The paper aims to compare the results obtained with the same laser source with a large beam and with small beams. These
latest were shaped from phase objects implemented to obtain several small beams from a single larger beam. The
consistency of the results from both sets of measurements is shown. It validates the assumptions made and the specific
mathematical treatments implemented to establish the link between the two approaches. It also validates and strengthens
the approach developed from the rasterscan procedure used to measure damage densities from the scanning with beams
of small dimensions. This shows that small beam tests are reasonably representative of tests carried out with large beams.
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 paper aims to compare the results obtained with the same laser source with a large beam and with small beams. These latest were shaped from phase objects implemented to obtain several small beams from a single larger beam. The consistency of the results from both sets of measurements is shown. It validates the assumptions made and the specific mathematical treatments implemented to establish the link between the two approaches. It also validates and strengthens the approach developed from the rasterscan procedure used to measure damage densities from the scanning with beams of small dimensions. This shows that small beam tests are reasonably representative of tests carried out with large beams.
Laser damage resistance is a key factor for the operation and the improvement of high power laser systems. Up today laser damage performance of optical components is mainly a defect related material characteristic. Metrology procedures have been developed to realize repeatable and accurate measurements of surface damage density due to nanosecond pulses. These measurement techniques were used to guide the improvement of surface damage resistance. Fractures must be eliminated from surfaces, in order not to suffer a damage growth phenomenon, whose exponential character will reduce the optical lifetime. High intensity hot spots due to beam modulations, spatial and/or temporal modulations, can also cause surface damage. Specific set-ups and experiments were carried out that allowed us to analyze and explain these phenomena: damage initiation mechanisms, damage growth and beam propagation inside the optical components. The presentation aims to highlight relevant progress on these topics (1) initiation mechanisms due to defects show two phases: a first incubation phase followed by the expansion one of the damage site ; it appears also that damage diameters are well correlated with the expansion fluence, (2) a refined bulk observation coupled to a fractal analysis allow the quantification of bulk damage and therefore to explain the laser damage growth and its dependence on beam parameters like pulse length and fluence (3) non-linear amplification of phase and amplitude modulations lead to the amplification of the spatial pre-existing small scale modulations that increase the laser energy density locally and finally the number of damage sites. The whole of results, damage initiation, damage growth and beam propagation, is discussed to the light of the laser damage observed on LMJ optics.
A millimetric aperture Nd:glass laser system has been designed and constructed at the CEA-CESTA. Its aim is to best mimic the laser conditions that can be found in inertial confinement fusion facilities. It is therefore used to study the main phenomena that prevents these lasers to work at their maximum power: the laser induced damage of the optical components. The combination of temporal and spatial modulators provides, every minute, a 6 J, 7 mm, 351 nm homogeneous beam at the fused silica sample location. This proceeding illustrates the capacity of the facility over two experiments: the study of damage initiation and the growth of laser damage sites on fused silica, up to millimetric scales
The growth of damage sites from micrometric to millimetric scales under high energy laser system conditions
have herein been investigated. In this realm, a saturation of the surface growth followed by the rapid expansion of
radial cracks has been observed. This observation contrasts with the previously reported exponential behavior1
(for pulse durations above 2 ns) and linear behavior (for pulse durations below 2 ns). The observation of the
longitudinal damage structure coupled with fractal analysis has shown that these shifts in growth behavior seem
to be correlated with changes in the damage morphology.
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