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International audience; Multilayer dielectric gratings (MLDG) are key optical components of Petawatt-class laser that are used to compress short pulses of high intensities. Laser-induced damage can occur on the top area of the components, typically arising in the pillars periodically etched. This phenomenon limits the power yielded by high power laser facilities such as PETAL (PETwatt Aquitaine Laser) laser facility. PETAL is expected to delivery pulses with a wavelength around 1053 nm, an energy around 3 kJ and a pulse duration between 0.5 and 10 ps. Coupled with LMJ (Laser MegaJoule), PETAL aims to study materials in extreme conditions to reproduce the environment in the heart of stars or planets, fusion by inertial confinement, particularly rapid ignition and shock ignition, and nuclear physics for medical proton therapy. In this study, we present a process to improve the laser-induced damage threshold of PETAL pulse-compression gratings in sub-picosecond regime by reducing the electric field intensity in the pillars. PETAL gratings have specific parameters of operation: Transverse Electric polarization, under vacuum, a period equal to 1780 lines per mm and diffraction efficiency higher than 95% for the -1st order. Theoretical designs are calculated with a code developed at the Fresnel Institute. The code solves Fresnel equations by using the differential method, Fast Fourier Factorization (FFF) and S matrix propagation algorithm. As a result, we obtain the distribution of the electric field and diffraction efficiency of any given diffraction order. First, starting with a given MLD mirror, we calculate an etching profile that maximizes the diffraction efficiency at the -1st order by taking into account the manufacturing constraints of future suppliers. Then, we optimize the mirror stack without changing the etching profile. We modify only the first top layers under the grooves. We obtained theoretical designs with the same etching profile and identical diffraction efficiency, associated with different electric field intensity values and expected different laser induced damage thresholds.

The Laser Megajoule facility, developed by the CEA is based on 176 Nd:glass laser beams focused on a micro -target positioned inside a 10-meter diameter spherical chamber. The facility will deliver a total energy of 1.4MJ of UV light at 0.35 μm and a maximum power of 400 TW. A specific petawatt beam, PETAL, offers a combination of a very high intensity beam, synchronized with the nanosecond beams of the LMJ. This combination allows expanding the LMJ experimental field in the High Energy Density Physics (HEDP) domain. Since October 2019, 56 beams are fully operational (7 bundle of 8 beams). The installation and the commissioning of new laser bundles and new plasma diagnostics around the target chamber are continuing, simultaneously to the realization of plasma experiments. A major project milestone has been achieved at the end of 2019, with the first experiment in the facility involving neutron production, through D-D reaction in a D2 capsule inside a gold rugby cavity. The next major milestones for LMJ will take place at the end of 2021 with the commissioning of the half LMJ (10 heating bundles of 8 beams and a specific bundle for plasma diagnostics purpose). The full presentation will describe the software environment used for the laser operation, the first results on the laser damages using our 3w optical components inspection system, the laser damages analysis software, the system of spot blocking, and the last performances obtained with the PETAL beam.

A novel original method is presented to detect and track laser damage sites on vacuum windows of the Laser MegaJoule (LMJ) facility. The method is based on spatial registration by Digital Image Correlation (DIC). It also involves corrections for gray level variations induced by variable lighting conditions. Using the present method, an efficient way is achieved to detect and follow laser damage sites as soon as they appear on the optical component. The developed tools offer the possibility of characterizing and predicting damage growth as a function of laser shot features.

We present the construction and the test of a generic neutrons detector for fusion experiments : OMEGA and NIF(USA), LMJ and APPOLON (FRANCE). The detector can be also used as a safety device on particle physics accelerator (ESS, Sweeden), Spiral2(France) etc.. . This detector is based on the technology CEA / IRFU MICROMEGAS detector. This diagnostic has been designed to achieve neutron spectroscopy in large γ background. Tests have been performed on the 60 beams, 30 kJ OMEGA laser system at the University of Rochester(LLE), and on LINAC4 accelerator at CERN during last november (Switzerland), and AMANDE(CEA). In Inertial Confinement Fusion experiment on facilities such as Laser MegaJoule (LMJ) in France and the National Ignition Facility (NIF) and OMEGA, LLE Rochester, USA we plan to achieve the ignition of capsules by compression deuterium-tritium (DT) or a deuteriumdeuterium (DD) filled target, and thus initiate a thermonuclear burn wave. In these experiments may be measured using neutrons output from the imploded capsule, like secondary and tertiary neutrons produced respectively in DD and DT targets. Measurement of these neutrons remains a challenge as the γ-rays and scattered neutrons induced by primary neutronsinteractions on the experimental hardware can blind detectors. The concept is based on the association of a Micromegas detector with a neutron-to-charged particle converter associated to a fast low noise electronics (

The Laser MegaJoule (LMJ) is a 176-beam laser facility, located at the CEA CESTA near Bordeaux (France). It is designed to deliver about 1.4 MJ of energy to targets, for high energy density physics experiments, including fusion experiments. A computational system, PARC has been developed and is under deployment to automate the laser setup process, and accurately predict laser energy, spatial and temporal shapes. PARC is based on MIRO computer simulation code. For each shot on LMJ, PARC determines the characteristics of the injection laser system required to achieve the desired main laser output and supplies post-shot data analysis and reporting. The presentation compares all characteristics (energy, spatial and temporal shapes, spot size, synchronism, wavefront correction and alignment on target) after amplification and after frequency conversion with predicted results or results computed with PARC.

