Search Results (44 total)

We report on a self-organizing, quasistable regime of laser proton acceleration, producing 1 GeV nanocoulomb proton bunches from laser foil interaction at an intensity of 7×10²¹  W/cm². The results are obtained from 2D particle-in-cell simulations, using a circular polarized laser pulse with Gaussian transverse profile, normally incident on a planar, 500 nm thick hydrogen foil. While foil plasma driven in the wings of the driving pulse is dispersed, a stable central clump with 1–2λ diameter is forming on the axis. The stabilization is related to laser light having passed the transparent parts of the foil in the wing region and enfolding the central clump that is still opaque. Varying laser parameters, it is shown that the results are stable within certain margins and can be obtained both for protons and heavier ions such as He²⁺.

We report on an electron accelerator based on few-cycle (8 fs full width at half maximum) laser pulses, with only 40 mJ energy per pulse, which constitutes a previously unexplored parameter range in laser-driven electron acceleration. The produced electron spectra are monoenergetic in the tens-of-MeV range and virtually free of low-energy electrons with thermal spectrum. The electron beam has a typical divergence of 5–10 mrad. The accelerator is routinely operated at 10 Hz and constitutes a promising source for several applications. Scalability of the few-cycle driver in repetition rate and energy implies that the present work also represents a step towards user friendly laser-based accelerators.

We demonstrate the amplification of a femtosecond signal pulse in an underdense plasma by a novel mechanism called superradiant amplification. The pulse is amplified by a counterpropagating few picosecond long pump pulse. In the superradiant regime, the ponderomotive forces exceed the electrostatic forces and arrange the plasma electrons to reflect the pump light into the signal pulse. We found a significant amplification in energy and intensity. The time structure of the amplified signal pulse carries intrinsic features of the superradiant regime. Sub-10-fs pulses of petawatt power appear feasible.

We investigate the influence of the laser prepulse due to amplified spontaneous emission on the acceleration of protons in thin-foil experiments. We show that changing the prepulse duration has a profound effect on the maximum proton energy. We find an optimal value for the target thickness, which strongly depends on the prepulse duration. At this optimal thickness, the rear side acceleration process leads to the highest proton energies, while this mechanism is rendered ineffective for thinner targets due to a prepulse-induced plasma formation at the rear side. In this case, the protons are primarily accelerated by the front side mechanism leading to lower cutoff energies.

The harmonic emission from thin solid carbon and aluminum foils, irradiated by 150 fs long frequency-doubled Ti:sapphire laser pulses at λ=395  nm and peak intensities of a few 10¹⁸   W/cm², has been studied. In addition to the harmonics emitted from the front side in the specular direction, we observe harmonics up to the 10th order, including the fundamental from the rear side in the direction of the incident beam, while the foil is still strongly overdense. The experimental observations are well reproduced by particle-in-cell simulations. They reveal that strong coupling between the laser-irradiated side and the rear side occurs via the nonlocal electron current driven by the laser light.

We propose that efficient acceleration of electrons in vacuum and underdense plasmas by an intense laser pulse can be triggered in the presence of another counterpropagating or intersecting laser pulse. This mechanism works when the laser fields exceed some threshold amplitudes for stochastic motion of electrons, as found in single-electron dynamics. Particle-in-cell simulations confirm that electron heating and acceleration in the case with two counterpropagating laser pulses can be much more efficient than with one laser pulse only. Two different diagnoses show that the increased heating and acceleration are caused mainly by direct laser acceleration rather than by plasma waves. In plasma at moderate densities such as a few percent of the critical density and when the underdense plasma region is large enough, the Raman backscattered and side-scattered waves can grow to a sufficiently high level to serve as the second counterpropagating or intersecting pulse and trigger the electron stochastic motion. As a result, even with a single intense laser pulse only in plasma, electrons can be accelerated to an energy level much higher than the corresponding laser ponderomotive potential.

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.

Scaling laws governing implosions of thin shells in converging flows are established by analyzing the implosion trajectories in the ( A,M) parametric plane, where A is the in-flight aspect ratio, and M is the implosion Mach number. Three asymptotic branches, corresponding to three implosion phases, are identified for each trajectory in the limit of A,M≫1. It is shown that there exists a critical value γ_cr  =  1+2/ν ( ν  =  1,2 for, respectively, cylindrical and spherical flows) of the adiabatic index γ, which separates two qualitatively different patterns of the density buildup in the last phase of implosion. The scaling of the stagnation density ρ_s and pressure P_s with the peak value M₀ of the Mach number is obtained.

