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Author(s): A. Rousse (corresponding author) [1]; C. Rischel [2]; S. Fourmaux [1]; I. Uschmann [3]; S. Sebban [1]; G. Grillon [1]; Ph. Balcou [1]; E. Förster [3]; J.P. Geindre [4]; [...]

We report on the cross-calibration of Thomson Parabola (TP) and Time-of-Flight (TOF) detectors as particle diagnostics, implemented on the most recent setup of the ALLS 100 TW laser-driven ion acceleration beamline. The Microchannel Plate (MCP) used for particle detection in the TP spectrometer has been calibrated in intensity on the tandem linear accelerator at the Université de Montréal. The experimental data points of the scaling factor were obtained by performing a pixel cluster analysis of single proton impacts on the MCP. A semi-empirical model was extrapolated and fitted to the data to apply the calibration also to higher kinetic energies and to extend it to other ion species. Two TOF lines using diamond detectors, placed at +6° and −9° with respect to the target-normal axis, were benchmarked against the TP spectrometer measurements to determine the field integrals related to its electric and magnetic dispersions. The mean integral proton numbers obtained on the beamline were about 4.1 × 1011 protons/sr with a standard deviation of 15% in the central section of the spectrum around 3 MeV, hence witnessing the high repeatability of the proton bunch generation. The mean maximum energy was of 7.3 ± 0.5 MeV, well in agreement with similar other 100 TW-scale laser facilities, with the best shots reaching 9 MeV and nearly 1012 protons/sr. The used particle diagnostics are compatible with the development of a high-repetition rate targetry due to their fast online readout and are therefore a crucial step in the automation of any beamline. [ABSTRACT FROM AUTHOR]

By extending the concept of thin film compression [Mourou et al., Eur. Phys. J. Spec. Top. 223(6), 1181 (2014)] to a thin plate, nonlinear post-compression from 24 fs to 13 fs of sub-petawatt laser pulses is demonstrated experimentally using a 1 mm-thick silica plate and chirped mirrors with a total anomalous dispersion of −50 fs2. The measurements were implemented with a specially designed dispersionless vacuum frequency-resolved optical gating, which is based on second harmonic generation of tested pulses in a 10 μm β-barium borate crystal glued on a 1 mm fused silica substrate. The used compression scheme is implemented in a geometry compatible with high power on-target experiment realization. [ABSTRACT FROM AUTHOR]

We show through experiments that a transition from laser wakefield acceleration (LWFA) regime to a plasma wakefield acceleration (PWFA) regime can drive electrons up to energies close to the GeV level. Initially, the acceleration mechanism is dominated by the bubble created by the laser in the nonlinear regime of LWFA, leading to an injection of a large number of electrons. After propagation beyond the depletion length, leading to a depletion of the laser pulse, whose transverse ponderomotive force is not able to sustain the bubble anymore, the high energy dense bunch of electrons propagating inside bubble will drive its own wakefield by a PWFA regime. This wakefield will be able to trap and accelerate a population of electrons up to the GeV level during this second stage. Three dimensional particle-in-cell simulations support this analysis and confirm the scenario. [ABSTRACT FROM AUTHOR]

In this paper, we report on bi-dimensional Particle-In-Cell simulations performed in order to reproduce the laser-driven proton acceleration obtained when a commercial 200 TW Ti:Sa Laser hits a solid target. The laser-to prepulse contrast was enhanced using plasma mirrors yielding to a main-to-prepulse contrast of ~1012. We varied the pulse duration from 30 fs to 500 fs and the target thickness from 30 nm to several tens of μm. The on-target laser energy was up to 1.8J leading to an intensity in excess of 1020W cm-2. A comparison between numerical and existing experimental data [S. Fourmaux et al., Phys. Plasmas 20, 013110 (2013)] is performed, showing a good agreement between experimental results and simulations which confirms that for ultra-thin targets there is an optimum expansion regime. This regime depends on the target thickness and on the laser intensity: if the target is too expanded, the laser travels through the target without being able to deposit its energy within the target. If the target is not sufficiently expanded, the laser energy is reflected by the target. It is important to note that maximum proton energies are reached at longer pulse durations (in the 100 fs regime) than what is currently the best compression pulse length for this type of lasers (typically 20-30 fs). This duration, around 50-100 fs, can be considered a minimum energy transfer time between hot electrons to ions during the considered acceleration process. [ABSTRACT FROM AUTHOR]

