Evaluation Of Hot Isostatic Pressed Beryllium For Low Scatter Cryogenic Optics

Abstract

Coarse and fine variants and standard grades of I-70A beryllium powder manufactured by Brush Wellman were consolidated by vacuum hot pressing (VHP) and by direct hot isostatic pressing (HIP) techniques. Five cm diameter flat mirrors were fabricated from 7.5 cm diameter by 7.5 cm long billets. All billets had densities of essentially 100% after HIP'ing. The mirror blanks were polished bare and the resulting surface evaluated for surface roughness and surface scatter at 0.51, 3.39, and 10.6 μm. The general trend noted was that better optical properties were obtained with direct HIP'ed coarse powder (since designated as 0-50 material). Surface roughnesses of 15 to 20 angstroms rms were obtained. The coarse powder was then used to fabricate a 0.5 meter diameter solid HIP'ed blank. This blank was polished as an F/1.5 sphere, scatter measurements taken, and the figure evaluated at cryogenic temperatures. Distortion of 0.19 waves rms @0.63 μm was seen at 97 K, corresponding to a homogeneity of ~ 26 ppb/K rms.