Patent GB-764681-A (GB)
Title: Improvements in travelling wave tubes
Inventor: ROBINSON NORMAN WRIGHT
IPC: H01J23/26
Filed: 1952-01-01
Abstract: 764,681. Travelling-wave tubes. MULLARD RADIO VALVE CO., Ltd. Dec. 5, 1952 [Jan. 1, 1952], No. 34/52. Class 39(1). In a travelling-wave tube of the helix type the helix is carried by an insulating support disposed inside the helix, the shape of the helix being independent of the shape of the insulating support the insulating support contacting the helix at one or a few points per turn. The support may be an insulating, preferably glass, quartz or ceramic, helix 16, Fig. 1, supported by members located by the interior of a precision bore outer glass tube constituting the tube envelope or a straight quartz, glass or ceramic tube whose cross section is a regular figure of 3-10 sides, e.g. a hollow square or a hollow or solid triangle or strips secured together to form a hollow triangle, Figs. 4-7. The insulating tube in these cases is carried by end members located by the precision bore glass envelope. In further forms, the support may be a ceramic strip twisted about one longitudinal edge or about its longitudinal centre line, Figs. 8, 9 (not shown) so that the points of support lie on one or two helices ; the strip is formed by simultaneous extrusion and twisting. In Fig. 1 the insulating helix may be made by winding glass rod on a mandrel to a helix of slightly too large external diameter, filling the spaces between the turns with pitch, wax, thermoplastic or thermosetting resin, grinding the helix down to the correct size, and removing the filler and mandrel. The glass helix is now inserted into conductive cylinders 6, 10 and conductive helix 17 is welded to these cylinders. The insulating and conductive helices are in contact at about one point per turn of the conductive helix, suitable pitch ratios lying between 15: 1 and 5: 1. The annular beam passes outside the helices. Tubes 6, 10 are carried by anode disc 4 via two tongues (not shown) and by discs 7, 11 and 12 ; the discs have flanges. Discs 4, 7, 11, 12 are located by the precision bore glass envelope. The external circuits may be waveguides, or coaxial lines, Fig. 3 (not shown) in which case the ends of the conductive helix gradually increase in pitch and finally become the inner conductors of the coaxial lines. The insulating helix may also be made by winding a glass rod on a rod of tungsten plated with copper and subsequently dissolving out the copper with nitric acid to allow removal of the helix ; alternatively the mandrel rod may have a stainless steel surface. The mandrel may be a tube instead of a rod. The conductive helix is wound on a separate mandrel. The insulating helix may alternatively be reinforced by an insulating rod passed down its centre. If the locus of the points of support is a helix this helix may have the same sense as or opposite sense to the conductive helix. The tube may be used as an amplifier or oscillator. Prior art devices are referred to in which the helix is supported by external means such as quartz or glass tubing of circular cross section which may or may not have dimples, or by three insulating rods, or by an internal insulating rod into which the helix is sealed over a small arc of each turn, is referred to. The Provisional Specification states that the insulating support which may be as shown in any of the embodiments described therein or as in Specifications 664,663 or 691,399 may be outside instead of inside the helix. If a conductive helix is supported by an external insulating helix, the latter may be reinforced with an outer sheath. This is achieved by suspending the mandrel and insulating helix vertically, a glass tube is passed over the insulating helix, partially evacuated and heated so that it collapses on to the insulating helix. The tube may have less material in the form of colloidal graphite in the central region of the helix.