Emmanuel D'Humieres

Physics of giant electromagnetic pulse generation in short-pulse laser experiments.

In this paper we describe the physical processes that lead to the generation of giant electromagnetic pulses (GEMPs) at powerful laser facilities. Our study is based on experimental measurements of both the charging of a solid target irradiated by an ultra-short, ultra-intense laser and the detection of the electromagnetic emission in the GHz domain. An unambiguous correlation between the neutralization current in the target holder and the electromagnetic emission shows that the source of the GEMP is the remaining positive charge inside the target after the escape of fast electrons accelerated by the ultra-intense laser. A simple model for calculating this charge in the thick target case is presented. From this model and knowing the geometry of the target holder, it becomes possible to estimate the intensity and the dominant frequencies of the GEMP at any facility.

Dynamic model of target charging by short laser pulse interactions.

A model providing an accurate estimate of the charge accumulation on the surface of a metallic target irradiated by a high-intensity laser pulse of fs-ps duration is proposed. The model is confirmed by detailed comparisons with specially designed experiments. Such a model is useful for understanding the electromagnetic pulse emission and the quasistatic magnetic field generation in laser-plasma interaction experiments.

X-ray amplification from a Raman free-electron laser.

We demonstrate that a mm-scale free-electron laser can operate in the x-ray range, in the interaction between a moderately relativistic electron bunch, and a transverse high intensity optical lattice. The corrugated light-induced ponderomotive potential acts simultaneously as a guide and as a low-frequency wiggler, triggering stimulated Raman scattering. The gain law in the small signal regime is derived in a fluid approach, and confirmed from particle-in-cell simulations. We describe the nature of bunching, and discuss the saturation properties. The resulting all-optical Raman x-ray laser opens perspectives for ultracompact coherent light sources up to the hard x-ray range.

Gigagauss-scale quasistatic magnetic field generation in a snail-shaped target.

A simple setup for the generation of ultra-intense quasistatic magnetic fields, based on the generation of electron currents with a predefined geometry in a curved snail (or escargot) target, is proposed and analyzed. Particle-in-cell simulations and qualitative estimates show that gigagauss scale magnetic fields may be obtained with existent laser facilities. The described mechanism of the strong magnetic field generation may be useful in a wide range of applications, from laboratory astrophysics to magnetized inertial confinement fusion schemes.

Pair creation in collision of γ-ray beams produced with high-intensity lasers.

Direct production of electron-positron pairs in two-photon collisions, the Breit-Wheeler process, is one of the basic processes in the universe. However, it has never been directly observed in the laboratory because of the absence of the intense γ-ray sources. Laser-induced synchrotron sources emission may open a way to observe this process. The feasibility of an experimental setup using a MeV photon source is studied in this paper. We compare several γ-ray sources and estimate the expected number of electron-positron pairs and competing processes by using numerical simulations including quantum electrodynamic effects.

Characterization of laser-produced fast electron sources for fast ignition

Laser-driven fast electron beams generated in planar foils or double cone targets have been characterized by means of two-dimensional particle-in-cell simulations. The laser beam focusing by the cone walls reduces the fast electron beam radius and increases the electron mean kinetic energy. Nevertheless, the beam divergence is high in both types of targets and can be split into two components: the dispersion angle and the radial velocity. The fast electron energy spectrum presents a power law profile. The effects of the fast electron source characteristics on the energy deposition in an ideal precompressed inertial fusion target have been analysed. The radial velocity component of the fast electron beam significantly increases the energy required for ignition while the electron power law spectrum increases it slightly when compared with the standard exponential one.

Investigation of high intensity laser proton acceleration with underdense targets

In the last few years, intense research has been conducted on laser-accelerated ion sources and their applications. Recently, experiments have shown that a gaseous target can produce proton beams with characteristics comparable to those obtained with solid targets. In underdense laser proton acceleration, volume effects dominate the acceleration, while in target normal sheath acceleration, the electric field value is directly related to the electron surface density. Using Particle-In-Cell simulations, we have studied in detail the effect of an underdense density gradient on proton acceleration with high intensity lasers. Underdense laser ion acceleration strongly depends on the length, the shape and the amplitude of the density gradient and on the laser pulse shape. The accelerated proton beam characteristics in the shock-like regime are very promising.

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⟩∼8×1010u2009A/cm2 in the samples, the collisional and resistive stopping powers in warm and compressed Al are estimated to be 1.5u2009keV/μm and 0.8u2009keV/μm, respectively. By contrast, for cold and solid Al, the corresponding estimated values are 1.1u2009keV/μm and 0.6u2009keV/μm. Prospective numerical simulations involving high...

Development of the PETawatt Aquitaine Laser system and new perspectives in physics

The paper describes the preparation of the short-pulse high-energy laser PETawatt Aquitaine Laser (PETAL), which will be coupled to the Laser Megajoule (LMJ) laser of CEA. The LMJ/PETAL facility will be opened for the academic access of European researchers. In parallel, diagnostics are being developed within the PETALxa0+xa0 project and many physical problems are being addressed ranging from the study of the problems of radiation generation and activation issues to the problem of generation of large electromagnetic pulses.

Measuring hot electron distributions in intense laser interaction with dense matter

Retrieving the characteristics of hot electrons produced in the interaction between solid targets and ultra-intense (I?>?1018?W?cm?2) laser pulses is essential for achieving?progress in our understanding of the interaction physics, which is?key for optimizing numerous downstream applications. Until now, various methods have been used, direct or indirect, but no correlation and no assessment of their respective merits were performed. Here we compare results retrieved from four different diagnostics, direct or indirect, as well as local or non-local, i.e. spectrometry of electrons, spectrometry of the protons accelerated by the electrons and optical probing of these beams expanding into vacuum from the targets. We show that measurements obtained locally at the target rear surface are consistent with those far away from the target and that one can use the diagnostics of the co-moving proton beams to retrieve information about electrons.