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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.

X-ray framing cameras (XRFCs) are routinely used at the Laser MegaJoule facility in x-ray imaging plasma diagnostics around the target chamber. Most of these diagnostics are based on multilayer x-ray toroidal mirrors under grazing incidence. The absolute calibration of the XRFCs is expressly expected both to optimize the signal-to-noise ratio for the dynamic range for specific experiments and to quantitatively process the data. The purpose of this paper is to describe our technique to routinely calibrate these instruments in the sub-keV spectral range. The calibration presented in this work was carried out using the XRFC enclosed in a sealed “airbox” structure. This calibration relies on a Manson source recently upgraded to operate at high emission current (5 mA) with 10 kV accelerating voltage to work with a 1-m grazing-incidence Rowland circle monochromator. The framing camera sensitivity was absolutely determined over the 0.1–1.2 spectral range with an average uncertainty of 2.4% rms while operating in DC mode. Finally, we compare the results with a synchrotron source calibration previously obtained and a theoretical model.

Targets experimented on the Laser Megajoule (LMJ) facility are composed of amorphous hydrogenated carbon capsules. Some of them present rippled surface features like sinusoidal functions. Other experimented targets are hohlraum-containing capsules. The main difficulty when analyzing the machined capsules is to characterize the feature’s orientation and the sinusoidal shape featured in the capsule thickness by laser machining. For the capsule enclosed by the hohlraum, the main challenge is to characterize the capsule centering inside the assembled hohlraum. X-ray tomography is used to realize measurement, and obtained results are presented in this paper.

The Laser MegaJoule (LMJ) is a high-power laser dedicated to laser-plasma experiments. At the beginning of the project in the mid-1990s, an optical metrology laboratory was created at CEA to help accomplish all the steps in the construction of this laser. This paper proposes an overview of the capabilities of this metrology laboratory in four main fields: surface imperfections, photometry, laser damage measurement, and wavefront measurement. The specificities for high-power laser optics in each domain are highlighted as well as the specific features that make our instruments unique.

While ITER holds the spotlight in the field of controlled fusion this success should not hide the fact that several different kinds of technology are being explored in the quest to achieve nuclear fusion on Earth. In this chapter we are going to look at “alternative” projects, such as the National Ignition Facility in the United States and the Laser Megajoule in France, investigating inertial confinement fusion (ICF). In addition, a dozen fusion-related startups supported by private money have recently emerged and are moving fast in this competitive field. Fusion has indeed attracted high-profile investors over the last few years. Several small companies have entered the still embryonic market of fusion reactors, such as Alpha Energy (recently renamed TAE Technologies) in California, Helion Energy in Seattle, LPPFusion in New York, General Fusion in Canada, Tokamak Energy, First Light Fusion, and Applied Fusion Systems in the United Kingdom, and a new company set up by MIT in Boston called Commonwealth Fusion Systems. They are all exploring new concepts. The total investment made in these entrepreneurial fusion projects is estimated to be about USD1.5 billion. In any case these stories seem to support Bill Gates’ view declaring in February 2016: “We need a massive amount of research into thousands of new ideas—even ones that might sound a little crazy—if we want to get to zero emissions by the end of this century. What we need to get that probability [of a breakthrough] up to be very high is to take 12 or so paths to get there … Like carbon capture and sequestration is a path. Nuclear fission is a path. Nuclear fusion is a path. Solar fuels are a path. For every one of those paths, you need about five very diverse groups of scientists who think the other four groups are wrong and crazy.” The proliferation of these public and private initiatives can only be welcomed. Dynamism and opportunities in a scientific field are measured by the research effort that accompanies them and by related indicators such as the number of publications and patents. From this point of view fusion is a powerful driver of scientific research and technological development.

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 needs optical components which are coated by sol gel with a post-treatment. This process induces crazing. We present the characterizations to understand the phenomenon and suggest leads to solve this problem.