We have developed a laser wakefield generated X-ray probe to directly measure the temporal evolution of the ionization states in warm dense aluminum by means of absorption spectroscopy. As a promising alternative to the free electron excited X-ray sources, Betatron X-ray radiation, with femtosecond pulse duration, provides a new technique to diagnose femtosecond to picosecond transitions in the atomic structure. The X-ray probe system consists of an adjustable Kirkpatrick-Baez (KB) microscope for focusing the Betatron emission to a small probe spot on the sample being measured, and a flat Potassium Acid Phthalate Bragg crystal spectrometer to measure the transmitted X-ray spectrum in the region of the aluminum K-edge absorption lines. An X-ray focal spot size of around 50 μm was achieved after reflection from the platinum-coated 10-cm-long KB microscope mirrors. Shot to shot positioning stability of the Betatron radiation was measured resulting in an rms shot to shot variation in spatial pointing on the sample of 16 μm. The entire probe setup had a spectral resolution of ∼1.5 eV, a detection bandwidth of ∼24 eV, and an overall photon throughput efficiency of the order of 10-5. Approximately 10 photons were detected by the X-ray CCD per laser shot within the spectrally resolved detection band. Thus, it is expected that hundreds of shots will be required per absorption spectrum to clearly observe the K-shell absorption features expected from the ionization states of the warm dense aluminum. [ABSTRACT FROM AUTHOR]

Using a laser plasma accelerator, experiments with a 80 TW and 30 fs laser pulse demonstrated quasi-monoenergetic electron spectra with maximum energy over 0.4 GeV. This is achieved using a supersonic He gas jet and a sharp density ramp generated by a high intensity laser crossing pre-pulse focused 3 ns before the main laser pulse. By adjusting this crossing pre-pulse position inside the gas jet, among the laser shots with electron injection, more than 40% can produce quasi-monoenergetic spectra. This could become a relatively straight forward technique to control laser wakefield electron beams parameters. [ABSTRACT FROM AUTHOR]

We report K α x-ray production with a high energy (110 mJ per pulse at 800 nm before compression/15 mJ at 400 nm after compression), high repetition rate (100 Hz), and high pulse contrast (better than 10−9 at 400 nm) laser system. To develop laser-based x-ray sources for biomedical imaging requires to use high-energy and high-power ultrafast laser system where compression is achieved under vacuum. Using this type of laser system, we demonstrate long-term stability of the x-ray yield, conversion efficiency higher than 1.5×10−5 with a Mo target, and the x-ray spot size close to the optical focal spot. This high-repetition K α x-ray source can be very useful for x-ray phase-contrast imaging. [ABSTRACT FROM AUTHOR]

Recent progress in high intensity ultrafast laser systems provides the opportunity to produce laser plasma x-ray sources exhibiting broad spectrum and high average x-ray flux that are well adapted to x-ray absorption measurements. In this paper, the development of a laser based x-ray absorption near edge structure (XANES) beamline exhibiting high repetition rate by using the Advanced Laser Light Source (ALLS) facility 100 Hz laser system (100 mJ, 35 fs at 800 nm) is presented. This system is based on a broadband tantalum solid target soft x-ray source and a grazing incidence grating spectrometer in the 1–5 nm wavelength range. To demonstrate the high potential of this laser based XANES technique in condensed matter physics, material science, or biology, measurements realized with several samples are presented: VO2 vanadium L edge, Si3N4 nitrogen K edge, and BPDA/PPD polyimide carbon K edge. The characteristics of this laser based beamline are discussed in terms of brightness, signal to noise ratio, and compared to conventional synchrotron broadband x-ray sources which allow achieving similar measurements. Apart from the very compact size and the relative low cost, the main advantages of such a laser based soft x-ray source are the picosecond pulse duration and the perfect synchronization between this x-ray probe and a laser pulse excitation which open the way to the realization of time resolved x-ray absorption measurements with picosecond range time resolution to study the dynamics of ultrafast processes and phase transition. [ABSTRACT FROM AUTHOR]

The 2D projection phase-contrast imaging performance of the ultrafast laser-based x-ray (ULX) source has been investigated. The potential of such a novel x-ray source has been assessed by imaging a reference object (Contrast Detail Evaluation phantom) in the in-line holography geometry and by applying a simple 1D numerical model to the data analysis. The results indicate that the ULX is a promising technique for 2D projection phase-contrast imaging and for implementation of phase-contrast micro-Computed Tomography (μ-CT). This is because by using high contrast laser pulse ULX simultaneously provides a very small x-ray source size along with a high average x-ray flux. In addition, due to the ultrashort x-ray burst duration, ULX might allow practical implementation of ultrafast phase-contrast stroboscopy and time-of-flight based electronic scatter rejection. This technique is also of interest for time resolved radiography to follow shock waves and radiative fronts propagating in an opaque matter. [ABSTRACT FROM AUTHOR]

