Permanent magnet focus unit with integral astigmatism corrector

Abstract

A focusing unit for a cathode ray tube comprises a plurality of permanent magnets, each having a longitudinal axis, and a plurality of coils. A form holds the magnets in a first annular array at substantially equally spaced intervals in which the magnet axes are substantially parallel to a longitudinal axis defined by the first annular array and holds the coils in a second annular array at positions spaced angularly between the magnets. Annular flanges of high magnetic permeability are disposed over longitudinally opposite ends of the magnets. At least one annular winding is disposed substantially adjacent to and inwardly from the first array of the magnets. In one embodiment, the coils are helical coils disposed in substantial circumferential alignment with the first annular array of the magnets. The helical coils each having a substantially is radially aligned longitudinal axis. In another embodiment, the plurality of coils are spaced radially between the first array of the magnets and the at least one annular winding. Each of these coils defines a substantially radially aligned longitudinal axis. In each embodiment, the plurality of coils of the second array are coupled in two interleaved sets for generating two quadripole focusing fields for correcting astigmatic aberrations of an electron beam in the cathode ray tube.