Laser Megajoule Research Articles

Laser Megajoule performance status

The Laser Megajoule (LMJ) is among the most energetic inertial confinement fusion laser facilities in the world, together with the National Ignition Facility (NIF) in the USA. The construction of the facility began back in 2003, and the first photons were emitted by the laser bundle #28 in 2014. Today, 11 laser bundles consisting of 88 large aperture 0.35×0.35m2 laser beams are in operation, delivering daily up to 330 kJ of energy at the wavelength of 351 nm on a target placed in the center of a 10 m diameter vacuum chamber. In this paper, we describe the laser system and its operational performances. We also detail the first laser campaigns carried out to prepare an increase of energy and power on the target. These campaigns, along with the completion of additional bundles mounting, will bring LMJ performance to 1.3 MJ thanks to 22 bundles in operation.

Prediction of Nanoscale Water Meniscus Shape between Deliquescent KDP Crystal Optics and AFM Probe for Water-Dissolution Repairing.

KDP (KH2PO4) crystal optics are the key elements for megajoule laser facilities. Nanoscale surface defects would cause laser-induced damage when the optics are irradiated by a high-fluence laser (over 10 J/cm2). Dip-pen nanolithography (DPN) could be used to repair the nanoscale surface defects in the KDP optics by the water meniscus. The high humidity required for high-efficiency and soft KDP surfaces penetrated by the AFM probe brings challenges for accurately predicting the water meniscus shape to evaluate the effectiveness of the DPN water-dissolution repairing. The multisolutions of the Young-Laplace and Kelvin equations also lead to the wrong water meniscus shape. A theoretical model that takes the high humidity and the penetration of the AFM probes into account is developed. The parametrization Young-Laplace equations are adopted for the zero contact angle of the water films, and the AFM probe is treated as the combination of the cone and sphere for the water meniscus whose size is larger than the AFM tip radius under high humidity. The penetration of the AFM probe is modeled by Hertz theory. Both the water films (3.3 nm thickness at 99% relative humidity) and indentations (1.46 nm depth at 300 nN contact force) are non-negligible for the nanoscale water meniscus between the KDP surface and the AFM probe. Moreover, the rough-fine two-step method is proposed to lock the correct solution of the Young-Laplace and Kelvin equations. The effectiveness of the proposed model is verified by comparison with reported ESEM images and pull-off forces. In addition, the overgrowth dots on the KDP surface are compared with the water meniscus. The linear growth of the water meniscus would cause the linear growth of the overgrowth dot, which proves the proposed model could be used to guide the DPN water-dissolution repairing for the nanoscale surface defects in the KDP optics.

First indirect drive inertial confinement fusion campaign at Laser Megajoule

The first indirect drive Inertial Confinement Fusion (ICF) experiments on the Laser Megajoule facility were carried out with approximately 150 kJ of laser energy distributed on 48 beams (12 quads) arranged in two cones. The target consisted of a gold vacuum rugby-shaped hohlraum and a plastic capsule located at its center, filled with deuterium gas fuel. The arrangement of the 12 quads is such that the laser irradiation on the wall generated a three-dimensional (3D) x-ray flux around the capsule creating 3D deformations on the imploding plastic shell. This constraint forced the design of a robust target (relatively thin ablator, around 40 μm) driven by a short laser pulse (3 ns) that delivered about 1011 neutrons. Full-integrated 3D radiation hydrodynamics simulations allowed both the target definition and the data interpretation (mainly radiation temperature, x-ray images, and neutron yield). 3D calculations and experiments compare well.

Experimental investigation of the interplay between optical and plasma smoothing induced on a laser megajoule beamline

Experimental investigation of the interplay between optical and plasma smoothing induced on a laser megajoule beamline

Transparent Films Thickness Mapping Highlighting the Viscosity Effect of Elastic Layers Made by Sol–Gel Process with an In-House Ellipsometer

New optical coatings are currently developed to mitigate the shockwave generated by nanosecond lasers in high-power laser systems such as the MegaJoule laser (LMJ). These shockwaves seem responsible for the damage growth observed on optical components. A possible solution for shockwave mitigation is using ormosil (organically modified silicate) coatings made by the sol–gel method with thicknesses of a few microns. Unfortunately, the sol–gel solution exhibits a viscous behavior, and thus, the deposited layers are heterogeneous in thickness. An experimental ellipsometer has been designed to measure this heterogeneity and highlight the viscoelastic properties of the layers responsible for self-healing effects that were observed when these layers were scratched. This ellipsometer allows us to know the refractive index of the coating and therefore its density. Density and thickness are the two essential parameters for determining the speed of sound and the modulus of elasticity of the layer, which indicate the ability of the layer to attenuate more or less elastic waves or shock waves.

