GB1304893A - - Google Patents

Abstract

1304893 Velocity-modulated tubes; cathoderay tubes; magnets J A MELNIKOV 5 Feb 1970 5504/70 Headings H1D and H1P An adjustable magnetic lens-for use, for example, in a forward or backward wave travelling-wave tube, a klystron or a cathoderay tube-includes an annular permanent magnet, axially magnetized in sectors, with adjacent sectors magnetized in opposite directions, and polepieces at each end of the magnet, each having a shape such that it can be arranged to extend only over the ends of all those sectors which have the same polarity, the magnet and the polepieces being rotatable with respect to one another. In the simplest arrangement (Fig. 1a), the magnet 1 is divided into two 180 degree sectors of opposite polarity, each soft magnetic polepiece 2 being in the form of a half disc with a central hub to facilitate rotation, whereby simultaneous rotation of the two polepieces through 360 degrees provides a field at a point along the axis which varies continuously from a given intensity in one sense along the axis, through zero, to an equal intensity in the opposite sense. Division of the magnet into a larger number of sectors (Fig. 1 to 1e, not shown) increases axial symmetry, while independent rotation of the two polepieces permits different field variations to be produced on the two sides of the magnet; if the two polepieces are angularly displaced through 180 degrees and the diameter of the polepieces is less than that of the magnet (Fig. 1c, not shown), the field resembles that of a radially magnetized annular magnet. Each ferrite magnet of a travelling-wave tube periodic focusing system, Fig. 5, is formed of a fixed inner annulus 3, axially magnetized in the same direction throughout, and a rotatable outer annulus 4, divided into two oppositely polarized sectors, adjacent magnets being separated by fixed polepieces 3 having a reduced diameter over a 180 degree sector, and which together with copper spacer rings 6 form the tube envelope, and project radially inward to form both a "field straightener" system and a coupled-cavity slowwave structure. Rotation of the outer sections 4 of the magnets permits adjustment to provide an accurately periodic field. A variety of arrangements of the magnets in a periodic system is disclosed (Fig. 6, and Figs. 8 to 13, not shown).