Signatures of resonantly driven laser-wakefield excitation by a pulse train generated by an optical delay mask.

Electron plasma waves can be efficiently excited by a resonant train of ultrashort pulses, spatially separated by a plasma wavelength. Generating a pulse train from a single amplified ultrashort pulse may be challenging when dealing with large beams. Here we discuss a pulse splitting technique using a simple delay mask that can be adapted to large diameter petawatt beams. We show via detailed numerical simulations that unique signatures of electrons accelerated by a resonantly excited wakefield can be obtained from realistic focused double-pulse trains obtained from a single-region delay mask.

Bremsstrahlung cannon design for shock ignition relevant regime.

We report on the optimization of a BremsStrahlung Cannon (BSC) design for the investigation of laser-driven fast electron populations in a shock ignition relevant experimental campaign at the Laser Megajoule-PETawatt Aquitaine Laser facility. In this regime with laser intensities of 1015 W/cm2-1016 W/cm2, fast electrons with energies ≤100 keV are expected to be generated through Stimulated Raman Scattering (SRS) and Two Plasmon Decay (TPD) instabilities. The main purpose of the BSC in our experiment is to identify the contribution to x-ray emission from bremsstrahlung of fast electrons originating from SRS and TPD, with expected temperatures of 40 keV and 95 keV, respectively. Data analysis and reconstruction of the distributions of x-ray photons incident on the BSC are described.

Optical and spectroscopic study of a supersonic flowing helium plasma: energy transport in the afterglow.

Flowing plasma jets are increasingly investigated and used for surface treatments, including biological matter, and as soft ionization sources for mass spectrometry. They have the characteristic capability to transport energy from the plasma excitation region to the flowing afterglow, and therefore to a distant application surface, in a controlled manner. The ability to transport and deposit energy into a specimen is related to the actual energy transport mechanism. In case of a flowing helium plasma, the energy in the flowing afterglow may be carried by metastable helium atoms and long-lived helium dimer ions. In this work a systematic investigation of the optical and spectroscopic characteristics of a supersonic flowing helium plasma in vacuum and its afterglow as function of the helium gas density is presented. The experimental data are compared with numerical modeling of the plasma excitation and helium dimer ion formation supported by a Computational Fluid Dynamic simulation of the helium jet. The results indicate that the plasma afterglow is effectively due to helium dimer ions recombination via a three-body reaction.

Micron-scale mapping of megagauss magnetic fields using optical polarimetry to probe hot electron transport in petawatt-class laser-solid interactions.

The transport of hot, relativistic electrons produced by the interaction of an intense petawatt laser pulse with a solid has garnered interest due to its potential application in the development of innovative x-ray sources and ion-acceleration schemes. We report on spatially and temporally resolved measurements of megagauss magnetic fields at the rear of a 50-µm thick plastic target, irradiated by a multi-picosecond petawatt laser pulse at an incident intensity of ~1020 W/cm2. The pump-probe polarimetric measurements with micron-scale spatial resolution reveal the dynamics of the magnetic fields generated by the hot electron distribution at the target rear. An annular magnetic field profile was observed ~5 ps after the interaction, indicating a relatively smooth hot electron distribution at the rear-side of the plastic target. This is contrary to previous time-integrated measurements, which infer that such targets will produce highly structured hot electron transport. We measured large-scale filamentation of the hot electron distribution at the target rear only at later time-scales of ~10 ps, resulting in a commensurate large-scale filamentation of the magnetic field profile. Three-dimensional hybrid simulations corroborate our experimental observations and demonstrate a beam-like hot electron transport at initial time-scales that may be attributed to the local resistivity profile at the target rear.

Investigation on target normal sheath acceleration through measurements of ions energy distribution.

