasma

The propagation of circularly polarized quadruple Gaussian laser beam in underdense magnetoplasma has been investigated. The effect of externally applied axial magnetic field on self-focusing of a laser beam is seen in the presence of relativistic nonlinearity. The nonlinearity arises due to the relativistic increase in mass of electrons which modifies the plasma frequency and hence the refractive index profile. Self-focusing of quadruple Gaussian laser beam in the presence of an externally applied magnetic field is found substantially improved in the case of extraordinary mode and reverse is true for the ordinary mode. Our results reveal that the propagation of the quadruple laser beam can be studied in three different regimes i.e. steady divergence, oscillatory divergence, and self-focusing regime depending on the initial point. For initial point not lying on the critical curve, the beam width parameter will either increase or decrease. The beam is more focussed at lower intensity in both cases viz. extraordinary and ordinary mode.

Self-focusing of an intense laser pulse interacting with a periodic lattice of metallic nanoparticle

The motivation for the present work is the study of self-focusing of an intense laser beam propagating through a periodic array of metallic nanoparticle. Using a perturbative method, a wave equation describing the nonlinear interaction of a laser beam with nanoparticles is derived. Evolution of laser spot size with the Gaussian profile for the circular and linear polarizations is considered. It is found that, in the same intensity, the linear polarization in a special interval of frequency resonantly acts better than the circular one.

Effect of spherical aberration on laser beam self-focusing in the atmosphere.

The effect of spherical aberration on the laser beam self-focusing in the atmosphere to assist delivering powerful laser beams from orbit to the ground is studied. It is found that for the small orbital mirror size, a Gaussian beam with negative spherical aberration might be more strongly compressed without filamentation than that without spherical aberration, and its spot size on the ground can be reduced well below the diffraction limit. However, for the large orbital mirror size, spherical aberration will result in an increase of receiver size on the ground.

Density Transition Based Self-Focusing of cosh-Gaussian Laser Beam in Plasma with Linear Absorption* *Supported by a Financial Grant from CSIR, New Delhi, India, under Project No. 03(1277)/13/EMR-II

Density transition based self-focusing of cosh-Gaussian laser beam in plasma with linear absorption has been studied. The field distribution in the plasma is expressed in terms of beam width parameter, decentered parameter, and linear absorption coefficient. The differential equation for the beam width parameter is solved by following Wentzel–Kramers–Brillouin (WKB) and paraxial approximation through parabolic wave equation approach. The behaviour of beam width parameter with dimensionless distance of propagation is studied at optimum values of plasma density, decentered parameter and with different absorption levels in the medium. The results reveal that these parameters can affect the self-focusing significantly.

Significant enhancement in self-focusing of high-power laser beam through dense plasmas by ramp density profile

The effect of the ramp density profile on relativistic self-focusing of a cosh-Gaussian (ChG) laser beam in dense plasmas is investigated. For this purpose, a modified refractive index of inhomogeneous cold quantum plasma (ICQP) with the inclusion of quantum correction in the relativistic regime is considered. Contrary to earlier works on self-focusing of laser beams through quantum plasmas, higher-order terms in the expansion of the dielectric function and the eikonal are taken into account. In this situation, the envelope equation governing the beam-width parameter is derived in the cylindrical coordinate system. Particularly, the self-focusing feature and the sensitivity of decentered parameter b for the ChG laser beam in the ICQP are also illustrated by utilizing a higher-order paraxial theory. The results are presented graphically and discussed.

Effect of light absorption on relativistic self-focusing of Gaussian laser beam in plasma

In the present paper, we have studied the effect of light absorption on relativistic self-focusing of Gaussian laser beam propagating through plasma. We have obtained the differential equation for beam width parameter by using parabolic equation approach under the usual WKB and paraxial approximations. The effect of light absorption on the behavior of beam-width parameter, normalized intensity and plasma dielectric function is presented graphically and discussed. It shows that absorption play an important role in weakening the relativistic self-focusing of laser beam during propagation in plasma and gives reasonably interesting results.