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The Laser Megajoule (LMJ) is among the most energetic inertial confinement fusion laser facilities in the world, together with the National Ignition Facility (NIF) in the USA. The construction of the facility began back in 2003, and the first photons were emitted by the laser bundle #28 in 2014. Today, 11 laser bundles consisting of 88 large aperture 0.35×0.35 m 2 laser beams are in operation, delivering daily up to 330 kJ of energy at the wavelength of 351 nm on a target placed in the center of a 10 m diameter vacuum chamber. In this paper, we describe the laser system and its operational performances. We also detail the first laser campaigns carried out to prepare an increase of energy and power on the target. These campaigns, along with the completion of additional bundles mounting, will bring LMJ performance to 1.3 MJ thanks to 22 bundles in operation.

Chirped pulse amplification has been widely implemented in high power laser chains. It consists of a set of diffraction gratings used to stretch and compress short laser pulses. In the case of high power laser chains, the compression stage is followed by the transport mirror in order to carry the laser beam to its final target. In such laser chains, laser beams propagate over a complex set of optical components and understanding the propagation of phase noise turns out to be of crucial importance. Phase modulation can induce laser damage on the final optical components. Here, we study the impact of phase modulation induced by the different diffraction gratings of the Petawatt Aquitaine Laser (PETAL) compressor on the downstream over-intensities, in particular on the transport mirror. This work allows us to quantify the impact of phase modulation for every single grating element in the compression stage, and to estimate the quantity of laser induced damage sites on transport optics for a specific laser shot.

In the framework of high-power lasers, surface defects on optics can generate strong light intensification and induce damage sites on downstream optics. To evaluate this intensification during high-energy laser shots, a three-step method is proposed. First, a dedicated measurement bench is designed to measure the intensification induced by defects on a wide variety of optics, including amplifier slabs, KDP crystals, mirrors, gratings, and vacuum windows, for propagation distances up to 2000 mm. A multi-resolution single-beam multiple-intensity reconstruction phase retrieval algorithm is then used to reconstruct a model of the defect, in both amplitude and phase, from a set of intensification measurements. Finally, the impact of the modeled defect on downstream optics is evaluated with a simulation of the high-power laser system. This method is experimentally validated through a case study of damage identified on one of the Laser Mégajoule (LMJ) beams, characterized with the method presented in this paper. The long-distance impact on the LMJ beam is estimated by simulation and compared to a direct near-field measurement.

In order to smooth the focal spot of high-power energetic lasers, pulses are phase-modulated. However, due to propagation impairments, phase modulation is partly converted into power modulation. This is called frequency modulation to amplitude modulation (FM-to-AM conversion). This effect may increase laser damage and thus increase operating costs. For the first time, to the best of our knowledge, we have studied the impact of the Kerr effect in this process. We have shown that when the Kerr effect is followed by a dispersive transfer function, a dramatic increase of FM-to-AM conversion may occur for a particular kind of FM-to-AM conversion that we have named "anomalous." Hence, we should remove or compensate for one of the items of the sequence: phase modulation, anomalous FM-to-AM conversion, Kerr effect, or the dispersive function. We have assessed all these solutions, and we have found an efficient inspection method to avoid anomalous FM-to-AM conversion.

Laser beams can carry multi-scale properties in space and time that ultimately impact their quality. The study of their evolution along complex optical sequences is of crucial interest, especially in high-intensity laser chains. For such analysis, results obtained with standard numerical methods strongly depend on the sampling. In this paper, we develop an analytic model for a sinusoidal phase modulation inside a sequence of first-order optics elements based on the Wigner matrix formalism. A Bessel decomposition of the Wigner function gives pseudo-Wigner functions that obey the general ABCD matrix law transformation without approximations and sampling considerations. Applied to a Gaussian beam, explicit expressions are obtained for the projections of the Wigner function in the sub-spaces and give a powerful tool for analyzing the laser beam properties. The formalism is established in the spatial and temporal domains and can be used to evaluate the impact of the phase noise on the beam properties and is not limited to small modulation depths. For the sake of illustration, the model is applied to the Talbot effect with the analysis of the propagation in the spatial and phase-space domains. A comparison with full numerical calculations evidences the high accuracy of the analytic model that retrieves all the features of the diffracted beam.

Cutaneous diphtheria is uncommon in Europe. In this study, we report a case of imported cutaneous infection due to a non-toxigenic but tox gene-bearing (NTTB) strain of Corynebacterium diphtheriae . The NTTB strains are recognized as emerging pathogens across Europe, and physicians and bacteriologists should be aware of the circulation of these strains.