F. Amiranoff

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.

Search for stimulated photon-photon scattering in vacuum

Abstract:We have searched for stimulated photon scattering in vacuum at a center of mass photon energy of 0.8 eV. The QED contribution to this process is equivalent to four wave mixing in vacuum. No evidence for scattering was observed. The corresponding upper limit of the cross-section is .

Acceleration of injected electrons in a laser wakefield experiment

An electron plasma wave (EPW) has been excited by a short laser pulse (5 J, 400 fs) via the laser wakefield (LWF) mechanism. At the LWF quasi-resonance condition, the 3 MeV injected electrons have been accelerated with a maximum energy gain of 1.5 MeV. The maximum longitudinal electric field is estimated to be 1.5 GV/m. It has been observed that electrons deflected during the interaction, can scatter on the walls of the experimental chamber and fake a high energy signal. A special effort has been given in the electron detection to separate the accelerated electrons signal from the background noise. The experimental data are confirmed with numerical simulations, demonstrating that the energy gain is affected by the EPW radial electric field. The duration of the EPW inferred by the number of accelerated electrons and by the numerical simulations is of the order of 1–10 ps.

Suprathermal and relativistic electrons produced in laser–plasma interaction at 0.26, 0.53, and 1.05 μm laser wavelength

Very energetic electrons produced in laser–plasma interactions at 0.26, 0.53, and 1.05 μm laser wavelength have been measured. The targets were 1.5 μm plastic foils and the laser intensity was around 1015 W/cm2. Detailed measurements of the electron distribution performed at a 0.26 μm laser wavelength exhibit an angular distribution strongly peaked along the laser axis at the highest energies (above 200 keV). Electrons up to 1.3 MeV have been observed in the 1.05 μm experiments. The hot temperatures inferred from the measured energy distributions are of the order of 100 keV in the 1.05 μm experiments, and 50 keV in the 0.53 μm/0.26 μm experiments. The experimental electron emission features are discussed with a special focus on the relation between the linear as well as the nonlinear electron plasma wave generation mechanisms and the maximum energy reached by an electron trapped in this wave.

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.

Laser wakefield: Experimental study of nonlinear radial electron oscillations

The plasma electron density oscillation produced in the wake of a narrow (beam waist≪plasma wavelength) ultrashort laser pulse is measured by frequency-domain interferometry with a temporal resolution much better than the electron plasma period, and a spatial resolution across the laser focal spot. The absolute density perturbation is observed to be maximum when the pulse duration equals half the plasma period. The relative density perturbation varies from a few percent at high density to 100% at low density. For nonlinear oscillations we measure the increase of the electron plasma frequency predicted for radial oscillations [J. M. Dawson, Phys. Rev. 113, 383 (1959)]. The damping of the oscillations is observed. It is very rapid (a few periods) when the oscillation is nonlinear. Comparison with the code WAKE [P. Mora and T. M. Antonsen, Jr., Phys. Rev. E 53, R2068 (1996)] indicates that the gas ionization creates a steep radial density gradient near the edge of the focus and that the electrons oscillating...

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, 1017 W/cm2–2×1019 W/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 1017 W/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 2×1019 W/cm2 in the lowest density case. The relativistic electrons accelerated in the forward direction appear to be correlated with the F-SRS. The exper...

Interaction of an ultra-intense laser pulse with a nonuniform preformed plasma

The propagation of an ultra-intense laser pulse in a preformed plasma channel was investigated experimentally. Different regimes of propagation were observed when the pulse duration was varied. For a long pulse and powers lower than the critical power for self-focusing,PL/PC<1 (I0=2×1017W/cm2), the laser pulse was guided by the preformed plasma channel over three Rayleigh lengths (4 mm) and a longitudinal plasma wave was generated by envelope self-modulation of the pulse. For a short pulse and PL/PC≫1, the interaction was dominated by self-focusing and Raman instabilities. Numerical simulations were run for the latter case, giving results comparable to the experiment. The simulations were also used to investigate the dynamics of the instabilities at high power. They showed that strong Raman side scattering first occurs at the beginning of the interaction and is then followed by self-focusing and envelope self-modulation.

Coupling between electron and ion waves in Nd‐laser beat‐wave experiments

The beating between two colinear Nd‐YLF and Nd‐YAG lasers in a homogeneous plasma generates intense relativistic plasma waves associated with a high longitudinal electric field of the order of 1 GV/m. It is shown that these electron waves couple with ion waves in the regime of modulational instability. Electric field amplitude and saturation time obtained by Thomson scattering are in agreement with theoretical predictions taking this mechanism into account.

Extending plasma accelerators: guiding with capillary tubes

In order to extend plasma accelerators, the laser beam has to be guided inside gas or plasma over a distance of the order of the dephasing length, which is typically much larger than the diffraction length z/sub R/ of the laser. A capillary tube can be used as a waveguide for high-intensity laser pulses over distances well in excess of z/sub R/. Experimental demonstration of monomode guiding over 100 z/sub R/ of 10/sup 16/ W/cm/sup 2/ pulses has been obtained in evacuated capillary tubes (45-70-/spl mu/m inner diameter). A drop of transmission has been observed when the intensity of the amplified spontaneous emission (ASE) is high enough to ionize the capillary tube entrance. Propagation in helium gas-filled (10-40 mbar) capillary tubes has been studied at intensities up to 10/sup 16/ W/cm/sup 2/; a plasma column with on-axis density of the order of 10/sup 17/ cm/sup -3/ has been created on a length of the order of 4 cm. The use of a capillary tube for an extended accelerator is discussed for the ease of linear, resonant excitation of plasma waves by laser wakefield.