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 advent of high-power lasers facilities such as the National Ignition Facility (NIF), and the Laser Megajoule (LMJ) in the near future, opens a new era in the field of High Energy Density Laboratory Astrophysics. These versatile laser facilities will provide unique platforms to study the rich physics of nonlinear and turbulent mixing flows. The extended laser pulse duration could be harnessed to accelerate targets over much larger distances and longer time periods than previously achieved. We report on the first results acquired on NIF with the ablative Rayleigh–Taylor Instability (RTI) platform. A 20-ns X-ray drive is tailored to accelerate planar modulated samples into the highly-nonlinear bubble merger regime. Based on the analogy between flames front and ablation front, highly nonlinear RTI measurements at ablation front can provide important insights into the initial deflagration stage of thermonuclear supernova of Type Ia. We also report on an innovative concept used to create even longer drive on multi-beam laser facilities. The multi-barrel hohlraum (Gattling Gun) approach consists, here, of three adjacent cavities, driven in succession in time. This novel concept has been validated on the Omega EP laser system. The three cavities were irradiated with three 6–10 ns pulse UV beams and a 30 ns, 90 eV X-ray radiation drive was measured with the time-resolved X-ray spectrometer μDMX. This concept is promising to investigate the pillar structures in the Eagle Nebula or for photoionization studies which require a steady light source of sufficient duration to recreate relevant physics.

The advent of high-power lasers facilities such as the National Ignition Facility (NIF), and Laser Megajoule (LMJ) in the near future opens a new era in the field of High Energy Density Laboratory Astrophysics. The LMJ, keystone of the French Simulation Program, is under construction at CEA/CESTA and will deliver 1.5 MJ with 176 beamlines. The first physics experiments on LMJ will be performed at the end of 2014 with 2 quadruplets (8 beams). The operational capabilities (number of beams and plasma diagnostics) will increase gradually during the following years. We describe the current status of the LMJ facility and the first set of diagnostics to be used during the commissioning phase and the first experiments. The PETAL project (PETawatt Aquitaine Laser), part of the CEA opening policy, consists in the addition of one short-pulse (500 fs to 10 ps) ultra-high-power, high-energy beam (a few kJ compressed energy) to the LMJ facility. PETAL is focalized into the LMJ target chamber and could be used alone or in combination with LMJ beams. In the later case, PETAL will offer a combination of a very high intensity multi-petawatt beam, synchronized with the nanosecond beams of the LMJ. PETAL, which is devoted to the academic research, will also extend the LMJ diagnostic capabilities. Specific diagnostics adapted to PETAL capacities are being fabricated in order to characterize particles and radiation yields that can be created by PETAL. A first set of diagnostics will measure the particles (protons/ions/electrons) spectrum (0.1–200 MeV range) and will also provide point projection proton-radiography capability. LMJ/PETAL, like previously the LIL laser [X. Julien et al., Proc. SPIE 7916 (2011) 791610], will be open to the academic community. Laboratory astrophysics experiments have already been performed on the LIL facility, as for example radiative shock experiments and planetary interiors equation of state measurements.

The project of Inertial Confinement Fusion as the Laser MegaJoule (LMJ) needs numerous optical components. The lenses and windows are coated by sol gel to increase their optical transmission in order to have a high laser damage threshold. These coatings need a post-treatment to increase their adhesion to the substrate. This process induces crazing when the layer is thick (antireflective layer at 1053 nm). We are going to present the several characterizations made to understand the phenomenon and to suggest a few leads to solve this problem.

The physics of compressible turbulence in high energy density (HED) plasmas is an unchartered experimental area. Simulations of compressible and radiative flows relevant for astrophysics rely mainly on subscale parameters. Therefore, we plan to perform turbulent hydrodynamics experiments in HED plasmas (TurboHEDP) in order to improve our understanding of such important phenomena for interest in both communities: laser plasma physics and astrophysics. We will focus on the physics of supernovae remnants which are complex structures subject to fluid instabilities such as the Rayleigh–Taylor and Kelvin–Helmholtz instabilities. The advent of megajoule laser facilities, like the National Ignition Facility and the Laser Megajoule, creates novel opportunities in laboratory astrophysics, as it provides unique platforms to study turbulent mixing flows in HED plasmas. Indeed, the physics requires accelerating targets over larger distances and longer time periods than previously achieved. In a preparatory phase, scaling from experiments at lower laser energies is used to guarantee the performance of future MJ experiments. This subscale experiments allow us to develop experimental skills and numerical tools in this new field of research, and are stepping stones to achieve our objectives on larger laser facilities. We review first in this paper recent advances in high energy density experiments devoted to laboratory astrophysics. Then we describe the necessary steps forward to commission an experimental platform devoted to turbulent hydrodynamics on a megajoule laser facility. Recent novel experimental results acquired on LULI2000, as well as supporting radiative hydrodynamics simulations, are presented. Together with the development of LiF detectors as transformative X-ray diagnostics, these preliminary results are promising on the way to achieve micrometric spatial resolution in turbulent HED physics experiments in the near future.

