Showing papers on "Filamentation published in 1977"
TL;DR: In this article, it was shown that resonance absorption together with ponderomotive force effects such as filamentation and density profile steepening strongly influence the laser light absorption, while inverse bremsstrahlung absorption is not the dominant absorption mechanism.
Abstract: High-intensity (1013–1017 W/cm2) 1.06 μm laser light absorption experiments with spherical and planar targets suggest that inverse bremsstrahlung absorption is not the dominant absorption mechanism. Evidence is presented that resonance absorption together with ponderomotive force effects such as filamentation and density profile steepening strongly influence the laser light absorption.
49 citations
TL;DR: In this article, the authors analyzed the steady-state self-focusing and filamentation of high-power whistlers in a magnetoplasma and found that the dominant nonlinearity arises because of nonuniform heating and redistribution of electrons and causes self-focus only when ω≳ωc/2 (ω and ωc being the wave and electron cyclotron frequencies, respectively).
Abstract: Using the earlier expressions for the nonlinear dielectric tensor of a magnetoplasma, the authors have analyzed the steady‐state self‐focusing and filamentation of high‐power whistlers. On the short time scale (t≪τe, τe being the energy relaxation time), the nonlinearity arises because of the ponderomotive force and is responsible for self‐focusing up to very low frequencies. On the time scale of energy relaxation time, the dominant nonlinearity arises because of nonuniform heating and redistribution of electrons and causes self‐focusing only when ω≳ωc/2 (ω and ωc being the wave and electron cyclotron frequencies, respectively). When the power of the whistler equals the critical power for self‐focusing, it propagates without convergence or divergence and also guides a second weak whistler to propagate without convergence or divergence. These results are in quantitative agreement with the experiments of Stenzel. At very high powers, self‐focusing of whistlers is not possible by any of the two mechanisms; i...
38 citations
TL;DR: In this paper, the authors studied the filamentation of a laser beam in a collisional plasma when the nonlinearity in the dielectric constant arises due to nonuniform heating and redistribution of electrons.
Abstract: In this paper we have studied the filamentation of a laser beam in a collisional plasma when the nonlinearity in the dielectric constant arises due to nonuniform heating and redistribution of electrons. For small‐scale fluctuations [r0< (mi/m)1/2 λm, where r0 is the beam radius, mi is the ion mass, m is the electron mass, and λm is the electron mean free path], the energy balance of electrons is mainly determined by thermal conduction. The growth rate of perturbation decreases rapidly with decreasing size of perturbation. For large‐scale fluctuations, the energy balance is mainly through collisions and the growth rate decreases with the scale length of perturbation. Maximum growth rate is obtained for some optimum value of scale length. The growth rate also has a maximum for some optimum value of intensity of the main beam.
19 citations
Journal Article•
9 citations
8 citations
TL;DR: In this article, the spatial perturbation of plasma density and internal field perpendicular to the direction of propagation was investigated when a high-power pulsed microwave is incident on the plasma whose plasma frequency is close to the applied one.
Abstract: The spatial perturbation of plasma density and internal field perpendicular to the direction of propagation are experimentally investigated, when a high-power pulsed microwave is incident on the plasma whose plasma frequency is close to the applied one. For the microwave power above a certain threshold, the electron density and the electric field show a temporally growing perturbation which increases with the incident power. The properties of the perturbation, including its growth rate, are discussed and compared with the theory for the filamentation instability.
6 citations
Journal Article•
Abstract: With the development of a large magnetized plasma source it has become possible to investigate space plasma physics problems in the laboratory. First, the nonlinear effects associated with the excitation of a large amplitude whistler wave have been explored. It is found that the radiation pressure of the wave and thermal effects give rise to a field-aligned density depression in which the wave becomes completely trapped. Hyperfine filaments with diameters small compared with the parallel wavelength are observed. Second, the stability of oblique whistler waves in the presence of an electron beam has been studied. A broadband whistler instability is observed and identified as a Cherenkov interaction between beam electrons and whistlers propagating near the resonance cone. These observations confirm the present model for the generation of VLF hiss in the aurora.
4 citations
TL;DR: In this paper, it was shown that stationary turbulence consisting of an ensemble of small amplitude lower hybrid wave packets becomes unstable against adiabatic perturbations along the external magnetic field.
3 citations
TL;DR: In vitro studies show that in extracts from either spcr or ts+ strains, or in the presence of ethanol, translational restriction is relieved, suggesting that the suppression of tif phenotypes could involve the translation process.
Abstract: SummaryA recent study showed that in E. coli T44 (λ) carrying the tif-1 mutation, elevated temperature and adenine can interfere with the translation process. The present study shows that the expression of tif phenotypes (thermoinduction and filamentation) is suppressed by factors which affect ribosomal function. Ethanol suppresses thermoinduction and, in some spcr mutants, both thermoinduction and filamentation are suppressed. An unknown factor(s) in yeast extract suppresses both thermoinduction and filamentation. In thermoresistant ‘revertant’ (ts+), the expression of the ts+ phenotype is suppressed by yeast extract, ethanol, guanosine+cytidine and by the addition of a spcr mutation. This indicates that this phenotype could be due to suppressor mutations, and the interaction between factors affecting ribosomal function and the ts+ phenotype suggests that the suppression of tif in the ts+ strains could operate on the ribosomal level. In vitro studies show that in extracts from either spcr or ts+ strains, or in the presence of ethanol, translational restriction is relieved, suggesting that the suppression of tif phenotypes could involve the translation process.
