Patent GB-771673-A (GB)
Title: Improvements in or relating to electric travelling wave amplifiers
Inventor: CLARKE GRAHAM MORLEY
IPC: H01J23/26
Filed: 1953-08-14
Abstract: 771,673. Travelling wave tubes. M-O VALVE CO., Ltd. Aug. 9,1954 [Aug. 14, 1953], No. 22522/53. Class 39(1). A travelling wave amplifier has a multi-start helix 15, Fig. 1, or 22, Fig. 2, comprising a number of coaxial helices having the same diameter and pitch and with interleaved turns the helices being so coupled to the input and output means that for an input wave having a wavelength equal to twice the helix circumference per turn, the phase of the wave at any point on any one helix is intermediate between the phases of those points on adjacent helices which lie on a straight line through the point and parallel to the common axis of the helices and so that the waves on the helices are additive in the output means. In one form the multi-start helix 15, Fig. 1, is coupled to the inner conductors of coaxial lines 1, 2 by conical members 14, 16 each having a number of slots equal in number to the number of helices which form helical transformations of gradually increasing pitch. There may be four helices and four slots. The slots are preferably evenly spaced around the axis and the helices are preferably evenly interleaved at least over the part where interaction with the beam takes place. The helix may be stiffened by longitudinal glass rods (not shown). The outer conductors of the coaxial line feeders are sealed to the main envelope portion by cones 7, 8 and annular glass seals 9, 10 which house the gun 11 and collector 12 respectively ; the annular beam passes outside the helix. Conductors 13, 17 may be solid or hollow but in the latter case must have a diameter below the cut-off diameter. In another embodiment, Fig. 2, the helix end couplings comprise conical conductors 23, 24 slotted as in Fig. 1 and housing cylinders 25, 26 which extend into rectangular waveguides 27, 28. The cathode 33 extends along the axis of the helix and carries a current, additional to the heater current, flowing through it to set up a circular magnetic field; it is coated for only the first half of its length and contains a heater within the coated region one end of the heater being connected to the conductor. Members 25, 33 and 26, 33 form coupling coaxial lines. A potential difference which may be D.C. or pulses is set up between helix 22 and the inner conductor 33 ; the magnetic field forming current flowing in the cathode may also be pulsed or sinusoidal, in synchronism with the pulsed radial field. Annular chokes 38, 39 in conjunction with the closed ends 29, 30 of the guides prevent escape of energy and give good impedance matching. The chokes can be replaced by conducting plates which extend radially from 33 across the waveguide aperture but spaced axially from the walls of the aperture by a small distance. Cathode conductor 33 which may be internally water cooled is sealed to envelope portion 21 by metal collars 36, 37 glass sleeves 34, 35 and ring flanges on the guides ; the helix may be water cooled tubing. It is shown that at a frequency where the circumference per turn is <#>/2 the electromagnetic field is the same as for a single helix whose circumference per turn is <#>/n where n is the number of interlaced helices. The mean circumference of coaxial lines 25, 33 ; 26, 33 is less than the shortest operating wavelength to eliminate " waveguide " modes along the helix having periodic circumferential variations. The Provisional Specification states in addition that the invention can be applied to forms comprising two concentric helices, each helix being multi start and the beam passing between the two concentric helices. Specification 646,546 is referred to.