Showing papers on "Relativistic plasma published in 2012"

Harmonic Generation from Relativistic Plasma Surfaces in Ultrasteep Plasma Density Gradients

Abstract: Harmonic generation in the limit of ultrasteep density gradients is studied experimentally. Observations reveal that, while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale lengths (${L}_{p}/\ensuremath{\lambda}l1$), the absolute efficiency of the harmonics declines for the steepest plasma density scale length ${L}_{p}\ensuremath{\rightarrow}0$, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the relativistic oscillating mirror was estimated to be in the range of ${10}^{\ensuremath{-}4}--{10}^{\ensuremath{-}6}$ of the laser pulse energy for photon energies ranging from 20--40 eV, with the best results being obtained for an intermediate density scale length.

Relativistic Reconnection and Particle Acceleration

Abstract: This chapter mainly deals with magnetic reconnection and particle acceleration in relativistic astrophysical plasmas, where the temperature of the current sheet exceeds the rest mass energy and the Alfven velocity is close to the speed of light. Magnetic reconnection now receives a great deal of interest for its role in many astrophysical systems such as pulsars, magnetars, galaxy clusters, and active galactic nucleus jets. We review recent advances that emphasize the roles of reconnection in high-energy astrophysical phenomena.

On the Role of the Accretion Disk in Black Hole Disk-Jet Connections

Abstract: Models of jet production in black hole systems suggest that the properties of the accretion disk—such as its mass accretion rate, inner radius, and emergent magnetic field—should drive and modulate the production of relativistic jets. Stellar-mass black holes in the “low/hard” state are an excellent laboratory in which to study disk–jet connections, but few coordinated observations are made using spectrometers that can incisively probe the inner disk. We report on a series of 20 Suzaku observations of Cygnus X-1 made in the jet-producing low/hard state. Contemporaneous radio monitoring was done using the Arcminute MicroKelvin Array radio telescope. Two important and simple results are obtained: (1) the jet (as traced by radio flux) does not appear to be modulated by changes in the inner radius of the accretion disk and (2) the jet is sensitive to disk properties, including its flux, temperature, and ionization. Some more complex results may reveal aspects of a coupled disk–corona–jet system. A positive correlation between the reflected X-ray flux and radio flux may represent specific support for a plasma ejection model of the corona, wherein the base of a jet produces hard X-ray emission. Within the framework of the plasma ejection model, the spectra suggest a jet base with v/c � 0.3 or the escape velocity for a vertical height of z � 20 GM/c 2 above the black hole. The detailed results of X-ray disk continuum and reflection modeling also suggest a height of z � 20 GM/c 2 for hard X-ray production above a black hole, with a spin in the range 0.6 a 0.99. This height agrees with X-ray time lags recently found in Cygnus X-1. The overall picture that emerges from this study is broadly consistent with some jet-focused models for black hole spectral energy distributions in which a relativistic plasma is accelerated at z = 10–100 GM/c 2 . We discuss these results in the

Strong terahertz radiation from relativistic laser interaction with solid density plasmas

Abstract: We report a plasma-based strong THz source generated in intense laser-solid interactions at relativistic intensities > 10(18) W/cm(2). Energies up to 50 mu J/sr per THz pulse is observed when the laser pulses are incident onto a copper foil at 67.5 degrees. The temporal properties of the THz radiation are measured by a single shot, electro-optic sampling method with a chirped laser pulse. The THz radiation is attributed to the self-organized transient fast electron currents formed along the target surface. Such a source allows potential applications in THz nonlinear physics and provides a diagnostic of transient currents generated in intense laser-solid interactions. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4729874]

Argon plasma modeling with detailed fine-structure cross sections

Abstract: Our recently reported fully relativistic distorted-wave electron-impact cross sections from the ground and metastable states of argon to various excited fine-structure levels are incorporated in a collisional-radiative model to obtain the population densities for the 3p54s and 3p54p (1s and 2p) fine-structure manifolds for low temperature argon plasmas. Excitation cross sections from the two 3p54s J = 1 resonance levels, 1s2 and 1s4, to the higher lying 2p fine-structure manifold as well as for transitions among individual levels of the 1s and 2p manifolds are also calculated and included in the present model which were not fully considered in any earlier model. Our results for the population densities of the 1s and 2p levels show good agreement with recent measurements. The variation of population densities of all the 1s and 2p levels with electron temperature and density are presented. We have also calculated and compared the intensities for the 750.38 nm (2p1 → 1s2) and 696.54 nm (2p2 → 1s5) lines with...