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.

Scratches at the surface of fused silica optics can be detrimental for the performance of optical systems. A carbon dioxide (CO2) laser is an interesting tool to remove those scratches because it can melt efficiently the silica in a rapid and localized way, without generating debris. In this article, we propose a new process for optical fabrication, which uses a CO2 laser to remove scratches between polishing and finishing steps. This is a linear process with no iterative polishing operations for scratch removal. This process is applied on an optic representative of laser megajoule facility production. Indeed, we succeed in removing a 10 μm deep scratch and we demonstrate that this laser operation increases the laser damage threshold by a factor of three in fluence.

The Alternative Energies and Atomic Energy Commission (CEA-CESTA, France) built a specific double crystal monochromator (DCM) to perform calibration of x-ray cameras (CCD, streak and gated cameras) by means of a multiple anode diode type x-ray source for the MegaJoule Laser Facility. This DCM, based on pantograph geometry, was specifically modeled to respond to relevant engineering constraints and requirements. The major benefits are mechanical drive of the second crystal on the first one, through a single drive motor, as well as compactness of the entire device. Designed for flat beryl or Ge crystals, this DCM covers the 0.9–10 keV range of our High Energy X-ray Source. In this paper we present the mechanical design of the DCM, its features quantitatively measured and its calibration to finally provide monochromatized spectra displaying spectral purities better than 98%.

The “Commissariat a l’energie atomique et aux energies alternatives” (CEA) studies and designs advanced X-ray diagnostics to probe dense plasmas produced at the future Laser MegaJoule (LMJ) facility. Mainly for X-ray imaging with high spatial resolution, different types of multilayer mirrors were developed to provide broadband X-ray reflectance at grazing incidence. These coatings are deposited on two toroidal mirror substrates that are then mounted into a Wolter-type geometry (working at a grazing angle of 0.45°) to realize an X-ray microscope. Non-periodic (depth graded) W/Si multilayer can be used in the broad photon energy range from 2 keV to 22 keV. A third flat mirror can be added for the spectral selection of the microscope. This mirror is coated with a Mo/Si multilayer for which the d-spacing varies in the longitudinal direction to satisfy the Bragg condition within the angular acceptance of the microscope and also to compensate the angular dispersion due to the field of the microscope. We present a study of such a so-called Gobel mirror which was optimized for photon energy of 10.35 keV. The three mirrors were coated using magnetron sputtering technology by Xenocs SA. The reflectance in the entire photon energy range was determined in the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the synchrotron radiation facility BESSY II in Berlin.

Optical coatings for fusion-class laser systems pose unique challenges, given the large substrate sizes, the high intensities incident on the coatings, and the system-focusing requirements, necessitating a well-controlled optical wavefront. Significant advancements have taken place in the past 30 years to achieve the coating capabilities necessary to build laser systems such as the National Ignition Facility, Laser Mégajoule, OMEGA EP, and OMEGA. This work summarizes the coating efforts and advancements to support such system construction and maintenance.

Direct-drive target designs below self-ignition threshold are proposed for the laser megajoule in the context of shock-ignition. Two distinct initial aspect ratios are considered and laser pulses are shaped following a classical Kidder's law in order to achieve an implosion velocity of 300 km/s, an in-fight adiabat close to unity and to maximize the peak areal density. The pulse shapes are adjusted to arrange shock timing at the inner side of the DT fuel. The robustness of the laser pulse is addressed by the means of random variations around the initial Kidder's laws. Correlation matrices show no significant correlations between laser parameters. An admissible envelope of laser pulse is given for both designs in order to warrant more than 80% of the best peak areal density. Variations of laser drive power produce variations of implosion velocities in the range 250–370 km/s. Self-ignition threshold is achieved and thermonuclear energy are produced in the range 3 kJ–27 MJ. Finally, the random procedure shows that it is possible to improve the first deterministic optimization and the laser pulses are given.