Full-scale optic designed for onsite study of damage growth at the Laser MegaJoule facility.

Large fusion scale laser facilities aim at delivering megajoules laser energy in the UV spectrum and nanosecond regime. Due to the extreme laser energies, the laser damage of final optics of such beamlines is an important issue that must be addressed. Once a damage site initiates, it grows at each laser shot which decreases the quality of the optical component and spoil laser performances. Operation at full energy and power of such laser facilities requires a perfect control of damage kinetics and laser parameters. Monitoring damage kinetics involves onsite observation, understanding of damage growth process and prediction of growth features. Facilities are equipped with cameras dedicated to the monitoring of damage site growth. Here we propose to design and manufacture a dedicated full size optical component to study damage growth at increased energy, on the beamline, i.e. in the real environment of the optics on a large laser facility. Used for the first time in 2021, the growth statistics acquired by this approach at the Laser MegaJoule (LMJ) facility provides a new calibration point at a fluence less than 5 J cm-2 and a flat-in-time pulse of 3 ns.

Cloud and microjet mix: A possible source of yield limitation of the National Ignition Facility targets

The National Ignition Facility (NIF) megajoule laser at the Lawrence Livermore National Laboratory (LLNL) has produced a significant amount of useful physics results related to inertial confinement fusion since it began operating in 2010. However, the achievement of its primary goal of generating ignition in a thermonuclear plasma has proven elusive, and measured yields were far below those expected under ideal illumination and target structure conditions. The reason for the limited yields has not been explained in detail. This work proposes and shows support for the possibility that the low yields may be, at least partly, the result of ejecta from the interior of the target containment and driving shell.

Time-resolved near backscatter imaging system on Laser MegaJoule.

The newly operating near-backscattering imaging (NBI) system on the Laser MegaJoule (LMJ) is briefly described with emphasis on the temporally resolved measurements and their synchronization with the LMJ laser pulse through target shots taken as part of the diagnostic commissioning campaign. The NBI measures the stimulated Brillouin and Raman scattered light around two quadruplets (one inner and one outer) of the upper LMJ hemisphere. The temporal resolution is achieved with a unique system: a specifically designed wide-open optical lens images 40 points of a diffuser onto an array of optical fibers with the scattered light recorded on a multiplexed photodiode array.

Cylindrical implosion platform for the study of highly magnetized plasmas at Laser MegaJoule.

Investigating the potential benefits of the use of magnetic fields in inertial confinement fusion experiments has given rise to experimental platforms like the Magnetized Liner Inertial Fusion approach at the Z-machine (Sandia National Laboratories) or its laser-driven equivalent at OMEGA (Laboratory for Laser Energetics). Implementing these platforms at MegaJoule-scale laser facilities, such as the Laser MegaJoule (LMJ) or the National Ignition Facility (NIF), is crucial to reaching self-sustained nuclear fusion and enlarges the level of magnetization that can be achieved through a higher compression. In this paper, we present a complete design of an experimental platform for magnetized implosions using cylindrical targets at LMJ. A seed magnetic field is generated along the axis of the cylinder using laser-driven coil targets, minimizing debris and increasing diagnostic access compared with pulsed power field generators. We present a comprehensive simulation study of the initial B field generated with these coil targets, as well as two-dimensional extended magnetohydrodynamics simulations showing that a 5T initial B field is compressed up to 25 kT during the implosion. Under these circumstances, the electrons become magnetized, which severely modifies the plasma conditions at stagnation. In particular, in the hot spot the electron temperature is increased (from 1 keV to 5 keV) while the density is reduced (from 40g/cm^{3} to 7g/cm^{3}). We discuss how these changes can be diagnosed using x-ray imaging and spectroscopy, and particle diagnostics. We propose the simultaneous use of two dopants in the fuel (Ar and Kr) to act as spectroscopic tracers. We show that this introduces an effective spatial resolution in the plasma which permits an unambiguous observation of the B-field effects. Additionally, we present a plan for future experiments of this kind at LMJ.