We demonstrate the feasibility of phase-contrast imaging with an ultrafast laser-based hard x-ray source. Hard x rays are generated during the interaction of a high-intensity femtosecond laser pulse (10 TW, 60 fs, 10 Hz) focused onto solid target in a very small spot (3 μm diam). Such a novel x-ray source has a number of advantages over other sources previously used for phase-contrast imaging: It is very compact and much cheaper than a synchrotron, it has higher power and better x-ray spectrum control than a microfocal x-ray tube, and it has much higher repetition rate than an x-pinch source. The Kα line at 17 keV produced using a solid Mo target, and the in-line imaging geometry have been utilized in this study. Phase-contrast images of test objects and biological samples have been realized. The characteristics of the images are the significant enhancement of interfaces due to an x-ray phase shift that reveal details that were hardly observable, or even undetectable, in absorption images and suppression of optically dense structures well defined in the absorption images. Our study indicates that the absorption and the phase-contrast images obtained with an ultrafast laser-based x-ray source provide complementary information about the imaged objects, thus enriching our arsenal of research tools for laboratory or clinic-based biomedical imaging. [ABSTRACT FROM AUTHOR]

Interaction of intense Ti:sapphire laser with solid targets has been studied experimentally by measuring hard x-ray and hot electron generation. Hard x-ray (8–100 keV) emission spectrum and Kα x-ray conversion efficiency (ηK) from plasma have been studied as a function of laser intensity (1017–1019 W/cm2), pulse duration (70–400)fs, and laser pulse fluence. For intensity I>1×1017 W/cm2, the Ag ηK increases to reach a maximum value of 2×10-5 at an intensity I=4×1018 W/cm2. Hot electron temperature (KTh) and ηK scaling laws have been studied as a function of the laser parameters. A stronger dependence of KTh and ηK as a function of the laser fluence than on pulse duration or laser intensity has been observed. The contribution of another nonlinear mechanism, besides resonance absorption, to hard x-ray enhancement has been demonstrated via hot electron angular distribution and particle-in-cell simulations. [ABSTRACT FROM AUTHOR]

Quasimonoenergetic electron beams with maximum energy >0.5 GeV and 2 mrad divergence have been generated in pure nitrogen gas via wakefield acceleration with 80 TW, 30 fs laser pulses. Long low energy tail features were typically observed due to continuous ionization injection. The measured peak electron energy decreased with the plasma density, agreeing with the predicted scaling for electrons. The experiments showed a threshold electron density of 3×1018cm-3 for self-trapping. Our experiments suggest that pure Nitrogen is a potential candidate gas to achieve GeV monoenergetic electrons using the ionization induced injection scheme for laser wakefield acceleration. [ABSTRACT FROM AUTHOR]

Terahertz time-domain spectroscopy is used to measure the complex terahertz conductivity of a nanogranular vanadium dioxide (VO2) thin film as a function of temperature through the metal-insulator transition. The Drude-Smith model provides a good fit to the observed terahertz conductivity, revealing a metallic state that forms via switching of individual nanograins and strong carrier confinement within the nanograins due to scattering off grain boundaries. Furthermore, the directly applied Drude-Smith model provides a more accurate description of the measured terahertz conductivity in this material than either Bruggeman or Maxwell-Garnett effective medium theories. [ABSTRACT FROM AUTHOR]

We study the photoinduced insulator-metal transition in VO2, correlating its threshold and dynamics with excitation wavelength. In single crystals, switching can only be induced with photon energies above the 670 meV gap. This contrasts with the case of polycrystalline films, where formation of the metallic state can be initiated also with photon energies as low as 180 meV, which are well below the bandgap. Perfection of this process may become conducive to schemes for optical switches, limiters, and detectors operating at room temperature in the mid-infrared. [ABSTRACT FROM AUTHOR]

Single-phase vanadium dioxide (VO2) thin films have been grown on Si3N4/Si substrates by means of a well-controlled magnetron sputtering process. The deposited VO2 films were found to exhibit a semiconductor-to-metal transition (SMT) at ∼69 °C with a resistivity change as high as 3.2 decades. A direct and clear-cut correlation is established between the SMT characteristics (both amplitude and abruptness of the transition) of the VO2 films and their crystallite size. [ABSTRACT FROM AUTHOR]