The year 2024 will mark the 10th anniversary of the commissioning of the Laser Megajoule (LMJ) Inertial Confinement Fusion Laser first bundle of 8 laser beams. About 400 experiments on target were carried out in this period of time. Currently 11 bundles are in operation at half energy and power for experiments delivering up to 330kJ UV on target. The full system will be completed within few years. In this paper, we take the opportunity of the anniversary to look back from the beginning. We detail an historical review of the laser system development carried out at CEA DAM since 1962. We illustrate the different laser systems built and how they evolved as laser technology evolved with some pioneering results from frequency conversion to pulse compression. We then detail the LMJ design and architecture and focus on laser damage management in the different sub-assemblies of the system. We review the performance reached at half/energy and power with half bundles completed. We finally show the laser campaign performed since 2021 to test the energy/power increase on a few numbers of bundles, preparing the LMJ operation at full performance.
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 September 2021, a major project milestone has been achieved with the commissioning of the half LMJ (88 beams are fully operational with 10 heating bundles of 8 beams and a specific bundle for plasma diagnostics purpose). 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. Another project milestone has been achieved at the end of 2021, with a dedicated laser experiment in the facility to explore the Power-Energy Diagram.
High energy laser facilities designed for fusion experiments, such as Laser MegaJoule or National Ignition Facility, are limited by laser-induced damage on their final optics. Accurate and early detection of damage growth is required for successful operation of such facilities. Since the image resolution is about the size of damage sites to monitor, diameter measurements are not sufficient to meet the objectives of damage growth quantification. An accurate size quantification of damage sites is based on light scattering measurements after time-consuming calibrations on the facility. An optical model is proposed to perform a simple and fast calibration of the measurements by numerical simulation. The model is based on light scattering measurements of several damage sites combined with optical simulations of the lighting system.
Laser MegaJoule (LMJ) is a high energy laser facility designed for fusion experiments. To track final optics damage, laser damage monitoring is carried out using images acquired by a camera. To prepare for the LMJ full energy/power operation, the damage models based on the phenomenological laws established in the laboratories are validated by experimental campaigns dedicated to performance. The very high quality of LMJ optics surfaces makes damage highly unlikely. In order to take the greater benefit of these performance campaigns, carried out on a reduced number of laser shots and components, a matrix of nearly 1000 damage sites is initiated offline on one optics. Precisely measured on a metrology bench before and after the campaign, this component was on LMJ facility during a performance campaign at the end of 2021. Very useful for the calibration of the LMJ monitoring camera, it also provided data to set LMJ laser damage models at higher energy level.
Final fused silica optics of high energy fusion class laser facilities are one of the components that limit the UV laser energy available for experiments. These final optics suffer from laser-induced damage. Some solutions are available to limit laser damage growth and to increase optics lifetime. However, to use them, it is necessary to be able to detect damage initiation as soon as possible, and to follow damage growth efficiently. An imaging system and a lighting source make the observation of laser damage sites possible after each laser shot without removing the optical components. Laser damage detection algorithms exist but they are not sufficiently efficient to provide reliable monitoring of damage growth over time because of small repositioning fluctuations of the optical system. An effective solution based on digital image correlation and brightness/contrast corrections is proposed to detect and follow laser damage sites as soon as they initiate in an automatic way. The effectiveness of the presented method is compared to the widely used method that is based on the analysis of local signal-to-noise ratio.
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 pétawatt 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.
The Laser MegaJoule (LMJ) is a 176-beamlines facility, located at the CEA CESTA near Bordeaux (France). It is designed to deliver about 1.4 MJ of ultraviolet laser energy on targets set in vacuum chamber, for high energy density physics experiments, including fusion experiments. The commissioning of the seven first bundles of height beams is achieved since November 2019 and the commissioning of next bundles is on the way. For performance requirements, it is important to follow final optics behavior. Moreover, for questions of manufacturability, ease of maintenance and cost, the understanding and the improvement of vacuum windows laser damage resistance are of main importance. The MDCC (Center Chamber Diagnostic System) is thus operating since November 2018 on the LMJ facility. It consists in a high resolution CCD camera combined with a predefined focus set of optics. The resolution of this system is about 100μm with a working distance of 8 m. This system can perform 3 functions: damage detection on the vacuum window surface, the measurement of the spatial profile on the vacuum window plane and of final optics transmission.
The Laser MegaJoule (LMJ) is a 176-beam laser facility, located at the CEA CESTA Laboratory 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. The commissioning of the two first bundles of 8 beams was achieved in December 2016 and commissioning of next bundles is on the way.
A computational system, PARC has been developed and is under deployment to automate the laser setup process, and accurately predict laser energy and temporal shape. PARC is based on the computer simulation code MIRO. 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 energy end temporal shapes measured after amplification and after frequency conversion with results computed with PARC. For most of the LASER shots, both measurement and computed results agree within five percent accuracy.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.