GB726823A - Improvements in or relating to magnetic circuits for travelling wave apparatus - Google Patents

Abstract

726,823. Magnets. WESTERN ELECTRIC CO., Inc. Dec. 17, 1952 [Dec.29, 1951], No. 31982/52. Class 35. [Also in Group XL (a)] Travelling wave apparatus is provided with a magnetic circuit for focusing comprising a pair of polepieces energized by a magnet and defining an air gap in which the delay line is positioned, and a " flux guide " of magnetic material extending between the polepieces and effectively encompassing the air gap, the flux guide being of varying thickness so as to focus the beam in a desired manner. In one form, Fig. 1, a travelling wave tube 16 having heat radiator 20 is positioned within and supported by two U-shaped magnets 10, 11 with common cylindrical polepieces 12, 13. The gun 17 is surrounded by a magnetic shield within the tube and the assembly is positioned within polepiece 12. The tube construction may be as described in Specification 697,550, [Group XL (a)], in which the gun shield is outside the tube envelope. 23, 24 are input and output wave-guides and may assist in supporting the tube. The flux guide may be two magnetic (steel) plates 27; they are linearly tapered from each end to the centre and are wide compared to their separation and serve to make the flux lines straight and the field uniform as described with reference to Figs. 2, 3 (not shown). A cylindrical flux guide is theoretically better than two plates but has the disadvantages that (a) large apertures therein would be necessary for the input and output wave-guides or, (b) if coaxial terminals are employed for coupling the input and output the tube is not accessible for inspection. Impingement of the electrons on the helix near the collector, due to space charge radial debunching, is prevented by using a strong magnetic field; the field should, however, be weaker along most of the helix to allow the electrons to pass close to the helix where the interaction is most favourable. Thus the flux guide cylinder or plates may be thickest near the gun and thinnest near the collector, Figs. 5, 6 (not shown). The thickness variation may be exponential or linear. In another form, Fig. 7 (not shown), the form of Fig. 1 is modified by shaping the right hand half of the flux guide to deviate from the linear increase in thickness by an exponentially increasing amount so that the field increases exponentially from the midpoint to the collector. Permanent or electromagnets may be used. The variation in thickness of the flux guide may be modified for example in devices where small signal theory does not hold.