The "Crossed-circle" periodic permanent magnet focusing structure for high power traveling wave tubes

An alternate technique of focusing beams previously requiring high cost platinum cobalt magnets is described. The "crossed-circle" structure using Alnico V bar magnets significantly reduces the intrinsic cost of focusing high density electron beams. The periodic "crossed-circle" focusing assembly consists of two rows of magnet, pole-piece, magnet sets perpendicular to each other and radial to the axis of the tube: the "cross". The flux return path between the bar magnets is a steel shell of appropriate thickness: the "circle". The end view of the structure exhibits a similarity to a four pole dc motor. A discussion of 1 Kw pulsed X and C band traveling-wave tubes using this focusing structure will be presented. The tubes provide greater than 1 Kw from 7 to 11 Gc and from 4 to 8 Gc respectively. The tubes are grid controlled and operate at duty cycles up to .01. PPM magnetic compression of the beam, by a factor of 2 to 1, which permits the same gun to be used on both tubes, will also be described.