We propose a mechanism that leads to efficient acceleration of electrons in plasma by two counterpropagating laser pulses. It is triggered by stochastic motion of electrons when the laser fields exceed some threshold amplitudes, as found in single-electron dynamics. It is further confirmed in particle-in-cell simulations. In vacuum or tenuous plasma, electron acceleration in the case with two colliding laser pulses can be much more efficient than with one laser pulse only. In plasma at moderate densities, such as a few percent of the critical density, the amplitude of the Raman-backscattered wave is high enough to serve as the second counterpropagating pulse to trigger the electron stochastic motion. As a result, even with one intense laser pulse only, electrons can be heated up to a temperature much higher than the corresponding laser ponderomotive potential.

Electron-positron and γ-photon production by high-intensity laser pulses is investigated for a special target geometry, in which two pulses irradiate a very thin foil (10–100 nm < skin depth) with same intensity from opposite sides. A stationary solution is derived describing foil compression between the two pulses. Circular polarization is chosen such that all electrons and positrons rotate in the plane of the foil. We discuss the laser and target parameters required in order to optimize the γ photon and pair production rate. We find a γ-photon intensity of 7×10²⁷/sr s and a positron density of 5×10²²/cm³ when using two 330 fs, 7×10²¹ W/cm² laser pulses.

The stagnation pressure p_s of imploding cylindrical ( n  =  2) and spherical ( n  =  3) shells is found to scale as p_s/p₀∝M₀^{2(n+1)/(γ+1)}, where M₀ is the Mach number of the imploding shell and p₀ its maximum pressure. The result holds approximately for Mach numbers in the range 2<M₀<25 relevant for inertial confinement fusion capsules and is of key importance for their ignition energy scaling. It is derived analytically on the basis of similarity solutions for an ideal gas with adiabatic exponent γ.

We study the angular distributions of fast electrons, ions, and bremsstrahlung x/ γ-rays generated during the interaction of an ultrashort intense laser pulse with solid targets. A relation is found on the angular directions for fast electrons and ions as a function of the particle's kinetic energy, experienced Coulomb potential changes, and the incident angle of the laser pulse. It is valid independent of the acceleration mechanisms and the polarization of the laser pulse, as confirmed by particle-in-cell simulations. The angular distribution of bremsstrahlung x/ γ-rays is presented to show explicitly its correlation with the corresponding angular distributions of electrons.

Filamented transport of laser-generated relativistic electron beams in a plasma is studied with reference to fast ignition of fusion targets. The study is based on transverse two-dimensional particle-in-cell simulation. Coalescence of current filaments and related ion dynamics are found to determine beam stopping and ion heating.

The interaction of ultrashort subpicosecond laser pulses with initially cold and solid matter is investigated in a wide intensity range (10¹¹ to 10¹⁷ W/cm²) by means of the hydrodynamic code MULTI-FS, which is an extension of the long pulse version of MULTI [R. Ramis, R. Schmalz, and J. Meyer-ter-Vehn, Comput. Phys. Commun. 49, 475 (1988)]. Essential modifications for the treatment of ultrashort pulses are the solution of Maxwell’s equations in a steep gradient plasma, consideration of the nonequilibrium between electrons and ions, and a model for the electrical and thermal conductivity covering the wide range from the solid state to the high temperature plasma. The simulations are compared with several absorption measurements performed with aluminum targets at normal and oblique incidence. Good agreement is obtained by an appropriate choice of the electron-ion energy exchange time (characterized by 10 to 20 ps in cold solid Al). In addition we discuss the intensity scaling of the temperature, of the pressure, and of the density, where the laser energy is deposited in the expanding plasma, as well as the propagation of the heat wave and the shock wave into the solid. For laser pulse durations >~150 fs considered in this paper the amount of isochorically heated matter at solid density is determined by the depth of the electron heat wave in the whole intensity range.

We have measured angularly resolved and absolutely calibrated spectra of the multi-MeV electrons produced by relativistic self-channeling in a high-density gas jet. Using 200 fs laser pulses with P_L  =  1.2 TW, we have investigated the electron spectrum dependence on the plasma electron density in the range of 3×10¹⁹–4×10²⁰ cm^-3. The experimentally obtained results are closely reproduced by three-dimensional particle-in-cell simulations. A detailed analysis shows that the self-modulated laser wake field, although active, cannot explain the experimental energy spectrum. The bulk of the fast electrons are produced by direct laser acceleration at the channel betatron resonance.

Filamentary ionization tracks have been observed via optical probing inside Al-coated glass targets after the interaction of a picosecond 20-TW laser pulse at intensities above 10¹⁹  W/cm². The tracks, up to 700 μm in length and between 10 and 20 μm in width, originate from the focal spot region of the laser beam. Simulations performed with 3D particle-in-cell and 2D Fokker-Planck hybrid codes indicate that the observations are consistent with ionization induced in the glass target by magnetized, collimated beams of high-energy electrons produced during the laser interaction.

