Laser Beam In Underdense Plasma Research Articles

Enhanced self-focusing of q-Gaussian laser beam in underdense plasma under the combined effect of density ramp and axial magnetic field

Enhanced self-focusing of q-Gaussian laser beam in underdense plasma under the combined effect of density ramp and axial magnetic field

Combined effect of plasma density ramp and wiggler magnetic field on self-focusing of q-Gaussian laser beam in underdense plasma

Combined effect of plasma density ramp and wiggler magnetic field on self-focusing of q-Gaussian laser beam in underdense plasma

Numerical study of self-focusing and filament formation of intense vortex beams in plasmas

We have studied the self-focusing and filamentation of vortex laser beams propagating in underdense plasmas with different values of the topological charge and initial laser powers. The self-focusing dynamics of vortex laser beams is closely related to the topological charge, which has attracted widespread attention. Based on the paraxial approximation of the Helmholtz equation, the steady-state solution of vortex beams propagating in underdense plasmas is deduced, and the expression for critical power of vortex laser beams is obtained. Furthermore, using the split-step Fourier method to solve the wave equation, we analyze the numerical images of vortex laser beams propagating in underdense plasma under different parameters. The simulation results show that the critical power for self-focusing is positively correlated with the topological charge of vortex laser beams. When the initial laser power is high enough, the vortex laser beam will first be focused into a thin ring, and then the modulation amplitude increases continuously, which eventually leads to the ring structure breaking into filaments. The number of filaments has an integer multiple relationship with the topological charge. In the process of filamentation, the radius and the maximum light intensity of vortex laser beam both change drastically. After the filamentation process is completed, the vortex laser beam continues to propagate with a new topological type. We further increase the incident laser power and find that the number of filaments of the vortex laser beam increases. The increased number of filaments is the value of its topological charge at each time. With the development of filament instability, higher-order modulation instability can be excited in the later stage, and the intensity of filaments will exhibit angular modulation. Our results show that in compared with the standard Gaussian beam, the propagation behavior of vortex laser beams in underdense plasmas is much more stable under the same power, wavelength and plasma parameters. The propagation characteristics of vortex laser beams are helpful to the theoretical and experimental study of stimulated backward Raman amplification of ultra-strong vortex beam in underdense plasmas.

Generation of second harmonics of relativistically self-focused q-Gaussian laser beams in underdense plasma with axial density ramp

An investigation on frequency doubling of intense laser beams through the phenomenon of second harmonic generation (SHG) in underdense plasmas has been presented. In order to increase the efficiency of S.H.G the density profile of plasma has been considered in the shape of upward ramp. When laser beam with frequency $$\omega _0$$ propagates through plasma, it makes the plasma electrons to oscillate at pump frequency. These oscillations of plasma electrons in the presence of thermal velocity generate a plasma wave at frequency $$\omega _0$$ . The generated plasma wave beats with the pump beam to double its frequency. Variational theory has been adopted to find semi analytical solution of the wave equation for the slowly varying envelope of the laser beam. By using hydrodynamic fluid model of plasma, nonlinear current density for SHG has been obtained. Emphasis are put on investigation of the effect of various laser and plasma parameters on propagation dynamics of pump beam and the power of generated second harmonics.

Interaction features of two ultra-intense laser pulses self-trapped in underdense plasmas

The interaction of two parallel relativistic laser beams in underdense plasmas is investigated by considering the evolution of their wave envelopes. The energy transfer between the two lasers is given by an expression based on the evolution of the total laser power in a regime without beam mixing. It is shown that how the energy is transferred depends nonlinearly on the initial phase difference of the lasers, and the result of the interaction depends on the laser intensity, spot radius, and their separation distance. The results are verified by direct numerical solution of the relativistic nonlinear Schrödinger equations for the laser envelopes as well as particle-in-cell simulation. The study and results should be helpful for understanding the energy transfer behavior of multiple co-propagating laser beams in underdense plasmas.

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.

Second harmonic generation by propagation of a p-polarized obliquely incident laser beam in underdense plasma

An analytical study of second harmonic generation due to interaction an intense, p-polarized laser beam propagating obliquely in homogeneous underdense plasma, in the mildly relativistic regime, has been presented. The efficiency of the second harmonic radiation as well as its detuning length has been obtained and their variation with the angle of incidence is analyzed. It is shown that, for a given plasma electron density, the second harmonic efficiency increases with the angle of incidence while the detuning length decreases. The second harmonic amplitude vanishes at normal incidence of the laser beam.

