Patent GB-796460-A (GB)
Title: Improvements in or relating to travelling wave tubes
IPC: H01J23/28
Filed: 1956-07-02
Abstract: 796,460. Travelling wave tubes. COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. July 2, 1956 [July 12, 1955], No. 20456/56. Class 39(1). A travelling wave tube has an interdigital delay line comprising hairpin shaped fingers 15, Figs. 2, 3 with an intermediate cross piece 16 to form a closed contour conductively connected to and supported by conductive bars 13, 12 the rectangular electron beam produced by a gun with indirectly heated cathode 2 and focussing electrode 6 and accelerating electrode 8 having rectangular apertures, passing through the channel defined by the aligned closed contours of the fingers 15, and further conductive strip electrodes 20, 22 insulated from the delay line are arranged between the finger apertures and the inner faces of the delay line bars 13, 12, bar 13 being highly positive, the adjacent strip 20 being at approximately at cathode potential, bar 12 being at a low and strip 22 at a high positive potential with respect to the cathode, so that an electrostatic focusing lens is produced. The fingers may have curved ends and curved intermediate portions as shown or both the ends and the intermediate portions can be straight, Fig. 4 (not shown). The additional electrodes 20, 22 are L-shaped and together with grids 24, 25 at cathode potential which prevent secondary emission from the copper fingers from bombarding the metal or insulating envelope, and the delay line bars 12, 13 are assembled by two pairs of threaded bolts and nuts 43, 58 with intermediate insulators 44 of alumina or ceramic. The first two fingers 15 and L-strips 20, 22 adjacent the cathode form a cylindrical convergent electrostatic lens to prevent the beam edges spreading in a parabolic manner due to space charge, and each subsequent pair of fingers and L-strips functions similarly. Suitable potentials are V 13  > V 8 , V 13  = 800V, V 12  LV 8 , V 12  = 0-100V, V 22  = V 26  = V 13 , V 20  = V 24  = Vzs = Vk V 8  = 400. V 13  may be adjusted within the range 100-1200V if V 12  is correspondingly adjusted each time. The L-strips 20, 22 have a row of spaced windows 21, parallel to the inner faces of delay line bars 12, 13, the hairpin fingers extending through each window. The embodiment of Figs. 2, 3 is a backward wave oscillator, the output being coupled to the finger adjacent to the cathode and a lossy coating being provided on the last two fingers at the other end of the delay line. The envelope is of copper and has a metal base 38 brazed thereto the electrodes being screwed or bolted to a metal plate 41 screwed to base 38. The focusing and accelerating electrode assembly is however secured to plate 41 by a strap 59. Holes 47, 48 in delay line bars 12, 13 allow for differential thermal expansion and the output coaxial line 50 lies in a channel extending along the whole length of delay line bar 13, the central conductor of the coaxial line being brazed to the finger adjacent to the cathode. In a modified construction of Figs. 2, 3 the fingers comprising simple hairpins are mounted in grooves in supports projecting from the delay line bars, the faces of the supports being concave to complete the closed contour for passage of the beam. The invention may be applied to " linear " travelling waves other than backward wave oscillators. The electrostatic lens eliminates the need for a magnetic focusing field. Specification 699,893 is referred to.