The Laser MegaJoule (LMJ) is a French high power laser that requires thousands of large optical components. For all those optics, scratches, digs and other defects are severely specified. Indeed, diffraction of the laser beam by such defects can lead to dangerous “hot spots” on downstream optics. With the help of a near-field measurement setup, we make quantitative comparison between simulated and measured near-fields of reference objects (such as circular phase steps). This leads to a better understanding which parameters impact the diffracted field. In this paper, we proposed to study two types of reference objects: phase disks and phase rings. We actually made these objects by CO2 laser ablation. The experimental setup to observe the diffracted intensity by these objects will be described and calibrated. Comparisons between simulations and measurements of the light propagation through these objects show that we are able to predict the light behavior based on complete phase measurement of these objects.

The Laser Megajoule (LMJ) is a French large scale laser facility dedicated to inertial fusion and plasma physics research. LMJ front-ends are based on fiber laser technology at nanojoule range [1]. Scaling the energy of those fiber seeders to the millijoule range is a way to upgrade LMJ’s front ends architecture and could also be used as seeder for lasers for ELI project for example. However, required performances are so restrictive (optical-signal-to-noise ratio higher than 50 dB, temporally-shaped nanosecond pulses and spatial single-mode top-hat beam output) that such fiber systems are very tricky to build. High-energy fiber amplifiers In 2015, we have demonstrated, an all-fiber MOPA prototype able to produce a millijoule seeder, but unfortunately not 100% conform for all LMJ’s performances. A major difficulty was to manage the frequency modulation used to avoid stimulated Brillouin scattering, to amplitude modulation (FM-AM) conversion, this limits the energy at 170µJ. For upgrading the energy to the millijoule range, it’s necessary to use an amplifier with a larger core fiber. However, this fiber must still be flexible; polarization maintaining and exhibit a strictly single-mode behaviour. We are thus developing a new amplifier architecture based on an Yb-doped tapered fiber: its core diameter is from a narrow input to a wide output (MFD 8 to 26 µm). A S² measurement on a 2,5m long tapered fiber rolled-up on 22 cm diameter confirmed that this original geometry allows obtaining strictly single-mode behaviour. In a 1 kHz repetition rate regime, we already obtain 750 µJ pulses, and we are on the way to mJ, respecting LMJ performances. Beam delivery In LMJ architecture the distance between the nanojoule fiber seeder and the amplifier stages is about 16 m. Beam delivery is achieved with a standard PM fiber, such a solution is no longer achievable with hundreds of kilowatt peak powers. An efficient way to minimize nonlinear effects is to use hollow-core (HC) fibers. The comparison between the different fibers will be presented in the conference. Fiber spatial beam shaping Spatial beam shaping (top-hat profile) is mandatory to optimize the energy extraction in free-space amplifier. It would be very interesting to obtain a flat-top beam in an all-fiber way. Accordingly, we have design and realize a large mode area single-mode top-hat fiber able to deliver a coherent top-hat beam. This fiber, with larger MFD adapted to mJ pulse, will be implemented to perform the spatial beam shaping from coherent Gaussian profile to coherent top-hat intensity profile in the mJ range. In conclusion, we will present an all-fiber MOPA built to fulfil stringent requirements for large scale laser facility seeding. We have already achieved 750 µJ with 10 ns square pulses. Transport of high peak power pulses over 17 m in a hollow-core fiber has been achieved and points out FM to AM conversion management issues. Moreover, spatial beam shaping is obtained by using specifically designed single-mode fibers. Various optimizations are currently under progress and will be presented.

The Laser megajoule (LMJ) is a French large scale laser facility dedicated to inertial fusion research. Its front-ends are based on fiber laser technology and generate highly controlled beams in the nanojoule range. Scaling the energy of those fiber seeders to the millijoule range is a way explored to upgrade LMJ’s architecture. We report on a fully integrated narrow line-width all-fiber MOPA prototype at 1053 nm designed to meet stringent requirements of large-scale laser facilities seeding. We achieve 750 µJ temporally-shaped pulses of few nanoseconds at 1 kHz. Thanks to its original longitudinal geometry and its wide output core (26µm MFD), the Yb-doped tapered fiber used in the power amplifier stage ensures a single-mode operation and negligible spectro-temporal distortions. The transport of 30 kW peak power pulses (from tapered fiber) in a 17 m long large mode area (39µm) hollow-core (HC) fiber is presented and points out frequency modulation to amplitude modulation conversion management issues. A S² measurement of this fiber allows to attribute this conversion to a slightly multimode behavior (< 13dB of extinction between the fundamental mode and higher order modes). Other HC fibers exhibiting a really single-mode behavior (

