Showing papers on "Filamentation published in 1989"
TL;DR: Interferograms of an underdense, preformed plasma which is irradiated with a line-focused laser beam show pronounced density perturbations, which are interpreted as being emission from a local, nonlinear focus in theunderdense plasma, rather than from the entire length of a filament.
Abstract: Interferograms of an underdense, preformed plasma which is irradiated with a line-focused laser beam show pronounced density perturbations. The interferograms are compared to simultaneous, gated images of second-harmonic emission. We observe a correlation between the spatial positions of the density perturbations and the peak of the second-harmonic emission. We interpret these phenomena as being emission from a local, nonlinear focus in the underdense plasma, rather than from the entire length of a filament.
39 citations
TL;DR: In this paper, the onset of filamentation for compact vortex structures in two-dimensional incompressible flows is elucidated and an estimate for the filamentation time of an unstably perturbed Kirchhoff ellipse is presented.
Abstract: The onset of filamentation for compact vortex structures in two‐dimensional incompressible flows is elucidated. An estimate is presented for the filamentation time of an unstably perturbed Kirchhoff ellipse, obtained from a linear analysis of the geometry of the instantaneous corotating streamfunction.
33 citations
TL;DR: In this paper, the dielectric window opening in ridge waveguide lasers needed for the ohmic contact is shown to cause appreciable stress deformations of the effective permittivity which defines the waveguide properties of the laser.
Abstract: The dielectric window opening in ridge waveguide lasers needed for the ohmic contact is shown to be able to cause appreciable stress deformations of the effective permittivity which defines the waveguide properties of the laser. For edge force values in the range of 40 dyn/ mu m, the emission properties of the laser, namely the near field and the threshold current, start to change. The results indicate that for still higher values, filamentation begins to occur. The numerical results presented use a global self-consistent model of the semiconductor laser and finite-element method for stress-field calculations. >
25 citations
TL;DR: In this paper, the filamentation instability of finite amplitude left-hand circularly polarized Alfven waves was investigated taking into account the second-order density and magnetic field perturbations that are created by the Alfven wave pressure.
Abstract: The filamentation instability of finite amplitude left-hand circularly polarized Alfven waves has been investigated taking into account the second-order density and magnetic field perturbations that are created by the Alfven wave pressure. The minimum scale length and time over which the filamentation occurs, are found. Our results are applied to Alfven waves that should scatter cosmic rays in the interstellar medium (ISM).
23 citations
TL;DR: In this paper, the nonlinear interaction of a 0.5 μm laser light with long scalelength preformed plasmas was investigated and it was shown that strong nonlinearities were driven by filamentation of the laser beam.
20 citations
TL;DR: In this article, a convective instability that leads to the filamentation of large-amplitude Alfven waves and generates nonoscillatory spatially growing density fluctuations is investigated by using a two-fluid plasma model.
Abstract: A convective instability that leads to the filamentation of large‐amplitude Alfven waves and generates nonoscillatory spatially growing density fluctuations is investigated by using a two‐fluid plasma model. Based on linear perturbation analysis for a four‐wave parametric coupling process, the dispersion relations of the instability for both a right‐ and left‐hand circularly polarized Alfven pump are derived. They are then solved numerically for the dependences of the threshold fields and growth rates on the wavenumber k of the nonoscillatory mode, the beta of the plasma, and the frequency of the pumps, for both right‐ and left‐hand circularly polarized Alfven pumps. The relevance of the proposed instability with some observations in the space plasmas is discussed.
15 citations
TL;DR: In this paper, the authors investigated the filamentation instability of electromagnetic electron and ion cyclotron waves by including the combined effect of relativistic electron mass variation as well as static density and field-aligned magnetic field perturbations that are created by the radiation pressure.
Abstract: The filamentation instability of electromagnetic electron and ion‐cyclotron waves is investigated by including the combined effect of relativistic electron mass variation as well as static density and field‐aligned magnetic field perturbations that are created by the radiation pressure. Analytical expressions for the critical power at which the instability starts are presented. The relevance of this investigation to radio‐frequency wave heating of fusion plasmas is pointed out.
15 citations
TL;DR: In this paper, the filamentation instability of electromagnetic waves in semiconductors in a uniform magnetic field is investigated and the nonlinear responses of the medium, including the nonparabolic dispersion of the conduction carriers as well as the second-order density and magnetic field perturbations that are created by the radiation pressure, are taken into account.
Abstract: The filamentation instability of electromagnetic waves in semiconductors in a uniform magnetic field is investigated. The nonlinear responses of the medium, including the nonparabolic dispersion of the conduction carriers as well as the second-order density and magnetic field perturbations that are created by the radiation pressure, are taken into account. The spatial amplification rate and threshold field associated with the filamentation instability are presented. The relevance of our work to electromagnetic-wave propagation in a sample of narrow-band-gap semiconductors is pointed out.
