US2840745A - Gas filled discharge devices - Google Patents

Description

June 24, 1958 R. E. LAKE 'GAs FILLED DISCHARGE DEVICES Filed June 10, 1955 I INVEN'I'OR wew lc {all 0 O A-r-r aNeYa 2,840,745 Patented June 24, 1958 GAS FILLED DISCHARGE DEVICES Ronald Ernest Lake, Chelmsford, England, assignor to English Electric Valve Company Limited, London, England, a British company Application June 10, 1955, Serial No. 514,599

Claims priority, application Great Britain July 2, 1954 4 Claims. (Cl. 313-195) This invention relates to gas filled discharge devices and more specifically, though not exclusively, to hydrogen filled thyratrons.

Common causes of failure of hydrogen filled thyratrons are the occurrence of primary grid emission and rise in the anode take-over voltage. In general primary grid emission is due to the deposition on the grid of material which has been sputtered from the cathode during the life of the device. If such material is deposited on the grid it will usually emit thermionically owing to the relatively high tube temperatures reached in operation.

The invention is illustrated in and further explained in connection with the accompanying diagrammatic drawings in which Fig. 1 is a diagrammatic representation, provided for purposes of explanation, of a known thyratron electrode arrangement and Fig. 2 is a corresponding view of a preferred embodiment of the present invention.

Referring to Fig. 1 the electrode arrangement therein illustrated comprises the usual heated cathode structure 1, with encircling cylindrical heat shield 2, grid 3, and anode 4. A transverse cathode sputter shield 5 is provided next to the cathode structure, a grid sputter shield 6 being provided between the shield 5 and the grid 3, the latter being carried by the usual grid cylinder 8.

This well known arrangement leaves much to be desired from the point of view of preventing the deposition of sputtered material from the cathode on to the grid and if it is sought to obtain improvement in this respect by increasing the number of sputter shields or baflles another defect is encountered. This arises by reason of the fact that in order that the thyratron may fire below a specified grid trigger voltage (in practice usually about 150 volts) and at a low anode voltage (usually about 1000 volts) the grid must take current, i. e. ionization must be present in the region between the grid 3 and the shield 6 which marks the limit of penetration of the anode field. Merely putting shields or bafiies between the cathode and the grid to prevent material sputtered from the cathode from reaching the grid would, by restricting the conducting path between grid and cathode, tend to prevent the tube from firing below the specified trigger voltage and at the low anode voltage, i. e. the characteristic of the tube may be said to have become more positive.

The present invention seeks to provide simple and easily constructed tubes wherein improved protection is obtained against the deposition of sputtered material upon the grid without incurring the defect above mentioned.

According to this invention a hydrogen filled thyratron or like grid controlled gas filled discharge device comprises a heated cathode structure, a control grid, an anode, a grid sputter shield adjacent said grid, and, inserted in the cathode-grid discharge path, baffle means for preventing material sputtered from the cathode from reaching the grid and electrode means for focussing the grid-cathode discharge whereby a desired low trigger voltage is obtained and low anode take-over voltage is maintained despite the presence of the bafiie means.

Preferably the bafiie means and focussing means comprise twotapered members, one within the other, constituting a double funnel structure with its smaller diameter end adjacent the cathode structure, the smaller end of the inner member being closed to provide one bafiie and the smaller end of the outer member having an inward annular projection to provide another baffie which is nearer the cathode, the two tapered members providing a focussing electrode system to focus the cathodegrid discharge and maintain a low anode take-over voltage despite the presence of the bafiles. Preferably also the outer of the two said members is constituted by a bell mouth formed as an extension of the cathode heat shield and carrying the inner of said members. Preferably again the grid sputter shield is carried from a ring of large diameter than said shield. The shield may be above, below or in the plane of its carrying-ring as design considerations may dictate.