The Laser Megajoule facility, developed by the CEA is designed to provide the experimental capabilities to study high density plasma physics. The 176 Nd:glass laser beams of the facility will deliver a total energy of 1.4MJ of UV light at 0.35 μm and a maximum power of 400 TW. The laser beams are focused on a micro-target inside a 10-meter diameter spherical chamber. A first bundle of eight laser beams was officially commissioned in October 2014. Since then, several experimental campaigns have been carried out, to qualify LMJ experimental capability and to validate radiative hydrodynamics simulations. New target diagnostics were installed around the target chamber for that purpose. The installation of new bundles is continuing, simultaneously to the physics experiments. A second control room has been dedicated to the first steps of every bundle integration. In parallel with the assembly of the bundles, the laser process is continuously improving (low contrast beam profile, upgraded chamber center reference, improved focal spot profile). Second bundle commissioning has been achieved at the end of 2016 with some physics experiments using the 16 operational beams. The LMJ facility is now operational with two bundles (16 beams) and the bundle commissioning rhythm is increasing. The present performances meet the needed requirements for the physics experiments. We are currently integrating new bundles, increasing the LMJ laser beam energy, and installing new diagnostics in order to achieve the next configurations.

Since the first experimental campaign conducted in 2014 with mid field Gated X-ray Imager (GXI) and two quadruplets (20 kJ at 351 nm) focused on target, the Laser MegaJoule (LMJ) operational capability is still growing up. New plasma diagnostics have been implemented: a large field 2D GXI, two broadband x-ray spectrometers (called DMX and miniDMX), a specific soft x-ray spectrometer and a Laser Entrance Hole (LEH) imaging diagnostic. A series of experiments have been performed leading to more than 60 shots on target. We will present the plasma diagnostics development status conducted at CEA for experimental purpose. Several diagnostics are now under manufacturing or development which include a Streaked Soft X-ray Imager (SSXI), an Equation Of State (EOS) diagnostic suite (“EOS pack”), a Full Aperture BackScattering (FABS) diagnostic, a Near Backscattered Imager (NBI), a high resolution 2D GXI, a high resolution x-ray spectrometer, a specific set of two polar hard x-ray imagers for LEH characterization and a set of Neutron Time of Flight (NTOF) detectors. We describe here the diagnostics design and performances in terms of spatial, temporal and spectral resolutions. Their designs have taken into account the harsh environment (neutron yields, gamma rays, electromagnetic perturbations, debris and shrapnel) and the safety requirements.

The Laser MegaJoule (LMJ) is a French high power laser project that requires thousands of large optical components. The wavefront performances of all those optics are critical to achieve the desired focal spot shape and to limit the hot spots that could damage the components. Fizeau interferometers and interferometric microscopes are the most commonly used tools to cover the whole range of interesting spatial frequencies. Anyway, in some particular cases like diffractive and/or coated and/or aspheric optics, an interferometric set-up becomes very expensive with the need to build a costly reference component or a specific to-the-wavelength designed interferometer. Despite the increasing spatial resolution of Fizeau interferometers, it may even not be enough, if you are trying to access the highest spatial frequencies of a transmitted wavefront for instance. The method we developed is based upon laser beam diffraction intermediate field measurements and their interpretation with a Fourier analysis and the Talbot effect theory. We demonstrated in previous papers that it is a credible alternative to classical methods. In this paper we go further by analyzing main error sources and discussing main practical difficulties.

Relationships between subsurface damage (SSD) depth and peak to valley surface roughness (Rt ) have been widely studied and present a major interest for an easy assessment of the SSD depth. We seek the relation between SSD depth and other surface roughness parameters using the Abbott-Firestone curve on a large campaign of grinding tests (with different abrasive grain size, grinding speed and grinding mode). The results reveal that the Abbott-Firestone parameter Mr2 , which can be linked to the volume fraction of valley in the roughness profile, is more accurate than Rt for an assessment of the SSD depth and that the relationship between Mr2 and the SSD depth varies when changing the grinding mode. Keywords: subsurface damage, surface roughness, Abbott Firestone, fused silica glass 1. INTRODUCTION Many studies tried to define an empi rical relationship between subsurface damages and surface roughness because these damages under surface, mainly due to the manufacturing steps, limit greatly the lifetime of optical components in high energy laser applications such as Laser Megajoule (LMJ) or National Ignition Facility (NIF)

High-power, single longitudinal mode, single-polarization beam generates from a polarization maintaining double-clad Yb-doped silica fiber laser is more applicable for inertial confinement fusion (ICF) applications. The laser beamlets that converge on a target originate from a low power fiber laser in the master oscillator room (MOR). The laser seed signal amplifies through several cascaded amplifier chains in the front-end system. Today, the most famous fusion facilities, including National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL), Ligne d’Integration Laser- Laser Megajoule (LIL-LMJ) and OMEGA utilize fiber-based lasers. In this paper, we have compared various pumping modes comprising end-pumped master oscillator-power amplifier (MOPA) configuration and distributed sidepumped multifiber series fiber laser for scaling up the power of large-mode area (LMA) Yb:silica fiber laser for forward pumping regime.

The path to successful inertial confinement fusion (ICF) requires to observe and control the micro balloon deformations. This will be achieved using X-ray microscope among other diagnostics. A high resolution, high energy X-ray microscope involving state-of-the-art toroidal mirrors and multilayer coatings is described. Years of experiments and experience have led to a small-scale X-ray plasma imager that proves the feasibility of all the features required for a LMJ diagnostic: spatial resolution of 5μm, broad bandwidth, millimetric field of view (FOV). Using the feedback given by this diagnostic, a prototype for the Laser MegaJoule (LMJ) experiments has been designed. The experimental results of the first diagnostic and the concepts of the second are discussed.

