Relativistic Plasma Waves Research Articles

Speed and shape of solitary waves in relativistic warm plasma

Large-amplitude solitary waves are investigated in a relativistic plasma with finite ion-temperature. The mass of electron is also considered. The Sagdeev’s pseudopotential is determined in terms ofu, the ion speed. It is found that there exists a critical value ofu0, the value ofu at which (u′)2=0, beyond which the solitary waves cease to exist. The critical value also depends on the parameters likeν, the soliton velocity;μ, the electronion mass ratio orσ, the temperature ratio of ion to electron. This result reproduces our previous result [Czech. J. Phys., Vol. 54 (2004), No. 4, 489–496] when the ion temperature is neglected.

Relativistic effect on stimulated Raman scattering of a laser in plasma

An intense laser beam propagating in relativistic plasma undergoes stimulated Raman scattering (SRS), producing a relativistic plasma wave and an electromagnetic sideband. The amplitudes of the Langmuir wave and scattered electromagnetic sideband wave increase initially and decrease after attaining a saturation level, giving a periodic exchange of energy between the pump wave and the daughter waves. The relativistic effect plays an important role during SRS of a laser in plasma.

Laser–plasma accelerator: status and perspectives

Laser-plasma accelerators deliver high-charge quasi-monoenergetic electron beams with properties of interest for many applications. Their angular divergence, limited to a few mrad, permits one to generate a small gamma ray source for dense matter radiography, whereas their duration (few tens of fs) permits studies of major importance in the context of fast chemistry for example. In addition, injecting these electron beams into a longer plasma wave structure will extend their energy to the GeV range. A GeV laser-based accelerator scheme is presented; it consists of the acceleration of this electron beam into relativistic plasma waves driven by a laser. This compact approach (centimetres scale for the plasma, and tens of meters for the whole facility) will allow a miniaturization and cost reduction of future accelerators and derived X-ray free electron laser (XFEL) sources.

The generation of mono-energetic electron beams from ultrashort pulse laser–plasma interactions

The physics of the interaction of high-intensity laser pulses with underdense plasma depends not only on the interaction intensity but also on the laser pulse length. We show experimentally that as intensities are increased beyond 10(20) W cm(-2) the peak electron acceleration increases beyond that which can be produced from single stage plasma wave acceleration and it is likely that direct laser acceleration mechanisms begin to play an important role. If, alternatively, the pulse length is reduced such that it approaches the plasma period of a relativistic electron plasma wave, high-power interactions at much lower intensity enable the generation of quasi-mono-energetic beams of relativistic electrons.

Novel techniques of laser acceleration: from structures to plasmas

Compact accelerators of the future will require enormous accelerating gradients that can only be generated using high power laser beams. Two novel techniques of laser particle acceleration are discussed. The first scheme is based on a solid-state accelerating structure powered by a short pulse CO(2) laser. The planar structure consists of two SiC films, separated by a vacuum gap, grown on Si wafers. Particle acceleration takes place inside the gap by a surface electromagnetic wave excited at the vacuum/SiC interface. Laser coupling is accomplished through the properly designed Si grating. This structure can be inexpensively manufactured using standard microfabrication techniques and can support accelerating fields well in excess of 1 GeV m(-1) without breakdown. The second scheme utilizes a laser beatwave to excite a high-amplitude plasma wave, which accelerates relativistic particles. The novel aspect of this technique is that it takes advantage of the nonlinear bi-stability of the relativistic plasma wave to drive it close to the wavebreaking.

Measurements of plasma-wave generation using a short-pulse high-intensity laser beat wave

Experiments to examine the generation of relativistic plasma waves via a high-intensity short-pulse beat-wave scheme are described in detail. The pulse stretcher of the Vulcan chirped-pulse amplification (CPA) laser system was modified to produce two frequency, 3ps pulses focusable to intensities up to 1018Wcm−2. Short high-intensity pulses were used to avoid limitations to the plasma-wave amplitude due to the modulational instability. Two experiments were undertaken, at 3 and 10TW, with the generation of plasma waves diagnosed by measuring the sidebands produced in the spectrum of the forward scattered beam. A resonance in the sideband signal was observed for an initial plasma density higher than expected for the given beat frequency. This resonance shift can be attributed to transverse ponderomotive expulsion of plasma electrons from the laser focal region. A monotonically increasing background was also observed, which was due to nonresonant cross-phase modulation.

Solitary Waves in Relativistic Electromagnetic Plasma

Solitary waves in relativistic electromagnetic plasmas areobtained numerically. The longitudinal momentum of electrons hasbeen taken into account in the problem. It is found that in themoving frame with electromagnetic field propagating the solitarywaves can exist in both cases, where the vector potentialfrequency is larger or smaller than the plasma characteristic frequency.

