Patent GB-776659-A (GB)
Title: Space charge wave discharge tubes
IPC: H01J25/20
Filed: 1953-12-18
Abstract: 776,659. Space charge wave tubes. HUGHES AIRCRAFT CO. Dec. 18, 1953 [Dec. 18, 1952], No. 35353/53. Class 39(1). A space charge wave discharge tube comprises a unipotential drift tube the drift tube comprising first means for establishing a predetermined spatial capacitance from a point of maximum electron bunching to the next point of minimum electron bunching and further means for establishing a second different spatial capacitance from a point of minimum electron bunching to the next point of maximum electron bunching so as to effect a change in the amplitude of the beam perturbations, so as to effect signal amplification or attenuation or noise reduction. In one form, Fig. 1, the space charge wave is amplified by cylinders 56, 58, 60, 62 of alternately large and small diameters, the reduction factors and spatial capacitances likewise being large and small alternately as they depend on the diameters, and each having a length of <#P>/4 measured in the cylinders. Since #p is different for the cylinders of different diameter, cylinders 56, 60 must be shorter than 58, 62, and 58, 62 have a diameter below cut-off so that feedback is eliminated. The beam at the point of entry to cylinder 56 is maximum charge density modulated and debunches until when it leaves 56 it carries maximum A.C. velocity fluctuations. In 58 the beam bunches again to a greater degree than originally, and the above process is repeated in cylinders 60, 62. A fundamental space charge wave mode is preferably used, but higher modes can be used if desired. The D.C. potentials of tungsten helices 34, 38 and amplifying section 36 (helices shown at slightly different D.C. potentials) are about the same but that of the latter is adjustable for optimum gain. The helices have matched terminations at their free ends comprising coatings 52, 54 of carbon black on the tube envelope. Brillouin flow may be used and may be initiated with the aid of focusing coil 76. A mathematical discussion is given. In a modification, Fig. 6 (not shown) the large diameter cylinders 56, 60 are replaced by helices having a larger number of turns per unit length than helices 34, 38 and which are connected to cylinders 58, 62. The devices can be used to amplify or attenuate signals or to attenuate noise according as to whether the beam first passes through a small or large reduction factor section and as to whether the point of entry is at an A.C. current maximum or velocity maximum. The input and output helices can be replaced by cavity resonators. One arrangement for reducing noise comprises a helix of small pitch and large reduction factor connected to a cylinder of low reduction factor, both members having a length of <#p>/4. The noise reducing section is followed by a klystron or travelling wave amplifier, Fig. 7 (not shown). The position of the noise reducing section is such as to give minimum noise at the output guide. Alternatively the desired sections of high reduction factors may be provided by " inductive walls " comprising basically a slow wave material with a resistive coating on its inner surface adjacent to the beam and a highly conductive coating on its outer surface. The slow wave material may have (a) high dielectric and low permeability constant, such as titanium dioxide or calcium, strontium or barium titanates, processed to have ceramic characteristics, (b) high permeability constant such as ferrites, R,Fe2O3, where R is magnesium or copper. Two or more ferrite materials may be mixed together to increase the frequency response range, (c) artificially produced dielectric and magnetic characteristics such as metal particles acting as <#>/2 dipoles and distributed throughout a dielectric or magnetic material. The metal particles can be cut to different lengths and randomly distributed to increase the band width of response. In a specific example a tube of strontium titanate, the thickness of the walls of which lies between an even and an odd multiple of quarter signal wavelengths, or may be less than a quarter signal wavelength, has an inner coating of tin oxide, produced from tin chloride, and so thin as to give a surface resistivity of 10,000 ohms/ sq. cm., and an outer coating of silver formed by plating or evaporation. Two such tubes 144, 147, Fig. 8, are arranged with interposed conductive tubes 139, 141, each of these members having a length of <#P>/4. The inductive wall is designed so that it effectively reduces the " spatial capacitance " of the electron beam. A single inductive wall section may replace helix 116 in Fig. 7 (not shown). Specification 747,904 is referred to.