Derivation of preliminary specifications for transmitted wavefront and surface roughness for large optics used in inertial confinement fusion:

Abstract

In preparation for beginning the design of the National Ignition Facility (NIF) in the United States and the Laser Mega-Joule (LMJ) in France, we are in the process of deriving new specifications for the large optics required for these facilities. They are currently being evaluated through modeling and experimentation. These specifications will be ready for general release by the end of the year. Traditionally, specifications for transmitted wavefront and surface roughness of large ICF optics have been based on parameters which were easily measured during the early 1980s, such as peak-to-valley wavefront error (PV) and root-mean- square (rms) surface roughness, as well as wavefront gradients in terms of waves per cm. While this was convenient from a fabrication perspective, since the specifications could be easily interpreted by fabricators in terms which were understood and conventionally measurable, it did not accurately reflect the requirements of the laser system. In some cases, optics which were not adequate for a given application which was particularly sensitive to periodic errors, were fabricated acceptably in terms of the optics specifications. For the NIF and LMJ laser systems, we have availed ourselves of advances in metrology and interferometry and an enhanced understanding of laser system performance to derive specifications which are based on power spectral densities (PSDs). Such requirements can more accurately reflect the requirements of the laser system for m…