Laser plasma acceleration of electrons: Towards the production of monoenergetic beams

The interaction of high intensity laser pulses with underdense plasma is investigated experimentally using a range of laser parameters and energetic electron production mechanisms are compared. It is clear that the physics of these interactions changes significantly depending not only on the interaction intensity but also on the laser pulse length. For high intensity laser interactions in the picosecond pulse duration regime the production of energetic electrons is highly correlated with the production of plasma waves. However as intensities are increased the peak electron acceleration increases beyond that which can be produced from single stage plasma wave acceleration and direct laser acceleration mechanisms must be invoked. If, alternatively, the pulse length is reduced such that it approaches the plasma period of a relativistic electron plasma wave, high power interactions can be shown to enable the generation of quasimonoenergetic beams of relativistic electrons.

Effect of electron inertia on the speed and shape of ion-acoustic solitary waves in relativistic plasma

Ion-acoustic solitary waves are investigated in a cold collisionless relativistic plasma. Electron inertia is also considered. The Sagdeev’s pseudopotential is determined in terms of u, the ion speed and depends on v, the velocity of the wave. It is found that there exists a critical value of u1 (≠ 0), the value of u at which (u′)2 = 0, beyond which the solitary waves cease to exist. The critical value also depends on the relativistic parameter u0/c when u0 is the drift velocity of the ion and c is the speed of light.

Progress on plasma accelerators: from the energy frontier to tabletops

This paper reviews the concepts, recent progress and current challenges for realizing the tremendous electric fields in relativistic plasma waves for applications ranging from tabletop particle accelerators to high-energy physics. Experiments in the 1990s on laser-driven plasma wakefield accelerators at several laboratories around the world demonstrated the potential for plasma wakefields to accelerate intense bunches of self-trapped particles at rates as high as 100 GeV m−1 in millimetre-scale gas jets. These early experiments have been followed in the current decade by experiments that are advancing on several fronts—increasing the accelerated charge (to the several nanocoulomb level), producing higher transverse beam quality (to the mm mrad normalized emittance level) and accessing new physics regimes at higher laser power. Several groups are engaged in pursuing two key challenges for laser wakefield accelerators—producing beams with small energy spread and extending the interaction length from millimetres to centimetres and beyond. Major breakthroughs on both fronts have occurred in the past year. In parallel with the progress in laser-driven wakefields, particle-beam driven wakefield accelerators are making large strides. A series of experiments using the 30 GeV beam of the Stanford Linear Accelerator has demonstrated high-gradient acceleration of electrons and positrons in metre-scale plasmas as well as key scaling laws for a ‘plasma afterburner’, a concept for doubling the energy of a high-energy collider in a few tens of metres of plasma. In addition to wakefield acceleration, these and other experiments have demonstrated the rich physics bounty to be reaped from relativistic beam–plasma interactions. This includes plasma lenses capable of focusing particle beams to the highest energy density ever produced, collective radiation mechanisms capable of generating high-brightness x-ray beams, collective refraction of particles at a plasma interface and acceleration of intense proton beams from laser-irradiated foils.

Monoenergetic beams of relativistic electrons from intense laser-plasma interactions.

High-power lasers that fit into a university-scale laboratory can now reach focused intensities of more than 10(19) W cm(-2) at high repetition rates. Such lasers are capable of producing beams of energetic electrons, protons and gamma-rays. Relativistic electrons are generated through the breaking of large-amplitude relativistic plasma waves created in the wake of the laser pulse as it propagates through a plasma, or through a direct interaction between the laser field and the electrons in the plasma. However, the electron beams produced from previous laser-plasma experiments have a large energy spread, limiting their use for potential applications. Here we report high-resolution energy measurements of the electron beams produced from intense laser-plasma interactions, showing that--under particular plasma conditions--it is possible to generate beams of relativistic electrons with low divergence and a small energy spread (less than three per cent). The monoenergetic features were observed in the electron energy spectrum for plasma densities just above a threshold required for breaking of the plasma wave. These features were observed consistently in the electron spectrum, although the energy of the beam was observed to vary from shot to shot. If the issue of energy reproducibility can be addressed, it should be possible to generate ultrashort monoenergetic electron bunches of tunable energy, holding great promise for the future development of 'table-top' particle accelerators.

Gravitational waves in magnetized relativistic plasmas

We study the propagation of gravitational waves (GW's) in a uniformly magnetized plasma at arbitrary angles to the magnetic field. No a priori assumptions are made about the temperature, and we consider both a plasma at rest and a plasma flowing out at ultrarelativistic velocities. In the $3+1$ orthonormal tetrad description, we find that all three fundamental low-frequency plasma wave modes are excited by the GW's. Alfv\'en waves are excited by a \ifmmode\times\else\texttimes\fi{} polarized GW, whereas the slow and fast magnetoacoustic modes couple to the + polarization. The slow mode, however, does not interact coherently with the GW. The most relevant wave mode is the fast magnetoacoustic mode which in a strongly magnetized plasma has a vanishingly small phase lag with respect to the GW allowing for coherent interaction over large length scales. When the background magnetic field is almost, but not entirely, parallel to the GW's direction of propagation even the Alfv\'en waves grow to first order in the GW amplitude. Finally, we calculate the growth of the magnetoacoustic waves and the damping of the GW.