An experimental campaign aiming at investigating the ion acceleration mechanisms through laser-matter interaction in femtosecond domain has been carried out at the Intense Laser Irradiation Laboratory facility with a laser intensity of up to 2 × 10(19) W/cm(2). A Thomson parabola spectrometer was used to obtain the spectra of the ions of the different species accelerated. Here, we show the energy spectra of light-ions and we discuss their dependence on structural characteristics of the target and the role of surface and target bulk in the acceleration process.

Laboratory measurements of resistivity in warm dense plasmas relevant to the microphysics of brown dwarfs.

Since the observation of the first brown dwarf in 1995, numerous studies have led to a better understanding of the structures of these objects. Here we present a method for studying material resistivity in warm dense plasmas in the laboratory, which we relate to the microphysics of brown dwarfs through viscosity and electron collisions. Here we use X-ray polarimetry to determine the resistivity of a sulphur-doped plastic target heated to Brown Dwarf conditions by an ultra-intense laser. The resistivity is determined by matching the plasma physics model to the atomic physics calculations of the measured large, positive, polarization. The inferred resistivity is larger than predicted using standard resistivity models, suggesting that these commonly used models will not adequately describe the resistivity of warm dense plasma related to the viscosity of brown dwarfs.

3-D numerical simulation of Yb:YAG active slabs with longitudinal doping gradient for thermal load effects assessment.

We present a study of Yb:YAG active media slabs, based on a ceramic layered structure with different doping levels. We developed a procedure allowing 3D numerical analysis of the slab optical properties as a consequence of the thermal load induced by the pump process. The simulations are compared with a set of experimental results in order to validate the procedure. These structured ceramics appear promising in appropriate geometrical configurations, and thus are intended to be applied in the construction of High Energy Diode Pumped Solid State Laser (DPSSL) systems working in high repetition-rate pulsed regimes.

Evidence of resonant surface-wave excitation in the relativistic regime through measurements of proton acceleration from grating targets.

The interaction of laser pulses with thin grating targets, having a periodic groove at the irradiated surface, is experimentally investigated. Ultrahigh contrast (~10(12)) pulses allow us to demonstrate an enhanced laser-target coupling for the first time in the relativistic regime of ultrahigh intensity >10(19) W/cm(2). A maximum increase by a factor of 2.5 of the cutoff energy of protons produced by target normal sheath acceleration is observed with respect to plane targets, around the incidence angle expected for the resonant excitation of surface waves. A significant enhancement is also observed for small angles of incidence, out of resonance.

Spatially resolved analysis of Kα x-ray emission from plasmas induced by a femtosecond weakly relativistic laser pulse at various polarizations.

Spatially resolved K-shell spectroscopy is used here to investigate the interaction of an ultrashort laser pulse (λ=800 nm, τ=40 fs) with a Ti foil under intense irradiation (Iλ(2)=2×10(18)Wµm(2)cm(-2)) and the following fast electron generation and transport into the target. The effect of laser pulse polarization (p, s, and circular) on the Kα yield and line shape is probed. The radial structure of intensity and width of the lines, obtained by a discretized Abel deconvolution algorithm, suggests an annular distribution of both the hot electron propagation into the target and the target temperature. An accurate modeling of Kα line shapes was performed, revealing temperature gradients, going from a few eV up to 15-20 eV, depending on the pulse polarization. Results are discussed in terms of mechanisms of hot electron generation and of their transport through the preplasma in front of the target.

A novel technique for single-shot energy-resolved 2D x-ray imaging of plasmas relevant for the inertial confinement fusion.

A novel x-ray diagnostic of laser-fusion plasmas is described, allowing 2D monochromatic images of hot, dense plasmas to be obtained in any x-ray photon energy range, over a large domain, on a single-shot basis. The device (named energy-encoded pinhole camera) is based upon the use of an array of many pinholes coupled to a large area CCD camera operating in the single-photon mode. The available x-ray spectral domain is only limited by the quantum efficiency of scientific-grade x-ray CCD cameras, thus extending from a few keV up to a few tens of keV. Spectral 2D images of the emitting plasma can be obtained at any x-ray photon energy provided that a sufficient number of photons had been collected at the desired energy. Results from recent inertial confinement fusion related experiments will be reported in order to detail the new diagnostic.

