Electromagnetic instability in plasmas heated by a laser field.

Electromagnetic instability is investigated in homogeneous plasmas heated by a laser wave in the range α=v_{0}^{2}/v_{t}^{2}≤2, where v_{0} is the electron quiver velocity and v_{t} is the thermal velocity. The anisotropic electron distribution function that drives unstable quasistatic electromagnetic modes is calculated numerically with the Vlasov-Landau equation in the high ion charge number approximation. A dispersion relation of electromagnetic waves which accounts for further nonlinear terms on v_{0}^{2} from previous results is derived. In typical simulation with ion charge number Z=13, a temperature T=5keV, a density n=9.8×10^{20}cm^{-3}, and a laser wavelength λ_{laser}=1.06µm, growth rates larger than 10^{12}s^{-1} in the quasicollisionless wave-number range were found for α≥1. In the same physical conditions and in the mildly collisional range a growth rate about 10^{11}s^{-1} was also obtained. The extent of the growth wave-number region increases significantly with increasing α.

Anisotropic electron-distribution function in inverse-bremsstrahlung-heated plasmas.

The electron-distribution function in homogeneous plasmas heated by a high-frequency laser field is calculated in velocity space from the Vlasov-Landau equation. The kinetic model is valid for moderate laser intensity defined by the relevant parameter α=v_{0}^{2}/v_{t}^{2}<0.5 where v_{0} and v_{t} are the peak velocity of oscillation in the high-frequency electric field and the thermal velocity, respectively. The results obtained constitute an improvement of the results reported in the literature devoted to weak electric field intensities. The electron-distribution function is calculated solving the kinetic equation with the use of the Legendre polynomial expansion within the laser field dipole approximation. It results in an infinite set of equations for the isotropic component f_{0}(v) and the anisotropic components f_{n≥1}(v) that we have solved numerically with appropriate truncation. For the second anisotropy f_{2}(v), we found that its maximum increases from the weak electric field intensity (α<0.01) to a moderate one (α=0.5) by a factor f_{2max}(α=0.5)/f_{2max}(α=0.01)≈48. Applications to the radiation pressure, electromagnetic instabilities, and photoabsorption are also considered.

Nonsymmetric laser-pulse propagation in capillary tubes with variable radius.

A model describing the nonsymmetrical laser-pulse propagation in capillary tubes with inner radius smoothly varying with the capillary length is proposed. Using this model, the use of capillaries with specially profiled entrance sections (particularly cone matching elements) for improving the coupling of the laser energy into a capillary and avoiding capillary ablation is analyzed. It is shown that cone entrances with a sufficiently small angle (α_{cone}≲10 mrad) help decrease the longitudinal energy flux and ablation of the entrance face of a capillary, but do not remove the requirements on the precision of laser-pulse focusing necessary to obtain regular laser fields, characterized by a symmetrical intensity distribution, centered on the capillary axis. To achieve regular laser fields without strong transverse gradients, the angle between the laser and capillary axes has to be smaller than 1 mrad for capillaries with an inner radius of the order of tens of microns.

Fourier-limited seeded soft x-ray laser pulse.

We present what we believe to be the first measurement of the spectral properties of a soft x-ray laser seeded by a high-order harmonic beam. Using an interferometric method, the spectral profile of a seeded Ni-like krypton soft x-ray laser (32.8 nm) generated by optical field ionization has been experimentally determined, and the shortest possible pulse duration has been deduced. The source exhibits a Voigt spectral profile with an FWHM of 3.1+/-0.3 mA, leading to a Fourier-transform pulse duration of 4.7 ps. This value is comparable with the upper limit of the soft x-ray pulse duration determined by experimentally investigating the gain dynamics, from which we conclude that the source has reached the Fourier limit. The measured bandwidth is in good agreement with the predictions of a radiative transfer code, including gain line narrowing and saturation rebroadening.

Aberration-free laser beam in the soft x-ray range.

By seeding an optical-field-ionized population-inverted plasma amplifier with the 25th harmonic of an IR laser, we have achieved what we believe to be the first aberration-free laser beam in the soft x-ray spectral range. This laser emits within a cone of 1.34 mrad(1/e(2)) at a repetition rate of 10 Hz at a central wavelength of 32.8 nm. The beam exhibits a circular profile and wavefront distortions as low as lambda/17. A theoretical analysis of these results shows that this high beam quality is due to spatial filtering of the seed beam by the plasma amplifier aperture.