Enhanced acceleration of injected electrons in a laser-beat-wave-induced plasma channel.

Enhanced energy gain of externally injected electrons by a approximately 3 cm long, high-gradient relativistic plasma wave (RPW) is demonstrated. Using a CO2 laser beat wave of duration longer than the ion motion time across the laser spot size, a laser self-guiding process is initiated in a plasma channel. Guiding compensates for ionization-induced defocusing (IID) creating a longer plasma, which extends the interaction length between electrons and the RPW. In contrast to a maximum energy gain of 10 MeV when IID is dominant, the electrons gain up to 38 MeV energy in a laser-beat-wave-induced plasma channel.

Nonresonant beat-wave excitation of relativistic plasma waves with constant phase velocity for charged-particle acceleration.

The nonresonant beat-wave excitation of relativistic plasma waves is studied in two-dimensional simulations and experiments. It is shown through simulations that, as opposed to the resonant case, the accelerating electric fields associated with the nonresonant plasmons are always in phase with the beat-pattern of the laser pulse. The excitation of such nonresonant relativistic plasma waves is shown to be possible for plasma densities as high as 14 times the resonant density. The density fluctuations and the fields associated with these waves have significant magnitudes, facts confirmed experimentally using collinear Thomson scattering and electron injection, respectively. The applicability of these results towards eventual phase-locked acceleration of prebunched and externally injected electrons is discussed.

Collinear Thomson scattering diagnostic system for the detection of relativistic waves in low-density plasmas

A Thomson scattering technique is used to study relativistic plasma waves at plasma densities as low as 2×1015 cm−3. A spatial-spectral filter is utilized to simultaneously attenuate the probe light ∼109 times and collect the weak scattered light which is shifted 3–24 Å from the probe wavelength. The plasma waves, excited by a two-wavelength CO2 laser pulse with intensities up to 1015 W/cm2, are probed by a 532 nm laser pulse in a collinear geometry. Both the red- and blueshifted sidebands of Thomson scattered light are simultaneously resolved in time and frequency.

Large-amplitude plasma wave generation with a high-intensity short-pulse beat wave

A short-pulse laser beat wave scheme for advanced particle accelerator applications is examined. A short, intense (3-ps, >10(18)-W cm(-2)) two-frequency laser pulse is produced by use of a modified chirped-pulse amplification scheme and is shown to produce relativistic plasma waves during interactions with low-density plasmas. The generation of plasma waves was observed by measurement of forward Raman scattering. Resonance was found to occur at an electron density many times that expected, owing to ponderomotive displacement of plasma within the focal region.

Electron acceleration by a wake field forced by an intense ultrashort laser pulse.

Plasmas are an attractive medium for the next generation of particle accelerators because they can support electric fields greater than several hundred gigavolts per meter. These accelerating fields are generated by relativistic plasma waves-space-charge oscillations-that can be excited when a high-intensity laser propagates through a plasma. Large currents of background electrons can then be trapped and subsequently accelerated by these relativistic waves. In the forced laser wake field regime, where the laser pulse length is of the order of the plasma wavelength, we show that a gain in maximum electron energy of up to 200 megaelectronvolts can be achieved, along with an improvement in the quality of the ultrashort electron beam.

On the study of instability of ion-acoustic waves in relativistic plasma: Unbounded and bounded

The instability of ion-acoustic waves has been theoretically studied in a multicomponent plasma consisting of positive ions, negative ions and two-temperature isothermal electrons, and also considering the streaming of ions through the derivation of dispersion relation in an unbounded and bounded system involving a comparative study between the results obtained in the two cases. Results for the phase velocity and the instability have been graphically illustrated for the plasmas (O−,Ar+) and (Cl−,H+).

Low-frequency waves in asymmetric magnetized relativistic pair plasma

ABSTRA C T The properties of waves in a pulsar magnetosphere are considered in the most general case of a non-neutral, current-carrying pair plasma with arbitrary distribution functions for electrons and positrons. General dispersion relations are derived for a strong but finite magnetic field, including gyrotropic terms caused by the deviations from quasi-neutrality and the relative streaming of electrons and positrons. It is shown how the ellipticity of the wave polarization depends on the plasma parameters and angle of propagation. Two examples of plasma distributions are analysed numerically: a waterbag distribution and a piecewise distribution that models the numerical result for pair cascades. A possible application to the interpretation of the observed circular polarization of some pulsars is discussed.

Characterization of electron beams produced by ultrashort (30 fs) laser pulses

Detailed measurements of electron spectra and charges from the interaction of 10 Hz, 600 mJ laser pulses in the relativistic regime with a gas jet have been done over a wide range of intensities (1018–2×1019 W/cm2) and electron densities (1.5×1018–1.5×1020 cm−3), from the “classical laser wakefield regime” to the “self-modulated laser wakefield” regime. In the best case the maximum electron energy reaches 70 MeV. It increases at lower electron densities and higher laser intensities. A total charge of 8 nC was measured. The presented simulation results indicate that the electrons are accelerated mainly by relativistic plasma waves, and, to some extent, by direct laser acceleration.