Temperature Effect on Self-Focusing and Defocusing of Gaussian Laser Beam Propagation Through Plasma in Weakly Relativistic Regime

The propagation characters of an intense Gaussian laser beam in underdense plasma are investigated taking into account the combined effects of relativistic and ponderomotive nonlinearities. The nonlinear second-order differential equation of the dimensionless beamwidth parameter is obtained and numerically solved. The effect of initial electron temperature on self-focusing and defocusing of Gaussian laser beam propagation in plasma are studied. Moreover, the behavior and variation of modified electron density and laser beam intensity as a function of plasma length in both self-focusing and defocusing regimes are discussed.

Harmonic generation by the propagation of two-colour laser beams in an underdense plasma

AbstractAn analytical theory is developed for studying the phenomenon of generation of efficient odd and even high harmonics by the propagation of two-colour linearly polarized laser beams in a homogeneous underdense plasma. The wave equation governing the evolution of the amplitude of various harmonics driven by the current density at corresponding frequencies is set up. The ratio of the fundamental frequencies of the two laser beams is considered to be an arbitrary integer. A numerical evaluation of amplitudes of the third, fourth and fifth harmonics has been presented. It is seen that the third harmonic amplitude generated by the two-colour system is enhanced in comparison to that obtained by a single laser beam. The detuning distance for the former is also increased in comparison to the latter case.

Non-stationary self-focusing of intense laser beam in plasma using ramp density profile

The non-stationary self-focusing of high intense laser beam in under-dense plasma with upward increasing density ramp is investigated. The obtained results show that slowly increasing plasma density ramp is very important in enhancing laser self-focusing. Also, the spot size oscillations of laser beam in front and rear of the pulse for two different density profiles are shown. We have selected density profiles that already were used by Sadighi-Bonabi et al. [Phys. Plasmas 16, 083105 (2009)]. Ramp density profile causes the laser beam to become more focused and penetrations deeps into the plasma by reduction of diffraction effects. Our computations show more reliable results in comparison to the previous works.

Axial magnetic field generation by intense circularly polarized laser pulses in underdense plasmas

Axial magnetic field generation by intense circularly polarized laser beams in underdense plasmas has been studied with three-dimensional particle-in-cell simulations and by means of theoretical analysis. Comparisons between analytical models and simulation results have identified an inverse Faraday effect as the main mechanism of the magnetic field generation in inhomogeneous plasmas. The source of azimuthal nonlinear currents and of the axial magnetic field depends on the transverse inhomogeneities of the electron density and laser intensity. The fields reach a maximum strength of several tens of megagauss for laser pulses undergoing relativistic self-focusing and channeling in moderately relativistic regime. Ultrarelativistic laser conditions inhibit magnetic field generation by directly reducing a source term and by generating fully evacuated plasma channels.

Relative phase interactions of two copropagating laser beams in underdense plasmas at different intensities and spot sizes

The mutual interactions of two copropagating laser beams at a relative phase are studied using a two-dimensional fluid code. The interactions are investigated in underdense plasma at selected beam configurations and beam parameters for two separate nonlinearities, i.e., the ponderomotive and the relativistic nonlinearity. The selected beam configurations are introduced by different initial transverse spot size perturbations (finite and infinite) and different initial transversal intensity distributions (nonuniform and uniform) over those spot sizes and the selected beam parameters are given by different initial beam intensities relevant to each nonlinearity. In the ponderomotive nonlinearity, simulation results show that no mutual interactions are demonstrated between the copropagating beams regardless of the initial beam configurations and parameters. In nonlinear relativistic simulations, the mutual interactions between the beams are clearly observed, a mutual repulsion is formed in the presence of initial intensities that are nonuniformly distributed over finite spot sizes, and an effective strongly modulated mutual attraction takes places in the presence of initial intensities that are uniformly distributed over infinite spot sizes. Moreover, it is found in these simulations that increasing the initial beam intensities improves the attraction properties between the copropagationg beams.

