L. Volpe

Fast-electron transport in cylindrically laser-compressed matter

Experimental and theoretical results of relativistic electron transport in cylindrically compressed matter are presented. This experiment, which is a part of the HiPER roadmap, was achieved on the VULCAN laser facility (UK) using four long pulses beams (~4 × 50 J, 1 ns, at 0.53 µm) to compress a hollow plastic cylinder filled with plastic foam of three different densities (0.1, 0.3 and 1 g cm−3). 2D simulations predict a density of 2–5 g cm−3 and a plasma temperature up to 100 eV at maximum compression. A short pulse (10 ps, 160 J) beam generated fast electrons that propagate through the compressed matter by irradiating a nickel foil at an intensity of 5 × 1018 W cm−2. X-ray spectrometer and imagers were implemented in order to estimate the compressed plasma conditions and to infer the hot electron characteristics. Results are discussed and compared with simulations.

Collisional and collective effects in two dimensional model for fast-electron transport in refluxing regime

The relativistic laser-driven electron transport in partially or fully ionized matter has been investigated in many recent experiments. The high laser intensity achievable today (up to 1020 W/cm2) allows to generate electron current density above 1011 A/cm2. In this regime, electromagnetic effects start to be dominant over collisional ones. In this context, we have developed a simple 2D model for the fast electron transport accounting for (1) electric effects on the electron penetration range and (2) the electron refluxing in thin foils. We compare our model with those existing in literature and with some recent experimental results on fast electron transport in matter. The model predicts a maximum value for the electron penetration range in the region where the collisional and the resistive effects are comparable.

Proton radiography of cylindrical laser-driven implosions

A recent experiment was performed at the Rutherford Appleton Laboratory (UK) to study fast electron propagation in cylindrically compressed targets, a subject of interest for fast ignition. In this experiment, protons accelerated by a picosecond laser pulse have been used to radiograph a 220 µm diameter cylinder (10 µm wall filled with 0.1 g/cc foam), imploded with _ 200 J of green laser light in 4 symmetrically incident beams of wavelength and pulse length 1 ns. Point projection proton backlighting was used to measure the compression degree as well as the stagnation time. Results were also compared to those from a hard X-ray radiography diagnostics. Finally, Monte Carlo simulations of proton propagation in the cold and in the compressed targets allowed a detailed comparison with 2D numerical hydro simulations.

Unraveling resistive versus collisional contributions to relativistic electron beam stopping power in cold-solid and in warm-dense plasmas

We present results on laser-driven relativistic electron beam propagation through aluminum samples, which are either solid and cold or compressed and heated by laser-induced shock. A full numerical description of fast electron generation and transport is found to reproduce the experimental absolute Kα yield and spot size measurements for varying target thicknesses, and to sequentially quantify the collisional and resistive electron stopping powers. The results demonstrate that both stopping mechanisms are enhanced in compressed Al samples and are attributed to the increase in the medium density and resistivity, respectively. For the achieved time- and space-averaged electronic current density, ⟨jh⟩∼8×1010 A/cm2 in the samples, the collisional and resistive stopping powers in warm and compressed Al are estimated to be 1.5 keV/μm and 0.8 keV/μm, respectively. By contrast, for cold and solid Al, the corresponding estimated values are 1.1 keV/μm and 0.6 keV/μm. Prospective numerical simulations involving high...

Development of x-ray radiography for high energy density physics

We describe an experiment performed at the LULI laser facility using an advanced radiographic technique that allowed obtaining 2D, spatially resolved images of a shocked buried-code-target. The technique is suitable for applications on Fast Ignition as well as Warm Dense Matter research. In our experiment, it allowed to show cone survival up to Mbar pressures and to measure the shock front velocity and the fluid velocity associated to the laser-generated shock. This allowed obtaining one point on the shock polar of porous carbon.