By taking the National Ignition Facility(NIF) from the USA and the Laser Megajoule(LMJ) from the France for examples,this paper analyzes the general layout and general structure engineering features of Inertial Confinement Fusion(ICF) laser facility.NIF is designed with a U-shaped building layout,and it not only can provide an optimum laser experimental equipment configuration but also allow the equipment to attach a second target chamber in future.The bundles of LMJ are arranged in an in-line building.Its layout decreases the optical path-length between the output of each beamline and the focusing system also provides an option for a second target chamber exists.The paper suggests that the general layout of an ICF laser facility should meet the requirements of the physical experimental purposes and should be characterized by operation stability,good integration and higher maintainability.Furthermore,it should design convenient interfaces for future development.In overall design of the equipment,optical elements are packaged into a optical-mehanical assemblies,and replaceable units(LRUs) are assembled with kinematic mounts.The general structure of ICF design should match the arrangement of the laser beams in structural layout and configuration.Moreover,the structure design can not dispense with the function,stability and cleanliness.The assignments of general structure design should meet the requirements of the installation and localization,integration and debugging,operation and maintenance of the optical components and the physical/optical diagnostic equipment.

A prototype of a picosecond x-ray streak camera has been developed and tested by Commissariat a l’Energie Atomique et aux Energies Alternatives to provide plasma-diagnostic support for the Laser Megajoule. We report on the measured performance of this streak camera, which almost fulfills the requirements: 50-μm spatial resolution over a 15-mm field in the photocathode plane, 17-ps temporal resolution in a 2-ns timebase, a detection threshold lower than 625 nJ/cm2 in the 0.05–15 keV spectral range, and a dynamic range greater than 100.

Streak cameras are widely used to record the spatio-temporal evolution of laser-induced plasma. A prototype of picosecond X-ray streak camera has been developed and tested by Commissariat a l’Energie Atomique et aux Energies Alternatives to answer the Laser MegaJoule specific needs. The dynamic range of this instrument is measured with picosecond X-ray pulses generated by the interaction of a laser beam and a copper target. The required value of 100 is reached only in the configurations combining the slowest sweeping speed and optimization of the streak tube electron throughput by an appropriate choice of high voltages applied to its electrodes.

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) facility located at CEA/CESTA started to operate in the early 2014 with two quadruplets (20 kJ at 351 nm) focused on target for the first experimental campaign. We present here the first set of gated x-ray imaging (GXI) diagnostics implemented on LMJ since mid-2014. This set consists of two imaging diagnostics with spatial, temporal, and broadband spectral resolution. These diagnostics will give basic measurements, during the entire life of the facility, such as position, structure, and balance of beams, but they will also be used to characterize gas filled target implosion symmetry and timing, to study x-ray radiography and hydrodynamic instabilities. The design requires a vulnerability approach, because components will operate in a harsh environment induced by neutron fluxes, gamma rays, debris, and shrapnel. Grazing incidence x-ray microscopes are fielded as far as possible away from the target to minimize potential damage and signal noise due to these sources. These imaging diagnostics incorporate microscopes with large source-to-optic distance and large size gated microchannel plate detectors. Microscopes include optics with grazing incidence mirrors, pinholes, and refractive lenses. Spatial, temporal, and spectral performances have been measured on x-ray tubes and UV lasers at CEA-DIF and at Physikalisch-Technische Bundesanstalt BESSY II synchrotron prior to be set on LMJ. GXI-1 and GXI-2 designs, metrology, and first experiments on LMJ are presented here.

International audience; RLCYC 75 is a new electrically calibrated laser calorimeter specially manufactured by Laser Metrology to calibrate energy diagnostics within the Laser MegaJoule (LMJ) facility. It consists of an optical cavity cooled by a hydraulic system. The system is designed to provide 1 m wavelength power laser measurements with uncertainty less than 1% at 2 kW and traceability to the International System of Units (SI). In this paper, the accuracy of RLCYC 75 measurements is studied. More precisely, three points are detailed: instrumentation uncertainty estimation, equivalence between optical and electrical supply and light absorption. To this end, electrical calibration campaigns and power laser measurement campaigns are conducted. Moreover, thermal and optical models are developed. Results show that RLCYC 75 design and instrumentation are efficient enough to reach the goal of relative uncertainty of about 1% at 2 kW. RLCYC 75 will become the 2 kW laser power primary standard for LMJ applications.

