Patent GB-869547-A (GB)
Title: Travelling wave electron discharge device
IPC: H01J25/38
Filed: 1958-03-07
Abstract: 869,547. Travelling wave tubes. STANDARD TELEPHONES & CABLES Ltd. March 7, 1958 [March 11, 1957], No. 7421/58. Class 39(1). A travelling wave amplifier tube comprises an elongated slow wave structure supported throughout its length by rigid support and alignment means, an electron gun unit at one end of the structure and an electron collector unit at the other end, a metal tube forming part of the evacuated envelope of the travelling wave tube mechanically securing together the electron gun and collector units independently of the rigid support and alignment means, the metal tube being deformable to correct for misalignment between the electron beam and the axis of the propagating structure and having a diameter less than that corresponding to waveguide cut-off at the highest operating frequency of the travelling wave tube, positioning elements on the electron gun and collector units receiving the respective ends of the rigid support and alignment means, and spring loading means acting axially between the electron gun unit and the adjacent end of the rigid support and alignment means. The envelope consists of end parts 2, 4, Figs. 3A, 3B, housing the gun and collector respectively, and a central part 3 of reduced diameter housing the tungsten or molybdenum helix 7, the central and end parts of the envelope being sealed together by cup shaped members 11, 12 and sleeves 13, 14, both cups and sleeves being of non magnetic metal, the cups facing inwardly towards one another and the sleeves 13, 14 being welded to the cups by weld beads 15, 16 respectively. The sleeves and cups 13, 14, 11, 12 are also brazed at 40, 67, and the cups are brazed to non magnetic metal sleeve 64 and welded to sleeve 65. The three envelope parts 2, 3, 4 are each formed by breezing premachined cups and tubes together and are then machined to obtain the required accurately dimensioned surfaces (not all surfaces need be accurately dimensioned, e.g. at the seals). The gun, delay line and collector are now inserted within their respective envelope parts. To effect the brazing the parts, which are self jigging, are loaded into a brazing fixture where they are retained by weights only. For an upside down braze the blanks are premachined to retain the breezing material. Further assembly consists in inserting the delay line into non magnetic retaining ring 36, Fig. 3B and coupling strip 83 is welded to the inner conductor 21 of a coaxial output feeder 17. Cylinder 65 which renders the cavity therearound non-resonant is now placed in position and brazed to 64 and coupling strip 66 is welded to inner conductor 32 of coaxial input feeder 26. End envelope portion 2 containing the gun and header 53 as a single internal subassembly is now moved up so that tube 13 is placed over cup 11 and non magnetic retaining ring 59 fits over helix assembly 6 with an interposed spring loading shock absorbing delay line support formed by washers 60, 61 of tungsten or molybdenum (washer 61 bears against the three ceramic support rods for the helix). Header 53 is now helium arc welded to 47 using weld bead 35 and cups 11, 12 to tubes 13, 14 respectively using weld beads 15, 16. After the tube has been assembled and the exhausted tube pinched off envelope portion 3 is bent at its central point to bring the gun and helix axes into coincidence, reinforcing strut 82 preventing fracture of the seals. The joints at 15, 16, 35 are easily machined off for repair, salvage or substitution of parts. Rings 36, 59 are machined to be concentric with 39, 62 so the delay line and solenoid are truly coaxial. The alignment of the envelope parts 2, 3, 4 is assisted by accurately formed surfaces on 11, 13; 12, 14. Magnetic focusing is achieved by a solenoid (or permanent magnet) 34 which is located by accurately machined surfaces 62 and 38, 39, 38 serving as an axial stop for the tube when it is inserted into the magnet by handle 73 and the solenoid having a latch to hold the tube in position. Envelope part 47 and disc 48 are of iron and form a magnetic shield round the gun, 48 serving also as a polepiece (and 19 as the other polepiece at the collector end). The electron gun is held in correct axial and radial position by springs 111 wrapped around L-shaped rods secured to "Kovar" (Registered Trade Mark) base pins 56a, 56b, 56c, 56d, which press metal spacer washer 58 against accurately machined surfaces 57 on envelope member 47. Header seal 55 and the seals in the concentric line feeders are of glass or ceramic. The focusing electrode 85, Fig. 11, has supporting tabs 88 fastened to strap 87 between dielectric tubes 86 and the indirectly heated cathode 90 which has a central hole 107 to allow passage of the positive ions (which do not therefore bombard the cathode) is supported by a metal foil cylinder 91 and a flanged cylinder 92. Dielectric tubes 86 pass through apertures in metal disc 58 and in the flange of 92 to locate the cathode and focusing electrode 85 in correct spaced relationship and are brazed to the walls of the apertures. A molybdenum disc 104 acts as a heat reflector and is carried by rods 105 from flanged cylinder 92. A zirconium getter 100 is a wire wound around a helical tungsten wire surrounding dielectric tube 101 which is mounted on lead 102 brazed to metal disc 58. Head 102 passes through dielectric tube 103 and is brazed to 56d at its upper end. The getter 100 is connected between 56d and 102 and the return circuit is via metal disc 58 and metal wall 47. Insulating tubes 86 are secured together by metallising their surfaces and brazing strap 87 thereto. The cathode heater has a central lead 94 connected via strap 95, secured to one tube 86, and flexible lead 96 to base pen 56a, the return heater lead being via cathode 90, sleeve 91, flanged cylinder 92 and flexible lead 97 to base pin 56b. The focusing electrode lead passes through a separate insulating tube and is integral with a flexible lead 98 connected to base pin 56c. The copper collector 9 is a hollow cylinder serving also as an exhaust tube having a pinched off and sealed (by cold welding) termination 41 and carries copper block 69 copper cooling fins 70 each having a plurality of circumferential spacer, strengthener and turbulence producing dimples 72 and inner spacing flanges 71 soldered to block 69. Fins 70 are brazed or soldered to copper block 69 and parts 71, 72 are secured to the adjacent fins by painting them with eutectic silver solder powder and brazing. The collector 9 is sealed to the remainder of the tube by glass seal 45 and by stepped metal cylinders 43, 44. The collector is made long to collect the spreading electron beam without forming hot spots. The coaxial feeders have flexible portions 23, 26 (or 23, 26 may be rigid) and are brought out obliquely as shown in Fig. 3B and secured in insulating grommets 74 of a separator 73. Sleeve 64 of non magnetic metal has a diameter below cut off so that it cannot function as a hollow pipe waveguide and cup 12 and tube 14 are eccentrically positioned to allow passage of input coaxial feeder 26. The helix couplers are microstrip transmission lines, each comprising a strip of triangular shape, Figs. 4 6 (not shown). The base of the tube (53) is potted with plastic or silicon rubber. All non magnetic parts of the tube are of stainless steel or copper-nickel alloy. In a modified embodiment Figs. 14, 15 (not shown) the flexible coaxial line feeders are shorter than in Figs. 3A, 3B and are connected to an external coaxial line having large diameter conductors by means of an intermediate coaxial line section having tapered inner and outer conductors. The joints generally are made by brazing or welding. Specification 777,224 is referred to.