Large Amplitude Electron Plasma Wave Research Articles

Multi-dimensional kinetic simulations of laser radiation absorption and electron acceleration in inhomogeneous underdense plasma

The sub-relativistic laser beam interaction with an underdense plasma is investigated via two-dimensional (2D) numerical simulations with respect to the laser polarization direction. Different parametric instabilities dominate the interaction depending on the propagation direction of the daughter waves in the simulation plane with respect to the laser field polarization and laser propagation direction. In the plane containing the laser electric field (p-polarization) the interaction is dominated by the two-plasmon decay instability and the beating of large amplitude electron plasma waves produces periodic ion density perturbations suppressing stimulated Raman scattering in the quarter critical density zone. A stronger absorption and heating of hot electrons is observed in the case where laser polarization is perpendicular to the simulation plane (s-polarization). Furthermore, by comparing a plane laser wave with a narrow beamlet, the effect of the initial transverse laser profile is proven to play an important role in exciting the filamentation instability, which competes with stimulated Brillouin scattering and affects the laser absorption and hot electron generation. A dedicated three-dimensional simulation indicates that a 2D simulation with p-polarization produces a more reliable results while the case of s-polarization overestimates the laser absorption and hot electron generation.

Suppression of autoresonant stimulated Raman scattering in transversely weakly magnetized plasmas

We investigate the behaviour of backward stimulated Raman scattering (BSRS) in inhomogeneous plasma with a transverse external magnetic field via 1D and 2D particle-in-cell simulations and theoretical analysis. The external magnetic field causes nonlinear damping of large-amplitude electron plasma waves (EPWs), and significantly limits the reflectivity of stimulated Raman scattering (SRS) even if is very weak. For a single resonance point, limits the kinetic nonlinear frequency shift, impedes the autoresonance behaviour of SRS, and narrows the resonance region. We also exhibit distorts the distribution function of electrons evidently, and alters the wavefronts of EPWs in high dimensional cases. The simulation results indicate a moderate can significantly reduce the trapped electron induced gain of SRS reflectivity, and make the BSRS reflectivity stay at its linear convective saturation level steadily.

Raman Scattering Instability of Transverse Electromagnetic Waves in Degenerate Spin Polarized Plasma

Raman scattering instability (RSI) is investigated in a magnetized quantum electron–ion plasma by applying the separate spin evolution-quantum hydrodynamic (SSE-QHD) model. The governing differential equations of large amplitude transverse electromagnetic waves (TEM) and electron plasma waves (EP) are derived in spin polarization effect. The nonlinear interaction of spin polarized TEM and EP waves is studied by using the standard technique of phasor matching to formulate general dispersion relation of RSI, which admits both three-wave decay and modulational instabilities. It is found that the growth rates of the obtained instabilities are a strong function of amplitude of TEM and quantum effects such as Bohm potential, electron spin effect and spin dependence Coulomb exchange interaction. Here, the growth rate of three-wave decay instability is found to increase with the increase of spin effect, whereas for oscillatory modulational instability the growth rate is observed to occur at a particular wavelength.

Enhanced second harmonic generation of Hermite–Gaussian laser beam in plasma having density transition

Under the combined effect of relativistic and ponderomotive nonlinearity, resonant second harmonic generation of an intense Hermite–Gaussian (H–Gaussian) laser beam in a plasma having upward density ramp profile has been theoretically investigated. Under WKB approximation and with the use of the moment theory approach the coupled differential equations are derived which govern the spot size of the H–Gaussian laser beam. The ponderomotive force creates the density gradient in the background plasma electrons which is responsible for the excitation of the electron plasma wave. The large-amplitude electron plasma wave interacts with the fundamental beam which produces coherent radiations with double the frequency of the incident beam. In addition, the Gouy phase shift is observed for the H–Gaussian laser beam in plasma under the effect of density transition. The analysis shows the vital role of the different modes of the H–Gaussian laser beam and density ramp on the efficiency of generated harmonics and self-phase shift of laser beams in plasma.

Combined effects of temperature and collisions on large amplitude electron oscillations in non-relativistic plasma

The combined effects of temperature and electron collision frequency with immobile ion on the propagation of large amplitude electron plasma wave in an unmagnetized non-relativistic plasma is studied. Two different temperature profiles are considered for one dimensional wave propagation. The variations of wave amplitude and its period are investigated under the simultaneous effects of temperature and collision parameters. The increase or decrease in the wave amplitude and its period depends not only on temperature and collision parameters, but it also depends on the initial conditions.

Moving electron density gratings induced in the beat-wave field of two counterpropagating laser pulses

The production and physical characteristics of moving electron density gratings (MEDGs) are investigated by analytical model and particle-in-cell (PIC) simulations. When the frequency difference of two counterpropagating pump pulses is equal to the electron plasma frequency, large amplitude electron plasma wave is resonantly excited in the plasma. As a result, electrons are excited to form a moving, gratinglike structure in the interfering region of pump pulses, while the ions keep at rest. The spatial period and velocity of the MEDG are obtained by solving the electron density modulation equation, which agrees well with one-dimensional PIC simulation results. When a short signal laser pulse is scattered on the MEDG, the frequency of the backscattering wave shifts due to the stimulated Raman scattering.