13 citations
TL;DR: In this article, a weakly nonlinear analysis of vorticity interface perturbations, and spectral stability calculations using the full equations of motion, together show that steady progressive waves are unstable to general subharmonic perturbation in the range 0.094 < d/λ < 1.7, where d is the mean layer thickness and λ is the primary wavelength.
Abstract: We study the instability of finite-amplitude waves on uniform vortex layers of finite thickness bounded by a plane rigid surface. A weakly nonlinear analysis of vorticity interface perturbations, and spectral stability calculations using the full equations of motion, together show that steady progressive waves are unstable to general subharmonic perturbations in the range 0.094 < d/λ< 1.7, where d is the mean layer thickness and λ is the primary wavelength. The relevance of this instability to ultimate interface filamentation is tested by performing several numerical contour-dynamical simulations of the nonlinear interface evolution for initial disturbances consisting of the finite amplitude wave plus eigenfunctions obtained from the spectral calculations. The results indicate that within the band of unstable wavelengths, small perturbations to the steady non-uniform flow given by the finite amplitude wave motion (vortex equilibrium) are able to grow in magnitude, until at a time t, the wave extremum encounters a hyperbolic critical point of the velocity field after which filamentation occurs. Arguments are put forward based on the unsteady simulations with the purpose of identifying the preferred frame of reference for viewing the kinematical events controlling the filamentation process. An estimate for tis then made, and the mechanism of filamentation found is discussed in relation to the recently proposed nonlinear-cascade mechanism of Dritschel (1988a).
12 citations
TL;DR: In this article, the authors made measurements of the spatial profile of a high power laser beam after propagating through a large underdense plasma and observed that the filamentation process observed was driven by the ponderomotive force rather than thermal effects.
10 citations
TL;DR: In this paper, a feasibility study and first experimental results of filamentation in preformed, long scale length, very underdense laser plasmas were reported, using schieren diagnostics using an optical probe beam.
TL;DR: In this paper, a radially modulated 0.35 μm laser beam has been inferred in plasmas produced from solid targets, and it was shown that the breakup is more severe in gold plasms compared to glass or aluminum plasmos and occurs at rather modest laser intensities of ∼5x10 12 W/cm 2.
TL;DR: In this article, the filamentation instability in a collisional, flowing plasma is discussed and it is shown that collisional absorption and finite thermal conductivity affect the growth rate and lead to a deflection of the filaments with respect to the impinging beam.
Abstract: The filamentation instability is discussed in a collisional, flowing plasma. It is shown that collisional absorption and finite thermal conductivity affect the growth rate and lead to a deflection of the filaments with respect to the impinging beam. In contrast to the noncollisional case, growth and deflection are also present in the supersonic case.
TL;DR: In this paper, the effects of flowing plasma on laser light filamentation was reexamined and extended to the thermal-conduction-driven filamentation mechanism, and formulas with a broad range of applicability were developed to predict filamentation growth lengths, thresholds, and fastest growing modes.
Abstract: The effects of flowing plasma on laser light filamentation is reexamined [Phys. Fluids 25, 2302 (1982)] and extended to the thermal‐conduction‐driven filamentation mechanism. Formulas with a broad range of applicability are developed to predict filamentation growth lengths, thresholds, and fastest growing modes. Transverse flow velocities that are of order Cs are found to have large effects on the filamentation growth rates and thresholds. Maximum growth rates are typically increased by a factor of 10 for thermal filamentation, and a factor of 100 for ponderomotive filamentation, when compared to zero‐flow conditions. These fastest growing modes have much shorter perturbation wavelengths than those found in the zero‐flow case.
TL;DR: In this paper, self-localized solutions of the equation δψ − ψ + ψ 3 = 0 are given which correspond to periodic chains of the plane nonlinear vortex-type structures of an incompressible fluid.
TL;DR: In this article, it is predicted that the density fluctuations produced by the filamentation instability along the interplanetary magnetic field have wavelengths greater than, at least, a few earth radii.
Abstract: Intense magnetosonic waves, originally propagating at the right angle with the interplanetary magnetic field, can excite a purely growing mode along the interplanetary magnetic field together with two symmetric magnetosonic sidebands propagating obliquely across the magnetic field. This instability process leads to the filamentation of the magnetosonic pump waves. These two excited magnetosonic sideband modes propagate together perpendicularly across the magnetic field and, meanwhile, form a standing wave pattern along the magnetic field. The thresholds of this filamentation instability can be exceeded in the solar wind environment. It is predicted that the density fluctuations produced by the filamentation instability along the interplanetary magnetic field have wavelengths greater than, at least, a few earth radii. The polarization of the obliquely propagating magnetosonic waves excited by the filamentation instability is determined by the characteristics of the magnetosonic pump waves and the environmental plasmas.
TL;DR: In this article, a numerical and experimental study of the propagation of a laser beam in a performed plasma is presented, where the main consequence is the variation of the spatial laser intensity distribution inside the plasma and the lowering of the interaction length which can, consequently, modify thresholds and growth rates of parametric instabilities.
TL;DR: In this article, the onset of laser beam filamentation in a preformed nonuniform plasma is investigated by using a two-dimensional computer code in which time-dependent plasma hydrodynamics and heat transport effects are accounted for.