We present a 2D analysis of direct-drive shock ignition for the laser Megajoule. First, a target design is chosen in the HiPER-like target family generated by scale up and down of the original HiPER target. A first analysis is done considering the 1D fuel assembly and 2D shock ignition by means of the ring at polar angle of 33°2. The intensity profile is top-hat and calculations are done for several different radii. It is shown that larger the radius, lower the minimum spike power is. In addition, the intensity in each quad can stay below 4 × 1014 W/cm2 and is considered non crucial for parametric instabilities such as two plasmons. A 2D analysis of the fuel assembly is done in a second step by considering the two rings located at 49° and 59°5 and their symmetric by the equatorial plane symmetry. It is shown that low mode asymmetries are important at the stagnation and can significantly affect the areal density obtained. Finally, full 2D calculations of shock ignition is done, using all the beams of the LMJ and show that the spike power needed for ignition and gain is increased by a factor greater than 3 regarding the power needed in perfectly isotropic fuel assembly. This increase is mainly due to high level low mode asymmetries generated during fuel assembly.

We have determined the thermal criterion for the Laser Megajoule cryogenic target that leads to a uniform layer of deuterium-tritium as specified for ignition. Thermal models were created, and computational fluid dynamics software was used to calculate this criterion in a spherical geometry as a function of capsule thermal conductivity. According to the values obtained, we have studied the possibility of removing anticonvection baffles in the cryogenic target. Results are presented in this paper and indicate that anticonvection baffles can be removed only if the gas density inside the cryogenic target is reduced and/or if the hydrogen content is reduced.

Smooth and uniform solid deuterium-tritium (DT) layers inside a spherical shell are needed in order to achieve ignition on the Laser Megajoule (LMJ) facility. The thermal environment around the cap...

The Laser Megajoule (LMJ) cryogenic target is protected from ambient thermal radiation by a thermal shroud. When the cryotarget, held by the cryotarget positioner, is at the LMJ chamber center, the thermal shroud has to be removed just before the shot to allow the laser beams to reach the laser entrance hole of the cavity.The shroud remover, PET, will have to disconnect the thermal shroud from the cryogenic target base without disturbing the target base temperature regulation (˜18 K ± 2 mK), which guarantees the needed cryogenic target conditions to reach the ignition.The shroud withdrawal is divided into two successive phases: a slow withdrawal for the thermal disconnection between shroud and target base and a fast withdrawal for a quick extraction of the shroud out of the laser beamways pointing onto the cavity. The slow shroud withdrawal must be handled within 30 min to respect laser pointing stability. After the final target alignment at the chamber center, the shroud must be ejected 0.5 m awa...

The High Power laser Energy Research facility (HiPER) is a European project dedicated to demonstrating the feasibility of producing energy by laser-driven inertial confinement fusion. A first design of the fast ignition cryogenic target has been established. It is composed of a thin-walled microshell with an inserted gold cone and filled with deuterium-tritium (DT) fuel by means of a capillary (conically guided capsule). After assembly, targets must be tight at cryogenic temperatures (16 to 19.6 K).In order to evaluate the manufacturing feasibility of a single-shot target prototype, a program has been adapted from the Laser Megajoule (LMJ) cryogenic target fabrication know-how. Target component study for HiPER concerns a hollow gold cone (25-deg half-angle and ˜25-μm thickness), a thin polymeric microshell (2-mm diameter and 3- to 10-μm thickness), and a silica capillary (30-μm outer diameter).First gas-tight targets at 77 K have been produced (helium gas leak rate ˜1.4 × 10-11 Pa·m3/s). Major eff...

This paper presents a detailed assessment of the intermediate step design Laser Megajoule cryogenic target thermal behavior. Several simulations were carried out with a computational fluid dynamics code named FLUENT in order to better understand the effect of different parameters on this new design.

As part of the French Inertial Confinement Fusion program, Commissariat a l'Energie Atomique has developed cryogenic target assemblies (CTAs) for the Laser Megajoule (LMJ) and a program in two stages for the permeation filling of these CTAs: (a) the permeation filling studies with the Study Filling Station cryostats and (b) the design and manufacturing of the whole operational chain of CTA filling facilities. This paper deals with the description of both the cryogenic studying and the filling facilities for the LMJ targets.