International audience; A Petawatt facility called PETAL (PETawatt Aquitaine Laser) is under development near the LMJ (Laser MegaJoule) at CEA Cesta, France. PETAL facility uses chirped pulse amplification (CPA) technique. This system needs large pulse compression gratings exhibiting damage threshold of more than 4 J/cm2 in normal beam at 1.053μm and for 500fs pulses. In this paper, we present our recent progress and developments of such pulse compression gratings. We have shown in previous works that the enhancement of the near electric field inside the pillars of the grating drives the damage threshold. This was evidenced from a macroscopic point of view by laser damage testing. We herein demonstrate that damage morphology during damage initiation at the scale of the grating groove is also consistent with this electric field dependence. Some recent grating designs will also be detailed.

The LMJ (Laser MegaJoule) is dedicated to inertial confinement fusion. To perform this type of experiment, 160 square beams are frequency converted and focused onto a target filled with a deuterium tritium mixture. We propose to review how these beams are shaped along their propagation through the LMJ. Going upstream from the target to the laser source, specific optics has been designed to meet the beam shaping requirement. A focusing grating and a pseudorandom phase plate concentrate the energy onto the target. A deformable mirror controls and compensates the spatial phase defect occurring during the propagation through the main slab amplifiers. A liquid crystal cell shapes the beam in order to compensate the gain profile of the main amplifiers. It also protects the growth of damages that take place in the final optics of the chain. At last, a phase mirror generates a square flat top mode from a gaussian beam within a regenerative amplifier. All these optical components have one common principle: they control the phase of the spatial laser field.

This paper investigates hot spot characteristics generated by the superimposition of multiple laser beams in the Laser MegaJoule configuration. First, properties of speckle statistics are studied in the context of the superimposition of several laser beams. The case of a single quadruplet is studied. Values of the speckle width and of the speckle length as well as of the abundance of the speckles are compared to the results given by numerical simulations. Application to the speckle patterns generated in the Laser MegaJoule configuration in the zone where all the beams overlap is then presented. The case of three different polarizations is investigated: P polarization, S polarization and the case of Double Polarization smoothing (DPS). It is found that the sizes of the speckles and their abundance depend on the choice of the polarization and that DPS seems to be the best option.

The Laser Integration Line (LIL) was first designed as a prototype to validate the concepts and the laser architecture of the Laser MegaJoule (LMJ). The LIL facility is a 4-beam laser representing a quad structure of the LMJ. A set of test campaigns were conducted to safely ramp up laser performance. The main goal was to measure quad-specific features such as beam synchronization and focal spot (size, smoothing contrast ratio or irradiation nonuniformity) versus the LMJ requirements. Following the laser commissioning, the LIL has become a major instrument dedicated to the achievement of plasma physics experiments for the French Simulation Program and was also opened to the academic scientific community. One of the attributes of the LIL facility is to be very flexible to accommodate the requests of plasma physicists during campaigns. The LIL is constantly evolving to best meet the needs of target physicists. Changes made or planned are either to improve the quality of laser beams, or to increase the LIL Energy-Power operating space. To optimize preparation and design of shot campaigns, the LIL performance status has been elaborated. It gives information about the characteristics of the laser in terms of near field and far field, defines the steps to maintain performance, explains how the facility responds to the request, details settings (smoothing, shaping of the focal spot, energy, temporal pulse shaping, beam pointing) and gives the limits in energy and power. In this paper, an overview of the LIL performance is presented.

This paper describes the alignment system developed on the Laser Megajoule facility, allowing to focus the laser beams and to point the plasma diagnostics on the target. After an overview of the main laser components and alignment architecture, we detail some major equipments as the 6 tele-microscopes used to align the target, the continuous phase plate within the final optics assembly, the plasma diagnostic green pointer and the common reference which is the cornerstone of the chamber center alignment. Finally we present some results obtained on the telemicroscope prototype and a photometric prototype of the common reference. The expected performance of the alignment system will also be discussed.

This paper describes the synchronisation system under development on the Laser Megajoule (LMJ) in order to synchronize the laser quads on the target to better than 40ps rms. Our architecture is based on a Timing System (TS) which delivers trigger signals with jitter down to 15ps rms coupled with an ultra precision timing system with 5ps rms jitter. In addition to TS, a sensor placed at the target chamber center measures the arrival times of the 3ω nano joule laser pulses generated by front end shots.