Quasi-matched propagation of ultra-short, intense laser pulses in plasma channels

Abstract: The propagation of an ultrashort and relativistically intense laser pulse in a preformed plasma channel is investigated. The nonlinear paraxial wave equation describing the laser propagation in the plasma is solved both analytically and numerically. For any arbitrary temporal laser pulse profile with a given power (less then the critical power for self-focusing) and any prescribed transverse density profile in the channel, we determine the laser intensity distribution along the pulse ensuring quasi-matched propagation, neglecting non-paraxial effects. For the case of a Gaussian laser with an initially uniform spot throughout the pulse, we determine the optimal channel depth that minimizes laser evolution (e.g., minimizes spot size oscillations). The analytical and semi-analytical results obtained for both cases in the weakly relativistic regime are presented and validated through comparison with numerical simulations.

Effect of Temperature Anisotropy on Various Modes and Instabilities for a Magnetized Non-relativistic Bi-Maxwellian Plasma

Abstract: Using kinetic theory for homogeneous colli- sionless magnetized plasmas, we present an extended review of the plasma waves and instabilities and dis- cuss the anisotropic response of generalized relativis- tic dielectric tensor and Onsager symmetry proper- ties for arbitrary distribution functions. In general, we observe that for such plasmas only those modes whose magnetic-field perturbations are perpendicu- lar to the ambient magnetic field, i.e., B1 ⊥ B0 ,a re effected by the anisotropy. However, in oblique prop- agation all modes do show such anisotropic effects. Considering the non-relativistic bi-Maxwellian distri- bution and studying the relevant components of the general dielectric tensor under appropriate conditions, we derive the dispersion relations for various modes and instabilities. We show that only the electromag- netic R- and L- waves, those derived from them (i.e., the whistler mode, pure Alfven mode, firehose insta- bility, and whistler instability), and the O-mode are affected by thermal anisotropies, since they satisfy the required condition B1⊥B0. By contrast, the perpen- dicularly propagating X-mode and the modes derived from it (the pure transverse X-mode and Bernstein mode) show no such effect. In general, we note that the

Investigation of non-stationary self-focusing of intense laser pulse in cold quantum plasma using ramp density profile

Abstract: The authors have investigated the non-stationary self-focusing of Gaussian laser pulse in cold quantum plasma. In case of high dense plasma, the nonlinearity in the dielectric constant is mainly due to relativistic high intense interactions and quantum effects. In this paper, we have introduced a ramp density profile for plasma and presented graphically the behavior of spot size oscillations of pulse at rear and front portions of the pulse. It is observed that the ramp density profile and quantum effects play a vital role in stronger and better focusing at the rear of the pulse than at the front in cold quantum plasmas.

Regions for Brillouin seed pulse growth in relativistic laser-plasma interaction

Abstract: Parametric plasma processes received renewed interest in the context of generating ultraintense and ultrashort laser pulses up to the exawatt-zetawatt regime. For Brillouin scattering and seed pulse amplification at high intensities, the strong coupling regime is of special interest. The intensity-driven low-frequency modes depend on the amplitudes of the laser fields. It is investigated here how these modes develop in the relativistic regime. Then, a unified treatment of Raman and Brillouin processes becomes necessary. Assuming circular polarization, it is shown that with increasing intensity an overlap of the originally different Raman, Brillouin, and modulational instability branches occurs. Numerical simulations with a linearized Maxwell-fluid code confirm the analytically predicted behavior.

Coupled modes in magnetized dense plasma with relativistic-degenerate electrons

Abstract: Low frequency electrostatic and electromagnetic waves are investigated in ultra-dense quantum magnetoplasma with relativistic-degenerate electron and non-degenerate ion fluids. The dispersion relation is derived for mobile as well as immobile ions by employing hydrodynamic equations for such plasma under the influence of electromagnetic forces and pressure gradient of relativistic-degenerate Fermi gas of electrons. The result shows the coexistence of shear Alfven and ion modes with relativistically modified dispersive properties. The relevance of results to the dense degenerate plasmas of astrophysical origin (for instance, white dwarf stars) is pointed out with brief discussion on ultra-relativistic and non-relativistic limits.

Stability of relativistic plasma-vacuum interfaces

Abstract: We study the plasma-vacuum interface problem in relativistic magnetohydrodynamics for the case when the plasma density does not go to zero continuously, but jumps. In the vacuum region, we consider the Maxwell equations for electric and magnetic fields. We show that a sufficiently large vacuum electric field can make the planar interface violently unstable. By using a suitable secondary symmetrization of the vacuum Maxwell equations, we find a sufficient condition that precludes violent instabilities. Under this condition, we derive an energy a priori estimate in the anisotropic weighted Sobolev space for the variable coefficients linearized problem for nonplanar plasma-vacuum interfaces.