Abstract: The onset of laser beam filamentation in a preformed nonuniform plasma is investigated by using a two‐dimensional computer code in which time‐dependent plasma hydrodynamics and heat transport effects are accounted for. Laser beam propagation is modeled using a paraxial wave approximation—refraction, diffraction, absorption, and ponderomotive force effects are accounted for. The simulations reveal details by which multifilamentary plasma and laser beam structures evolve in the region near the critical density surface. For the particular case of a 1.05 μm laser beam with an intensity of 1015 W/cm2 incident on a long scale length preformed plasma, a consistent treatment of the ponderomotive forces and of the plasma dynamics is found to be essential in determining the evolution of the filamentary structures.
TL;DR: In this article, the authors reported a new type of instability in XeCl discharge lasers, which is induced by the marginal level of preionization as buffer gas pressure p and Δ t increases.
TL;DR: In this paper, the relativistic σω-model was used to detect the filamentation instability of ω-meson decay in a collision of heavy nuclei with a few tens of GeV/nucleon.
01 Dec 1989
TL;DR: In this article, the authors studied the instability growth in accelerated plasmas of Z pinches and liners and calculated the spectra of instability growth rates σm,k(t) and showed that the presence of an axial magnetic field creates a window of stability in the space of flow parameters.
Abstract: Perturbation growth in accelerated plasmas of Z pinches and liners is studied. The spectra of instability growth rates σm,k(t) are calculated. The properties of these spectra allow one to explain filamentation and stratification of plasmas observed in experiments. Presence of an axial magnetic field is shown to create a ’window of stability’ in the space of the flow parameters.
TL;DR: In this article, the detailed behavior of a beam of light propagating through a fully ionized plasma is studied near the critical density, where the authors consider the proper motion of the plasma in an intense electromagnetic field.
Abstract: The detailed behaviour of a beam of light propagating through a fully ionized plasma is studied near the critical density. Transient effects in the laser beam and plasma are included by taking into account, in a self-consistent manner, the proper motion of the plasma in an intense electromagnetic field. Parameters of the laser beam and plasma conditions are studied that prove most beneficial in providing a uniform deposition of energy. The behaviour of the light beams near the critical density at the three wavelengths 1054 nm, 527 nm and 351 nm are compared.
TL;DR: In this article, a new technique was described for the observation of filamentation in laser-produced plasmas, and experiments showed a very rapid growth of filaments which can continue to propagate through considerable depths of transparent material.
08 Aug 1989
TL;DR: In this article, the linear growth rates for stimulated Raman scattering and beam filamentation parametric instabilities for high power radiation beams in adisturbed ionospheric space plasma environment were investigated.
Abstract: SUMMARY AND CONCLUSIONWe have computed the linear growthrates for stimulated Raman scattering andbeam filamentation parametric instabilitiesfor high power radiation beams in adisturbed ionospheric space plasmaenvironment. We have considered both 1pmand 10pm laser radiation and plasmaparameters typical of a disturbedionospheric environment.For both the high and low plasmadensity case we find growth times forstimulated Raman scattering (SRS) to be onthe order of y-1 K 10- 4-10 -3 sec for radiation beams with power densities vo /c 210- with vo the plasma electron quivervelocity and c the speed of light. Thesegrowth times yicld typical e- folding growthlengths L = cy- on the order of 100 km.For disturbed space plasma dimensions on theorder of several thousands of kilometers, itcan be expected that many e- folding lengthswould be present over which SRS can grow.In this case, stimulated Raman scattering ofhigh power beam energy is expected to bestrong with associated degradation in beamquality and propagation. At lower beampower densities vo /c S 10 -5, the growthrates (times) or SRS would becorrespondingly smaller (larger).In addition we have also computed thegrowth rates of another parametric processdriven by the beam, the filamentationinstability. This instability can result inbeam break -up and filamentation due toinhomogeneities in the background plasma obeam.
22 May 1989
TL;DR: In this article, the dispersion relation of the instability has been derived and solved numerically for the dependence of the threshold field and spatial growth rate on the wave number k of the nonoscillatory mode, the beta of the plasma, and the frequency of the magnetosonic pump.
Abstract: Summary form only. The ISEE 1 and 2 spacecraft have detected both magnetosonic waves and Alfven waves propagating upstream in the high-speed solar wind. The magnetosonic wave is a hybrid and elliptically polarized mode that can propagate obliquely across the magnetic field. It becomes decoupled from the Alfven wave when the direction of its propagation is exactly perpendicular to the ambient magnetic field. The filamentation instability of the magnetosonic wave that propagates perpendicular to the magnetic field has been analyzed. The authors have extended this analysis to the general case of convective filamentation instabilities of obliquely propagating magnetosonic waves. The dispersion relation of the instability has been derived and solved numerically for the dependence of the threshold field and spatial growth rate on the wave number k of the nonoscillatory mode, the beta of the plasma, and the frequency of the magnetosonic pump. >