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Abstract

A rapid random accessed electron beam memory system comprises a disc mounted for rotation and supporting an information storage medium. An electron gun is mounted for movement across the disc. The gun has ultra-compactness and extremely low mass, yet is capable of developing a finely focused electron beam probe at high beam current densities. The gun comprises a low-mass field emission cathode, the cathode having an emitting tip and being adapted to receive a predetermined electrical potential to form a high brightness electron source at the tip. An electrostatic focus lens forms a real image of the electron source in the vicinity of the storage medium. The lens comprises a first electrode adapted to receive a predetermined second electrical potential which is positive relative to the potential on the tip and has a value effective to extract electrons from the tip. The electrode defines a relatively small aperture for deforming the diameter of an electron beam which is formed. A second electrode located downbeam of the first electrode is adapted to receive an adjustable third, focusing, electrical potential which is negative relative to the second electrical potential, and said second electrode having an aperture which is larger than the first electrode aperture. A third electrode located downbeam of the second electrode is adapted to receive a fourth, accelerating, electrical potential which is positive relative to the third potential for accelerating the beam, the third electrode having an aperture which is also larger than said first electrode aperture. The second, third and fourth electrical potentials are selected to establish beam-focusing fields between said first and second and between said second and said third electrodes. The gun has an ultra-low mass in order to make feasible rapid random accessing of any area of the storage medium.

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