Selection and measurement of microsphere laser targets based on refraction

Abstract

The hollow glass or plastic microspheres used as laser targets range in diameter from a few micrometers to several hundred micrometers and can be selected from batches of commercially available microspheres. Preliminary selection of the very few that appear satisfactory for further testing can be accomplished by a practical nondestructive technique based on their refractive properties; when they are transilluminated and viewed through a microscope focused upon the surface upon which they rest, deviations from sphericity and uniform wall thickness appear as deviations from the ideal pattern of a perfectly symmetrical bright circular disk surrounded by a black ring of uniform thickness; wall defects appear as anomalies in this pattern. The method is shown to be soundly based on optical principles and theory. A model of the illumination distribution, including intensity profiles of model microspheres with specified characteristics, is compared with the rigorous measurements necessary for final selection of laser targets. The ratio between the inside diameter of the black ring and the outside diameter of the microsphere is a function of wall thickness, which can be calculated accurately on this basis. The same relationship also permits determination of the density of DT gas in a gas-filled sphere or the thickness of a layer of solid DT on the inner surface. Either microspheres or their photographs can be rapidly scanned and digitized by a computer-controlled microdensitometer; data points can be recorded for 1-μm intervals in the X and Y directions and can be displayed as density contours, density profiles, or isometric density profiles. Another convenient instrument for viewing microspheres as well as for rigorous measurements is a color-isodensity television system with a microscope attachment.