Patent GB-668725-A (GB)
Title: Improvements in or relating to electron discharge devices
Inventor: BECK ARNOLD HUGH WILLIAM
IPC: H01J23/27
Filed: 1948-06-07
Abstract: 668,725. Travelling-wave tubes. STANDARD TELEPHONES & CABLES, Ltd. June 7, 1948, No. 15300/48. Class 39 (i). [Also in Group XL (b)] In a travelling-wave amplifier the electromagnetic wave transmission path comprises a helix formed as a coating on the inner surface of a tube of insulating material. In one form, Fig. 1, the device comprises a wave-guide 1 serving as part of the envelope of the device loaded with solid dielectric 3 and having an outer metal wall 2. The helix 10 is a coating on the inner wall of the dielectric 3. The dielectric 3 is tapered in the vicinity of input and output waveguides 4, 5, to provide correct matching. Guides 4, 5 may be of rectangular section, and guide 1 may be of cylindrical or rectangular section. The helical coating eliminates wall charges. The dielectric 3 may be of high permittivity material such as that described in Specification 648,291 or of lower permittivity material such as glass. In the latter case the helical coating is coupled to the input and output guides 4, 5 by suitable means (not shown). By using a dielectric and a helix in combination in this manner the electromagnetic wave velocity is reduced to below that when using a helix alone by a factor equal to the square root of the permittivity of the dielectric, so that the beam potential may be reduced to a few hundred volts. If on the other hand the dielectric is a thin tube of low-loss glass it functions merely as a mechanical support for the helix and the wave velocity is not appreciably modified by the dielectric. In another form, Fig. 2, the glass envelope wall 12 has a helix 20 coated thereon interiorly, and has a colloidal graphite coating 23 on the exterior to suppress parasitic oscillations. The glass tube 11 is sealed at either end to glass bulbs 13, 14 via disc seals 18, 19, the helix being connected to the discs via matching stubs 21, 22 which may be lengths of wire sealed at the helix ends into the glass tube or coated continuations of the helix. Input and output wave-guides (not shown) are attached at 18, 19. The helix in all cases may be of low-loss material, such as silver or copper, or of high-loss material such as iron, nickel, or colloidal carbon. The helical coating may be formed by the colloidal process described in Specification 646,921, [Group II]. Alternatively a wire of the desired material is inserted in the tube where it is supported by molybdenum springs; it is then heated in vacuo to evaporate the material on to the tube wall. The device is then placed in a lathe and part of the coating is removed by cutting an internal thread. The pitch of the helix so formed may be different at either end from the main part to facilitate matching to the input and output wave-guides. The device may be used as an oscillator with standing waves instead of an amplifier. Reference is made to the pulling out of the ends of the helices of prior art devices where they are connected to stubs, and to prior art devices using loaded wave-guides in which some inert gas is allowed to remain in the tube for ion focusing.