A mechanism for generating large (>1 GeV/m) accelerating wakes in a plasma is proposed. Two slightly detuned counterpropagating laser beams, an ultrashort timing pulse and a long pump, exchange photons and deposit the recoil momentum in plasma electrons. This produces a localized region of electron current, which acts as a virtual electron beam, inducing intense plasma wakes with phase velocity equal to the group velocity of the short pulse. Modulating the pumping beam generates periodic accelerating structures in the plasma (“plasma linac”) which can be used for particle acceleration unlimited by the dephasing between the particles and the wake. An important difference between this type of plasma accelerator and the conventional wakefield accelerators is that this type can be achieved with laser intensities I≪10¹⁸ W/cm².

An initially short ( <1/ω_p) laser pulse can be superradiantly amplified by a counterpropagating long low-intensity pump while remaining ultrashort. This superradiant amplification occurs if the frequency of the pulse is lower than that of the pump, and the initial pulse intensity is sufficiently high. Numerical simulations indicate that the short pulse can be amplified to an intensity hundreds of times the pump intensity, with the pump depletion as high as 40%. This implies that the long pump is efficiently time compressed without frequency chirping and pulse stretching, making the superradiant amplification an interesting alternative to chirped-pulse amplification.

Short pulse laser ablation of semiconductors and metals is studied by means of ultrafast time-resolved microscopy. The characteristic stages of the conversion of solid material into hot fluid matter undergoing ablation are identified. Initially metallic material transforms during the expansion into a transparent state with a high index of refraction.

We report the observation of neutrons released from d(d,n)³He fusion reactions in the focus of 200 mJ, 160 fs Ti:sapphire laser pulses on a deuterated polyethylene target. Optimizing the fast electron and ion generation by applying a well-defined prepulse led to an average rate of 140 neutrons per shot. Furthermore, the production of a substantial number of MeV γ rays could be observed. The occurrence of neutrons and γ rays is attributed to the formation and explosion of a relativistic plasma channel in the laser focus, which is confirmed by numerical calculations.

Two spatially separated toroidal magnetic fields in the megagauss range have been detected with Faraday rotation during and after propagation of a relativistically intense laser pulse through preionized plasmas. Besides a field in the outer region of the plasma oriented as a conventional thermoelectric field, a field with the opposite orientation closely surrounding the propagation axis is observed, in conditions under which relativistic channeling occurs. A 3D particle-in-cell code was used to simulate the interaction under the conditions of the experiment.

We report the first simultaneous measurement of the reflectivity and optical emission of a strong (4–8 Mbar) shock front emerging at a free surface of a solid. Planar shock waves were driven by thermal x rays from a laser-heated cavity. The inferred model-independent brightness temperature of the shock front in silicon turns out to be significantly below the expected Hugoniot temperature. We find that our data cannot be explained within the two-temperature model which assumes instantaneous metallization of silicon in the density jump.

Laser hole boring and relativistic electron transport into plasma of 10 times critical density is studied by means of 2D particle-in-cell simulation. At intensities of I₀λ²  =  10²⁰ W cm^-2 μm², a channel 12λ deep and 3λ in diameter has formed after 200 laser cycles. The laser driven electron current carries up to 40% of the incident laser power. When penetrating the overdense region, it breaks up into several filaments at early times, but is channeled into a single magnetized jet later on. These features are essential for fast ignition of targets for inertial confinement fusion (ICF).

Optical signals from shock waves emerging at a free surface of metals are expected to yield information about the equation of state and the transport and relaxation properties of hot dense plasmas. We present the results of optical measurements on planar shock waves (velocity ≃22 km/s, pressure ≃8 Mbar) in solid aluminum which were generated by exposing a miniature sample to intense thermal x rays from a laser-heated cavity. The reflectivity of the free surface of the sample for the light from a probe laser (λ=532 nm) and the absolute value of its optical emission were simultaneously registered with a 7-ps temporal resolution. For interpretation we used a two-temperature hydrodynamic code which includes the electron heat conduction and electron-ion relaxation and accounts for the nonequilibrium shock structure. The underlying self-consistent model for the equation of state and the transport coefficients of a metal over the relevant range of thermodynamic parameters are described in some detail. The reflectivity decay signal, which yields direct information on the effective collision frequency in the unloading material, and the emission peak, which is sensitive to the heat conductivity and dielectric permittivity of the hot and dense plasma behind the shock front, are well reproduced by the simulations. The emission signals are, however, longer than predicted, possibly due to the residual surface roughness in the experiment. On a longer time scale of 1–2 ns, the emission signal is well described by a simple radiation transport model with the Kramers-Unsöld opacity.

Relativistic self-channeling of a picosecond laser pulse in a preformed plasma near critical density has been observed both experimentally and in 3D particle-in-cell simulations. Optical probing measurements indicate the formation of a single pulsating propagation channel, typically of about 5 μm in diameter. The computational results reveal the importance in the channel formation of relativistic electrons traveling with the light pulse and of the corresponding self-generated magnetic field.