Patent GB-701034-A (GB)
Title: Improvements in or relating to travelling wave amplifiers
IPC: H01J25/38
Filed: 1949-12-09
Abstract: 701,034. Travelling-wave tubes. WESTERN ELECTRIC CO., Inc. Dec. 9, 1949 [Dec. 10, 1948], No. 31668/49. Class 39(1) [Also in Group XL(b)] In a travelling wave amplifier in which an electron beam travels linearly and generally parallel to the wave transmission path a D.C. field is set up transversely of the beam to assist in focusing and to collect positive ions from the beam which would otherwise accumulate along the beam and cause noise in the output circuit. In one form, Fig. 1, the helix 10 is supported by ceramic rods 11, and is surrounded by a metal sleeve 12 which is maintained at a negative D.C. potential relative to the helix. A magnetic focusing coil 24 is provided. The rods 11 are fitted at their ends into notches in metal collars 13, 14 which have connectors 15, 16 attached to the helix 10. Loss material 21 is formed as a coating on the inwardly facing surfaces of the rods 11. The device has indirectly heated cathode 2, beam forming electrode 6 and accelerating electrode 7 with grid 8. Input and output waveguides 17, 18 are provided. All the conducting material of the tube structure is non-magnetic. The helix 10 may be forty wavelengths long at the centre frequency of operation. In another form, Fig. 2, the helix 40 is wound on a ceramic support 39 and is surrounded by an electrode. 56 which may have a positive or negative D.C. potential with respect to the helix so that the positive ions are collected by the helix 40 or by electrode 56 respectively. The annular indirectly heated cathode has emissive material 33 in a groove thereof and projects an annular beam between the helix 40 and the electrode 56. The envelope 31 which is of non-magnetic conducting material such as copper, bronze, aluminium or molybdenum finally collects the electrons, or alternatively a separate collecting electrode may be provided. The input and output are coupled to the helix via coaxial lines 42, 43; 50, 51 provided with glass or ceramic seals 44, 52. Similar seals 37 attached to metal eyelets 36 are provided for other electrode leads. The input coaxial line 45, 46 includes a choke 47 to provide a high impedance connection between the helix and the battery 49, and a by-pass condenser 48 is also provided. The ends of the helix may be tapered in diameter and also in pitch if desired so as to provide a good impedance match with the input and output coaxial lines. Alternatively input and output waveguides coupled as in Fig. 1 may be used. Loss material 41 is formed as a coating on member 39 at the centre portion thereof. In another form, Fig. 3 (not shown), an annular beam is projected in the space between two concentric helices. The outer helix rests against the inner wall of the envelope and the inner helix is wound on an axial ceramic support carried at either end from the envelope by pins. Loss material is formed as a coating on the inner surface of the envelope and on the axial ceramic support, near the centre of the helix. Balanced input and output two conductor transmission lines are provided. The input circuit includes two choke coils and a by-pass condenser. The helices are maintained at different D.C. potentials, either the inner or the outer one having the higher positive potential. The helices are wound to have about the same phase velocities so that as their diameters are different the spacings between turns are in general also different. The double helix structure will support either a longitudinal wave or a transverse wave in which the field mid-way between the helices is mainly longitudinal or mainly transverse; the two waves have slightly different phase velocities and either wave may be utilized for amplification. Efficient coupling for the desired mode may be attained during design or manufacture by adding or subtracting wire from one end of the helix. For example if the coupling is initially good for the longitudinal mode, adding or subtracting a half free space wavelength of wire at one end of the helix makes the coupling good for the transverse mode. The helices are preferably wound in opposite senses rather than in the same sense since this makes the separation in wave velocity between the two modes greater and consequently it is easier to select the desired mode by adjusting the beam velocity. Specifications 652,155 and 669,473 are referred to.