Patent GB-588186-A (GB)
Title: Improvements in vacuum or low-pressure electric discharge apparatus
IPC: H01J25/587
Filed: 1941-08-22
Abstract: 588,186. Magnetrons. RANDALL, J. T., BOOT, H. A. H., and WRIGHT, C. S. Aug. 22, 1941, No. 10707. [Class 39 (i)] In a magnetron of the type having a plurality of coupled hollow resonators of the same natural frequency surrounding and having gaps opening into the anode-cathode space, for example of the kind described in Specification 588,185, secondary emission is utilized for the production of the whole or major part of the electron stream from the cathode. To obtain primary electrons (a) a separate filamentary cathode may be arranged adjacent the main cathode, (b). the main cathode may be heated to a sufficient temperature to give a limited thermionic emission from the whole surface or from the ends only, (c) the ionization of residual gases by the high voltage may be relied on, or (d) a source of ionizing radiation such as radioactive material may be arranged near or within the apparatus. As a number of electrons will return to the main cathode during operation under the action of the magnetic .field, the source of primary electrons need not be maintained after the discharge has been initiated. The return of electrons may be increased by electrostatic means, such as grid. The heating of the main cathode may be direct, indirect or by the discharge only. In the latter case, cooling may be necessary to prevent excessive heating ; in any case it is desirable to cool the cathode to prevent emission of gases. As secondary emission materials may be used layers of oxides of thorium, aluminium or calcium, or barium silicate ; these may be so thin as to show the Malter effect. A base of aluminium, alone or coated on copper, may be oxidized by cleaning in caustic soda and then heating in boiling water. A beryllium-copper alloy may be used, as a thin layer of beryllium oxide forms thereon. Ordinary thermionic materials may also be used such as barium and strontium oxides, or a thorium oxide coating on a molybdenum base. A tantalum base is also suitable. For high emission under less severe conditions, a caesium on oxygen cathode may be used. The primary emission materials may be oxides of barium, strontium or thorium, or plain tungsten. In one form, the main cathode 1, Fig. 1, has an internal heater 5 stretched between end-plates 5<1> which also support a primary filament cathode 2 tensioned.by a spring 4 at one end and passing through an insulating bush 3 at the other. In a complete tube, the anode 6, Fig. 3, is a block containing a number of cylindrical resonators 7 communicating by narrow gaps to a central space containing a hollow cathode 12, through, which cooling water is passed by tubes 13 at top and bottom, which also act as cathode leads. The primary cathode 17 is supported at one end from cathode 12 and at the other end by lead 18. Energy is taken off from one of the resonators by loop 11. The resonators are coupled by closed end-spaces as in Specification 588,185. In a modification, the primary cathode 17, Fig. 7, is situated in a groove 22 machined or pressed in the hollow cathode 12. The cathodes may also be of disc form and the resonators may be radial slots. According to the Provisional Specification the invention is applicable to magnetrons, thyratrons, X-rav and cathode-ray tubes. The caesium layer may be on aluminium or silver oxide.