Magnetically guided fast electrons in cylindrically compressed matter.

Fast electrons produced by a 10 ps, 160 J laser pulse through laser-compressed plastic cylinders are studied experimentally and numerically in the context of fast ignition. K(α)-emission images reveal a collimated or scattered electron beam depending on the initial density and the compression timing. A numerical transport model shows that implosion-driven electrical resistivity gradients induce strong magnetic fields able to guide the electrons. The good agreement with measured beam sizes provides the first experimental evidence for fast-electron magnetic collimation in laser-compressed matter.

Directional bremsstrahlung from a Ti laser-produced x-ray source at relativistic intensities in the 3-12 keV range.

Front and rear side x-ray emission from thin titanium foils irradiated by ultraintense laser pulses at intensities up to ≈5 × 10(19) W/cm2 was measured using a high-resolution imaging system. Significant differences in intensity, dimension, and spectrum between front and rear side emission intensity in the 3-12 keV photon energy range was found even for 5 µm thin Ti foils. Simulations and analysis of space-resolved spectra explain this behavior in terms of directional bremsstrahlung emission from fast electrons generated during the interaction process.

Tracking propagation of ultrashort intense laser pulses in gases via probing of ionization.

We use optical interferometry to study the propagation of femtosecond laser pulses in gases. We show the measurements of propagation in a nitrogen gas jet and we compare the results with propagation in He under the same irradiation conditions. We find that in the case of nitrogen, the detailed temporal structure of the laser pulse can be tracked and visualized by measuring the phase and the resulting electron-density map. A dramatically different behavior occurs in He gas jets, where no details of the temporal structure of the laser pulse are visible. These observations are explained in terms of the ionization dynamics of nitrogen compared to helium. These circumstances make N2 gas sensitive to variations in the electric field and, therefore, allow the laser-pulse temporal and spatial structures to be visualized in detail.

Intense gamma-ray source in the giant-dipole-resonance range driven by 10-TW laser pulses.

A gamma-ray source with an intense component around the giant dipole resonance for photonuclear absorption has been obtained via bremsstrahlung of electron bunches driven by a 10-TW tabletop laser. 3D particle-in-cell simulation proves the achievement of a nonlinear regime leading to efficient acceleration of several sequential electron bunches per each laser pulse. The rate of the gamma-ray yield in the giant dipole resonance region (8

Experimental characterization of picosecond laser interaction with solid targets.

We have characterized the plasma produced by a picosecond laser pulse using x-ray spectroscopy. High-resolution high-sensitivity spectra of K -shell emission from a Ti plasma have been obtained, showing a strong contribution from multiply ionized ions. Hydrodynamic and collisional-radiative codes are used to extract the plasma temperature and density from these measurements. We show that our measurements can provide benchmarks for particle-in-cell (PIC) simulations of preplasma conditions in ultraintense laser-matter interactions.

Novel x-ray multispectral imaging of ultraintense laser plasmas by a single-photon charge coupled device based pinhole camera.

Spectrally resolved two-dimensional imaging of ultrashort laser-produced plasmas is described, obtained by means of an advanced technique. The technique has been tested with microplasmas produced by ultrashort relativistic laser pulses. The technique is based on the use of a pinhole camera equipped with a charge coupled device detector operating in the single-photon regime. The spectral resolution is about 150 eV in the 4-10 keV range, and images in any selected photon energy range have a spatial resolution of 5 microm. The potential of the technique to study fast electron propagation in ultraintense laser interaction with multilayer targets is discussed and some preliminary results are shown.

Femtosecond interferometry of propagation of a laminar ionization front in a gas.

