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

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 0.35×0.35m 2 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.

Assessing low laser damage densities of fused silica optics by means of a functional raster scan protocol

The knowledge of the laser damage resistance of fused silica optics for their use in high-power lasers is of primary importance for the operation and maintenance of these facilities. Among the control procedures developed, one of the most relevant to date is the raster scan procedure [Lamaignère et al., Rev. Sci. Instrum. 78, 103105 (2007)]. This procedure is used to determine the damage density of optical components as a function of fluence. To date, this procedure takes into account all triggered damage sites, regardless of their size and/or morphology. We have added a step to this procedure, which consists in irradiating again all the damage sites with a series of shots to ascertain their ability to grow. This allows us to estimate the densities of growing damage sites, which are most critical for the safe operation of lasers. This pragmatic approach can be considered a functional test procedure. By applying this procedure to large optical areas, we were then able to measure extremely low damage densities (∼10-4 damage cm-2) over a wide range of fluences [0-18 J cm-2], corresponding to fluences to which the optics are irradiated during the operation of high-power lasers. We can then determine the damage law of a given set of optical components. This reference law makes it possible, on the one hand, to predict the lifetime of the optics. On the other hand, any deviation can then be analyzed with regard to this reference law. Thanks to the determination of confidence intervals, this functional procedure can also be used to delimit the reference law by upper and lower bounds.

Estimation of laser-induced damage depth from surface image features

In laser damage experiments, damage initiation and growth are typically monitored by imaging the surface of the tested fused silica sample, ignoring their bulk morphology. The depth of a damage site in fused silica optics is considered to be proportional to its equivalent diameter. However, some damage sites experience phases with no diameter changes but growth in the bulk independently from their surface. A proportionality relationship with the damage diameter does not accurately describe the growth of such sites. In the following, an accurate estimator for damage depth is proposed, which is based on the hypothesis that the light intensity scattered by a damage site is proportional to its volume. Such an estimator, using the pixel intensity, describes the change of damage depth through successive laser irradiations, including phases in which depth and diameter variations are uncorrelated.