Search Results (13 total)

Recently achieved high intensities of short laser pulses open new prospects in their application to hole boring in inhomogeneous overdense plasmas and for ignition in precompressed DT fusion targets. A simple analytical model and numerical simulations demonstrate that pulses with intensities exceeding 10²²  W/cm² may penetrate deeply into the plasma as a result of efficient ponderomotive acceleration of ions in the forward direction. The penetration depth as big as hundreds of microns depends on the laser fluence, which has to exceed a few tens of GJ/cm². The fast ions, accelerated at the bottom of the channel with an efficiency of more than 20%, show a high directionality and may heat the precompressed target core to fusion conditions.

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 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.

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.

Isochoric heating of matter by intense heavy ion beams promises to become a fruitful approach to warm dense matter studies. For heating times that are long on the hydrodynamic time scale of the target response a tamped target is essential. The proposed dynamic confinement provides homogeneous target heating by a low-Z tamper, which allows one to apply powerful x-ray scattering diagnostics. To demonstrate the potential of the method, heating of a hydrogen sample with the SIS-18 beam at GSI Darmstadt is investigated numerically. The intense x-ray bursts for diagnostics can be provided by the PHELIX laser currently installed at GSI. In the optimized heating regime, density variations can be reduced to a level of 15% from the initial density value.

High-order harmonic generation from a solid target surface has been investigated using femtosecond laser pulses focused to intensities greater than 10¹⁸ W/cm². The experiments show that the harmonics are very intense, with a conversion efficiency that is one or two orders of magnitude larger than that of harmonics generated in gases. Beside the observation of presently the shortest wavelength harmonics from femtosecond-laser solid target interaction, i.e., down to 22 nm, an anomaly has been observed in the harmonic spectrum. In contrast to the expected well-known continuous “roll off” of the high-harmonic orders, the harmonic intensity decreases with the increase of harmonic order, but in between shows minima which are significantly less intense than the neighboring harmonics. Furthermore, the order of the harmonic minima depend on target material. Additional calculations using numerical kinetic particle simulations and a simpler oscillating mirror model show that the physical origin of these modulations is an intricate interplay of resonance absorption and ponderomotive force which leads to a complex electron density profile evolution. Furthermore, this is emphasized by a spectral line analysis of the harmonics. In agreement with the theory, broad lines have been observed and, in particular for the harmonics in the minima, a complex interference structure is present.

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.

Hot electrons generated upon interaction of p-polarized 130 fs laser pulses with copper and penetrating into the target material are characterized with respect to their energy distribution and directionality. “Experimental” data are obtained by comparing the rear-side x-ray emission from layered targets with Monte Carlo electron-photon transport simulations. Theoretical electron energy distributions are derived by means of a one and a half–dimensional particle-in-cell code. Both sets of data consist of a two-temperature distribution of electrons propagating in a direction almost perpendicular to the target surface. The “experimental” data contain a considerably higher population of the lower temperature electrons. The discrepancy is explained by the intensity distribution of the laser spot. The results are used to design an experiment for demonstrating photopumping of cobalt with copper Kα radiation. A 10 μm copper foil is backed with 1 mm of polyethylene (PE) followed by 10 μm of cobalt, the rear-side Kα emission of which is measured. The PE layer prevents fast electrons from reaching the cobalt. Comparing the cobalt Kα emission with that of nickel, which is not photopumped by copper Kα shows enhancement by almost a factor of 2.

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 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.

We report the polarization measurement of a very high order (59th and 61st order, ∼17 nm) harmonic. A Mo-Si multilayer mirror is used as a polarizer with reflectivity of 60% and polarization analyzing power close to unity around a wavelength of 17 nm. We observe that for these high harmonics in Ne there is no rotation of the polarization ellipse with respect to the fundamental laser polarization, even though the harmonics are in the plateau. For linear and slightly elliptical laser polarization the harmonics still remain linearly polarized. These findings are supported by a full theoretical simulation, which includes spatiotemporal integration and phase matching of the emitted harmonic radiation.

The interaction of 2 ps FWHM laser pulses with solid targets at intensities 10¹⁶-10¹⁸ W/cm² is studied for two essentially different cases of laser front steepness resulting in preplasma and preplasma-free interaction. Red and blue frequency shifts of a backscattered fundamental wave and its second harmonic depending on the incident laser pulse shape are observed. They are associated with Doppler shifts corresponding to inward or outward movement of the critical density surface. A model including ponderomotive force was developed to explain the experimental results.