Large-amplitude Electromagnetic Waves Research Articles

Relativistic laser-plasma interactions in the quantum regime

We consider nonlinear interactions between a relativistically strong laser beam and a plasma in the quantum regime. The collective behavior of electrons is modeled by a Klein-Gordon equation, which is nonlinearly coupled with the electromagnetic wave through the Maxwell and Poisson equations. This allows us to study nonlinear interactions between arbitrarily large-amplitude electromagnetic waves and a quantum plasma. We have used our system of nonlinear equations to study theoretically the parametric instabilities involving stimulated Raman scattering and modulational instabilities. A model for quasi-steady-state propagating electromagnetic wave packets is also derived, and which shows possibility of localized solitary structures in a quantum plasma. Numerical simulations demonstrate collapse and acceleration of electrons in the nonlinear stage of the modulational instability, as well as possibility of the wake-field acceleration of electrons to relativistic speeds by short laser pulses at nanometer length scales. Our study is relevant for understanding the localization of intense electromagnetic pulses in a quantum plasma with extremely high electron densities and relatively low temperature.

Stochastic heating of electrons by a large-amplitude extraordinary wave in plasma

Stochastic heating of plasma electrons by a large-amplitude electromagnetic wave propagating across a strong external magnetic field is studied theoretically and numerically. An analytic estimate of the threshold wave amplitude at which heating begins is obtained. The dependence of the average electron energy on the magnetic field and plasma density is investigated using particle-in-cell simulations.

Parametric decay of X-mode radiation into electron Bernstein and lower hybrid waves in a plasma

A large-amplitude electromagnetic wave, propagating perpendicular to ambient magnetic field in the X-mode, is shown to be susceptible to parametric decay into a short-wavelength electron Bernstein wave and a lower-hybrid wave. The density perturbation associated with the lower-hybrid wave couples to the oscillatory velocity due to the pump to produce a density perturbation driving the Bernstein wave. The Bernstein wave and the pump exert a low-frequency ponderomotive force on electrons, driving the sideband.

Magnetization of a quantum plasma by photons

It is shown that the ponderomotive force of large-amplitude electromagnetic waves (photons), which includes the electron spin current and exchange potential contributions in a quantum plasma, can generate magnetic fields. The present result can account for the magnetic fields in dense compact astrophysical objects and in the next generation laser–solid density plasma interaction experiments.

Full-scale simulation study of stimulated electromagnetic emissions: The first ten milliseconds

AbstractA full-scale numerical study is performed of the nonlinear interaction between a large-amplitude electromagnetic wave and the Earth's ionosphere, and of the stimulated electromagnetic emission emerging from the turbulent layer, during the first 10 milliseconds after switch-on of the radio transmitter. The frequency spectra are downshifted in frequency and appear to emerge from a region somewhat below the cutoff of the O mode, which is characterized by Langmuir wave turbulence and localized Langmuir envelopes trapped in ion density cavities. The spectral features of escaping O-mode waves are very similar to those observed in experiments. The frequency components of Z-mode waves, trapped in the region between the O- and Z-mode cutoffs show strongly asymmetric and downshifted spectra.

Dispersion relations of nonlinearly coupled electromagnetic and electrostatic waves in relativistic plasmas

In the present work large amplitude electromagnetic waves in cold plasmas at relativistic intensities are studied analytically in a plane wave geometry. Special attention is drawn to the nonlinear coupling of electromagnetic and electrostatic modes. In the framework of the Akhiezer–Polovin model, periodic and more general quasiperiodic waves are taken into account. For small densities a two-time-scale method is used to calculate an analytical solution up to the fourth order in the plasma density. Nonlinear dispersion relations are calculated for coupled waves, taking into account the full plasma response for linear as well as for circular polarization. In the presence of a large amplitude electrostatic wave, the results show a major difference from the commonly considered dispersion relation for electromagnetic waves. Finally, the solutions of the Akhiezer–Polovin model are compared with particle-in-cell simulations.

Parametric coupling of low frequency whistler to Alfven wave in a plasma

The parametric decay of a large amplitude electromagnetic wave in the ion cyclotron range of frequency into a compressional Alfven wave and an electromagnetic sideband wave in a magnetized plasma is investigated. The pump wave propagates in the direction of ambient magnetic field whereas the decay waves propagate at oblique angles. When the pump wave is left circularly polarized the decay is not permitted kinematically as the momentum of pump photon always exceeds the sum of momenta of the decay wave photons. For the right circularly polarized whistler mode pump the decay is permitted with sideband nearly right circularly polarized. The density perturbation associated with the Alfven wave couples with the pump driven oscillatory velocities of ions and electrons to produce a current driving the sideband. The sideband and the pump exert pondermotive force on ions and electrons that drive the Alfven wave. The frequency and growth rate of the Alfven wave increase with the normalized pump frequency. The threshold power density, determined by the collisional damping rates of the decay waves is rather modest.