Transverse Effects in Collective Atomic Recoil Lasing

We investigate transverse effects in collective atomic recoil lasing (CARL), where a cold atomic sample is lightened by a far detuned laser beam resonant with the internal atomic transition. The gradient force of the scattered radiation field produces a collective self-focusing on the atoms, which could be observed in a Bose-Einstein condensate stored in a bidirectional optical ring cavity or in the superradiant CARL-BEC regime.

A Proposed 100-kHz fs Laser Plasma Hard X-Ray Source at the ELI-ALPS Facility

Femtosecond lasers are a proven source of hard X-rays for ultrafast probing applications; however, such X-ray sources do not offer significant advantages over accelerator-based photon sources. In this paper, the use of the proposed 100-kHz two-cycle ALPS-HR laser at the ELI-ALPS facility to drive a femtosecond hard X-ray source is investigated and target-related issues for high repetition rates are also discussed. A particle-in-cell simulation is used to determine the optimal plasma scale length for the investigated laser parameters to generate the necessary hot electron spectrum. Electron transport and photon creation are simulated using a Monte Carlo method in liquid droplet targets. The generated photon characteristics are postprocessed to build the synthetic X-ray spectrum and temporal pulse shape. The X-ray source characteristics are compared with other similar laser plasma X-ray sources. The parameters for the preliminary experiments are also discussed.

Three-Dimensional Simulations of Cylindrical Target Implosion Imaging Using Laser-Driven Proton Source

Many experiments, based on the road map of the European High Power laser Energy Research facility project, were performed to study fast electron transport in compressed matter in the context of fast ignition approach to inertial confinement fusion. The generation of high intensity beams from laser-matter interaction extends the possibility to use protons as a diagnostic to image imploding targets in these experiments. The analysis of experimentally obtained proton images requires a careful analysis and accurate numerical simulations using both hydrodynamic and Monte Carlo (MC) codes. An experiment has been performed in 2008 at Rutherford Appleton Laboratory to study fast electron propagation in cylindrical imploding targets illuminated by four laser pulses. In this paper, we present new simulation results in 3-D geometry. Three-dimensional density map is generated by running the 3-D version of the MULTI code. Proton radiography images are then simulated using the MC code MCNPX.

Proton radiography of a laser-driven cylindrical implosion

A recent experiment was performed at the Rutherford Appleton Laboratory (UK) to study fast electron propagation in cylindrically compressed targets, a subject of interest for fast ignition. This experiment was performed in the framework of the experimental road map of the Hiper project (the European High Power laser Energy Research facility Project). In this experiment, protons accelerated by a pecosecond laser pulse have been used to radiograph a 220 μm‐diameter, 20 μm‐wall cylinder filled with 0.1 g/cc foam, imploded with ∼200 J of green laser light in 4 symmetrically incident beams of pulse length 1 ns. Point projection proton backlighting was used to measure the compression degree as well as the stagnation time. Results were compared to those from hard X‐ray radiography. Finally, Monte Carlo simulations of proton propagation in the cold and in the compressed targets allowed a detailed comparison with 2D numerical hydro simulations.

X-ray diagnostics of fast electrons propagation in high density plasmas obtained by cylindrical compression

We report on X-ray diagnostics results from an experiment on fast electrons propagation in cylindrically compressed targets. It was performed on the VULCAN TAW laser facility at RAL (UK) using four long pulses (1ns, 70 J each at 2ω) to compress a cylindrical polyimide target filled with CH foam at 3 different initial densities. The cylindrical geometry allows us to reach temperatures and densities higher than those obtained in planar geometry compression. 2D hydrodynamic simulations predicted a core density range from 4 to 8 g/cm3 and a core temperature from 30 eV up to 175 eV at maximum compression. An additional short laser pulse (10 ps, 160 J at ω) was focused on a Ni foil at one of the cylinder edges in order to generate a fast electrons current propagating along the compressed target. A X-ray radiography diagnostic was implemented in order to estimate the core plasma conditions of the compressed cylinder. Moreover two Bragg X-ray spectrometers collected the Kα fluorescence from the target so as to determine the variations of fast electrons population during the compression.