Abstract
A grating-inspection system and a damage-analysis method have been developed to measure in situ laser-induced damage on a 1.5-m tiled-grating assembly of the OMEGA EP pulse compressor during a 15-ps, 2.2-kJ energy ramp. The beam fluence at which significant damage growth occurred was determined. This is the first report on beam fluence versus laser-induced-damage growth of meter-sized multilayer-dielectric-diffraction gratings. This result was correlated to the damage-probability measurement conducted on a small grating sample and is consistent with the fluence, corresponding to 100% damage probability.
Highlights
Grating-Damage Inspection System The grating-based pulse compressor of the petawatt-class, short-pulse OMEGA EP laser consists of four sets of tiled-grating assemblies, each measuring 141 cm # 43 cm (Refs. 1 and 16)
An on-shot near-field fluence map was measured for each high-energy shot, and a grating-inspection system (GIS) image was obtained after each shot
The grating surface scanning image after each high-energy shot was correlated to the on-shot laser-beam fluence map to determine the relation between damage growth and beam fluence
Summary
Understanding the in-situ laser-induced–damage threshold of large-aperture multilayer-dielectric-diffraction (MLD) gratings is paramount for petawatt-class laser facilities to reach design energies.[1,2,3,4,5,6,7,8] Until now, short-pulse damage testing has been performed only on small-scale samples.[9,10,11,12,13,14,15] No vacuumdamage test data are available on large-scale MLD gratings, and it has not been proven that one can transfer the results of the small samples to full-aperture MLD gratings. This article reports on the performance and findings of a vacuumcompatible grating-inspection system (GIS) that was deployed to detect in-situ damages of large-aperture gratings between high-energy shots. The following sections (1) describe the mechanism and characterization of the inspection system; (2) introduce the methodology for detecting grating damage and the analysis method for determining the laser-beam fluence causing damage growth; (3) discuss the accuracy of the determined laser-beam fluence; (4) compare the damage-test result of a large-aperture MLD grating to the damage-probability measurement conducted on a small-grating witness sample; and (5) present conclusions. The GIS consists of a line-shape illumination generator and an imaging system. A point source from a fiber-based, 1053-nm continuous-wave laser is projected to form a lineshape illumination pattern on the surface of grating 4 using a multi-element lens assembly.
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