Magnetization of Rydberg plasmas by electromagnetic waves

AbstractIt is shown that the ponderomotive force of a large-amplitude electromagnetic wave in Rydberg plasmas can generate quasi-stationary magnetic fields. The present result can account for the origin of seed magnetic fields in the ultracold Rydberg plasmas when they are irradiated by the high-frequency electromagnetic wave.

Magnetization of a warm plasma by the nonstationary ponderomotive force of an electromagnetic wave

It is shown that magnetic fields can be generated in a warm plasma by the nonstationary ponderomotive force of a large-amplitude electromagnetic wave. In the present Brief Report, we derive simple and explicit results that can be useful for understanding the origin of the magnetic fields that are produced in intense laser-plasma interaction experiments.

Large-amplitude electromagnetic waves in relativistic plasmas

The propagation of linearly polarized large-amplitude electromagnetic waves in relativistic plasmas is studied in the framework of the Akhiezer-Polovin-model. Different forms of the basic equations are reviewed and important solutions are presented for small and critical plasma densities. The well-known periodic solutions are generalized to quasiperiodic solutions taking account of additional electrostatic oscillations.

Oscillating two stream instability of electromagnetic pump in the ion cyclotron range of frequency in a plasma

An analytical formalism of oscillating two stream instability of a large amplitude electromagnetic wave in the ion cyclotron range of frequency in a plasma is developed. The instability produces electrostatic ion cyclotron sidebands and a driven low frequency mode. The nonlinear coupling arises primarily due to the motion of ions and is strong when the pump frequency is close to ion cyclotron frequency and the oscillatory ion velocity is a significant fraction of acoustic speed. For propagation perpendicular to the ambient magnetic field, the X-mode pump wave produces flute type perturbation with maximum growth rate at some specific wavelengths, which are three to four times larger than the ion Larmor radius. For propagation at oblique angles to ambient magnetic field, the ion cyclotron O-mode, the growth rate increases with the wave number of the low frequency mode.

Generation of magnetic fields by the ponderomotive force of electromagnetic waves in dense plasmas

AbstractWe show that the non-stationary ponderomotive force of a large-amplitude electromagnetic wave in a very dense quantum plasma with streaming degenerate electrons can spontaneously create d.c. magnetic fields. The present result can account for the seed magnetic fields in compact astrophysical objects and in the next-generation intense laser–solid density plasma interaction experiments.

Generation of magnetic fields by the non-stationary ponderomotive force of electromagnetic waves in plasmas with streaming electrons

It is shown that the non-stationary ponderomotive force of large amplitude electromagnetic waves in plasmas with streaming electrons can spontaneously create magnetic fields. The present result may account for the magnetic fields in laser-produces plasmas, in cosmic plasmas, as well as in galactic and inter-galactic spaces.

A comparison between electron orbits for both an axial magnetic field and an ion-channel guiding in a FEL with an electromagnetic wave wiggler

AbstractThe operation of a free-electron laser (FEL) with electromagnetic wave wiggler in the presence of an ion-channel guiding as well as an axial guide magnetic field is considered and compared. Theoretical studies of electron trajectories and dispersion relations in a combined ion electrostatic field as well as large-amplitude backward-propagating electromagnetic waves are analyzed. The large-amplitude wave acts like a magnetostatic wiggler in a FEL. The results of a numerical study are presented and discussed. It is shown that in the wiggler pumped ion-channel free-electron laser (WPIC-FEL), electron orbits and dispersion relation are time-dependent, and over time, electron orbits while oscillating bear a periodic motion.

Wakefield Acceleration by Radiation Pressure in Relativistic Shock Waves

A particle acceleration mechanism by radiation pressure of precursor waves in a relativistic shock is studied. For a relativistic, perpendicular shock with the upstream bulk Lorentz factor of $\gamma_1 \gg 1$, large amplitude electromagnetic (light) waves are known to be excited in the shock front due to the synchrotron maser instability, and those waves can propagate towards upstream as precursor waves. We find that non-thermal, high energy electrons and ions can be quickly produced by an action of electrostatic wakefields generated by the ponderomotive force of the precursor waves. The particles can be quickly accelerated up to $\epsilon_{\rm max}/\gamma_1 m_e c^2 \sim \gamma_1$ in the upstream coherent wakefield region, and they can be further accelerated during the nonlinear stage of the wakefield evolution. The maximum attainable energy is estimated by $\epsilon_{\rm max}/\gamma_1 m_e c^2 \sim L_{\rm sys}/(c/\omega_{pe})$, where $L_{\rm sys}$ and $c/\omega_{pe}$ are the size of an astrophysical object and the electron inertial length, respectively.

