GB578273A - Improvements in electron optical systems - Google Patents

Abstract

578,273. Electron lenses. BRITISH THOMSON - HOUSTON CO., Ltd., and GABOR, D. March 3, 1943, No. 3510. [Class 39 (i)] A dispersing electron, lens for correcting spherical aberration consists of an axially symmetrical electronic space charge confined within a limited region by combined electric and magnetic fields. The electron beam to be focused travels along the axis 17, Fig. 4, and passes first through a convergent magnetic lens 2. It then passes into the region within magnetic coil 4 in which electrons supplied by annular auxiliary cathode 6 are confined within the space marked 7 by the combined effect of the magnetic field, the positive electrode 5 and negatively charged end electrodes which as shown are the ends of coil casing 3. It is shown that, provided the cathode 6 is situated on or near the line of field 8, Fig. 5, which cuts the axis 17, there will be set up two rotating streams of electrons travelling in opposite directions and forming a highly concentrated space charge which, near the axis, increases with radial distance from the axis. The potential at the axis falls to zero and in order to prevent hollowing out of the beam, and the accumulation of excess energy electrons, electrons are removed from the region at a controlled rate by slightly positive electrode 16; the charge density ultimately falls to zero at the edges of the region and no electrons are collected by anode 5. The magnetic fields of the rotating streams combine to oppose and weaken the field of coil 4 at the axis. As the lens has a negative spherical aberration, it combines with ordinary lenses having a positive aberration to produce a compound lens of low spherical aberration and short focal length, suitable for electron microscopes. The Provisional Specification states that chromatic aberration may also be corrected.