Patent GB-1490463-A (GB)
Title: Thermionic cathode and method of making same
IPC: H01J23/065
Filed: 1976-01-27
Abstract: 1490463 Cathode materials and processing; klystrons, travelling wave tubes; thermionic valves VARIAN ASSOCIATES Inc 27 Jan 1976 [31 Jan 1975] 03173/76 Heading H1D A thermionic cathode in a grid-controlled electron source, e.g. for a Klystron or travelling wave tube, or for a tetrode or triode valve, emits electrons preferentially through the apertures of the grid to avoid over-heating, the emitting areas of the cathode being separated by non-emitting areas coated with a deposited layer of non-emitting material. The cathode for a Klystron or travelling wave tube (Fig. 1, not shown) may comprise a concave impregnated porous body with depressed dimples, e.g. spherical sections in its surface as in Fig. 2 or with a smooth surface as in Fig. 4, in each case with the bands 30 aligned with the grid conductors 28 and coated with non-emitting material 33, separated from the emitting impregnant by a sealing layer 32 to avoid interaction. In making the cathode of Fig. 2 a porous button, e.g. of W, is temporarily impregnated with Cu or thermoplastic material, removed after machining the button concave, Fig. 3a. The surface may then be sealed in a grid pattern by laser welding, or the entire surface may be sealed by deposition, e.g. of W from WF cracked on the hot substrate, Fig. 36. The body is then impregnated with emitter, e.g. Ba aluminate, and the surface of the of the sealing layer is covered with a non-emitting layer such as Zr, Ti, C, or MoC or other metallic carbide by vapour deposition, vacuum evaporation or sputtering, Fig. 3c. Finally the dimples are machined, exposing the emitter impregnant, Fig. 3d. Alternatively a smooth cathode as Fig. 4 may be made as a Figs 5a-g. the concave body is made as above but is then masked, the mask 40 being solid along the grid lines; an inert powder, e.g. Ba Co 3 , is laid down through the mask apertures, Fig. 5c, and on removal of the mask, Fig. 5d, a sealing layer 32<SP>1</SP>, is coated over the entire surface, Fig. 5e, followed by a non emissive coating 33<SP>1</SP>, Fig. 5f. Brushing removes the powder and the overlying layers, exposing the emitting surface 26, Fig. 5g: in a planar triode, Fig. 6, the cathode may be made by the same process, having round W wires 28<SP>11</SP> and an opposing anode 3<SP>11</SP>, or the surface may be longitudinally grooved, e.g. of cylindrical section, corresponding to the process of Figs. 2, 3. The cathode of a similar triode, Fig. 7, may be made from a slab of solid Ni 10<SP>111</SP>, on which a non-emitting layer 33<SP>11</SP> is deposited (the sealing layer 32 being unnecessary), after which grooves 26 are machined: the entire surface is coated with emitter, e.g. mixed Ba-Sr-Ca carbonates, which is then scraped from the lands, allowing the emitting areas in the grooves to be smoothed. Heating converts the carbonates to the emitting mixed oxides. The electron source of a Klystron or travelling wave tube (Fig. 1, not shown), projecting a convergent beam through an annular anode where it forms a pencil beam, comprises the porous W cathode of Figs 2, 3 with grid conductors corresponding to the non- emitting areas. The cathode is supported in vacuum in an alumina tube formed by expansion matching Fe-Co-Nl sleeves to the anode block on one side and to a stainless steel end plate on the other, which plate carries a tubular cathode support of W impregnated with Cu. This carries, on telescoped sleeves of Mo-Re alloy, the cathode and also the grid of Mo-Re lying in front of the cathode and supported for cooling on a beryllia ring mounted on the end of the cathode support; an annular focusing electrode connected to the grid is mounted on the the front of the beryllia ring. A radiant heater is mounted behind the cathode block, with emerging connecting leads.