Nanosecond Laser-Induced Damage (LID) in potassium dihydrogen phosphate (KH 2 PO 4 or KDP) remains anissue for light-frequency converters in large-aperture lasers such as NIF (National Ignition Facility, in USA) andLMJ (Laser MegaJoule, in France). In the “nal optic assembly, converters are simultaneously illuminated bymultiple wavelengths during the frequency conversion. In this con“guration, the damage resistance of the KDPcrystals becomes a crucial problem and has to be improved. In this study, we propose a re“ned investigationabout the LID mechanisms involved in the case of a multiple wavelengths combination. Experiments based on anoriginal pump-pump set-up have been carried out in the nanosecond regime on a KDP crystal. In particular, theimpact of a simultaneous mixing of 355 nm and 1064 nm pulses has been experimentally studied and comparedto a model based on heat transfer, the Mie theory and a Drude model. This study sheds light on the physicalprocesses implied in the KDP laser damage. In particular, a three-photon ionization mechanism is shown to beresponsible for laser damage in KDP.Keywords: KDP crystal, multi-wavelength laser-induced damage, wavelength combination eects

The laser Megajoule (LMJ) project was launched in 1995 by the French Atomic Energy Commission and is aimed at developing a facility to achieve inertial confinement fusion. The LMJ architecture is based on 176 laser beamlines. To provide these 176 high-powered beams when required for subsequent operations, one of the main issues consists in reliably aligning the laser amplifier sections. This paper provides an overview of the image processing techniques developed to identify and measure the beam centering and pointing directions. These techniques have been developed and tested thoroughly against sets of up to 450 images representing both nominal and extreme conditions acquired during the initiation and power rising of the Laser Integration Line (LIL). This facility is fully consistent with the LMJ requirements, a complete laser chain with 4 beamlines. After presenting the basic design principles, we focus on the demonstrated performances measured.

In this paper, we report on the latest experimental results on UV-visible multichannel streak cameras for pulse shape measurement on CEA Laser Megajoule facility. Two main characteristics of these instruments were studied: linear dynamic range and signal-to-noise ratio. With a standard PHOTONIS P820PSU bilamellar streak tube working at -10kV, a linear dynamic range of 240 was measured simultaneously in 7 channels at a 25 ns sweep duration. This value is adequate for the measurement of a high contrast laser pulse. Nevertheless, the signal-to-noise ratio at the bottom of the channel dynamic range is not sufficient to reach the requirements on the power measurement accuracy (better than few percents rms). This would require that the maximum output peak current of the P820 streak tube (0.25 mA @ - 10 kV) be improved by a factor of 5. Finally, the streak cameras that we deploy will have to be fully calibrated.

During the development of the laser megajoule (LMJ), a high power laser facility dedicated to DT fusion, CEA has made important efforts to understand and improve laser induced damage threshold of fused silica optics at the wavelength of 351 nm. For several years, with various industrials and academics partners, we have focused on optimizing the grinding, lapping and polishing processes to increase materials performance. In this paper, we describe our efforts in various fields: subsurface damage characterization, lapping process simulation, diamond grinding and lapping machine instrumentations, ... Our concern is to control and manage the material removal at each step of the process in order to reduce the cracks region extension and thus to diminish the damage density.

The lifetime of silica optics in high power laser facility as the Laser MegaJoule (LMJ) is typically limited by the initiation of surface damages and their subsequent growth. To prevent this problem, a mitigation technique is used: it consists in a local melting of silica by CO2 laser irradiation on the damage site. Because of the difficulty to produce efficient mitigated sites with large depth, the characterization of damage site to mitigate is very important. In this context, confocal microscopy appears to be an efficient solution to detect precisely cracks present under the damage site.

This paper presents a statistical method for determining the dimensions, tolerance and specifications of components for the Laser MegaJoule (LMJ). Numerous constraints inherent to a large facility require specific tolerances: the huge number of optical components; the interdependence of these components between the beams of same bundle; angular multiplexing for the amplifier section; distinct operating modes between the alignment and firing phases; the definition and use of alignment software in the place of classic optimization. This method provides greater flexibility to determine the positioning and manufacturing specifications of the optical components. Given the enormous power of the Laser MegaJoule (over 18 kJ in the infrared and 9 kJ in the ultraviolet), one of the major risks is damage the optical mounts and pollution of the installation by mechanical ablation. This method enables estimation of the beam occultation probabilities and quantification of the risks for the facility. All the simulations were run using the ZEMAX-EE optical design software.

This paper presents recent studies on the mechanical support system of Laser MegaJoule large dimension optical components. The dimensions of these optical components and their orientation resulted in considerable distortions due to gravity. An original and very simple method based on an isostatic mounting together with industrial springs located on the edges of the component is presented. Springs are used to compensate for gravity distortion. This system provides improved support performance at very low cost (for example, PV less than 0.5μm for a 610mm * 430mm * 80mm silicate optic at 45° from the vertical). The support systems are optimized with IDEAS finite element analysis software and validated with experimental measurements.

The Laser MegaJoule (LMJ) final optics assembly: - performs the frequency conversion (from 1053nm to 351nm), - focuses 4 laser beams, - addresses any point of the focal volume. This paper introduces recent studies concerning the new optomechanical design of this system. The following topics are presented: - the new optical frame design - the 3 axis movement system - the interface between the optical Line Replaceable Unit (LRU) and the structure (TTT system) - the principal optic LRU presentation.