Laser-driven plasma waves in capillary tubes.

The excitation of plasma waves over a length of up to 8 cm is demonstrated using laser guiding of intense laser pulses through hydrogen-filled glass capillary tubes. The plasma waves are diagnosed by spectral analysis of the transmitted laser radiation. The dependence of the spectral redshift-measured as a function of filling pressure, capillary tube length, and incident laser energy-is in excellent agreement with simulation results. The longitudinal accelerating field inferred from the simulations is in the range of 1-10 GV/m.

Absolute time-resolved x-ray laser gain measurement.

We present the first direct measurement of the time evolution of the gain of a soft x-ray laser amplifier. The measurement is based on the injection of a seed pulse, obtained by high-order harmonic generation, into an x-ray laser medium. Strong amplification occurs when the seed pulse is synchronized with the gain period. By precisely varying the delay between the x-ray laser plasma creation and the seed pulse injection, the actual temporal evolution of the soft x-ray amplifier gain is obtained with a subpicosecond resolution.

Demonstration of a collisionally excited optical-field-ionization XUV laser driven in a plasma waveguide.

We describe the first demonstration of a collisionally excited optical-field-ionization laser driven within a waveguide. Lasing on the 4d(9)5d-4d(9)5p transition at 41.8 nm in Xe8+ was observed to be closely correlated to conditions under which the pump laser pulses were guided well by a gas-filled capillary discharge waveguide. Simulations of the propagation of the pump laser radiation show that gain was achieved over essentially the whole 30 mm length of the waveguide.

Eigenmodes for capillary tubes with dielectric walls and ultraintense laser pulse guiding.

The properties of the eigenmodes of a capillary tube are examined in the context of ultrashort intense laser pulse guiding. The dispersion relation for the eigenmodes of a cylindrical hollow waveguide is derived and the family of eigenmodes EH(nus) is shown to be a solution of the wave equation up to the first order under the condition k(0)a >>1, where k(0) is the light wave number and a the capillary tube radius. The expressions of the fields for the eigenmodes are given at zero and first order of a small parameter equal to the ratio of the perpendicular to longitudinal wave number and the absorbed intensity at the wall is estimated.

Nonlinear propagation of short intense laser pulses in a hollow metallic waveguide.

The propagation of a short intense laser pulse in the femtosecond range in a hollow metallic waveguide gives rise to heating of the metallic wall. The temperature of the degenerate electron gas in the wall is increased during the pulse duration and this heating affects the propagation and dissipation of the laser pulse. Analytical and numerical analysis shows that, as the dissipation is increased, the leading edge of the pulse decreases more slowly than the rear, resulting in a pulse shortening.

[Invasions of Paederus sabaeus (Coleoptera Staphylinidae) in central Africa. 1. Entomological and epidemiological aspects]. / Invasions de Paederus sabaeus (Coléoptère Staphylinidae) en Afrique centrale. 1. Aspects entomologiques et épidémiologiques.

In May 1993, at the end of the rainy season, outbreaks of Paederus sabaeus (Coleoptera, Staphylinidae) were recorded in Brazzaville (Congo), Kinshasa (Zaire), Franceville and Libreville (Gabon) and even in Bangui (CAR) at the North of the equator. A short review of previous outbreaks in Africa and on vesicant substances is given by the authors. These beetles are attracted to neon lights and they rest on the walls or on the skin of the occupants. When the insects are crushed on the bare skin their haemolymph liberate pederine and related vesicant components which provocate dermatitis. The insects disappeared spontaneously after three to four weeks.

Identification of widespread pollution in the southern hemisphere deduced from satellite analyses.

Vertical profiles of ozone obtained from ozonesondes in Brazzaville, Congo (4 degrees S, 15 degrees E), and Ascension Island (8 degrees S, 15 degrees W) show that large quantities of tropospheric ozone are present over southern Africa and the adjacent eastern tropical South Atlantic Ocean. The origin of this pollution is widespread biomass burning in Africa. These measurements support satellite-derived tropospheric ozone data that demonstrate that ozone originating from this region is transported throughout most of the Southern Hemisphere. Seasonally high levels of carbon monoxide and methane observed at middle- and high-latitude stations in Africa, Australia, and Antarctica likely reflect the effects of this distant biomass burning. These data suggest that even the most remote regions on this planet may be significantly more polluted than previously believed.