Bernstein mode coupling to cyclotron harmonic radiation in a plasma

An electron Bernstein wave in the presence of an ion-acoustic wave of suitable wave number converts itself into cyclotron harmonic electromagnetic radiation in a plasma. The Bernstein wave imparts oscillatory velocity to electrons that couples with the density perturbation associated with the sound wave to produce a nonlinear current, driving the electromagnetic wave at sum (or difference) frequency. Conversely a large amplitude electromagnetic wave at a cyclotron harmonic can parametrically excite an electron Bernstein wave that may effectively heat the electrons.

Stimulated scattering instabilities of electromagnetic waves in an ultracold quantum plasma

The nonlinear interaction between a large-amplitude electromagnetic wave (EMW) and low-frequency electron and ion plasma oscillations in a quantum plasma is considered. By using the Maxwell equations and the quantum hydrodynamic equations, a system of equations, which shows the nonlinear couplings between the EMWs and the electrostatic plasma oscillations, is derived. The nonlinear mode coupled equations are then Fourier analyzed to obtain the dispersion relation which exhibits decay and modulational instabilities. The relevance of such an investigation of stimulated scattering of light off plasmons in ultracold quantum plasmas is noted.

Two types of PSBL ion beam observed by Geotail: Their relation to low frequency electromagnetic waves and cold ion energization

Two types of the plasma sheet boundary layer (PSBL) crossing during a geomagnetically active period are analyzed in detail to investigate relationship between the ion beams, low-frequency (0.01–2 Hz) electromagnetic waves, and cold ion energization in PSBL using the Geotail data. In response to changes in the solar wind conditions and auroral activity inferred from the AL index, the properties of PSBL ions changed dramatically: During the less disturbed period, we observed counter-streaming thermalized beams with no significant wave activity in the low-frequency range. The thermalized beams may be generated from the distant magnetotail and have undergone thermalization before reaching the Geotail position of X ∼ −30 R E. During the periods of enhanced auroral activity with southward IMF, collimated freshly injected earthward beams are observed together with the large-amplitude electromagnetic waves having a peak around 0.025 Hz (∼0.1 Ω ci). Cold ions are energized during the periods of large-amplitude low-frequency fluctuations. It is suggested that fresh beams from the near-Earth neutral line (NENL) play an important role in the cold ion energization.

Absolute and convective nature of Raman instability in relativistic hot plasmas

The modulational and Raman instabilities are investigated in the relativistic regime, for a large amplitude electromagnetic wave propagating in a hot plasma of arbitrary density. Temporal growth rates and the number of unstable branches are determined when the plasma temperature, density, and the amplitude of the wave vary. The convective or absolute nature of the Raman instabilities is then determined. The present paper extends previous results by Guérin et al. [Phys. Plasmas 2, 2807 (1995)] and Couairon and Mora [Phys. Plasmas 8, 3434 (2001)]. It is shown that for any temperature, the Raman instability is convectively unstable at low density and low intensity, and absolutely unstable when the intensity or the density exceeds a threshold that we determine as a function of the temperature. In contrast with the cold plasma case, the threshold for absolute instability does not coincide with the merging of branches found by a classical temporal stability analysis.

The state of pulsar theory

I summarize the status of pulsar theory, now 35 years after their discovery. Although progress has been made in understanding the relevant processes involved, there are several widely held misconceptions that are inhibiting further advances. These include the idea that plasma “must” be accelerated from the magnetic polar caps (the basis for the “Hollow Cone Model”) and the idea that winds would be driven away by centrifugal forces, with large amplitude electromagnetic waves playing no role whatsoever. However, recent theoretical work is converging on a picture that closely resembles the latest HST and Chandra images, providing hope for the future. No less than three groups have recently confirmed the early Krause-Polstorff–Michel simulations showing that the fundamental plasma distribution around a rotating neutron star consists of two polar domes and an equatorial torus of trapped nonneutral plasma of opposite sign charges. Unless a lot of new physics can be added, this distribution renders the Goldreich–Julian model irrelevant (i.e., along with most of the theoretical publications over the last 33 years).