On the breaking of a plasma wave in a thermal plasma: II. Electromagnetic wave interaction with the breaking plasma wave

Abstract: In thermal plasma, the structure of the density singularity formed in a relativistically large amplitude plasma wave close to the wavebreaking limit leads to a refraction coefficient with discontinuous spatial derivatives. This results in a non-exponentially small above-barrier reflection of an electromagnetic wave interacting with the nonlinear plasma wave.

Relativistic model on pulsar radio emission and polarization

Abstract: We have developed a relativistic model for pulsar radio emission and polarization by taking into account a detailed geometry of emission region, rotation, and modulation. The sparks activity on the polar cap leads to plasma columns in the emission region and modulated emission. By considering relativistic plasma bunches streaming out along the rotating dipolar field lines as a source of curvature radiation, we have deduced the polarization state of the radiation field in terms of the Stokes parameters. We have simulated a set of typical pulse profiles and analyzed the role of viewing geometry, rotation, and modulation in the pulsar polarization profiles. Our simulations explain most of the diverse behaviors of polarization generally found in pulsar radio profiles. We show that both the "antisymmetric" and "symmetric" types of circular polarization are possible within the framework of curvature radiation. We also show that the "kinky" nature in the polarization position angle traverses might be due to the rotation and modulation effects. The phase lag of the polarization position angle inflection point relative to the phase of core peak depends upon the rotationally induced asymmetry in the curvature of source trajectory and modulation.

Experimental demonstration of a compact high efficient relativistic magnetron with directly axial radiation

Abstract: A compact relativistic magnetron with axial microwave radiation is experimentally investigated. Under the modified magnetic field distribution, only connecting a special horn antenna in the axial direction, the relativistic magnetron can stably radiate high power and high efficiency microwaves. The total length of the device is ∼0.3 m, and the volume is ∼0.014m3. In such working condition that the applied voltage is 539 kV and the magnetic field is ∼0.38 T, the output microwave power is ∼1.24 GW. Correspondingly, the total efficiency is about 34.1%. The radiating frequency is 2.35GHz, which is in agreement with the π mode frequency of the theoretical expectation.

Ion acoustic solitary waves in high relativistic plasmas with superthermal electrons and thermal positrons

Abstract: Propagation of ion acoustic waves in plasmas containing superthermal electrons, thermal positrons and high relativistic ions is investigated. It is shown that the Korteweg-de Vries (KdV) equation describes the nonlinear waves in such plasmas. The effects of relativistic ions and superthermal electrons on the soliton identifications are discussed.

Compact cryogenic source of periodic hydrogen and argon droplet beams for relativistic laser-plasma generation

Abstract: We present a cryogenic source of periodic streams of micrometer-sized hydrogen and argon droplets as ideal mass-limited target systems for fundamental intense laser-driven plasma applications. The highly compact design combined with a high temporal and spatial droplet stability makes our injector ideally suited for experiments using state-of-the-art high-power lasers in which a precise synchronization between the laser pulses and the droplets is mandatory. We show this by irradiating argon droplets with multi-terawatt pulses.

Quantum ring solitons and nonlocal effects in plasma wake field excitations

Abstract: A theoretical investigation of the quantum transverse beam motion for a cold relativistic charged particle beam travelling in a cold, collisionless, strongly magnetized plasma is carried out This is done by taking into account both the individual quantum nature of the beam particles (single-particle uncertainty relations and spin) and the self consistent interaction generated by the plasma wake field excitation By adopting a fluid model of a strongly magnetized plasma, the analysis is carried out in the overdense regime (dilute beams) and in the long beam limit It is shown that the quantum description of the collective transverse beam dynamics is provided by a pair of coupled nonlinear governing equations It comprises a Poisson-like equation for the plasma wake potential (driven by the beam density) and a 2D spinorial Schrodinger equation for the wave function, whose squared modulus is proportional to the beam density, that is obtained in the Hartree's mean field approximation, after disregarding the exchange interactions The analysis of this pair of equations, which in general exhibits a strong nonlocal character, is carried out analytically as well as numerically in both the linear and the nonlinear regimes, showing the existence of the quantum beam vortices in the form of Laguerre-Gauss modes and ring envelope solitons, respectively In particular, when the relation between the plasma wake field response and the beam probability density is strictly local, the pair of the governing equations is reduced to the 2D Gross-Pitaevskii equation that allows one to establish the conditions for the self focusing and collapse These conditions include the quantum nature of the beam particles Finally, when the relation between the plasma wake field response and the beam probability density is moderately nonlocal, the above pair of equations permits to follow the spatio-temporal evolution of a quantum ring envelope soliton Such a structure exhibits small or violent breathing, but it remains very stable for long time

