Search

Abstract: In the fast ignition approach to inertial confinement fusion (ICF), the ignition is expected to be triggered by an ultra-high intensity laser-produced fast electron beam, propagating and releasing its energy at the core of the shock-compressed deuterium–tritium (DT) capsule. Therefore the study of fast electron generation and transport is crucial for fast ignition science. As a consequence of fast electron transport, K-shell ionization of the medium atoms is produced with the subsequent emission of a K-alpha photon. The imaging of K-α sources is therefore very useful to obtain both quantitative and qualitative information on fast electron transport. Spherically bent crystals can provide 2-D spatially resolved and mono-energetic images of the K-α source and are widely used in several fast ignition oriented experiments. [ABSTRACT FROM AUTHOR]

Collimated transport of ultrahigh intensity electron current was observed in cold and in laser-shocked vitreous carbon, in agreement with simulation predictions. The fast electron beams were created by coupling high-intensity and high-contrast laser pulses onto copper-coated cones drilled into the carbon samples. The guiding mechanism--observed only for times before the shock breakout at the inner cone tip--is due to self-generated resistive magnetic fields of ~0.5-1 kT arising from the intense currents of fast electrons in vitreous carbon, by virtue of its specific high resistivity over the range of explored background temperatures. The spatial distribution of the electron beams, injected through the samples at different stages of compression, was characterized by side-on imaging of hard x-ray fluorescence. [ABSTRACT FROM AUTHOR]

Energy loss in the transport of a beam of relativistic electrons in warm dense aluminum is measured in the regime of ultrahigh electron beam current density over 2 × 1011 A/cm2 (time averaged). The samples are heated by shock compression. Comparing to undriven cold solid targets, the roles of the different initial resistivity and of the transient resistivity (upon target heating during electron transport) are directly observable in the experimental data, and are reproduced by a comprehensive set of simulations describing the hydrodynamics of the shock compression and electron beam generation and transport. We measured a 19% increase in electron resistive energy loss in warm dense compared to cold solid samples of identical areal mass. [ABSTRACT FROM AUTHOR]

Simultaneous experimental measurements of copper Kα imaging and the maximum target normal sheath acceleration proton energies from the rear target surface are compared for various target thicknesses. For the T-cubed laser (≈4 J, 400 fs) at an intensity of ≈2 × 1019 W cm-2, the hot electron divergence is determined to be θHW HM≈22° using a Kα imaging diagnostic. The maximum proton energies are measured to follow the expected reduction with increasing target thickness. Numerical modeling produces copper Kα trends for both signal level and electron beam divergence that are in good agreement with the experiment. A geometric model describing the electron beam divergence reproduces the maximum proton energy trends observed from the experiment and the fast electron density and the peak electric field observed in the numerical modeling. [ABSTRACT FROM AUTHOR]

Fast-electron beam stopping mechanisms in media ranging from solid to warm dense matter have been investigated experimentally and numerically. Laser-driven fast electrons have been transported through solid Al targets and shock-compressed Al and plastic foam targets. Their propagation has been diagnosed via rear-side optical self-emission and Kα X-rays from tracer layers. Comparison between measurements and simulations shows that the transition from collision-dominated to resistive field-dominated energy loss occurs for a fast-electron current density ~5 × 1011 A cm−2. The respective increases in the stopping power with target density and resistivity have been detected in each regime. Self-guided propagation over 200μm has been observed in radially compressed targets due to ~1kT magnetic fields generated by resistivity gradients at the converging shock front. [ABSTRACT FROM PUBLISHER]

The effect of target material on fast-electron transport is investigated using a high-intensity (0.7 ps, 1020 W/cm2) laser pulse irradiated on multilayered solid Al targets with embedded transport (Au, Mo, Al) and tracer (Cu) layers, backed with millimeter-thick carbon foils to minimize refluxing. We consistently observed a more collimated electron beam (36% average reduction in fast-electron induced Cu Kα spot size) using a high- or mid-Z (Au or Mo) layer compared to Al. All targets showed a similar electron flux level in the central spot of the beam. Two-dimensional collisional particle-in-cell simulations showed formation of strong self-generated resistive magnetic fields in targets with a high-Z transport layer that suppressed the fast-electron beam divergence; the consequent magnetic channels guided the fast electrons to a smaller spot, in good agreement with experiments. These findings indicate that fast-electron transport can be controlled by self-generated resistive magnetic fields and may have important implications to fast ignition. [ABSTRACT FROM AUTHOR]

The double laser pulse approach to relativistic electron beam (REB) collimation in solid targets has been investigated at the LULI-ELFIE facility. In this scheme two collinear laser pulses are focused onto a solid target with a given intensity ratio and time delay to generate REBs. The magnetic field generated by the first laser-driven REB is used to guide the REB generated by a second delayed laser pulse. We show how electron beam collimation can be controlled by properly adjusting the ratio of focus size and the delay time between the two pulses. We found that the maximum of electron beam collimation is clearly dependent on the laser focal spot size ratio and related to the magnetic field dynamics. Cu-Kα and CTR imaging diagnostics were implemented to evaluate the collimation effects on the respectively low energy (≤100 keV) and high energy (≥MeV) components of the REB. [ABSTRACT FROM AUTHOR]

We studied the transport of an intense electron beam produced by high intensity laser pulses through metals and insulators. Targets were irradiated at two different intensities, 1017 W/cm2 and 1019 W/cm2, at the laser facility Xtreme Light XL-III in Beijing, a Ti:Sa laser source emitting 40 fs pulses at 800 nm. The main diagnostic was Cu-Kα fluorescence imaging. Images of Kα spots have been collected for those two laser intensities, for different target thickness, and for different materials. Experimental results are analyzed taking into account both collisional and collective effects as well as refluxing at the edge of the target. The target temperature is evaluated to be Tc ∼ 6 eV for intensity I = 1017 W/cm2 (for all the tested materials: plastic, aluminium, and copper), and Tc ∼ 60 eV in aluminium and 120 eV in titanium for intensity I = 1019 W/cm2. [ABSTRACT FROM AUTHOR]