Search Results (44 total)

We report experiments demonstrating enhanced coupling efficiencies of high-contrast laser irradiation to nanofabricated conical targets. Peak temperatures near 200 eV are observed with modest laser energy (10 J), revealing similar hot-electron localization and material heating to reduced mass targets (RMTs), despite having a significantly larger mass. Collisional particle-in-cell simulations attribute the enhancement to self-generated resistive (∼10 MG) magnetic fields forming within the curvature of the cone wall, which confine energetic electrons to heat a reduced volume at the tip. This represents a different electron confinement mechanism (magnetic, as opposed to electrostatic sheath confinement in RMTs) controllable by target shape.

The characteristics of fast electrons laser accelerated from solids and expanding into a vacuum from the rear target surface have been measured via optical probe reflectometry. This allows access to the time- and space-resolved dynamics of the fast electron density and temperature and of the energy partition into bulk (cold) electrons. In particular, it is found that the density of the hot electrons on the target rear surface is bell shaped, and that their mean energy at the same location is radially homogeneous and decreases with the target thickness.

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.

Fast adiabatic plasma heating of a thin solid target irradiated by a high intensity laser has been observed by an optical fast interferometry diagnostic. It is driven by the hot electron current induced by the laser plasma interaction at the front side of the target. Radial and longitudinal temperature profiles are calculated to reproduce the observed rear-side plasma expansion. The main parameters of the suprathermal electrons (number, temperature, and divergence) have been deduced from these observations.

Proton beams laser accelerated from thin foils are studied for various plasma gradients on the foil rear surface. The beam maximum energy and spectral slope reduce with the gradient scale length, in good agreement with numerical simulations. The results also show that the j×B mechanism determines the temperature of the electrons driving the ion expansion. Future ion-driven fast ignition of fusion targets will use multikilojoule petawatt laser pulses, the leading part of which will induce target preheat. Estimates based on the data show that this modifies by less than 10% the ion beam parameters.

We report on strong nonuniformities in target heating with intense, laser-produced proton beams. The observed inhomogeneity in energy deposition can strongly perturb equation of state (EOS) measurements with laser-accelerated ions which are planned in several laboratories. Interferometric measurements of the target expansion show different expansion velocities on the front and rear surfaces, indicating a strong difference in local temperature. The nonuniformity indicates at an additional heating mechanism, which seems to originate from electrons in the keV range.

We demonstrate that in ultraintense ultrafast laser-matter interaction, the interplay of laser-induced oscillating space-charge fields with laser E and B fields can strongly affect whether the interaction is relativistic or not: stronger laser fields may not in fact produce more relativistic plasma interactions. We show that there exists a regime of interaction, in the relation of laser intensity and incident angle, for which the Brunel effect of electron acceleration is strongly suppressed by AC gyromagnetic fields, at a frequency different from the laser field. Analytically and with 1.5D particle-in-cell modeling, we show that from gyromagnetic effects, even in the absence of usual J×B second-harmonic contributions, there are strong effects on the harmonic emission and on the generation of attosecond pulses.

We present a general expression for the maximum ion energy observed in experiments with thin foils irradiated by high-intensity laser pulses. The analytical model is based on a radially confined surface charge set up by laser accelerated electrons on the target rear side. The only input parameters are the properties of the laser pulse and the target thickness. The predicted maximum ion energy and the optimal laser pulse duration are supported by dedicated experiments for a broad range of different ions.

We report on first measurements of the transverse characteristics of laser-produced energetic ion beams in direct comparison to results for laser accelerated proton beams. The experiments show the same low emittance for ion beams as already found for protons. Additionally, we demonstrate that the divergence is influenced by the charge over mass ratio of the accelerated species. From these observations we deduced scaling laws for the divergence of ions as well as the temporal evolution of the ion source size.

We present a new mechanism for high-order harmonic generation by reflection of a laser beam from an overdense plasma, efficient even at moderate laser intensities (down to Iλ²≈4×10¹⁵  W cm^-2 μm²). In this mechanism, a transient phase matching between the electromagnetic field and plasma oscillations within a density gradient leads to the emission of harmonics up to the plasma frequency. These plasma oscillations are periodically excited in the wake of attosecond electron bunches which sweep across the density gradient. This process leads to a train of unevenly spaced chirped attosecond pulses and, hence, to broadened and chirped harmonics. This last effect is confirmed experimentally.

The acceleration of multi-MeV protons from the rear surface of thin solid foils irradiated by an intense (∼10¹⁸  W/cm²) and short (∼1.5  ps) laser pulse has been investigated using transverse proton probing. The structure of the electric field driving the expansion of the proton beam has been resolved with high spatial and temporal resolution. The main features of the experimental observations, namely, an initial intense sheath field and a late time field peaking at the beam front, are consistent with the results from particle-in-cell and fluid simulations of thin plasma expansion into a vacuum.

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.

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 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.

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.

Monomode guiding over 100 Rayleigh lengths (10 cm) of high intensity ultrashort laser pulses ( 10¹⁶ W/cm², 120 fs) has been demonstrated in hollow dielectric capillary tubes (45–70 μm internal diameter) without inner wall damage. Analytical predictions for coupling conditions and damping length are confirmed experimentally for tubes under vacuum. With 5 to 40 mbar of He gas in the tube, when laser ionization occurs the energy and duration of the transmitted pulse decrease while its spectrum is broadened.