The Effect of Slurry Composition on the Surface Quality of Potassium Dihydrogen Phosphate Crystals Finished By Fluid Jet Polishing.

Potassium dihydrogen phosphate (KDP) is an indispensable optical crystal utilized in fusion-class laser systems for frequency conversion and polarization control. Fabrication of a freeform KDP optic in which the exit surface possesses areas of varying thickness is currently under exploration as a method to spatially alter polarization states of the laser beam and improve beam irradiation on the targets. Due to the properties of the crystal such as water solubility, softness, brittleness, and sensitivity to thermal shock, however, there are limited methods of fabricating freeform surfaces with acceptable laser-induced–damage thresholds (LIDTs) and optical quality.

Fluid jet polishing (FJP) is one such method that can meet this challenge, yet improvements to the process are needed to produce higher-quality surfaces with more robust LIDTs. The FJP method consists of pumping a polishing slurry (abrasive particles suspended in a liquid medium) through a nozzle toward a surface to generate a sub-aperture removal spot. The simplicity allows for flexibility of the slurry, which can be specialized for the surface to be polished, such as KDP. The current slurry employs alumina abrasive particles suspended in mineral oil. Although, the method produces surface defects, such as cracks, scratches, and deep pits. Additionally, the high viscosity of the slurry is a limiting factor for the pumping pressures and nozzle diameters that are desired. In this work, we studied the FJP performance of new slurry candidates, employing alumina particles of varying sizes and hardness dispersed in a low-viscosity mineral oil. We found a clear correlation of particle size and hardness on surface quality, which can provide guidance for further slurry optimization.

Fabrication of a freeform, potassium dihydrogen phosphate (KDP) wave plate by fluid jet polishing (FJP) is under active investigation at the University of Rochester’s Laboratory for Laser Energetics (LLE). Implementation of such a freeform optic on the OMEGA Laser System would provide dramatically improved laser-beam uniformity for fusion experiments. The current FJP process produces a variety of defects such as cracks and pits that reduce optical surface quality and laser-damage resistance. This work investigates alternative slurry compositions for improving the FJP method. Slurries with varying abrasive particles and surfactants were used in the FJP process and generated different surface finishes. The surfaces were characterized by microscopy and interferometry to measure surface roughness and defects. The results, which demonstrated a clear correlation of particle size and hardness to surface quality, will provide guidance for further optimization work.