The laser Megajoule (LMJ) project was launched in 1995 by the French Atomic Energy Commission and is aimed at developing a facility to achieve inertial confinement fusion. The LMJ architecture is based on 240 laser beamlines to be focused onto a millimetre size target. To meet this challenge, one of the main issues consists in a proper control of the quality of the beam wavefront at the output of the laser. For this purpose, the LMJ makes use of a large-size adaptive mirror (n amed M1 in the actual laser baseline), with 400 by 400 mm² useful aperture. This paper provides an overview of the M1 prototype, using 39 mechanical actuators. This mi rror was made by the French company CILAS© together with the help of ISP© for the mechanical actuators. CEA, Cilas an d ISP have joined their efforts to complete the overall design. The prototype has been tested on the Laser Integration Line (LIL). This facility is fully consistent with the LMJ requirements, a complete laser chain which comprises 4 beamlines. After presenting the basic design principles, we focus on the demonstrated performance measured on a dedicated setup, as well as on LIL facility. Keywords: deformable mirror, adaptive optics, astatic actuator, mechanical actuator, wavefront control, wavefront correction, high power laser

We review the development of a new glass formulation and manufacturing technology for a neodymium-doped phosphate based laser glass used in the LLNL National Ignition Facility (NIF) and the French Laser MegaJoule (LMJ). The glass development process built on both accumulated experience and the utilization of glass science principles, and the resultant new glass offers superior laser properties in combination with improvements in physical properties to enhance manufacturing yield. Essentially in parallel, a continuous melting production line was also conceived, designed and operated to meet both the schedule and cost targets of the NIF. Prior to 1997, phosphate laser glasses were manufactured by a discontinuous pot-melting process with limited production rate and associated high costs. The continuous melting process met several technical challenges, including producing glass with low residual water content and absence of inclusions which become damage sites when used in the NIF laser system.

Powerful laser systems of new generation are created nowadays - National Ignition Facility (NIF, USA), Laser Megajoule (LMJ, France) and ISKRA-6 (Russia). The most important problem after power characterization is formation of radiation of the necessary quality. For this purpose the use of adaptive optical systems (AOS) is provided in all three facilities. The present work is mainly devoted to characteristics definition and working off of the most problem AOS elements - wavefront sensor and wide-aperture deformable mirror for "ISKRA-6".

The lifetime of optical components submitted to high laser fluences is degraded under organic contaminant environment. The molecular background of the Ligne d'Integration Laser (LIL), prototype of the future Laser Megajoule, might reduce the laser damage threshold of exposed fused silica surfaces. This paper reports the interaction effects between pure model contaminant deposits and a pulsed 1064 nm laser radiation on the coming out of mirror damage. The experimental setup allowed us to condense nanolayers of model contaminants on optics, the deposit impacts were then investigated by Laser Induced Damage Threshold (LIDT) tests in Rasterscan mode. In order to highlight physical processes emphasizing the emergence of optics damage, we characterized the irradiated deposit using interferometric microscopy analysis and spectrophotometric analysis. The challenge was to determine physical and phenomenological processes occurring during the irradiation of a pure contaminant deposit with a 1064 nm pulsed laser and to study the impact of this model contaminant on the LIDT of dielectric SiO 2 /HfO 2 mirrors.

Correctly determining the lifetime of optical components is a major issue in the operation of high power laser facilities such as the Laser Megajoule developed by the Commissariat a l'Energie Atomique (CEA). Laser damage that occurs at the surface is a main cause of optical aging, and may lead to dramatic degradation of the focal spot. To estimate the effect of such defects, we measured and calculated the distortion of the focal spot induced by "model defects." These "model defects" are circular silica dots randomly distributed on a silica substrate. The experiments were conducted in the ANTALIA facility at the Centre d'Etude Scientifique et Technique d'Aquitaine (CESTA). We performed numerical calculations of beam propagation with the Miro software, developed by the CEA. We obtain a remarkable correlation between measurements and simulations in the central part of the focal spot for large defects. However, experimental noise and measurement dynamics become serious problems when we confine our attention to smallerdefects (

This paper presents recent studies of the propagation of high-power laser beams like Laser Integration Line (LIL) and Laser Megajoule (LMJ) beams when interactions occur with environmental pollution particles. The studies are mainly achieved with the CEA-DAM MIRO beam propagation code. The highest intensifications in the downstream propagation are obtained for phase objects such as dielectric particles rather than for amplitude objects such as metallic particles. Dramatic amplifications of Kerr nonlinear effects both inside the component and at its rear-surface can occur depending on the particle size.

In this paper we report several studies of the propagation of the Laser Integration Line (LIL) and Laser Megajoule (LMJ) beams when interactions occur with optical components defects. These studies are mainly achieved on a numerically predictive basis with the CEA MIRO beam propagation code. The flaws that we considered are located at the front or rear surface of the optical components. These surface flaws correspond to engineered defects such as scratches and to surface damage resulting from laser-induced growth process, from mitigation process or from target interactions debris. Results account for the possible downstream impacts of flaws at the rear-surface of the optics and from one component to another along the laser chain. In particular, the influence on the LIL/LMJ end-of-line focal spot intensity and size is predicted.

Ongoing laser fusion experiments like the "Laser MegaJoule Project" and the "National Ignition Facility" have created a strong demand for large optical lenses of special grade fused silica. The required lens dimension poses several challenges to the manufacturing process. One of the key issues is to provide a suitable measurement technique, which is capable to fulfill the extreme demands for characterizing the optical homogeneity of those large fused silica blanks. We report on our first results achieved with an interferometer system that was installed to explore the potential and feasibility of stitching interferometry. Although the principle of stitching is well known, it had to be adapted to the special "oil-on measurement technique" that is necessary to characterize lens blanks without expensive surface polishing.

