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An apparatus has been developed and built at the National Bureau of Standards to study low-density (~4014particles/cm3or 40-4 mm Hg) gas flow.1, 2This is an electron—optical schlieren technique which depends on the scattering of electrons in collision with individual gas molecules or atoms to form a dark field image of the gas. The apparatus consists of a bakeable stainless steel low-pressure chamber and other chambers containing brass and glass electron optical components which are not baked. The low-pressure chamber (~10-8 mm Hg) is connected to three other chambers (~10-6 mm Hg) through two small apertures and a slit. Metal gaskets are used on the bakeable chamber. The system is pumped by four oil diffusion pumps each with a liquid nitrogen trap. A small liquid helium trap (~1 1.) will be used to decrease the residual pressure from 10-7 mm Hg to 10-8 mm Hg or better.

A procedure was developed and used to obtain the optimal shape of corrected aspheric geodesic lenses for F-numbers ranging from 3 to 15. This optimization procedure allows the (constant) refractive index, lens radius, and focal length to be used as independent design parameters. Results of the increase in size of the first diffraction spot and shift in the focal plane caused by perturbations in a lens’s shape have been included for several lenses with F-numbers between 3 and 10.

From Abstract: "This document is the final report of research carried on in the Electron Physics Section of the National Bureau of Standards during the period from February 1, 1955 to March 1962 in developing an electron optical method for the visualization of low-pressure gas flow."