On self-focusing of an ultrashort intense relativistic laser pulse in a plasma
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Filamentation
Self-focusing
Modulational Instability
Relativistic plasma
1407_46The generation of nonlinear plasma wakefields by an intense, short laser pulse and the relativistic optical guiding of intense laser pulses in plasmas are studied with a nonlinear, self-consistent model of laser-plasma interactions. Nonlinear steepening and period lengthening of the plasma waves are observed, and expressions are obtained for various nonlinear wakefield quantities. Relativistic focusing with the self-consistent plasma response shows that laser pulse fronts and laser pulses shorter than a plasma wavelength, 2(pi) c/(omega) (rho ), are not relativistically guided and will continuously erode due to diffraction.
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We report the spectral signatures induced by ultrafast laser excitation of Xe. There is no significant difference in the number of observable Xe transitions regardless of the focusing condition, suggesting the possibility for filamentation-based remote sensing of Xe.
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A brief introduction to the interaction of ultra‐intense laser pulses with plasmas is presented. A number of interesting phenomena due to relativistic electron dynamics are reviewed in simple calculations. These relativistic effects range from nonlinear frequency shifts of light waves to penetration of overdense plasmas, filamentation and self‐focusing. Finally, computer simulations are used to illustrate several strongly nonlinear effects in such plasmas, including heating by the oscillating ponderomotive force and relativistic filamentation.
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The transverse modulational instability, or filamentation, of two collinear waves is investigated, using a coupled nonlinear Schrödinger-equation model. For infinite media it is shown that the presence of the second laser field increases the growth rate of the instability and decreases the scale length of the most unstable filaments. Systems of two copropagating waves are shown to be convectively unstable and systems of two counterpropagating waves are shown to be absolutely unstable, even when the ratio of backward- to forward-wave intensity is small. For two counterpropagating waves in finite media, the threshold intensities for the absolute instability depend only weakly on the ratio of wave intensities. The general theory is applied to the pondermotive filamentation of two light waves in homogeneous plasma.
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Filamentation has been examined in laser-produced plasmas in which magnetic fields are present, using a two-fluid model of the plasma. Configurations in which the low-frequency perturbation is in turn parallel and perpendicular to the magnetic field have been considered and the instability examined in the two cases. In the latter it is shown that the instability is quenched by large enough fields, while for the ion-acoustic perturbation the behavior is more complex.
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Filamentation
Degeneracy (biology)
Momentum (technical analysis)
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Filamentation
Self-focusing
Modulational Instability
Relativistic plasma
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Many practical applications of ultrashort, high power laser pulses require the laser pulse to be focused to a high intensity and remain relatively collimated over large distances in plasmas. Such applications include x-ray lasers [1], laser-plasma-based electron accelerators [2] and laser-induced nuclear fusion schemes [3]. Self-focusing and self-channeling of laser pulses by relativistic and ponderomotive mechanisms [4] are laser-plasma processes which can accomplish this feat.
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In this paper we study the influence of the magnetised thermal conductivity on the propagation of a nanosecond $10^{14} \mathrm{Wcm}^{-2}$ laser in an underdense plasma by performing simulations of a paraxial model laser in a plasma with the full Braginskii magnetised transport coefficients. Analytic theory and simulations show the shortening of the self-focal length of a laser beam in a plasma as a result of the reduction of the plasma thermal conductivity in a magnetic field. Furthermore the filamentation of a laser via the thermal mechanism is found to have an increased spatial growth rate in a magnetised plasma. We discuss the effect of these results on recent magnetised inertial fusion experiments where filamentation can be detrimental to laser propagation and uniform laser heating. We conclude the application of external magnetic fields to laser-plasma experiments requires the inclusion of the extended electron transport terms in simulations of laser propagation.
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The filamentation instability triggered when two counter streaming plasma shells overlap appears to be the main mechanism by which collisionless shocks are generated. It has been known for long that a flow aligned magnetic field can completely suppress this instability. In a recent paper [PHYSICS OF PLASMAS 18, 080706 (2011)], it was demonstrated in two dimensions that for the case of two cold, symmetric, relativistically colliding shells, such cancellation cannot occur if the field is not perfectly aligned. Here, this result is extended to the case of two asymmetric shells. The filamentation instability appears therefore as an increasingly robust mechanism to generate shocks.
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Robustness
Weibel instability
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