Patent GB-811116-A (GB)
Title: Improvements in or relating to travelling wave velocity modulation devices
Inventor: CLARKE GRAHAM MORLEY
IPC: H01J25/16
Filed: 1955-07-21
Abstract: 811,116. Velocity modulated tubes. M.O. VALVE CO. Ltd. July 13, 1956 [July 21, 1955], No. 21126/55. Class 39(1). A travelling wave klystron has input and output waveguides 1, 2, Fig. 1, each loaded with a series of inductive elements 3 and a series of discrete electron beams projected transversely across the guides centrally between pairs of adjacent inductive elements the beams being velocity modulated by an input wave travelling along the guide 1 from left to right and after being bunched in drift tubes 8 delivering energy to the output guide 3 to excite a travelling wave the amplitude of which builds up by interaction with the successive beams as it travels along the output guide. The inductive elements are inductive over the whole frequency band between the cutoff frequency of the guides and the upper cutoff frequency of the filter formed by the guides loaded by the inductive elements they each comprise two thin rectangular metal plates 18, 19, Fig. 2, are identical and regularly spaced at corresponding positions along the guides, the cross sectional dimensions of the guides also being identical. A series of identical electron sources 4 project beams midway between pairs of adjacent inductive elements. The reverse wave in the output builds up only at frequencies where the phase change between successive electron beams is n# radions which are arranged to be at the ends of the design pass band. The input signal may have a restricted bandwidth to exclude these frequencies. If on the other hand there is only one beam for every n the section (a section being taken between mid points of adjacent inductive elements), the reverse wave will build up in the output for n-1 ranges of frequencies between the ends of the pass band as well as the end pass band frequencies. If a narrow band width can be tolerated the reflectionless termination 15 can be removed and the reverse output wave fed to a suitable junction to combine it with the forward output wave. In another modification the impedance of one of the waveguides is varied along its length by varying the nature of the inductive elements, e.g. if the impedance of the output guide is high for the initial beams so the forward wave builds up rapidly to a maximum desirable value and the waveguide impedance is then progressively reduced by varying the values of the inductive elements so power in the forward wave builds up but the voltage does not. Variation of the impedance level in the output waveguide in this manner requires the impedance level to vary in the input guide to synchronise the phase velocities of the waves in the two guides but this decreases the efficiency of velocity modulation for the later beams. A high coupling coefficient without decrease in impedance is obtained by forming the guide with a metal ridge 17 in the centre of the broad face of the rectangular guide, the height of the ridge being chosen to maximise the impedance and coupling coefficient and metal tubes 20 which project only slightly out of the side of the ridge and holes 21 being provided for passage of the beam. The drift tube lengths are such as to give optimum bunching and the beam current is usually the same in all beams which are focused by electromagnets.