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.
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.
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.
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.
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