Collisionless distribution function for the relativistic force-free Harris sheet

Abstract: A self-consistent collisionless distribution function for the relativistic analogue of the force-free Harris sheet is presented. This distribution function is the relativistic generalization of the distribution function for the non-relativistic collisionless force-free Harris sheet recently found by Harrison and Neukirch [Phys. Rev. Lett. 102, 135003 (2009)], as it has the same dependence on the particle energy and canonical momenta. We present a detailed calculation which shows that the proposed distribution function generates the required current density profile (and thus magnetic field profile) in a frame of reference in which the electric potential vanishes identically. The connection between the parameters of the distribution function and the macroscopic parameters such as the current sheet thickness is discussed.

Energetics of runaway electrons during tokamak disruptions

Abstract: In a tokamak disruption, a substantial fraction of the plasma current can be converted into runaway electrons. Although these are usually highly relativistic, their total energy is initially much smaller than that of the pre-disruption plasma. However, following a suggestion by Putvinski et al. [Plasma Phys. Controlled Fusion 39, B157 (1997)], it is shown that as the post-disruption plasma drifts toward the first wall, a non-negligible part of the energy contained in the poloidal magnetic field can be converted into kinetic energy of the runaway electrons. This process is simulated numerically, and it is found that in an ITER-like tokamak runaway electrons can gain kinetic energies up to about 70 MJ by this mechanism.

Analytic solution for kinetic equilibrium with respect to β-processes in nucleon plasmas with relativistic pairs

Abstract: An analytic solution is obtained for kinetic equilibrium with respect to β-processes in nucleon plasmas with relativistic pairs. The nucleons (n, p) are assumed to be nonrelativistic and nondegenerate (their mass is taken to be infinite), while the electrons and positrons are assumed to be ultrarelativistic because of a high temperature (T > 6∙109 K) or high density (ρ > μ106 g/cm3), or both, where μ is the number of nucleons per electron. The analysis can be simplified because of the analytic relationship of the density and chemical potential of the electrons in an ultrarelativistic plasma, as well as by the use of a modified gaussian method for calculating the Fermi functions. The electron chemical potential and the number of nucleons per initial electron are calculated as functions of ρ and T.

Collisional effects on the oblique instability in relativistic beam-plasma interactions

Abstract: The general oblique instability for a relativistic electron beam propagating through a warm and resistive plasma is investigated fully kinetically by a variable rotation method. Analysis shows that the electrostatic part of the oblique instability is attenuated and eventually stabilized by collisional effects. However, the electromagnetic part of the oblique instability (EMOI) is enhanced. Since the current-filamentation instability as a special case of the EMOI has a larger growth rate, it becomes dominant in the collisional case as shown in our two-dimensional particle-in-cell simulations. While the beam diverges in the collisionless case, it can become magnetically collimated in the collisional case due to stabilization of the electrostatic instabilities when the initial beam spreading angle is less than certain magnitude such as a dozen degrees. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4736980]

Ion acoustic soliton in relativistic degenerate electron-positron-ion plasma

Abstract: Large amplitude ion-acoustic (IA) soliton in a fully relativistic plasma consisting of relativistic cold ions and relativistic degenerate electrons and positrons are investigated It is shown that the features of IA soliton are strengthened increasingly with relativistic effects The relativistic degeneracy of electrons and positrons acts in opposite way on the properties of the IA soliton The latter becomes more flatten in the first case and narrower in the second one

Chorus wave amplification: A free electron laser in the Earth's magnetosphere

Abstract: A new theoretical model for whistler-mode chorus amplification in the Earth’s magnetosphere is presented. We derive, based on the free-electron laser mechanism in a high-gain amplifier, a new closed set of self-consistent relativistic equations that couple the Hamiltonian equations for particles with Maxwell’s equations. We demonstrate that these equations predict, through a cubic equation, whistler amplification levels in good agreement with those observed in the Earth’s magnetosphere.