An Estimation Method of Surface Defects of LMJ Microshells

This paper presents the development of a radiographic characterization method for microshells. In the Laser MegaJoule (LMJ) framework, microshells are tiny plastic spheres used in inertial fusion laser experiments. For this work, these microshells were characterized using low-energy radiography. In the microshell radiographs, phase contrast was noted at the edges of the microshells. The origin of this phenomenon has been identified as sharp variation of gray-scale amplitude due to refraction. Our theoretical model links pixel information with microshell geometry and is used for contour detection and characterization. Finally, an estimation of surface defects described by spherical harmonics is calculated.

Absolute x-ray calibration of a gated x-ray framing camera for the Laser MegaJoule facility in the 0.1 keV-1 keV spectral range.

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.

Metrology of Acquisition Chains and Signal Processing of LMJ Experiments

Since the first experiment in 2014, more and more plasma diagnostics are being deployed on the Laser MegaJoule (LMJ) facility manufactured by C.E.A/D.A.M. These diagnostics aim at measuring radiations or particles emitted during laser experiments to study high-energy physics, especially inertial confinement fusion (ICF). Different types of sensors surround the LMJ target chamber and realize the conversion of the quantities of interest to an electric signal. The signal is then transmitted via coaxial cables, acquired by a broadband oscilloscope, and digitally post-processed. Each step of this typical acquisition chain adds measurement errors and increases the global uncertainty. First, a numerical model of the digitizer alongside a specific hardware system designed to perform its metrology in situ will be presented. It computes errors sources such as offset, gain and skew, and provides a measurement of the effective number of bits (ENOB) of the digitizer. The experimental characterization of the electrical chain via its transfer function measurement will also be detailed. Finally, the numerical methods deployed to handle the inverse problem, based on deconvolution processes, will be introduced, including future developments exploiting Bayesian inferences and statistical approaches.

Focusing aspherical optical components for next generation megajoule laser system

The idea of creating a laser thermonuclear installation was considered back in 1960s by Basov N and Krokhin O. [1]. Facility for conducting research near DT-fuel ignition threshold is being commenced at the moment [2], assembly of the laser installation interaction chamber has already been completed. To focus laser sight on the target, off-axis aspherical lenses are used as part of the final optical module (FOM). These lenses are manufactured using the developed technology of forming off-axis aspherical surfaces of optical parts. Combined use of shaping technology and stand for monitoring angular deviations of the surfaces of FOM lenses, allowed us to achieve high quality off-axis aspheric optical elements both in terms of standard deviation of the surface shape, as well as the value of the scattering circle.

Erratum to: Calculation of Direct Drive Targets for Megajoule Laser Facilities with Radiation in the Second and Third Harmonics of Nd Laser

The numerical results for cryogenic direct drive targets of megajoule facilities with radiation in the second and third harmonics of a Nd laser are presented. The calculations were performed with the 1D radiation hydrodynamics code ERA with the laser light absorption model that takes into account stimulated Brillouin scattering (SBS), generation of fast electrons in the processes of two-plasmon decay (TPD), and stimulated Raman scattering (SRS). The verification of the developed models was carried out on the basis of the comparison with experiments performed at the OMEGA and NIF facilities. The ignition margin (WQ) of nonuniform fusion targets with an allowance for energy losses due to radiation transfer and heat conduction from the hot spot was the objective of the target optimization. The calculations showed that SBS and target heating by fast electrons generated in TPD and SRS fatally reduce WQ of targets with a CH ablator for the megajoule laser with wavelength λ = 0.53 µm. The possibilities of decreasing these effects by replacing a CH ablator with a glass ablator and reducing the laser intensity upon increasing the target aspect ratio are considered. However, in both cases, WQ remains substantially below unity for the laser with wavelength λ = 0.53 µm. The ignition margin increases by a factor of ∼2 upon transition from the second to the third harmonic of a Nd laser. A glass ablator almost eliminates fast electrons in calculation with the laser wavelength λ = 0.35 µm. In this case, if SBS is reduced by a factor of 3–4 via shifting the laser emission lines in the neighboring channels by Δμ ≈ 10–20 A, the ignition margin WQ ∼ 2 and a fusion energy yield of ∼50 MJ are obtained in the 1D calculation for a laser energy of ∼2 MJ and the third harmonic of a Nd laser.

Requirements for a 4ω Thomson scattering system on megajoule scale laser facilities.