Studies of stimulated Raman backscattering and associated nonlinear laser–plasma interactions

Coupled theoretical and computational work is presented aimed at understanding and modelling stimulated Raman backscattering (SRBS) relevant to laser–plasma interactions in large-scale, homogeneous plasmas. With the aid of a new code for simulating and studying the nonlinear coupling in space–time of a large number of modes, and an Eulerian Vlasov–Maxwell code for studying the evolution of large amplitude electron plasma waves, we report results and their interpretations to elucidate the following five observed, nonlinear phenomena associated with SRBS: coupling of SRBS to Langmuir decay instabilities (LDIs); effect of ion-acoustic damping on SRBS; cascading of LDI; stimulated Raman scattering cascades; and stimulated electron acoustic wave scattering (SEAS).

Neutrino-driven wakefield plasma accelerator

A classical fluid description is used to investigate the nonlinear interaction between neutrino bursts and a relativistic collisionless cold unmagnetized plasma. It is shown that during the interaction large amplitude electron plasma waves are excited in such a way that charged particles trapped in this high gradient potential can be accelerated to extremely high energies.

Electric Field Formation in Laboratory Plasmas.

Laboratory investigations on the formation of space-related electric fields have been performed using collisionless magnetized-plasma flows. In a set of experments, potential differences are externally applied along magnetic-field lines, different plasmas come into contact with each other under the currentless condition, a high-speed plasma flow is injected into a region of converging manetic-field lines, large-amplitude electron plasma waves are generated in a uniform magnetic field, and local electron-cyclotron resonances occur in various kinds of nonuniform magnetic fields. It is found that the elcetric field is formed in the region of density gradient, a U-shaped double layer (DL) is accompaned by ion-cyclotron oscillations, a DL is formed in the currentless system, and that an electron local-plug results in the formation of a potential dip and hump with a scale length of the order of Debye length. Electron- and ion-acceleration electric fields with a scale lengths of a magnetic-field gradient are also found to be formed by passing ions and electrons through nonuniform magnetic fields, respectively.

Plasma Potential Formation Due to Localized Radio-Frequency Electric Fields

A plasma potential formation is investigated in a collisionless magnetized plasma flow in the presence of localized radio-frequency electric fields. An extremely large-amplitude electron plasma wave is generated by applying an external rf potential to a ring exciter surrounding the plasma column. Steep changes of electron density, temperature and potential along the plasma flow are observed in the region around the wave generation, where the waves are localized because of enhanced wave damping. The phenomena drastically change in accordance with the characteristics of electron-wave propagation. The local potential hill and subsequent large potential drop observed are attributed to the ponderomotive force on electrons and the nonlinear electron acceleration due to enhanced wave damping, respectively. This claim is supported by calculations based on one-dimensional Poisson's equation including spatial variations of the charged particles' velocity distribution functions which are strongly affected by the loc...

Resonant and quasi-resonant excitation of plasma waves by means of sequences of laser pulses

The possibility of generating large amplitude electron plasma waves by means of n short laser pulses in a cold collisionless electron plasma is investigated in detail on the basis of a fluid relativistic model. The problem of the 'resonant' interaction between a synchronized sequence of laser pulses and the plasma is solved analytically up to the wavebreaking limit. The potentialities of the 'quasi-resonant' interaction are investigated numerically paying attention to the sensitivity of the evolution of the system to the number and to the length of the pulses, the distance between successive pulses, and their shape. The possibility of performing photon acceleration experiments is also demonstrated.

Momentum diffusion in current drive by electron plasma waves

Fast electron acceleration and velocity diffusion are investigated in wavepackets formed by large-amplitude waves with relativistic phase velocities. In tokamaks, such waves are generated, e.g. in current drive by free-electron lasers where large-amplitude electron plasma waves or lower hybrid waves are generated. At small field intensities, the velocity diffusion in wavepackets with Gaussian-like envelopes is found to be well described by the quasilinear approximation. At large field intensities, trapping effects become important, and deviations from quasilinear diffusion are found. It is demonstrated that these deviations may have a significant effect on the velocity distribution of the electrons. A wave-induced runaway-like phenomenon of relativistic electrons is discussed.

Large amplitude plasma wave excitation by means of sequences of short laser pulses.

The generation of a large amplitude electron plasma wave by means of n short laser pulses is studied in the quasistatic approximation using a one-dimensional relativistic cold plasma model. The problem is reduced to considering the coupled equations for the vector and the scalar potentials. Preliminary results are obtained by means of a linear stability analysis of the system and by the integration of the Poisson equation for constant pulses. The energetics and time evolution of the excitation process are described by the numerical integration of the relevant equations.

