Patent GB-729812-A (GB)
Title: Improvements in or relating to high-frequency electron-discharge tubes
IPC: H01J25/12
Filed: 1950-09-20
Abstract: 729,812. Velocity-modulated tubes. SPERRY CORPORATION. Sept. 20, 1950 [Sept. 22, 1949], No. 23061/50. Class 39 (1). In a velocity-modulation device the beam is converged to a minimum diameter by electrostatic focusing means and is prevented from diverging subsequently by a magnetic field set up between two pole pieces having apertures for the passage of the beam. The point of minimum cross-section of the beam is located within the first pole piece at a distance upstream of its downstream end approximately equal to the radius of the exit hole in the pole piece. In one form, Fig. 3, the tube has two cavity resonators 22, 23 separated by a drift tube 26 and blocks 49, 51 of soft iron form the magnetic pole pieces; the pole faces 52, 53 may be planar, as shown, or frusto-conical to minimize leakage flux. Alternatively the inner part of pole piece 49 adjacent aperture 54 may have a frusto-conical shape, as shown to achieve the same effect. The magnet may be of the permanent type and may comprise steel cradles fitting on the pole pieces 49, 51, alnico blocks carried by the cradles, and a steel bar connecting the alnico blocks, the various magnet parts being secured together and held against the pole piece by metal straps 17 which are tightened up by screws, Fig. 1 (not shown), but alternatively the permanent magnet apart from the pole pieces may be made in one piece. A centrally apertured block 21 between the pole pieces 49, 51 carries discs 27, 28 and tubular member 24 so as to form the cavity resonators and drift tube; members 21, 24, 27, 28 are of copper or of a copper-plated material. Molybdenum sleeves 32, 33 are provided at the resonator gaps, and have very thin walls, e.g. 0.005 inches, so as to minimize secondary emission from their end surfaces. To further assist in achieving this object, sleeves 32, 33 may have different internal diameters and if desired all four sleeves may have different internal diameters. The input and output couplings to rectangular waveguides 39 are by way of impedance matching and coupling slots 37, 38. Glass windows 44 are sealed to the outer ends of the guides to maintain the vacuum. In the sealing process the frames 43 carrying windows 44 are welded to flanges 42 by an atomic hydrogen weld. Frame 43 has annular slots 47, 48 to thermally isolate window 44 during the welding process and this construction also helps to prevent fracture of the window through external jarring of members 42, 43. Cooling fins 41 are provided on the guide walls. The pole piece 49 may have a positive potential and serve as an accelerating electrode. The cathode is indirectly heated and has a spherically concave emissive surface 59, and the electrons are focused by electrode 61 comprising cylinder 63 and disc 62 carried by cylindrical support 67. Edges 64, 66 of disc 62 and cylinder 63 respectively exert the major controlling effect on the beam, edge 64 being of smaller diameter than edge 66. An extension 57 of pole piece 49 serves to isolate the cathode and electrostatic focusing means from the magnetic focusing field. An iron skirt 74 surrounding the cathode prevents the formation of metallic deposits on glass bell 76. Extension 57 of the pole piece 49 is sealed to glass bell 76 via flanged members 77, 78 and carries skirt 74. The final collector comprises sleeve 91 which is of small inner diameter to prevent the formation of a virtual cathode, sleeve 81 of larger inner diameter and copper bell 82 which is pinched at one end after the device is evacuated. Sleeve 83 and end plate 84 protect bell 82 and cooling fins 86 spaced by rings 87 are provided. Collector member 81 is sealed to pole piece 51 via disc 88, glass ring 89 and pairs of flanged members 77, 78 ; glass ring 89 provides thermal isolation of the collector from the remainder of the tube structure. If battery 73 is replaced by a pulse generator giving large amplitude pulses, a pulsed output can be obtained from guide 36. The beam forming and focusing means are discussed in detail; the equipotential lines adjacent the cathode are concentric circles to concentrate the beam into a smaller crosssectional area than the emissive surface, and this is facilitated by providing a smoother grid across the left-hand face of block 49. Alternatively, if the smoother grid is not provided the aperture 54 must be made larger to allow for divergence of the beam due to the field distortion at edge 60. The focusing electrode 61 is electrically connected to the cathode. The resonator walls of copper or copper plated material include portions 92, Fig. 4, which are secured to block 93, for example along two opposite edges, and left free at the other two edges; they are folded at their middle portions and connected to slots at the ends of shafts 94 which are movable by threaded knurled rings 98 acting through bellows seals 96, 97 and threaded sleeve 102 for tuning. Nuts 103, 104 serve as limit stops. The walls 92 may be connected to an intermediate structure rather than directly to block 93. Annular slots 29, 31 permit pretuning by bowing of the discs 27, 28. In another embodiment, Fig. 6, the magnetic pole pieces 108, 109 produce a focusing field only in the drift tube 106; they carry sleeves 116, 117 and discs 111, 112, 113, 114. Annular slots (not shown) may be provided between discs 111, 112 and pole pieces 108, 109 to permit pretuning by bowing of the discs 111, 112. In both embodiments an electromagnet may be used instead of a permanent magnet. The frequency of the output resonator may be a harmonic of the frequency of the input resonator. The device may be converted into an oscillator by inserting a coupling loop between the two resonators. The invention may also be applied to reflex klystrons, velocity modulation tubes employing three or more resonators, and travelling wave tubes. Prior art klystrons with (a) a coil surrounding the drift space; (b) coils arranged around the resonators to converge the beam to cross-overs at both gaps, the drift tube being tapered; or (c) magnetic focusing means extending from behind the cathode to beyond the output resonator gap, are referred to. Reference has been directed by the Comptroller to Specification 674,758.