Referring to Fig. 2 the electrode system therein shown comprises a heated cathode structure 1, heat shield 2, grid 3, and anode 4, all :as in Fig. 1, except for the heat shield. This shield instead of being of simple cylindrical form as in Fig. 1 is extended into a bell mouth 2a to form an outer part of a double funnel structure, a narrow annular ring 2b being provided at the smaller end as shown. An inner tapered member 7 closed at its inner end is carried from this outer member, the two members forming a double funnel structure and being concentric and of the same taper angle. A grid sputter shield is shown at 6, this shield being carried by an annular ring 6a which is attached to the grid cylinder 8.

With the arrangement shown in Fig. 2 the ring 2b and closed face of the part 7 act as the main balfies to trap cathodic material sputtered by the cathode and though they restrict the discharge path there is substantially no increase of the required trigger voltage due to their presence because the two tapered surfaces refocus the discharge on to the grid. As will be seen sputtered cathode material can fall practically only on to components which are at cathode potential. The construction, despite its simplicity, has been found in experimental practice to give a high degree of freedom from grid emission during the life of the tube; to be readily designed to trigger at predetermined required voltages; and to exhibit stable anode take-over voltage.

While I have described my invention in one of its preferred embodiments, I realize that modifications may be made, and I desire that it be understood that no limitations upon my invention are intended other than those which may be imposed by the scope of the appended claims.

I claim:

1. A grid controlled gas filled discharge device com prising a heated cathode structure, an anode, a control gird positioned in the electron path between the cathode structure and the anode, a grid sputter shield adjacent said grid, and inserted in the cathode-grid discharge path, baflie means positioned between the cathode structure and control grid for preventing material sputtered from the cathode structure from reaching the grid, said baflie means being combined with electrode means for focussing the cathode discharge onto the control grid whereby a desired low trigger voltage is obtained and low anode take-over voltage is maintained despite the presence of the baffle means, and an envelope having an ionizable atmosphere sealed therein and containing all of the aforesaid components, and wherein the bafiie means and electrode means for focussing the cathode discharge comprise two tapered inner and outer members, one within the other, constituting a double funnel structure with its smaller diameter and adjacent the cathode structure, the smaller end of the inner member being closed to provide one bafileand the smaller end of the outer member having an inward annular projection to provide another baffle which is nearer the-cathode structure.

2. A device as set forth in claim 1 'wherein'there is a-cathode heat shield surrounding said cathode structure and wherein the outer of thetwo saidmembe'rs forms-a bell mouth as an extension of said cathode heat shield and carries the inner of said members.

3. A device as claimed in claim 1 wherein there is a cathode heat shield surrounding said cathode structure and wherein said grid sputter shield is carriedrfrom a'ringr of larger diameter than said cathode heat shield.

4. A grid controlled gas filled discharge device comprising a heated cathode structure, a cylindrical heat shield surrounding said cathode structure in spaced relation thereto and extending beyond said cathode structure and terminating in a frusto-conical bell mouth, an annular member located intermediate said'cylindrical heat shield and the bell mouth thereof and directed inwardly for restricting the discharge path from said cathode structure, an inner tapered member having a flat central portion extending transversely across the discharge path through said annular member in spaced relation to the plane of said ring, said inner tapered member having side walls 4 extending in spaced annular relation to said frusto-conical bell mouth for forming an outwardly directed conicalshaped annular path of electrons, a grid cylinder concentrically surrounding said bell mouth and extending along a longitudinal axis in opposite directions with respect thereto, a ring mounted interiorly of said grid cylinder and directed inwardly in a plane spaced transversely from the end of said bell mouth, a grid sputter shield carried by said ring in a position spaced therefrom, and away from said bell mouth, a control grid supported transversely of said grid cylinder in spaced relation to and away from said grid sputter shield andan anode mounted in spaced relation to said control grid, said annular member and the flat central portion of said inner tapered member constituting-bafiles for trapping cathodic material sputtered from the heated cathode structure from reaching the control grid and an envelope having an ionizable atmosphere sealed therein, said envelope containing allot the aforesaid components.

References Cited in the file of this patent UNITED STATES PATENTS 2,415,816 Depew Feb. 18, 1947 2,500,153 Cork Mar. 14, 1950 2,514,165 Ramsay July 4, 1950 2,650,998 Watrous Sept. 1, 1953

Images