With the arrival of megajoule class laser facilities, the features of laser-produced plasmas are evolving toward unprecedented high electron temperatures reached in the environment of a cm-scale indirect-drive Hohlraum for a few tens of nanoseconds. In this context, the need for in situ experimental characterization of the plasma parameters becomes critical in order to test hydrodynamics simulations in these novel conditions. Taking advantage of the progress achieved in the last 40 years, Thomson scattering has become a classic diagnostic in the characterization of laser produced plasmas. However, the many beam configuration of the megajoule scale experiments makes the measurements increasingly complex because the Thomson scattering signals produced by the 351 nm heaters themselves dominate the plasma emission around 263 nm, a wavelength range typically of interest when a 4ω Thomson probe is used. This paper reviews the requirements for and the potential of a 4ω Thomson scattering system to be operated on such 351 nm megajoule scale facilities in order to characterize the hot (Te > 3 keV) plasmas produced in the indirect-drive irradiation of a Hohlraum. It is found that the configuration of the diagnostic could be optimized in order to enable the detection of the ion acoustic resonances over a large domain of plasma parameters. The results for the electron plasma wave resonances are also given.

First experimental observation of a photoabsorption-edge induced shock by its coalescence onto a regular ablation-shock

Implosion experiments of an inertial confinement fusion (ICF) target on the laser megajoule (LMJ) and the National Ignition Facility require, for certain designs, a precise timing coalescence of four shocks at a specific point of the capsule, which strongly depends on the ablator equation of state. In experiments at the Ligne d'Intégration laser facility, a prototype for the LMJ, coalescence of two shocks was studied in a planar polystyrene (CH) sample in an indirect drive configuration. Shocks were driven by x-ray emission generated in a spherical hohlraum radiatively heated using a 12 ns duration laser pulse temporally shaped to produce two steps in the radiation temperature history that launches these two successive ablation-shock waves. Shock velocity was inferred from a Velocity Interferometer System for Any Reflector (VISAR). Shot performed with 10 ns long truncated laser pulses reduces VISAR blanking, which allows us, for the first time to our knowledge, to observe a photoabsorption-edge induced shock, edge-shock for short, which is a third shock out of the two-step radiation temperature history, coalescing with the second ablation shock. The accurate measurement of this shock with well-controlled x-ray drive should potentially help to constrain the equation of state and opacity of carbon in coronal plasma conditions since the behavior of this shock is very sensitive to both. Moreover, since they can drastically alter the speed of coalesced shocks (in keyhole experiments or ignition designs for ICF), measurements of these edge-shocks may also contribute to improving our ICF design capabilities.

General design and experiment for separated final optics assembly on high energy laser system

Final optics assembly (FOA) is paramount to high energy laser system. It took more than ten years for the National Ignition Facility (NIF) and Laser Megajoule Facility (LMJ) to optimize their FOA and improve output capability. However, the final optical damage limited the output capability of the NIF and LMJ. Contamination is the dominant factor which can dramatically reduce their damage threshold. Accordingly, in this work we propose a separated FOA which mainly locates in the atmosphere and is convenient for contaminants sources management. In addition, we adopts an on-line cleaning system to improve clean status of FOA and achieves longevity lifetime of final optics. The experiment result of the separated FOA demonstrates that even more than one thousand particle contaminants are generated by each laser shot of 6 J/cm2, the on-line cleaning system can take major of them away and maintain cleanliness of separated FOA at ISO Class 3. Our separated FOA can stably operate under a 351-nm laser shot of 6 J/cm2 (21.7 ns shaped-pulse), which is significant to improve the output capability of high energy laser system.

Towards megajoule x-ray lasers via relativistic four-photon cascade in plasma.

A theoretically highly efficient mechanism, operating at high laser intensities and powers, is identified for spectral transferring huge laser energies to shorter ultraviolet and x-ray wavelengths. With megajoule laser energies currently available at near-optical wavelengths, this transfer would, in theory, enable megajoule x-ray lasers, a huge advance over the millijoules x-ray pulses produced now. In fact, enabling even kilojoule x-ray lasers would still be a fantastic advance, and a more likely achievable one, considering practical experimental inefficiencies.

Overview of the Laser Mega Joule (LMJ) Facility and PETAL Project in France

Overview of the Laser Mega Joule (LMJ) Facility and PETAL Project in France

Influence of the Asynchronous Multibeam Irradiation a Spherical Fusion Target by Megajoule Laser Beams on the Efficiency of Thermonuclear Burning

Influence of the Asynchronous Multibeam Irradiation a Spherical Fusion Target by Megajoule Laser Beams on the Efficiency of Thermonuclear Burning