3 citations
TL;DR: In this paper, it was shown that the filamentation instability of a high frequency pump wave can have an exceptionally low threshold intensity in a magnetized plasma and that the excitation of a purely growing mode may have important consequences in the laser heating schemes of pinch devices.
3 citations
TL;DR: In this paper, the effect of the saturating nonlinearity of the pump wave on the spatial growth of the filamentation instability of a Gaussian plane electromagnetic beam in a collisionless magnetoplasma was investigated.
Abstract: Presents an investigation of the effect of the saturating nonlinearity of the pump wave on the spatial growth of the filamentation instability of a Gaussian plane electromagnetic beam in a collisionless magnetoplasma. The nonlinearity is caused by the redistribution of carriers by the ponderomotive force. The growth rate is not a monotonically increasing function of the beam intensity and the optimum size of perturbation increases with the beam intensity beyond a certain value of the beam intensity which is in contrast to the uniform plane beam case. The growth rate decreases with the distance of propagation due to the self-focusing effects and is enhanced with the increase of static magnetic field for extraordinary waves, but the reverse is true for ordinary waves.
06 Dec 1977
TL;DR: Argus as discussed by the authors is a twin beam, 20 cm output aperture, Nd:glass laser system that has delivered 4TW to a laser fusion target, which is based on the concepts that multiple spatial filtering can prevent beam fill factors.
Abstract: Argus is a twin beam, 20 cm output aperture, Nd:glass laser system that has delivered 4TW to a laser fusion target. This performance is based on the concepts that multiple spatial filtering can prevent beam fill factors. Damage to optics due to self focusing and filamentation does not occur on Argus. The only form of damage is induced by broadband radiation from xenon flashlamps interacting with contaminants on or in the Nd:glass. The severity of damage is measured by the fraction of the beam obscured by the damage sites. This averages 0.1% per surface or 0.75% per arm. The amount of damage does not appear to be strongly related to the number of amplifier firings and generally occurs during the first few firings.
01 Jan 1977
TL;DR: In this article, the self focusing of gaussian electromagnetic beams in short (t«τe), and long (t>τe) time scales has been investigated, where τe is the energy relaxation time of electrons with heavy particles and e 0=e(EE*=0).
Abstract: Using the earlier expressions for nonlinear dielectric constant (e=e 0+Φ(EE*) of isotropic and magnetoactive plasmas, we have investigated the self focusing of gaussian electromagnetic beams in short (t«τe), and long (t>τe) time scales; τe is the energy relaxation time of electrons with heavy particles and e 0=e(EE*=0). The wave equation has been solved in the paraxial ray and WKB approximations by expanding e as e=e a(EE* |r=0)+γr2. Unlike previous analyses1 no restriction has been put on the relative magnitudes of Φ and e 0. Consequently, the treatment is applicable beyond critical electron density. In a magnetoplasma, the self focusing of extraordinary mode has been studied and the treatment is applicable for all values of plasma frequency. Saturating effects of nonlinearity have been discussed in detail. Special attention to self focusing of whistlers and interpretation of Stenzel’s experimental results has been given. We have also studied the filamentation of a plane uniform whistler and the growth rate of filamentation instability is seen to be a saturating function of the intensity of the beam.
TL;DR: In this article, the generation of parametric instabilities in laser heating of plasmas confined in a linear magnetic device has been discussed and the nonlinearities are taken into account by the inclusion of a ponderomotive force.
Abstract: This paper discusses the generation of parametric instabilities in laser heating of plasmas confined in a linear magnetic device. The nonlinearities are taken into account by the inclusion of a ponderomotive force. We derive a generalized dielectric tensor and the corresponding dispersion relation which enables us to determine the instability thresholds and linearized growth rates. The cases where the parametrically excited low frequency wave propagates either parallel or perpendicular to the magnetic field are analyzed in detail. Propagation in an oblique direction is briefly described. As regards the laser heating schemes, the low frequency filamentation and the stimulated Brillouin scattering seem to pose a possible difficulty. Fortunately, several of the potential instabilities cannot be excited in the configuration considered.
TL;DR: In this paper, the nonlinear effects associated with the excitation of a large amplitude whistler wave have been explored, and it is found that the radiation pressure of the wave and thermal effects give rise to a field-aligned density depression in which the wave becomes completely trapped.
Abstract: With the development of a large magnetized plasma source it has become possible to investigate space plasma physics problems in the laboratory. First, the nonlinear effects associated with the excitation of a large amplitude whistler wave have been explored. It is found that the radiation pressure of the wave and thermal effects give rise to a field-aligned density depression in which the wave becomes completely trapped. Hyperfine filaments with diameters small compared with the parallel wavelength are observed. Second, the stability of oblique whistler waves in the presence of an electron beam has been studied. A broadband whistler instability is observed and identified as a Cherenkov interaction between beam electrons and whistlers propagating near the resonance cone. These observations confirm the present model for the generation of VLF hiss in the aurora.