Laser megajoule (LMJ) is a laser facility being built by the French nuclear science directorate, Commissariat a l'Energie Atomique. This facility is a large flashlamp pumped laser (1.8-MJ optical output). The pulsed power conditioning system of LMJ is based on a modular design of 480 modules (400-MJ energy capacitor bank). A new capacitor bank module (CBM) for the LMJ power conditioning system has been developed by the Institute of High Current Electronics (IHCE, Tomsk) in collaboration with International Technologies for High Pulsed Power (France). Two such modules were produced and tested in IHCE. The main parts of the CBM are the following: high-voltage power supplies, command control system, air conditioning system, capacitor block, and cables delivering the energy to a load. The parameters of the module are the following: total rated capacitance of 2900 (+4%) muF, charging voltage up to 24 kV, peak current amplitude of 240 kA, peak stored energy up to 864 kJ, pulsewidth at 10% of peak power of 360 mus, dimensions of 2.3 times 1.5 times 2.5 m3, and weight of ~2500 kg. In nominal regime (24-kV charging voltage, 40-m length cables), the energy delivered to flashlamps in each channel is 74 kJ (86% efficiency). No failures were observed in more than 1000 test shots, and all parameters of the CBM are well within the requirements. Detailed description of the module design, performance, and test results are stated in this paper.

The LASER MEGAJOULE (LMJ) will be an experimental fusion laser facility located at CEA-CESTA research center, close to Bordeaux. Fusion experiments will involve targets filled with tritium, requiring the design of containment devices to ensure worker protection against radiation exposure according to the French safety regulations. This paper reports the work performed by the LMJ safety team to design these confinement devices. At first, the paper describes the tritium inventory evaluated inside the facility. Then, specifications and allocations for the vacuum and air purge systems of the target bay are defined and explained. A spread model is described to evaluate the spatial repartition of the residual contamination in the chamber, concluding to an anisotropic process. Consequently, security factors have been defined according to the different locations inside the target bay. Definition of containment protections for the equipment of the target bay like fume hoods and glove boxes does not depend only on the effects of contamination (internal exposure hazards), but also on activation of materials (external exposure). Consequently, safety requirements have been defined in order to design containment devices. Finally, a specification of containment barriers performances is given (static and dynamic modes). Leakage of the barriers, negatives pressures, cleaning and conditioning are addressed.

International audience; The laser-induced damage of optical components of high-power laser systems such as the National Ignition Facility (NIF) (Murray, Proc SPIE 3492:1–10, 1998) and Laser MegaJoule (LMJ) (André, Fusion Eng Des 44:43–49, 1999) can have a significant impact on the operating costs of these laser systems since optimal shaping to carry out ignition is required. Laser-induced damage (LID) of fused silica appears on the exit surface, mostly at 3ω, and tends to grow exponentially with each laser shot. This damage affects the performance of optical components and limits their lifetime. The damage is due to thermal explosions of microdefects or microabsorbers, and these explosions emit a plasma that generates shock waves introducing cracks that increase the damage. This paper, thus proposes the deposit of an elastic layer on the exit surface to mitigate the shock waves and thereby decrease the extent of the damage volume. To this end, we have developed a mendable sol–gel-processed hybrid inorganic–organic layer. The hybrid layer is a nanocomposite made of silica and polydimethylsiloxane (PDMS). Their resultant mechanical properties can be tuned by adjusting the elastomer/silica ratio and catalysis conditions. The synthesis used to create these layers was investigated with an infrared spectrometer. The optical quality of the hybrid layers was checked by UV/visible/NIR spectrophotometry and thickness uniformity were mapped via reflectometry. Young’s modulus, Poison’s ratio, and the hardness have been measured in order to characterize the mechanical properties of these layers. The surface acoustic waves technique was used to determine Young’s modulus and Poison’s ratio of an optical layer obtained by a sol–gel process. The hardness has been measured with a homemade indenter. The results of first measurements of laser damage made with an excimer laser at 351 and 353 nm demonstrated a reduction of the laser-induced damage threshold with the PDMS/silica ratio.

SolGel coated fused silica samples were intentionally contaminated in a manner representative of the Laser Megajoule optics used in ISO 7 cleanrooms. They were then tested to assess the impact of this contamination on their LIDT.

