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The SEPAGE spectrometer (Spectrometre Electrons Protons A Grandes Energies) was realized within the PETAL+ project funded by the French ANR (French National Agency for Research). This plasma diagnostic, installed on the LMJ-PETAL laser facility, is dedicated to the measurement of charged particle energy spectra generated by experiments using PETAL (PETawatt Aquitaine Laser). SEPAGE is inserted inside the 10-meter diameter LMJ experimental chamber with a SID (Diagnostic Insertion System) in order to be close enough to the target. It is composed of two Thomson Parabola measuring ion spectra and more particularly proton spectra ranging from 0.1 to 20 MeV and from 8 to 200 MeV for the low and high energy channels respectively. The electron spectrum is also measured with an energy range between 0.1 and 150 MeV. The front part of the diagnostic carries a film stack that can be placed as close as 100 mm from the target center chamber. This stack allows a spatial and spectral characterization of the entire proton beam. It can also be used to realize proton radiographies.

Recent theoretical and experimental studies suggest the possibility of enhancing the efficiency and ease of laser acceleration of protons and ions using underdense or near critical plasmas through electrostatic shocks. Very promising results were recently obtained in this regime. In these experiments, a first ns pulse was focused on a thin target to explode it and a second laser with a high intensity was focused on the exploded foil. The delay between two lasers allowed to control the density gradient seen by the second laser pulse. The transition between various laser ion acceleration regimes depending on the density gradient length was studied. With a laser energy of a few Joules, protons with energies close to the energies of TNSA accelerated protons were obtained for various exploded foils configurations. In the high energy regime (~180 J), protons with energies significantly higher than the ones of TNSA accelerated protons were obtained when exploding the foil while keeping a good beam quality. These results demonstrate that low-density targets are promising candidates for an efficient proton source that can be optimized by choosing appropriate plasma conditions. New experiments were also performed in this regime with gas jets. Scaling shock acceleration in the low density regime to ultra high intensities is a challenge as radiation losses and electron positron pair production change the optimization of the shock process. Using large-scale Particle-In-Cell simulations, the transition to this regime in which intense beams of relativistic ions can be produced is investigated.

The 200TW laser system, (Ti:Sapphire CPA system) delivering 5J in 25fs pulse with a 10Hz repetition rate and a contrast ratio of 1:10^-11 at the fundamental 800nm frequency, is used at the Advanced Laser Light Source (ALLS) facility to develop new generation of x-ray and pulsed particle beam sources (electrons, protons, neutrons). Experimental results on the betatron emission and electron emission from high intensity (

Focusing a high intensity laser pulse, onto a thin foil target generates a plasma and energetic proton and ion beams from the target rear and front sides, propagating along the target normal. Such laser produced collimated and energetic protons beams are of high interest because of the wide range of applications: ion based fast ignitor schemes, probing of electromagnetic fields in plasma, isotopes production or hadron therapy. The 100 TW class laser system at the Advanced Laser Light Source facility, is used with an intensity close to 1019 W/cm2, to study protons acceleration with femtosecond laser pulses, ultra thin foil target and high contrast laser pulse ratio. To characterize the plasma expansion, we monitor it with an imaging technique using a femtosecond laser probe. In this configuration we were able to reach a proton critical energy of 12 MeV and to work with target foil thickness as small as 15 nm.

Collimated beams of energetic protons are produced by the interaction of short duration high intensity laser pulses with solid foils. This field has been the subject of many studies in the last decade. This interest is motivated by the wide range of application of such beams: ion based fast ignitor schemes, probing of electromagnetic fields in plasma, isotope production or hadron therapy. The recently commissioned 200 TW laser system (5 J, 25 fs, 1010 laser pulse contrast, 10 Hz repetition rate at 800 nm) at the Advanced Laser Light Source (ALLS) facility has been used to study proton acceleration with femtosecond laser pulses. The proton spectrum was characterized using a time of flight detector. Due to the high contrast of the laser pulse, foil targets as thin as 30 nm could be studied.

Energy Energy transfer between a long (3-10 ps) "pump" pulse and a short (400 fs) "seed" one, both at a wavelength of 1.057μm quasi counterpropagating in an underdense preformed plasma and produced from the ionization of a gas jet, was observed. Numerical simulations reveal that the energy transfer is due to the coupling involving ion acoustic waves excited in the Stimulated Brillouin Backscattering in the strong coupling regime. The plasma characteristics were tailored using a high-energy ionization laser beam and the plasma density was controlled using a Thomson scattering diagnostic. The energy exchange was observed for different gas (ion) types, pressures (plasma densities), polarization and intensities of the interacting beams.

