L. Gremillet

Filamented transport of laser-generated relativistic electrons penetrating a solid target

The paraxial propagation of a relativistic electron beam in a solid target is examined, within a three-dimensional model of particles interacting with the target electron return current via a diffusive electromagnetic field. Simulations of a modulated beam show amplification of the modulation seed, with growth rates comparing reasonably well with the linear analysis of the model. Scenarios of beam fragmentation are observed and discussed in more realistic conditions, when beam collisions on both target ions and electrons and the resulting solid heating and ionization are taken into account.

Subfemtosecond, coherent, relativistic, and ballistic electron bunches generated at ω0 and 2ω0 in high intensity laser-matter interaction

Harmonics of the laser light have been observed from the rear side of solid targets irradiated by a laser beam at relativistic intensities. This emission evidences the acceleration of subfemtosecond electron bunches by the laser pulse in front of the target. These bunches emit coherent transition radiation (CTR) when passing through the back surface of the target. The spectral features of the signal recorded for targets of thicknesses up to several hundred microns are consistent with the electrons being accelerated by both the laser electric field—via vacuum heating and/or resonance absorption,—and the v×B component of the Lorentz force. The spatial study of the radiation shows that the relativistic electrons causing the CTR radiation are coherent and propagate ballistically through the target, originating from a source with a size of the order of the laser focal spot.

Strong absorption, intense forward-Raman scattering and relativistic electrons driven by a short, high intensity laser pulse through moderately underdense plasmas

The propagation of a short and intense laser pulse (1.057 μm, 350 fs, 1017u2009W/cm2–2×1019u2009W/cm2) through preformed undercritical plasmas (≈5%–40% of nc) has been experimentally investigated on the 100-TW laser facility at the Laboratoire pour l’Utilisation des Lasers Intenses. The transmission and reflection of the 1 μm laser pulse, the forward- and backward-Raman (respectively, F-SRS and B-SRS) scattered light and the emission of fast electrons are reported. Significant absorption occurs in these plasmas, which is found to increase with the laser intensity. B-SRS is strongly driven at 1017u2009W/cm2 and gradually decreases at higher intensities. It is shown that the transmission is low and only weakly dependent on the laser intensity. In contrast, the forward Raman scattering continuously increases with the laser intensity, up to 7% of the incident energy at 2×1019u2009W/cm2 in the lowest density case. The relativistic electrons accelerated in the forward direction appear to be correlated with the F-SRS. The exper...

Fast electron energy deposition in a magnetized plasma: Kinetic theory and particle-in-cell simulation

The collisional dynamics of a relativistic electron jet in a magnetized plasma are investigated within the framework of kinetic theory. The relativistic Fokker–Planck equation describing slowing down, pitch angle scattering, and cyclotron rotation is derived and solved. Based on the solution of this Fokker–Planck equation, an analytical formula for the root mean square spot size transverse to the magnetic field is derived and this result predicts a reduction in radial transport. Some comparisons with particle-in-cell simulation are made and confirm striking agreement between the theory and the simulation. For fast electron with 1 MeV typical kinetic energy interacting with a solid density hydrogen plasma, the energy deposition density in the transverse direction increases by a factor 2 for magnetic field of the order of 1 T. Along the magnetic field, the energy deposition profile is unaltered compared with the field-free case.

Fast decompression of ultra-thin targets for high-energy, high-contrast laser pulses

In the laser‐plasma interaction process, for ultra‐high temporal contrast laser pulses, experimental measurements show that reducing the thickness of solid targets increases the laser‐to‐fast electrons energy conversion and the hot electron temperature. We have performed an experiment using the LULI 100 TW laser facility working in the chirped pulse amplification (CPA) mode at a wavelength λ0u2009=u20091.057u2009μm, pulse duration 320 fs, laser spot size FWHM ∼6 μm and intensity ∼1×1018u2009W/cm2 in which the laser pulses were temporal‐contrast enhanced by the use of two plasma mirrors. Shots were performed on Si3N4 aluminum coated targets of thickness 30 nm to 500 nm. Spectra of the laser‐accelerated electrons were recorded with a spectrometer and are compared to PIC simulations performed with the CALDER code. The simulations allow an insight into the electron heating process during the laser‐matter interaction.

Propagation In Matter Of Currents Of Relativistic Electrons Beyond The Alfven Limit, Produced In Ultra‐High‐Intensity Short‐Pulse Laser‐Matter Interactions

This paper reports the results of several experiments performed at the LULI laboratory (Palaiseau, France) concerning the propagation of large relativistic currents in matter from ultra‐high‐intensity laser pulse interaction with target. We present our results according to the type of diagnostics used in the experiments: 1) Kα emission and Kα imaging, 2) study of target rear side emission in the visible region, 3) time resolved optical shadowgraphy.

Generation and Transport of Fast Electrons in Laser Irradiated Targets at Relativistic Intensities

The transport of relativistic electrons in solid targets irradiated by a short laser pulse at relativistic intensities has been studied both experimentally and numerically. A Monte‐Carlo collision code takes into account individual collisions with the ions and electrons in the target. A 3D‐hybrid code takes into account these collisions as well as the generation of electric and magnetic fields and the self‐consistent motion of the electrons in these fields. It predicts a magnetic guiding of a fraction of the fast electron current over long distances and a localized heating of the material along the propagation axis. In experiments performed at LULI on the 100 TW laser facility, several diagnostics have been implemented to diagnose the geometry of the fast electron transport and the target heating. The typical conditions were: E1 ⩽ 20 J, λ = 1 μm, τ ≈ 300 fs, I ≈ 1018−5.1019W/cm2. The results indicate a modest heating of the target (typically 20–40 eV over 20 μm to 50 μm), consistent with an acceleration o...

Fast particles and hard photons generated by an ultraintense laser pulse

Two sets of computer results are discussed. By means of a 2D Maxwell-Vlasov coupling code, the generation of MeV-range electrons and ions is addressed as well as the subsequent X- ray production and neutron production provided by post- processing the fast particle distribution. With a 3D MHD- fluid target-fast beam coupling code, the propagation of electrons in dense matter is discussed, with emphasis on the target heating.

Study of Fast Electron Propagation in Ultra-Intense Laser Pulse Interaction with Solid Targets Using Rear Side Optical Self-Radiation and Reflectivity-Based Diagnostics

Experimental results are reported on transport through a solid target of fast-electrons created by an ultra-intense laser pulse interaction. In particular, the goal was to determine the heating induced in the material by the fast electrons. Such a study is of great interest within the context of the Fast Igniter [1] approach to Inertial Confinement Fusion, where the heating needed to ignite nuclear reactions is supposed to be achieved by a sub-ps fast electron bunch. Experimentally, the main point is therefore to observe the propagation geometry of the fast electron beam and to estimate the amount of energy which can be carried and deposed in dense matter by a given electron source. So far, theory and simulations have not yet provided a complete picture of the prop-agation phenomena. Therefore, experimental work is requried in order to understand and dis crimed the basic processes involved.

Effects of self-generated electric and magnetic fields in laser-generated fast electron propagation in solid materials: Electric inhibition and beam pinching

We present some experimental results which demonstrate the presence of electric inhibition in the propagation of relativistic electrons generated by intense laser pulses, depending on target conductivity. The use of transparent targets and shadowgraphic techniques has made it possible to evidence electron jets moving at the speed of light, an indication of the presence of self-generated strong magnetic fields.