Spallation caused by shock waves in optical components such as those used in the Laser MegaJoule facility during laser operation leads to material fracture during a Laser-Induced damage event. One solution may be to use a viscoelastic thin film on these components to mitigate spallation, but it must have excellent optical, mechanical, and resistance to laser damage properties. Among the viscoelastic materials investigated were Nafion and polydimethylsiloxane-based Ormosil. These materials, as thin films deposited on a fused silica substrate, were studied under nanosecond pulsed lasers at 1064 and 532 nm with different diagnostics in situ and post-mortem. In particular, the effect of the films on spallation was studied using the laser shock technique. Preliminary results showed that these thin films have interesting properties that could help to reduce mechanical damage to optical components.
Laser-assisted surface structuring was developed at CERN for the treatment of the inside wall of the vacuum system of the Large Hadron Collider (LHC). 50-µm-deep grooves were created by material ablation while the laser scanned the surface. A part of this material was redeposited as micrometer-size particle aggregates. This two-scale rugosity efficiently traps electrons. During the operation of the LHC, the surface is submitted to high electromagnetic forces and cooling cycles which might deteriorate its performances. Accelerations of the surface in the order of 350 000 g are expected to be induced by these electromagnetic forces. The LAser Shock Adhesion Test (LASAT), initially developed to assess the adhesion of coatings by spallation, was used to accelerate the surface of treated samples, in order to reproduce stress states similar to those generated by the electromagnetic forces. Pressure shock waves generated by nanosecond laser irradiation produce sharp velocity variations of the surface. Decelerations and, therefore, applied inertial forces were evaluated from the dynamics of the sample macroscopic surface, whose velocity evolution was measurement by VISAR (Velocity Interferometer System for Any Reflector) with a time resolution smaller than 1 ns. Once the test set-up was calibrated, the collect and the analysis of detached particles allowed the quantification of ejected material as a function of the applied mechanical stresses.
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