Pitch angle scattering of relativistic electrons from stationary magnetic waves: Continuous Markov process and quasilinear theory

Abstract: We develop a Markov process theory of charged particle scattering from stationary, transverse, magnetic waves We examine approximations that lead to quasilinear theory, in particular the resonant diffusion approximation We find that, when appropriate, the resonant diffusion approximation simplifies the result of the weak turbulence approximation without significant further restricting the regime of applicability We also explore a theory generated by expanding drift and diffusion rates in terms of a presumed small correlation time This small correlation time expansion leads to results valid for relatively small pitch angle and large wave energy density—a regime that may govern pitch angle scattering of high-energy electrons into the geomagnetic loss cone

Parametric decays in relativistic magnetized electron-positron plasmas with relativistic temperatures

Abstract: The nonlinear evolution of a circularly polarized electromagnetic wave in an electron-positron plasma propagating along a constant background magnetic field is considered, by studying its parametric decays. Relativistic effects, of the particle motion in the wave field and of the plasma temperature, are included to obtain the dispersion relation of the decays. The exact dispersion relation of the pump wave has been previously calculated within the context of a relativistic fluid theory and presents two branches: an electromagnetic and an Alfven one. We investigate the parametric decays for the pump wave in these two branches, including the anomalous dispersion zone of the Alfven branch where the group velocity is negative. We solve the nonlinear dispersion relation for different pump wave amplitudes and plasma temperatures, finding various resonant and nonresonant wave couplings. We are able to identify these couplings and study their behavior as we modify the plasma parameters. Some of these couplings are suppressed for larger amplitudes or temperatures. We also find two kinds of modulational instabilities, one involving two sideband daughter waves and another involving a forward-propagating electroacoustic mode and a sideband daughter wave.

Perpendicular propagating modes for weakly magnetized relativistic degenerate plasma

Abstract: Using the Vlasov-Maxwell system of equations, the dispersion relations for the perpendicular propagating modes (i.e., X-mode, O-mode, and upper hybrid mode) are derived for a weakly magnetized relativistic degenerate electron plasma. By using the density (n0=pF3/3π2ℏ3) and the magnetic field values for different relativistic degenerate environments, the propagation characteristics (i.e., cutoff points, resonances, dispersions, and band widths in k-space) of these modes are examined. It is observed that the relativistic effects suppress the effect of ambient magnetic field and therefore the cutoff and resonance points shift towards the lower frequency regime resulting in enhancement of the propagation domain. The dispersion relations of these modes for the non-relativistic limit (pF2≪m02c2) and the ultra-relativistic limit (pF2≫m02c2) are also presented.

Response to 'Comment on 'Solitary waves and double layers in an ultra-relativistic degenerate dusty electron-positron-ion plasma' '[Phys. Plasmas 19, 064703 (2012)]

Abstract: The investigation of the occurrence of nonlinear electrostatic waves (viz., solitary waves and double layers) in degenerate plasmas was the main concern of the article presented by Roy et al. [Phys. Plasmas 19, 033705 (2012)]. The equations of state used in the article were the limits explained by Chandrasekhar [Mon. Not. R. Astron. Soc. 170, 405 (1935)]. It was designated as “misleading” by some authors, which is opposed in this reply with explanation.

THE RELATIVISTIC WIND IN PWNe

Abstract: Pulsar Wind Nebulae (PWNe) are bubbles of relativistic plasma that form when the relativistic pulsar wind is confined by the SNR or the ISM. They have a broad band spectrum extending from from Radio to γ-rays, via synchrotron and Inverse Compton emission, and their study can provide important clues on the otherwise unobservable properties of the cold pulsar wind. The richness of emission features, revealed by recent observations, has driven a renewed interest in the theoretical modeling of these objects. In recent years an MHD paradigm has emerged. which has proved successful in explaining the richness of emission features observed in the X-ray band, and that has contributed to settle many old issues. PWNe are perhaps the best systems where relativistic dynamics can be investigated with high accuracy, given their proximity and the fact that they are persistent sources, and understanding their behavior is an essential step for the field of high energy astrophysics in general. I present here the current status of MHD models: what are the key ingredients, their successes, and open questions.

Acceleration of cone-produced electrons by double-line Ti-sapphire laser beating

Abstract: Acceleration of electrons is demonstrated in a beat wave scheme by using a prepulse-free short-pulse (150 fs) double-line Ti-sapphire laser. To inject electrons, we used a hybrid target composed of a cone-drilled plate and a gas jet, where the cone-produced electrons were accelerated via the forced plasma wave excited in the gas jet that was situated behind the plate. This resulted in an increase in slope temperature from 0.05 to 0.15 MeV. We find a correlation between the slope temperature and forced relativistic plasma wave. The wake amplitude is 15 GV/m at the resonant density of 2.5×1018 cm-3 in a hydrogen plasma. The wake acceleration models can explain the increase in slope temperature.