Minimizing Contamination to Multilayer Dielectric Diffraction Gratings Within a Large Vacuum System

摘要

The University of Rochester's Laboratory for Laser Energetics (LLE) has recently completed the construction of the OMEGA EP short-pulse, petawatt laser system. A major structure for OMEGA EP is the grating compressor chamber (GCC). This large (15,750-ft~3) vacuum chamber contains numerous optics used in laser-pulse compression of two 40-cm-sq-aperture, IR (1054-nm) laser beams. Critical to this compression, within the GCC, are eight sets (four per beamline) of tiled (e.g., three optical elements precisely held side by side to act as one element) multilayer-dielectric (MLD)-diffraction-grating assemblies (three gratings per assembly) that provide the capability for producing 2.6-kJ output IR energy per beam at 10 ps. The primary requirements for each of the 24 large-aperture (43-cm × 47-cm) gratings are a high diffraction efficiency greater than 95%, a peak-to-valley wavefront quality of less than λ/4 waves at 1054 nm, and a laser-induced-damage threshold greater than 2.7 J/cm~2 at 10-ps pulse width (measured at normal beam incidence). Degradation of grating laser-damage thresholds due to adsorption of contaminants must be prevented to maintain system performance. The presence of extrinsic contaminants (either particulate or molecular) in the vacuum system puts the MLD gratings at risk with respect to lowered damage threshold. A number of protocols have been developed and implemented at LLE to minimize MLD grating contamination and to characterize the performance of the GCC. In this paper, we describe component-cleaning procedures; the qualification, testing methods, and studies undertaken for materials intended for use within the chamber; as well as the necessary cleaning procedures to integrate and operate the MLD grating assemblies.