Brightness as a function of lens excitation

Abstract

The Langmuir limit, defined by B. D. Langmuir, was derived from Lambert’s law. For convex cathode or curved trajectories, beams with brightness higher than the Langmuir limit are obtainable. Measured brightness varies dramatically when we vary the excitation in the first lens of a system consisting of an electron gun and two magnetic lenses. The ratios of maximum to minimum brightness were measured as 717, 403, and 310 for beam energies of 10, 20, and 30 keV, respectively. These results indicate that the brightness was not conserved when lens excitation was varied. The maximum brightness for beam energies of 10, 20, and 30 keV were observed to be, respectively, 143, 208, and 92.1 times higher than the Langmuir limit.