Search Results (59 total)

The propagation in a rarefied plasma (n_e≲10¹⁵  cm^-3) of collisionless shock waves and ion-acoustic solitons, excited following the interaction of a long (τ_L∼470  ps) and intense (I∼10¹⁵  W cm^-2) laser pulse with solid targets, has been investigated via proton probing techniques. The shocks’ structures and related electric field distributions were reconstructed with high spatial and temporal resolution. The experimental results were interpreted within the framework of the nonlinear wave description based on the Korteweg–de Vries–Burgers equation.

Correct modeling of the electron-energy transport is essential for inertial confinement fusion target design. Various transport models have been proposed in order to extend the validity of a hydrodynamical description into weakly collisional regimes, taking into account the nonlocality of the electron transport combined with the effects of self-generated magnetic fields. We have carried out new experiments designed to be highly sensitive to the modeling of the heat flow on the Ligne d’Intégration Laser facility, the prototype of the Laser Megajoule. We show that two-dimensional hydrodynamic simulations correctly reproduce the experimental results only if they include both the nonlocal transport and magnetic fields.

The comparative efficiency and beam characteristics of high-energy ions generated by high-intensity short-pulse lasers (∼1–6×10¹⁹  W/cm²) from both the front and rear surfaces of thin metal foils have been measured under identical conditions. Using direct beam measurements and nuclear activation techniques, we find that rear-surface acceleration produces higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. Our observations are well reproduced by realistic particle-in-cell simulations, and we predict optimal criteria for future applications.

We have used point-projection K-shell absorption spectroscopy to infer the ionization and recombination dynamics of transient aluminum plasmas. Two femtosecond beams of the 100 TW laser at the LULI facility were used to produce an aluminum plasma on a thin aluminum foil (83 or 50 nm), and a picosecond x-ray backlighter source. The short-pulse backlighter probed the aluminum plasma at different times by adjusting the delay between the two femtosecond driving beams. Absorption x-ray spectra at early times are characteristic of a dense and rather homogeneous plasma. Collisional-radiative atomic physics coupled with hydrodynamic simulations reproduce fairly well the measured average ionization as a function of time.

The laminarity of high-current multi-MeV proton beams produced by irradiating thin metallic foils with ultraintense lasers has been measured. For proton energies >10  MeV, the transverse and longitudinal emittance are, respectively, <0.004  mm mrad and <10^-4  eV s, i.e., at least 100-fold and may be as much as 10⁴-fold better than conventional accelerator beams. The fast acceleration being electrostatic from an initially cold surface, only collisions with the accelerating fast electrons appear to limit the beam laminarity. The ion beam source size is measured to be <15   μm (FWHM) for proton energies >10  MeV.

We present a modeling study of x-ray line polarization in plasmas driven by high-intensity, ultrashort duration pulsed lasers. Electron kinetics simulations of these transient and nonequilibrium plasmas predict non-Maxwellian and anisotropic electron distribution functions. Under these conditions, the magnetic sublevels within fine structure levels can be unequally populated which leads to the emission of polarized lines. We have developed a time-dependent, collisional-radiative atomic kinetics model of magnetic sublevels to understand the underlying processes and mechanisms leading to the formation of polarized x-ray line emission in plasmas with anisotropic electron distribution functions. The electron distribution function consists of a thermal component extracted from hydrodynamic calculations and a beam component determined by PIC simulations of the laser-plasma interaction. We focus on the polarization properties of the He-like Si satellites of the Ly_α line, discuss the time evolution of polarized satellite spectra, and identify suitable polarization markers that are sensitive to the anisotropy of the electron distribution function and can be used for diagnostic applications.

Improving the temporal contrast of ultrashort and ultraintense laser pulses is a major technical issue for high-field experiments. This can be achieved using a so-called “plasma mirror.” We present a detailed experimental and theoretical study of the plasma mirror that allows us to quantitatively assess the performances of this system. Our experimental results include time-resolved measurements of the plasma mirror reflectivity, and of the phase distortions it induces on the reflected beam. Using an antireflection coated plate as a target, an improvement of the contrast ratio by more than two orders of magnitude can be achieved with a single plasma mirror. We demonstrate that this system is very robust against changes in the pulse fluence and imperfections of the beam spatial profile, which is essential for applications.

The evolution of laser-generated MeV, MA electron beams propagating through conductors and insulators has been studied by comparing measurement and modeling of the distribution of MeV protons that are sheath accelerated by the propagated electrons. We find that electron flow through metals is uniform and can be laser imprinted, whereas propagation through insulators induces spatial disruption of the fast electrons. Agreement is found with material dependent modeling.