The high-power Laser MegaJoule (LMJ) for inertial confinement fusion experiments that is currently under construction at CEA-CESTA in France will require a high number of large aperture Pockels cells and frequency converters made of potassium dihydrogen phosphate (KDP) and DKDP (Deuterated KDP). These optical components will be operated several times a year at fluences close to their Laser Induced Damage Threshold (LIDT) which may reduce significantly their lifetime and increase substantially the maintenance costs of the LMJ. In a global effort to reduce these costs we have designed the SOCRATE facility as a complete system for materials characterization, LIDT measurement and optics conditioning by laser to increase their lifetime. In this paper we examine the relevance of adapting the laser conditioning process to the bulk KDP quality. First the existence of heterogeneities in large KDP crystals is stressed; next the LIDTs in the different parts of the crystals using focused or collimated beams are compared. Finally we focus on the efficiency of the excimer conditioning process in the different growth sectors of KDP samples and demonstrate that for the current conditioning process the efficiency depends only weakly on the original material heterogeneities.

This paper presents an overview of a photometer developed by CEA to characterize large coated optics (up to 800 x 400 mm 2 ).

Commercial off-the-shelf (COTS) single-mode and multimode germanosilicate optical fibres are intended for use as part of control-command applications in the future Laser Megajoule facility. We evaluated their vulnerability to the radiative environment associated to this project by characterizing their transient radiation responses at infrared wavelengths (1064, 1310 and 1550 nm) after high dose rate exposure (> 10 8 Gy/s). Our results showed that these COTS fibres exhibit very different transient responses (10 -9 s to 1 s after pulse) and permanent damages (times > 1 s), depending on the wavelength of transmitted signal and on their compositions. Presented results could be used to define several choices to ensure the functionality of these applications for both shortest times after a shot (10- 9 to 10 3 s) and during the whole LMJ lifetime (30 years).

The French Commissariat a l'Energie Atomique (CEA) began the construction of the Laser Megajoule (LMJ), a 240-beam laser facility, at the CEA laboratory CESTA near Bordeaux. The LMJ will be a cornerstone of the CEA "Programme Simulation", the French analog to the US Stockpile Stewardship Program. The LMJ is designed to deliver 2MJ of 0.35 μm light to targets for high energy density physics experiments and to ultimately obtain ignition and propagating burn with DT targets in the laboratory. The Scientific conception and system design was completed in 1999 and was followed by the Demonstration of an Engineering Prototype which was achieved in early 2003 with operation of one beam of the Ligne d'Integration Laser (LIL) at CESTA, with 9.5 kJ of UV light (0.35 μm) in less than 9 ns from a single laser beam. The Ralization phase of the LMJ facility was initiated in March of 2003 with the construction start of teh building and the target chamber. This paper will present results from the commissioning phase of the LIL program in 2003 and 2004. The activation and commissioning of the full 8 beamlines of LIL over the next 2 years will be part of determining the final specifications and integration and commissioning plans for the LMJ which is expected to demonstrate first light performance through 240 beams by 2010.

The Ligne d’Integration Laser (LIL) is a prototype installation at scale 1 of one of the 30 lasers of future Laser Mega Joule. It is intended to validate technological choices made for LMJ and to prepare its exploitation. The facility will contain nearly 10.000 optics and over 4000 m 2 of mirrors. Cleanliness will be an essential matter in the facility since contamination of optics can reduce their laser damage threshold. Hence, airborne molecular contamination (AMC) has been sampled near optics in strategic places of the LIL. These samplings have shown high levels of organic compounds, notably in the amplifying section, which is expected to be th e most sensitive part in the LIL. Suspecting a local source of contamination, outgassing tests of typical materials const ituting the amplifying section have been carried out. Among them, one sealing material has been identified as a source of organic contamination near the optics. Effects of this pollution have been investigated by a measurement of laser damage threshold after intentional contamination of optics. This work shows the complexity of the outgassing contamination issue, since several steps are necessary to evaluate the effects of this contamination on optical surfaces: air samplings, identification of sources, outgassing tests, intentional contamination of optics and finally measurement of laser damage threshold.

For high power laser applications like the "Laser Megajoule" facility under construction in France, laser-induced damage threshold (LIDT) in fused silica is a limitation. CEA has made efforts to improve LIDT at the wavelength of 351 nm. Polishing and post polishing processes have been optimized. Laser damage sites density was decreased by several orders of magnitude by combining different fabrication steps. In order to further enhance optical laser resistance and to remove damaged sites on full-size optics, several small-beam raster scanning techniques have been studied and developed to condition fused silica optics. To stop the growth of damage sites, a continuous CO2 laser was used to re-melt them. Laser induced damage tests, performed on instrumented and automated facilities, are reported in order to check and illustrate the effectiveness of these treatments. Damage initiation studies as well as damage growth measurements are presented.

As part of LMJ project (Laser Megajoule), CEA has built the LIL - Ligne d'Integration Laser - a LMJ prototype. This prototype uses full sized optics (400x400 mm 2 ) with very tight specifications. SESO is one of the suppliers of optical components for this laser, among them filtering lenses - called L3 and L4 - used at 1053 nm (1ω), thin flat plates - continuous phase plates and debrishield - and thick windows, all used at 351 nm (3ω). All these optics are in fused silica and combine good wavefront specifications, very low roughness and no or few surface quality defects. Today, including spare parts, about 40 components have been produced. The purpose of this paper is to describe the facilities for grinding, polishing and finishing these optics. Computer Controlled Polishing robot for lenses Double side polishing machine for flat optics After a brief presentation of the specific metrology used, we give a detailed overview of the performances obtained on the produced components. This work is related to the LIL - LMJ project directed by CEA, France.

