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The formation and evolution of density channels created by the interaction of a short bright (600 fs, 5 3 10 18 W cm 22 mm 2 ) laser pulse with a preformed plasma are studied by means of experiments and 2D particle-in-cell (PIC) simulations. Hollow density channels are observed by interferometry, and a fast radial expansion ͑5 3 10 8 cm͞s͒ is measured. Magnetic fields around 50 MG inferred from Faraday rotation measurements suggest the occurrence of self-focusing. The PIC simulations support this hypothesis and show ion depletion during the laser pulse as well as radial expansion in agreement with experiments. [S0031-9007(98)05299-5] PACS numbers: 52.40.Nk, 52.25.Rv, 52.50.Jm, 52.70.Kz With the progress of compact short-pulse multiterawatt laser systems [1], it has become possible to explore the domain of relativistic laser pulse propagation and channel formation in an underdense plasma. These are topics of considerable interest for the fast ignitor (FI) concept [2], relevant to the inertial confinement fusion (ICF) studies. Indeed, the ultimate interest of this concept relies on the penetration of a short and bright laser pulse in the overdense core and the generation of the electrons that will ignite the fuel. However, to cross the underdense corona without energy losses, this pulse is planned to propagate into a hollow channel. Channel formation has been observed for several years with subrelativistic laser pulses [3]. Density depression in the channel and density increase on its walls are caused by the combined effects of transverse ponderomotive force, space-charge fields, and plasma heating. The channel formation process can be enhanced [4] by the cumulative effects of ponderomotive and relativistic self-focusing [5] which increase drastically the laser intensity, as shown recently in 3D simulations [6]. Moreover, the evacuated channel can act as a beam selfpropagation guide, as seen recently [7].In this Letter, we present direct measurements and 2D particle-in-cell (PIC) simulations that show the dynamics of subpicosecond relativistic laser pulse self-channeling in a fully ionized underdense plasma in the presence of a density gradient. The PIC simulations quantitatively reproduce the experimental results at early and long times. Therefore we rely on the PIC simulation to describe the channel formation during the laser pulse since this is not directly accessible by interferometry. These results are complementary to the ones of Borghesi et al.[7] as we put in evidence (i) the local dynamics of channel formation (ion depletion during the laser pulse), (ii) the local dynamics of channel evolution (radial expansion of the channel after the laser pulse), and (iii) the existence via Faraday rotation of relativistic electron currents accelerated in the forward direction during the laser pulse. Among other consequences, these results suggest the self-focusing of the laser pulse and a strong local heating of the plasma.The experiments are performed with the P102 CPA laser system [1] at CEA͞LV. A long creation la...

We present the experimental demonstration of a subaperture compression scheme achieved in the PETAL (PETawatt Aquitaine Laser) facility. We evidence that by dividing the beam into small subapertures fitting the available grating size, the sub-beam can be individually compressed below 1 ps, synchronized below 50 fs and then coherently added thanks to a segmented mirror.

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The Petawatt Aquitaine Laser (PETAL) facility was designed and constructed by the French Commissariat à l'énergie atomique et aux énergies alternatives (CEA) as an additional PW beamline to the Laser MegaJoule (LMJ) facility. PETAL energy is limited to 1 kJ at the beginning due to the damage threshold of the final optics. In this paper, we present the commissioning of the PW PETAL beamline. The first kJ shots in the amplifier section with a large spectrum front end, the alignment of the synthetic aperture compression stage and the initial demonstration of the 1.15 PW @ 850 J operations in the compression stage are detailed. Issues encountered relating to damage to optics are also addressed.

A multi-Petawatt high-energy laser PETAL coupled to the Ligne d'Intégration Laser (LIL) is under construction in the Aquitaine Region in France. This Petawatt laser will be dedicated to academic experiments in the fields of high energy density physics and ultra high intensity. Nd : glass laser chain coupled with the chirped pulse amplification (CPA technique allows delivery of high energy. Optical parametric CPA for pre-amplification and a new compression scheme will be implemented. PETAL is designed to deliver 3.6 kJ of energy in 500 fs on a target corresponding to 7.2 PW. The PETAL beam linked to the up to 60 kJ ns UV beams from the LIL will present new scientific research opportunities.

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High-energy petawatt lasers using the chirped-pulse amplification technique require meter-sized gratings to limit the beam fluence on the surface of the grating. An alternative, studied by many groups, is a mosaic grating consisting of smaller, coherently added gratings. We propose what we believe to be a new compression scheme consisting of beam phasing instead of grating mosaic phasing. This synthetic aperture compression scheme allows us to control the beam thanks to a unique segmented mirror equipped with three degrees of freedom. With this configuration, the beam is divided into small subapertures adapted to the classical grating size. After compression, these subapertures are coherently added before the focusing stage. Therefore the alignment processes are simplified.