Time-resolved K-shell x-ray spectra are recorded from sub-100 nm aluminum foils irradiated by 150-fs laser pulses at relativistic intensities of Iλ²=2×10¹⁸ W μm²/cm². The thermal penetration depth is greater than the foil thickness in these targets so that uniform heating takes place at constant density before hydrodynamic motion occurs. The high-contrast, high-intensity laser pulse, broad spectral band, and short time resolution utilized in this experiment permit a simplified interpretation of the dynamical evolution of the radiating matter. The observed spectrum displays two distinct phases. At early time, <~500 fs after detecting target emission, a broad quasicontinuous spectral feature with strong satellite emission from multiply excited levels is seen. At a later time, the He-like resonance line emission is dominant. The time-integrated data is in accord with previous studies with time resolution greater than 1 ps. The early time satellite emission is shown to be a signature of an initial large area, high density, low-temperature plasma created in the foil by fast electrons accelerated by the intense radiation field in the laser spot. We conclude that, because of this early time phenomenon and contrary to previous predictions, a short, high-intensity laser pulse incident on a thin foil does not create a uniform hot and dense plasma. The heating mechanism has been studied as a function of foil thickness, laser pulse length, and intensity. In addition, the spectra are found to be in broad agreement with a hydrodynamic expansion code postprocessed by a collisional-radiative model based on superconfiguration average rates and on the unresolved transition array formalism.

K-shell x-ray spectroscopy of sub-100 nm Al foils irradiated by high contrast, spatially uniform, 150 fs, Iλ²=2×10¹⁸   W μm²/cm², laser pulses is obtained with 500 fs time resolution. Two distinct phases occur: At ≤500  fs a broad feature comparable to the resonance transitions occurs due to satellites, and at ≥500  fs the resonance transitions dominate. Initial satellites arise from a large area, high density, low temperature (∼100  eV) plasma created by fast electrons. Thus, contrary to predictions, a short, high intensity laser incident on a thin foil does not create a uniform, hot dense plasma.

Collimated jets of carbon and fluorine ions up to 5  MeV/nucleon (∼100   MeV) are observed from the rear surface of thin foils irradiated with laser intensities of up to 5×10¹⁹   W/cm². The normally dominant proton acceleration could be surpressed by removing the hydrocarbon contaminants by resistive heating. This inhibits screening effects and permits effective energy transfer and acceleration of other ion species. The acceleration dynamics and the spatiotemporal distributions of the accelerating E fields at the rear surface of the target are inferred from the detailed spectra.

We present the results of a detailed study on the acceleration of intense ion beams by relativistic laser plasmas. The experiments were performed at the 100 TW laser at the Laboratoire pour L’Utilisation des Lasers Intenses. We investigated the dependence of the ion beams on the target conditions based on theoretical predictions by the target normal sheath acceleration mechanism. A strong dependence of the ion beam parameters on the conditions on the target rear surface was found. We succeeded in shaping the ion beam by the appropriate tailoring of the target geometry and we performed a characterization of the ion beam quality. The production of a heavy ion beam could be achieved by suppressing the amount of protons at the target surfaces. Finally, we demonstrated the use of short pulse laser driven ion beams for radiography of thick samples with high resolution.

We present the results of a benchmark experiment aimed at validating recent calculation techniques for the emission properties of medium and high-Z multicharged ions in hot plasmas. We use space- and time-resolved M-shell x-ray spectroscopy of a laser-produced gas jet xenon plasma as a primary diagnostic of the ionization balance dynamics. We perform measurements of the electron temperature, electron density, and average charge state by recording simultaneous spectra of ion acoustic and electron plasma wave Thomson scattering. A comparison of the experimental x-ray spectra with calculations performed ab initio with a non-local-thermodynamic-equilibrium collisional-radiative model based on the superconfiguration formalism, using the measured plasma parameters, is presented and discussed.

Absorption of L-M and L-N transitions of nickel has been measured using point projection spectroscopy. The x-ray radiation from laser-irradiated gold cavities was used to heat volumetrically nickel foils “tamped with carbon” up to 20 eV. Experimental spectra have been analyzed with calculations based on the spin-orbit split arrays statistical approach and performed for each ionic species Ni⁵⁺ to Ni¹¹⁺. Using a least-squares fit, this method provides an ion distribution broader than at local thermodynamic equilibrium, which is explained by spatial and temporal temperature gradients. A major improvement in the simulation of the absolute value of transmission is obtained with a resolved transition array statistical calculation that reproduces the experimental spectrum with the nominal areal mass density by taking into account the saturation of narrow lines.

We report on single-shot frequency-domain interferometric measurements showing space- and time-resolved ponderomotive electron density profile steepening of a short-scale-length ultraintense laser-produced plasma. The density gradient scale length is varied by applying a time-delayed laser prepulse. The measured absolute position of the critical density surface is found to be in agreement with one-dimensional hydrodynamic simulations for the range of scale lengths studied.

The ion-distribution dynamics of an expanding aluminum plasma produced by a nanosecond laser pulse at moderate intensity ( 10¹³  W cm^-2) is studied by point-projection x-ray absorption spectroscopy with unprecedented, picosecond, time resolution. We show that the ionic populations measured as a function of distance to the target and at different probing times differ markedly from those predicted by widely accepted collisional radiative models coupled to hydrodynamic simulations. We discuss the effects of radiation, conduction, and expansion cooling on the spatiotemporal ionic distribution evolution.