We use optical interferometry to investigate ultrafast ionization induced by an intense, ultrashort laser pulse propagating in a helium gas. Besides standard phase shift information, our interferograms show a localized region of fringe visibility depletion (FVD) that moves along the laser propagation axis at luminal velocity. We find that such a loss of visibility can be quantitatively explained by the ultrafast change of refractive index due to the field ionization of the gas in the laser pulse width. We demonstrate that by combining the post facto phase shift distribution with the probe pulse transit effect in the ionizing region, the analysis of the observed FVD yields significant information on the ultrafast dynamics of propagation of the ionization front in the gas.

An experimental model to study intrusive forces in rats.

We have developed an experimental model for the study of the response of the periodontal ligament and bone to intrusive orthodontics forces in rats. Thirty-five Wistar rats, body weight 250 gr., were grouped as follows: control (GI), 48 hs (GII), 96 hs (GIII), 7 days (GIV). A steel band was cemented to the first upper right molar with a welded spring running to the occlusal surface of the second molar to exert a vertical force of 15 gr. The left side of treated animals was also used as control. The animals were sacrificed at the corresponding treatment times. Radiographs of the left and right halves of the jawbones were taken with an ultrafine grain industrial film. Seven measurements of the width of the periodontal ligament were taken. The data reveal narrowing of the periodontal ligament at the peri-apex and the furcation area for the animals in Group II. Conversely, Group III and IV animals exhibited widening of the periodontal ligament, particularly in the animals submitted to the force for 7 days. Statistical analysis of the data by Student's paired t test showed that the difference for the latter group was statistically significant (p < 0.05). The data for the horizontal measurements do not afford conclusive results. The present results confirm the value of the model to study the response to vertical intrusive forces.

An experimental model to study intrusive forces in rats

We have developed an experimental model for the study of the response of the periodontal ligament and bone to intrusive orthodontics forces in rats. Thirty-five Wistar rats, body weight 250 gr., were grouped as follows: control (GI), 48 hs (GII), 96 hs (GIII), 7 days (GIV). A steel band was cemented to the first upper right molar with a welded spring running to the occlusal surface of the second molar to exert a vertical force of 15 gr. The left side of treated animals was also used as control. The animals were sacrificed at the corresponding treatment times. Radiographs of the left and right halves of the jawbones were taken with an ultrafine grain industrial film. Seven measurements of the width of the periodontal ligament were taken. The data reveal narrowing of the periodontal ligament at the peri-apex and the furcation area for the animals in Group II. Conversely, Group III and IV animals exhibited widening of the periodontal ligament, particularly in the animals submitted to the force for 7 days. Statistical analysis of the data by Student’s paired t test showed that the difference for the latter group was statistically significant (p < 0.05). The data for the horizontal measurements do not afford conclusive results. The present results confirm the value of the model to study the response to vertical intrusive forces.

An experimental model to study intrusive forces in rats.

We have developed an experimental model for the study of the response of the periodontal ligament and bone to intrusive orthodontics forces in rats. Thirty-five Wistar rats, body weight 250 gr., were grouped as follows: control (GI), 48 hs (GII), 96 hs (GIII), 7 days (GIV). A steel band was cemented to the first upper right molar with a welded spring running to the occlusal surface of the second molar to exert a vertical force of 15 gr. The left side of treated animals was also used as control. The animals were sacrificed at the corresponding treatment times. Radiographs of the left and right halves of the jawbones were taken with an ultrafine grain industrial film. Seven measurements of the width of the periodontal ligament were taken. The data reveal narrowing of the periodontal ligament at the peri-apex and the furcation area for the animals in Group II. Conversely, Group III and IV animals exhibited widening of the periodontal ligament, particularly in the animals submitted to the force for 7 days. Statistical analysis of the data by Students paired t test showed that the difference for the latter group was statistically significant (p < 0.05). The data for the horizontal measurements do not afford conclusive results. The present results confirm the value of the model to study the response to vertical intrusive forces.