In the context of high power laser systems, the laser damage resistance of fused silica surfaces at 351 nm in the nanosecond regime is a major concern. Under successive nanosecond laser irradiations, an initiated damage can grow which can make the component unsuitable. The localized CO2 laser processing has demonstrated its ability to mitigate (stopping) laser damage growth. In order to mitigate large damage sites (millimetric), a method based on fast microablation of silica has been proposed by Bass et al. [Bass et al., Proc. SPIE 7842, 784220 (2010)]. This is accomplished by scanning of the CO2 laser spot with a fast galvanometer beam scanner to form a crater with a typical conical shape. The objective of the present work is to develop a similar fast micro-ablation process for application to the Laser MegaJoule optical components. We present in this paper the developed experimental system and process. We describe also the characterization tools used in this study for shape measurements which are critical for the application. Experimental and numerical studies of the downstream intensifications, resulting of cone formation on the fused silica surface, are presented. The experimental results are compared to numerical simulations for different crater shape in order to find optimal process conditions to minimize the intensifications in the LMJ configuration. We show the laser damage test experimental conditions and procedures to evaluate the laser damage resistance of the mitigated sites and discuss the efficiency of the process for our application.

Recent progress in high-gain direct-drive inertial confinement fusion with the laser Megajoule is reviewed. A new baseline direct-drive target design is presented which implodes with a two-cones irradiation pattern of indirect-drive beam configuration and zooming. Perturbation amplitudes and correlated growth rates of hydrodynamic instabilities in the compressed core of a directly driven inertial confinement fusion capsule are analyzed in planar and spherical geometries, with and without heat conduction, in the unsteady state regime of the deceleration. Shock propagation in heterogeneous media is addressed in the context of first shock. The neutron and photon emissions of high-gain direct-drive target are characterized. Numerical interpretations of directly driven homothetic cryogenic D2 target implosion experiments on the Omega facility are presented.

A new solution for high-gain direct-drive fusion with the indirect drive beam irradiation of the Laser Megajoule is presented here. This solution uses 2 rings per hemisphere at polar angles of 49° and 59.5°. The 2D calculations with the hydrodynamics code FCI2 lead to a thermonuclear energy of 32 MJ with less than 1 MJ incident laser energy.

Petawatt lasers can generate extreme states of matter, making them unique tools for high-energy-density physics. Pressures in the gigabar regime can potentially be generated with cone-wire targets when the coupling efficiency is high and temperatures reach 2–4 keV (ref. 1). The only other method of obtaining such gigantic pressures is to use the megajoule laser facilities being constructed (National Ignition Facility and Laser MegaJoule). The energy can be transported over surprisingly long distances but, until now, the guiding mechanism has remained unclear. Here, we present the first definitive experimental proof that the heating is maximized close to the wire surface, by comparison of interferometric measurements with hydrodynamic simulations. New hybrid particle-in-cell simulations show the complex field structures for the first time, including a reversal of the magnetic field on the inside of the wire. This increases the return current in a spatially separated thin layer below the wire surface, resulting in the enhanced level of ohmic heating. There are a significant number of applications in high-energy-density science, ranging from equation-of-state studies to bright, hard X-ray sources, that will benefit from this new understanding of energy transport.

The Laser MegaJoule (LMJ) program plans to obtain Deuterium-Tritium (DT) mixture ignition leading to a fusion gain of ten. Cryogenic targets are hollow spheres whose interior is covered with a solid cryogenic fuel layer. The success of DT ignition depends on quality of the fuel layer uniformity. These targets must be cooled and kept at temperatures near the triple point (19.8 K) with a very good stability (+/-1 mK) for many hours, in the center of the 5 m radius experimental vacuum chamber with a position accuracy of a few microns. In order to validate our current device concepts, we have manufactured scale one prototypes to confirm all thermal and mechanical challenges, such as sharp thermal regulation, cooling autonomy and cryogenic target transfer.

The Laser Megajoule (LMJ) Cryotarget Positioner (PCC) will be used to set cryogenic targets in the vacuum chamber centre of this experimental facility for fusion by inertial confinement. In the Fre...

Smooth solid DT layer inside a plastic spherical shell are needed in order to achieve the ignition on the Laser MegaJoule (LMJ) facility. This paper follows our previous one1 and presents the up to date results concerning the quenching and breathing techniques. New analysis software has been made which allowed us to update the results already obtained for the breathing technique. New experiments done on quenching have leaded to obtain a smooth layer for the higher mode down to 2.3 K below the triple point. The time where the roughness NIF specifications are obtained while the temperature is 1,5 K below the triple point is around 6 seconds using the quenching technique. Roughness analyses are made on one axis view. We also discuss other experimental aspects, such as shadowgraphy and redistribution process.

In this paper, we report the results of thermal simulations made for the cryogenic target of Laser MegaJoule (LMJ). Thermal simulations are done with FLUENT for a new 3-D model which takes in accou...

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.