This article gives an overview of recent x-ray diffraction experiments with time resolutions down to 10^-13 s. The scientific motivation behind the development is outlined, using examples from solid state physics and biology. The ultrafast resolution may be provided either by fast detectors or short x-ray pulses, and the limitations of both techniques are discussed on the basis of state of the art experiments. In particular, it is shown that with present designs, high time resolution reduces the structural information attainable with high spatial resolution, thereby limiting feasible experiments on the ultrashort time-scale. The first experiment showing subpicosecond conformation changes was recently achieved with simple solids using an ultrafast laser-produced plasma x-ray source. The principles of this experiment are described in detail.

We report on shadowgraphic measurements showing the first space- and time-resolved snapshots of ultraintense laser pulse-generated fast electrons propagating through a solid target. A remarkable result is the formation of highly collimated jets ( <20-μm) traveling at the velocity of light and extending up to 1 mm. This feature clearly indicates a magnetically assisted regime of electron transport, of critical interest for the fast ignitor scheme. Along with these jets, we detect a slower ( ≈c/2) and broader (up to 1 mm) ionization front consistent with collisional hot electron energy transport.

We have studied the distribution function of the hot electrons produced during the interaction of a 120-fs, 60-mJ, 800-nm wavelength and a p-polarized laser pulse with bilayered Al/Fe targets. The main pulse interacts with a preformed plasma, obtained with a controlled prepulse, whose density gradient scale length has been measured. The electron distribution function is characterized by means of the Kα emission of the two materials of the target as a function of the Al-layer thickness. The low-energy region (<50 keV) of the hot-electron distribution function shows no dependency in shape on the gradient scale length, but only a variation in the total number of the generated electrons. The comparison between the experimental results and the particle-in-cell and Monte Carlo calculations of the electron distribution function and the Kα emission is gratifying.

Ultrashort pulse laser-solid interaction experiments with 4×10¹⁶ W/cm²,120 fs, 45° incidence angle, p-polarized pulses are theoretically analyzed with the help of 11 / 2-dimensional (11 / 2 D) particle-in-cell (PIC) simulations. The laser impinges upon preformed plasmas with a precisely controlled density-gradient scale-length. PIC electron distribution functions are used as an input to 3D Monte Carlo simulations to interpret measured electron distributions and Kα radiation emission. Satisfactory agreement between the experimental and simulation results is obtained for the measured absorption coefficient, the energy distribution of the back-scattered hot electrons, the hot-electron temperature in the bulk of the target, and the Kα yield, when the preplasma scale-length is varied.

The absorption coefficient of an ultrashort, high-intensity S-polarized laser pulse is calculated for plasmas with weakly anisotropic electron energy distribution functions and high gradients of electron density. In the limiting cases of normal and anomalous skin depth effects, the plasma kinetic equation coupled to the Maxwell equations can be solved analytically for different relations between the laser wavelength, the electron density gradient scale length, and the skin depth. For anisotropic electron distribution functions, we obtained an increase of the laser absorption coefficient for transverse to longitudinal electron temperature ratio greater than one.

We present measurements of the samarium (Z=62) M-shell absorption band around 11 Å (1.1 keV) at electron temperatures between 5 and 20 eV. Using point projection absorption spectroscopy, we measured the absolute transmission of an x-ray probe beam through a radiatively heated samarium plasma at different drive temperatures. The measurements show two principal absorption features corresponding to the spin-orbit-split 3d-4f transitions of Sm IV–Sm X ions. The influence of the heating on the spectral opacity shape was characterized by a shift of the absorption features to higher energy for a hotter plasma. The data are compared with calculations done with the superconfiguration code SCO for different plasma temperatures. The temperature and average ionization inferred from the analysis of the experiment are compared to radiative hydrodynamic simulations.

We have studied experimentally the angular and energy distribution of the suprathermal electrons produced during the interaction of a 120 fs, 50 mJ, 800 nm, P-polarized laser pulse on SiO₂ targets. A sharply collimated jet of electrons is observed in the laser specular reflection direction, in the plane of incidence, superimposed to an angularly uniform electron distribution. Electron energies are ≈20 keV for a laser intensity of 4×10¹⁶ W cm^-2 and 45° incidence angle. The electron jet is weaker and angularly broadened with the introduction of a laser prepulse controlling the electron density gradient scale length. Laser absorption and Kα line intensity measurements show a maximum for a prepulse delay of ≈6 ps with an electron energy rising to ≈180 keV. Gradient scale length measurements at this prepulse delay fit the laser absorption peak scaling obtained from standard resonant absorption theory.

A diagnostic is proposed for determining the density of laser-generated plasmas. The scheme exploits surface harmonics generated by short, intense laser pulses reflected from an overdense plasma layer. At sufficiently high intensities (Iλ²>10¹⁸ W cm^-2), the reflected spectrum contains harmonics well above the plasma frequency, corresponding to the maximum plasma density. The transmitted spectrum, on the other hand, exhibits a low-frequency cutoff for ω<ω_p, offering a simple means of deducing the plasma density.