Patent GB-724277-A (GB)
Title: Improvements to t.r. cells
Inventor: BRIDGES THOMAS JAMES
IPC: H01J17/04
Filed: 1951-11-30
Abstract: 724,277. High-frequency discharge apparatus. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Dec. 1, 1952 [Nov. 30, 1951], No. 28083/51. Class 39 (1). In a gas-filled T.R. cell comprising a pair of main electrodes, at least one of which is movable for discharge gap adjustment, and in which a glow discharge is established for priming purposes, the priming electrodes are duplicated to ensure steady maintenance of said glow discharge and to reduce the possibility of transition to an arc discharge which would tend to remove most of the free electrons and so delay ionization in the cell. The priming or keep-alive electrodes may be of non- oxidizing material such as rhodium or may have a non-oxidizing active surface. Alternatively they may be of high resistance material, for example a semi-conducting ceramic such as reduced titania. In one arrangement, Fig. 1, the T.R. cell comprises a cylindrical body 1 having an axial bore 3 the ends of which are hermetically sealed by glass or mica windows (not shown) to permit the passage of electromagnetic energy. A fixed main electrode 4 and a movable main electrode 5 are mounted in a further bore 2 in the body 1, the electrode 5 having a discharge point 8 which protrudes through and is brazed into the centre of a flexible diaphragm 11, movement of the electrode being effected by a tuning mechanism enclosed in the casing .9. Projecting through clearance holes 13 into the cavity 7 are two keep-alive electrodes 12 each comprising a metal rod 14 passing through a bore 15 in a sleeve 16, made of an alloy which will seal to glass, which is brazed into the body 1. Each sleeve 16 is sealed to a hollow glass insulator 19 through which the rod 14 extends to a metal end cap 20 for external electrical connection. The rods 14 can be uncoated since they are spaced from the sleeves 16, but are preferably provided with a sheathing 21 of glass or ceramic which is ground away at the end to expose the tapered point 12 of the rod 14. The points 12 need not project into the cavity 7 as shown but may be set back in the sleeves 16. One glass insulator 19<SP>1</SP> is made large enough to act as a reservoir for the gas filling which is preferably a mixture of argon and water vapour. In an alternative arrangement, Fig. -3, the two keep-alive electrodes are mounted side by side. Two thin wires 22 are welded to thicker wires 23 held by a glass bead 27 and extending through a glass insulator (not shown) to separate metal connecting caps 24. The wires 22 have glass powder coated on them by electrophoresis followed by heating to glaze the powder. An outer glass tube 25 is heated to softening point to bond to the inner coatings and is then drawn out to a tapered end 26 which is ground off to expose the ends of the wires 22. A further alternative comprises a coaxial construction in which one keepalive electrode is coated with glass and is held by a glass bead inside a hollow outer keep-alive electrode. An alternative form of T.R. cell comprises a length of hollow waveguide having two pairs of discharge electrodes spaced along its length. One electrode of each pair is solid and threaded externally for screw adjustment and the other electrode of each pair is hollow and surrounds a priming electrode. In any of the arrangements the priming or keep-alive electrodes have applied to them a steady potential or a radiofrequency potential of periodicity less than the recovery time of the cell.