Interactions of two co-propagating laser beams in underdense plasmas using a generalized Peaceman–Rachford ADI form

A generalized Peaceman–Rachford ADI form based on the regularized finite difference scheme is employed to study the nonlinear interactions of two co-propagating laser beams in underdense plasmas. A numerical scheme using the form is constructed for the solution of two coupled 2D time-dependent nonlinear Schrödinger equations for quasineutral plasmas in paraxial approximation. In the constructed scheme, the coupled equations are reduced to systems of nonlinear tridiagonal equations that are solved using the matrix factorization and a direct explicit iteration method. In the stability analysis, a time-varying control parameter is introduced for a conditionally stable solution at varying conditions. The scheme has a good agreement with a previous simulations at similar conditions, and in comparison with other algorithms, the scheme presents a better conservation characteristic for the photons power. Finally, the simulations results confirmed that the scheme is suitable to simulate the interactions of two co-propagating laser beams in underdense plasma and it can successively simulate the associated phenomena at different parameters and conditions.

Interactive dynamics of two copropagating laser beams in underdense plasmas.

The interaction of two copropagating laser beams with crossed polarization in the underdense plasmas has been investigated analytically with the variational approach and numerically. The coupled envelope equations of the two beams include both the relativistic mass correction and the ponderomotive force effect. It is found that the relativistic effect always plays the role of beam attraction, while the ponderomotive force can play both the beam attraction and beam repulsion, depending upon the beam diameters and their transverse separation. In certain conditions, the two beam centers oscillate transversely around a propagation axis. In this case, the ponderomotive effect can lead to a higher oscillation frequency than that accounting for the relativistic effect only. The interaction of two beams decreases the threshold power for self-focusing of the single beam. A strong self-trapping beam can channel a weak one.

Plasma wave and second-harmonic generation of intense laser beams due to relativistic effects

This article studies the relativistic self-focusing of an intense laser beam in underdense plasma. Due to relativistic self-focusing of the laser beam, a transverse intensity gradient is created, and this can generate a plasma wave at the pump wave frequency. The mechanism of the plasma wave, second-harmonic generation, and the parameters which govern the second-harmonic yield, have been studied in detail. For a typical laser with wavelength (λ=1.053 μm), intensity (1018 W cm−2) and a plasma having density (ωp/ω0=0.25), the second-harmonic intensity was found to be around (1014 W cm−2).

Self-focusing and merging of two copropagating laser beams in underdense plasma.

The propagation of two laser beams copropagating in underdense plasma has been studied numerically by solving their coupled envelope equations. It shows that two beams can merge each other, or split into three beams, or propagate with unstable trajectories, depending upon their power and initial beam separation. During the merging process, strong emission of radiation is observed. It also shows that the density cavitation channels due to the transverse ponderomotive force of the beams tend to trap them inside and prevent them from merging each other.

Ion wave response to intense laser beams in underdense plasmas

Numerical and theoretical studies of laser beam interaction with underdense plasmas involving ion wave instabilities are presented. The theoretical model that is used involves realistic distribution of laser intensity in a focal spot and a non-paraxial electromagnetic wave equation coupled to the ion acoustic wave equation in a two-dimensional geometry. Included among the important results is a weak correlation between backscattered stimulated Brillouin scattering (SBS) reflectivity and filamentation or self-focusing instabilities. The transmitted light demonstrates angular spreading and frequency shifts consistent with near-forward SBS. The role of filamentation and self-focusing on the transmitted light is also discussed.

Channeling of intense laser beams in underdense plasmas

A hydrodynamic simulation is used to show that intense laser pulses propagating in underdense plasmas create stable, long-lived, and completely evacuated channels. At low intensities, I=10{sup 17} W/cm{sup 2}, self focusing seriously distorts the temporal envelope of the pulse, but channeling still occurs. At high intensities, I=10{sup 19} W/cm{sup 2}, channeling can proceed over many diffraction lengths with significant distortion restricted to the leading edge of the pulse. {copyright} {ital 1997} {ital The American Physical Society}

Amplitude and phase modulation accompanying laser beam trapping in plasmas

The self-trapping of laser beams in underdense plasmas or the propagation of laser beams in preformed favorable electron density distributions can be accompanied by gross distortions of pulse shape and spectrum. A simple theory is used to estimate the magnitude of these effects.