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.
The Laser Mégajoule (LMJ) facility has about 40 large optics per beam. For 22 bundles with 8 beams per bundle, it will contain about 7.000 optical components. First experiments are scheduled at the end of 2014. LMJ components are now being delivered. Therefore, a set of acceptance criteria is needed when the optical components are exceeding the specifications. This set of rules is critical even for a small non-conformance ratio. This paper emphasizes the methodology applied to check or re-evaluate the wavefront requirements of LMJ large optics. First we remind how LMJ large component optical specifications are expressed and we describe their corresponding impacts on the laser chain. Depending on the location of the component in the laser chain, we explain the criteria on the laser performance considered in our impact analyses. Then, we give a review of the studied propagation issues. The performance analyses are mainly based on numerical simulations with Miró propagation simulation software. Analytical representations for the wavefront allow to study the propagation downstream local surface or bulk defects and also the propagation of a residual periodic aberration along the laser chain. Generation of random phase maps is also used a lot to study the propagation of component wavefront/surface errors, either with uniform distribution and controlled rms value on specific spatial bands, or following a specific wavefront/surface Power Spectral Distribution (PSD).
KEYWORDS: Diagnostics, Near field, Optical amplifiers, Plasma, Mirrors, Frequency conversion, Calibration, Picosecond phenomena, Space operations, Fusion energy
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.
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 (<500 micron) or to the diffuse light around the central part of the focal spot. We present some modifications of the ANTALIA experimental setup designed to overcome these problems.
French Megajoules laser (LMJ) and Ligne d'Integration Laser (LIL) are two high power lasers using large diffractive components for focal spot conditioning. Those components are a 420x470 mm2 focusing grating and a 398x383 mm2 continuous phase plate (CPP), both working at wavelength 0.351 μm. In order to control the tight specifications that have to meet those components, CEA and company Jobin-Yvon, in charge of their manufacturing, have developed specific setups. After a brief introduction describing specifications and requirements, we will present the different setups and analysis software used and finally discuss the different measurements that could be done.
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 mm2) 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.
The design of high power lasers such as the MEGAJOULE laser (LMJ) and its first prototype, the Laser Integration Line (LIL) requires optical components with very strict and diverse specifications over large apertures. Though technologies used for the fabrication of these components may be usually compatible with such specifications, fabrication processes are often restricted by our ability to measure the effective performances. In order to determine the effective quality of its components, CEA is equipped with a wide range of metrology devices, many of them were developed for the specific needs of LIL and LMJ programs. After a short description of the Megajoule laser, we will focus on two different metrology devices used in the characterization of its optical components: interferometry and photometry.
As part of the LIL (Ligne d'Integration Laser) and LMJ (Laser Megajoule) projects, CEA/CESTA and SAGEM company have carried out an accurate metrology of the back focus length of filtering lenses. These measurements should enable us to make up for a lack of focus adjustment on the laser and to correctly position these lenses. Three different lenses have been characterized: • Lenses L1 and L2 with a back focus length of 10 m (aperture of F/16). Uncertainty is estimated at ±1,3 mm. • Lens L3 with a back focus length of 26 m (aperture of F/43). Uncertainty is estimated at ±5,3 mm. To achieve these measurements, 4 methods have been used: (1) Measurement of the curvature radius of each side on a Coordinate Measuring Machine and index homogeneity characterization before polishing. (2) Interferometric measurement with a reference spherical mirror. (3) Interferometric measurement using a lens side as a reference spherical mirror with a flat mirror. (4) Interferometric measurement by closing the cavity at the top of the lens back side. Uncertainty calculation has been done for each method.
The design of high power lasers such as the MEGAJOULE laser (LMJ) and its first prototype, the Laser Integration Line (LIL) requires optical components with very strict and diverse specifications over large apertures. Though technologies used for the fabrication of these components may be usually compatible of such specifications, fabrication processes are often restricted by our ability to measure the effective performances. In order to determine the effective quality of its components and to help optimizing their production, CEA equipped with a wide range of metrology devices, many of them were developed for the specific needs of LIL and LMJ programs. In the same time CEA also supported the development of specific metrology means at its optics vendors to help in th fabrication process design. After a short description of the Megajoule laser, we will focus on the different metrology devices used in the characterization of its optical components non-exhaustively ranging from interferometry and photometry measurements to focal spot analysis.
A laser damage measurement campaign was realized on PHEBUS high power laser on two different high reflecting HfO2/SiO2 mirrors centered at the wavelength of 1.053 micrometers at 45 degrees incidence. The two tested mirrors were deposited using e-beam technology: a large 620 X 440 mm2 LIL mirror was made with an oxide HfO2 target, and a 100 mm-diameter mirror with a metal Hf target. The test were performed with a 40 mm wide beam. Damages were detected by light scattering on a separate facility. Macroscopic and microscopic images of the damages were taken. A statistical analysis of these data is proposed to compare the mirrors. It is also interesting to compare large beam damage data to small beam laboratory statistics.
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