Patent GB-892535-A (GB)
Title: Travelling wave electron tube apparatus
IPC: H01J23/087
Filed: 1958-07-07
Abstract: 892,535. Travelling wave tubes. VARIAN ASSOCIATES. July 7, 1958 [July 24, 1957], No. 21711/58. Class 39(1) In a travelling wave tube preferably a backward wave oscillator a permanent magnet 52, 53 or a solenoid is provided Fig. 9 for causing a longitudinal magnetic focusing field to be produced and has two polepieces 55, 56 each with a hollow cylindrical shim of magnetic material mounted thereon with the output waveguide 48 extending between the shims increasing the uniformity of the magnetic field along the beam path especially over the region of the output waveguide where the magnetic field is liable to vary most. The shims 63, 64 are mounted on (spot welded to) polepieces 55, 56 each having 3 pointed centring screws 62 for centring and aligning the discharge tube within the magnet system. The discharge tube also has polepieces 5, 31 forming parts of the envelope and the polepiece 31 serving as the collector. The external magnet parts, polepieces carrying the shims and polepiece retainers 57, 58 are held together by bolts 54 (see Fig. 9) and the output waveguide is taken through a flexible grommet 65. The shims 63, 64 may be of unequal lengths, as shown and may if desired be extended right up to either side of the output waveguide. The tube envelope body portion 1 is of Cu Ni alloy and surrounds a tape or wire helix 2 which is mounted by glazing in 3 sapphire rods 3 forming a pressfit in cylinder 1 the end of the helix adjacent the cathode being coupled to the output waveguide 43 by means of an annular member 39 having a raised block portion 41 to which the helix is brazed, and the vacuum being maintained by a ceramic window 47 which is sealed across a tapered section of the waveguide. The window is carried by frames 45, 46 of copper nickel alloy and brass frame. The end seals of the tube are effected by ceramic discs 34, 7 metal cups 6, 32, 33 and metal disc 35 and pinched off copper exhaust tube 36. The metal to metal tube envelope joints are effected by brazing. The indirectly heated cathode is mounted by three Mo washers 12, 13, 14 and has an annular emissive surface inner and outer focusing electrodes 24, 13 being provided followed by density modulating grid 18 of hexagonal shape brazed to adapter 17 carried by three metal tabs 15 from a frame 12 to which they are brazed but not in contact therewith so thermal expansion and contraction of 18 is permitted. In assembly the cathode mounting structure is slipped into the envelope with end cup 8 against 1 and held thereagainst by spring 11<SP>1</SP> abutting against metal washer 12; the gun structure as a whole being carried by three sapphire rods 9. The cathode is mounted from a ring 14 by 3 struts 20, Fig. 7 so as to allow for thermal expansion and contraction and outer focusing electrode is similarly mounted. The gun anode 28 acts as a beam ' shaver' i.e. it eliminates the outermost electrons of the beam and carries a grid 29. Electronic tuning is achieved by varying the voltage of anode 28. Collector structure 37 which may or may not be of magnetic material drains positive ions while 31 37<SP>1</SP> and 2 prevent the return of secondaries to the helix. In a modification, Fig. 12, a ring electrode negative with respect to the collector and helix e.g. at cathode potential drains the positive ions and also prevents secondary electrons from the collector from returning to the helix.