Decay and modulational instabilities of electron plasma waves in unmagnetized dusty plasmas

Nonlinear interaction between a large-amplitude electron plasma wave and low-frequency perturbations in dusty plasmas is considered. A nonlinear dispersion relation appropriate for three-wave decay interaction as well as modulational and filamentation instabilities is obtained. Analytical expressions for the growth rates and thresholds are presented. Also derived are the dynamical equations describing the long-term behaviour of modulationally unstable electron plasma waves in plasmas with fixed charged dust particulates.

On the maximum longitudinal electric field of a large amplitude electron plasma wave excited by a short electromagnetic radiation pulse

The excitation of large amplitude plasma oscillations by the ponderomotive force exerted by a short electromagnetic (e.m.) radiation pulse on the electrons, is described within a one-dimensional, relativistic, cold plasma model. The quasistatic approximation, which assumes that the fluid variables follow adiabatically the temporal evolution of the field variables, is assumed to be valid. For a fixed rectangular profile of the e.m. wavepacket, travelling with group velocity v g< c, the analytical solution of the problem is obtained. It is shown that, after the transit of the pulse, the coherent longitudinal electric field (travelling with phase velocity v φ = v g) can reach values higher than E MAX st =√2(m e cω pe /e)(γ φ−1) 1 2 , where γ φ=(1−v 2 φ/c 2) -1 2 , up to E MAX =√2(m e cω pe /e)(γ 2 φv φ/c) (3−v 2 φ/c 2−√5−2v 2 φ/c 2+v 4 φ/c 4) 1 2 . However, this is only a transient dynamics which leads inevitably to the wavebreaking at a distance of approximately one wavelength from the beginning of the pulse. Actually, only for E< E MAX st stationary plasma oscillations can be sustained.

Measurements of beat-wave-accelerated electrons in a toroidal plasma.

Electrons are accelerated by large amplitude electron plasma waves driven by counterpropagating microwaves with a difference frequency approximately equal to the electron plasma frequency. Energetic electrons are observed only when the phase velocity of the wave is in the range 3${\mathit{v}}_{\mathit{e}}$${\mathit{v}}_{\mathrm{ph}}$7${\mathit{v}}_{\mathit{e}}$ (${\mathit{v}}_{\mathrm{ph}}$ was varied 2${\mathit{v}}_{\mathit{e}}$${\mathit{v}}_{\mathrm{ph}}$10${\mathit{v}}_{\mathit{e}}$), where ${\mathit{v}}_{\mathit{e}}$ is the electron thermal velocity, (${\mathit{kT}}_{\mathit{e}}$/${\mathit{m}}_{\mathit{e}}$${)}^{1/2}$. As the phase velocity increases, fewer electrons are accelerated to higher velocities. The measured current contained in these accelerated electrons has the power dependence predicted by theory, but the magnitude is lower than predicted [B. I. Cohen et al., Nucl. Fusion 28, 1519 (1988)].

Excitation of large amplitude plasma waves

The basic principles of various methods for exciting large amplitude electron plasma waves are reviewed, with particular emphasis on the methods that pertain to plasma accelerators. We also discuss methods for producing dense plasmas suitable for accommodating the large electric fields possible in plasma waves.

Excitation of a large amplitude plasma wave by a short laser pulse

In the "Laser Wakefield Accelerator" scheme a large amplitude electron plasma wave is excited by a single laser pulse, with some important advantages on the beat-wave and particle wake-field schemes.For the case of very short laser pulses (femtoseconds), the plasma wave can possibly grow to even larger amplitudes due to Raman forward scattering inside the pulse broad frequency spectrum.

Multiple-fluid models for plasma wake-field phenomena.

In this paper we present various treatments of plasma wake-field phenomena which employ multiple-fluid models. These models generalize the one-dimensional, nonlinear, relativistic single-fluid model which has been used extensively in previous plasma wake-field calculations. Using a two-fluid model, we discuss the interaction of a low-energy continuous electron beam with wake-field-generated plasma waves. The phenomena of continuous-beam modulation and wave period shortening are discussed. The relationship between these effects and the two-stream instability is also examined. Also, using a three-fluid model, effects due to plasma electron temperature in nonlinear plasma wake-fields are examined and compared to previous work. Finally, the consequences of ion motion induced by large-amplitude electron plasma waves are calculated by including the fluid behavior of the ions.

The stability of the plasma-sheath with secondary emission

The potential and charge particle distributions are calculated for a collisionless sheath bounding a plasma adjacent to a wall which emits secondary electrons under both floating and current-carrying conditions. The corresponding conditions at the sheath-plasma boundary are determined in terms of the parameters secondary electron beam density/plasma density and beam velocity/plasma electron thermal speed. This 'steady state initial condition' is then examined for instability to excitation of the beam-plasma instability using particle-in-cell computer simulations. Results are given and summarized in the appropriate parameter space showing when the instability can be classified as weak, strong, or violent. The work is related to other work on the beam-plasma instability-theoretical, computational and experimental- and on large amplitude electron plasma waves.