In the field of thin film coatings, sol-gel (SG) process is an alternative to the conventional Physical Vapor Deposition (PVD) techniques. Sol-gel process is particularly competitive on large-area or fragile substates by taking advantage of various liquid phase deposition techniques performed at room temperature and atmospheric pressure, coupled with the versatility of organo-metallic chemistry. Developed by the French Commission for Atomic Energy (CEA) since 1985 for its former high-power lasers generation, optimized sol-gel coatings proved also very resistant to laser energy. In 1998, THALES Angenieux (TAGX) was selected by CEA to provide all the sol-gel coatings dedicated to the French Laser MegaJoule (LMJ) prototype, named Ligne d'Integration Laser (LIL). In cooperation with Saint-Etienne Pole Optique et Vision (POV), TAGX initiated the building of a sol-gel technological platform (SGPF) aimed at demonstrating the feasibility of production of optical and functional coatings on large area substrates. A technology transfer was performed by CEA (Le Ripault) to TAGX focusing on the manufacture mainly of single-layer antireflective coatings (SLAR), but also of multi-layer AR-coatings and of multi-layered highly reflective (HR)-coatings. Since beginning of 2001 and using SGPF equipments, TAGX successfully coated within specificaitons and schedule most of the 300 optics required for LIL activation. After this 2 years 1/2 production campaign in pre-industrial conditions, we can now analyse the advantages of each deposition technique used, the repeatability of the several processes, and the performance of the various coatings.

In the Laser MegaJoule indirect drive experiments, the time-averaged radiation asymmetry on a Deuterium-Tritium (DT) capsule must be minimized to achieve high-yield implosions. A two-dimensional model estimates the time-averaged effect of power imbalance, laser beam pointing and target fabrication errors on the final DT deformation, which is then submitted to an ignition threshold. As these errors will take random values from one LMJ shot to another, the robustness study aims at quantifying the probability of failing to reach ignition. Here, we focus on laser power imbalance. We distinguish two types of error sources in laser performance, according to whether they take long-time (more than the laser pulse duration) or short-time (less than the laser pulse duration) correlated values. Indeed, as the final DT deformation results from the whole laser pulse history, the failure probability depends on the error time-correlation. A 1D-time model of the laser beam power, from the front-end to the target, was developed to quantify the variations of the output power imbalance due to the source contributions. Taking into account this detailed time behavior, instead of modeling all errors as long-time correlated values, leads to cut the global effect of power imbalance on ignition probability by half.

One proposed capsule design for the Laser Megajoule facility is analyzed to determine surface finish specifications required to achieve ignition and propagated burn. We estimate the sensitivity of this capsule to hydrodynamic instabilities by means of direct two-dimensional simulations. Configuring multimode perturbations located at the DT ice and ablator surfaces, the fusion yield is predicted.

As part of the LMJ (Laser Megajoule) program, CEA is building the LIL laser with full size optics and LMJ requirements. SAGEM has been selected as the supplier of large optical components and coatings with very high laser- induced damage threshold. Including spare parts, about 100 mirrors 610*430 mm2 with LIDT-3ns>25 J/cm2 have to be produced. Using a 5 m3 vacuum chamber and the 100 J/cm2 mirror coating process developed at CEA-LETI, with Hafnium and SiO2 materials, we are now typically in a serial production phase. To date, about thirty mirrors have been delivered. This paper focuses on the acceptance tests performed after coating, at SAGEM then CEA: LIDT measurement and Raster-Scan on samples; reflectance mapping on CEA automatic photometer; reflected wavefront deformation with n 800 mm/1ω CEA interferometer.

We carried out a set of experiments on the Omega laser facility at Rochester with Laser MegaJoule like indirect drive irradiation. We studied the irradiation non-uniformity with the foam ball radiography technique and the implosion symmetry with (D2 + Argon) filed capsules core emission. Cylindrical 'Nova scale 1' thin wall hohlraums were used. Forty of the Omega beams, arranged in three cones on each side of the hohlraum were used to create the x-ray drive. Eight additional beams were used on a Ti source to radiograph the foam balls. The shaped laser pulse was about 3 ns duration. The radiation drive was measured on each shot. The images were recorded with a 5 micrometers resolution Gated X-ray Imager coupled to a CCD camera.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Research on optical communication behavior in radiative environments is a key point for the design of diagnostic links for the large physic instruments (Laser MegaJoule at CEA, Large Hadron Collider at CERN). For years, the radiation tolerance of several types of emitters (light emitting diode and laser diode (LD)) have been tested with promising results for the LDs. New technologies and devices (Vertical-Cavity Surface-Emitting Lasers (VCSEL)) have recently appeared as promising candidates to replace conventional edge emitting LDs. The shorter wavelength VCSELs (below 1 micrometers ) are well adapted for short distance data links, due to their low threshold current, high efficiency and large possibilities for integration.