We demonstrate the wave-front correction of the LULI 100TW, 300 fs/30 J high power laser facility for a sequence of shots. Excellent beam focusability close to diffraction limit has been obtained using an adaptive optic system, composed of a large aperture dielectric coated deformable mirror and a home-made shearing interferometer. This correction allows to produce reproducible focal spots with Strehl ratios close to 0.9 at a repetition rate of a shot every 20 minutes, despite of wave front distortions generated by cumulative thermal effects in the large disc amplifiers.

The control of coherence is a critical issue for the high-power lasers used in inertial confinement fusion (ICF). The level of coherence is an important parameter for the control of the light intensity distribution as well as the growth rate of parametric instabilities. Over the past few years, experimental and theoretical studies have evidenced the ability of an underdense plasma to reduce the spatial and temporal coherence of an intense laser beam prooagating through it. As any process affecting laser propagation, plasma-induced incoherence appears fundamental because it can impact on parametric instabilities. We present results obtained with the six-beam LULI laser facility, in the nanosecond regime, showing direct evidences of the reduction of spatial and temporal coherence of an initially RPP-smoothed laser beam after propagation through a preformed plasma. Plasma induced incoherence (PII) proceeds from several mechanisms which include self-focusing, filamentation and non-linear coupling between these mechanisms and forward stimulated Brillouin scattering (FSBS). Part of these experiments was dedicated to the understanding of the physical mechanisms involved in PII, as the break up of a single hot spot and the existence of ion acoustic waves having small wave vectors transverse to the interaction beam which are produced in the PII processes. The spatial and temporal characteristics of these waves give a unique access to the influence of PII on stimulated Brillouin and Raman scattering.

Within the context of inertial confinement fusion, experiments involving the "self-induced smoothing" of a laser beam propagating in an underdense semicollisional plasma are presented. These results, which reveal severe modifications of the focal spot intensity pattern via increased angular spreading and hot spot size reduction, are compared with three-dimensional simulations from the code PARAX.

We have implemented on one beam of the LULI six-beam high-energy (6×100 J, 1 ns) Nd:glass laser facility a closed-loop Adaptive Optics (AO) system to compensate for thermal distortions onto the wave front. Using the AO system composed of a dielectric coated deformable mirror and of a wave front sensor, we are able to improve the wave front quality in order to obtain a focal spot close to the diffraction limit. This allows not only to improve the reproducibility of the experiments but also to increase by at least two orders of magnitude the peak intensity as compared with what usual laser smoothing techniques can achieve.

We have studied the influence of the target properties on laser-accelerated proton and ion beams generated by the LULI multi-terawatt laser. A strong dependence of the ion emission on the surface conditions, conductivity, shape and material of the thin foil targets were observed. We have performed a full characterization of the ion beam using magnetic spectrometers, Thompson parabolas, radiochromic film and nuclear activation techniques. The strong dependence of the ion beam acceleration on the conditions on the target back surface was found in agreement with theoretical predictions based on the target normal sheath acceleration (TNSA) mechanism. Proton kinetic energies up to 25 MeV have been observed.

A model is presented, which describes the stimulated Brillouin scattering (SBS) of a randomized laser beam interacting with an underdense, expanding plasma. The model accounts for the self-focusing of speckles in the regime where plasma heating has an important effect on speckle self-focusing and on SBS threshold and reflectivity. The model exhibits a good agreement with SBS levels measured at the LULI multi-beam facility for a broad range of the laser and plasma parameters and both types of beam smoothing - RPP and polarization smoothing (PS). The model and experiments confirm that eh PS technique allows to control the SBS level more efficiently than RPP.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An experiment is performed which definitely demonstrates the occurrence of the Langmuir decay instability in the laser plasma interaction. Stimulated Raman scattering is used as a source of well-defined electron plasma waves. Both products of the instability are observed simultaneously using Thomson scattering of a short wavelength probe beam. Resolution of the Thomson scattering measurements in wavelength, wavenumber, time and space allow an unambiguous identification of the ion acoustic waves. Spatial and temporal correlation of these ion acoustic waves with the stimulated Raman scattering activity is observed whereas they appear to be anticorrelated in time and space with the stimulated Brillouin scattering activity.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.