US2044619A - Double shielded grid construction - Google Patents

Description

.Fune 16, 1936. o. w. LIVINGSTON' DOUBLE SHIELDED GRI D CONSTRUCTION Filed April 23, 1935 Fig.5.

W H km IPA/enter: Orrin W. Livingston, b2 7? 6x9 Hi5 Attorne g.

Patented June 16, 1936 ii? SA DOUBLE SHIELDED GRID CONSTRUCTION Orrin W. Livingston, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application April 23, 1935, Serial No. 17,765

8 Claims.

My invention relates to electron discharge devices of the type wherein a controllable dis-- charge takes place through an ionizable gaseous medium. More particularly my invention relates to improvements in electrode structure whereby the initiation or an electric discharge is able to be controlled accurately and dependably.

It has previously been proposed to provide shielding elements so arranged as to protect the grid member of a gas-filled tube from deposits of electron emitting material and electrostatic and heat radiation effects. Such an arrangement is disclosed and claimed, for example, in my co-pending application, Serial No. 704,503, assigned to the same assignee as the present application.

To adapt the principles set forth in my abovementioned application, Serial No. 704,503, to the requirements of large scale production it is desirable to incorporate them in a tube having a concentric electrode assembly as distinguished from the less compact end to end construction. While there are few theoretical objections to such an adaptation, in practice it has proven a problem of some magnitude to devise concentric electrode parts which perform the multiple shielding functions required and yet do not unduly complicate the process of assembly or make the finished structure too intricate or fragile.

It is, therefore, an object of my present invention to provide an electrode assembly which will make possible shielding of the control grid from deposition of electron emitting material as well as inter-electrode heat radiation and electro static effects and will additionally be suitable for a concentric mounting construction.

It is a further object of my invention to provide concentric shielding electrodes of such design as to meet the requirements of gaseous discharge tube operation and, at the same time, permit a compact and rigid assembly of parts.

Other and further objects if my invention will be apparent upon considering the following specification taken in connection with the appended claims and the accompanying drawing, in which Fig. l is an elevation partly in section of the tube embodying my invention with external circuit connections illustrating the use of the same, while Fig. 2 is a perspective view partially cut away to show in detail a form and arrangement of the electrode parts. Fig. 3 is an elevation partly in section of a modification of which my invention is capable, while Fig. 4 is a perspective view more fully disclosing the constructional details of such modification.

In tubes of this type, containing a thermionic cathode, an anode and a gaseous ionizing medium at a pressure sufiicient to support an arclike discharge, the flow of a discharge current between the cathode and anode depends on the presence of substantial ionization. To prevent the formation of positive ions and the initiation of a discharge, it is possible to use a control electrode or grid held at some predetermined potential. This operates to diminish the velocity of the electrons drawn from the cathode by the anode potential to an extent suificient to prevent ionization of the gas from taking place. As the voltage of the control member or grid is varied, it will be found that for a given condition of the tube there is some critical voltage which will just permit the tube to start. Once current flow has commenced, however, the presence of large numbers of ions serves to neutralize to a large extent the electrostatic effect of the grid and to make cessation of current dependent chiefly on a discontinuance or reversal of anode potential.

This characteristic property of the gaseous tube gives it unique value as an electronic valve or relay and has led to its application in widespread fields. Due to the fact that many of the uses to which this type of tube is put demand exacting accuracy and dependability of operation, it is of extreme importance that the critical grid voltage which will cause breakdown of the tube shall not vary either with time or with conditions of operation. Unfortunately, however, a number of factors are always present which tend to make unlikely this desired uniiormity of operation. Among them is the wellknown phenomenon of grid emission, which consists of the generation of electrons from the surface of the grid member. With a clean cold grid this effect is of slight importance, but if the grid becomes coated with impurities such, for example, as an electron emitting oxide, and is raised to an elevated temperature by radiation from the cathode or anode, a sufiicient emission may take place to cause breakdown of the tube. This is, of course, due to the fact that the emitted rence in gaseous tubes as at present constructed due in part to the fact that the usual cathode, coated with electron emissive material, is apt to have its surface substances thrown ofi in considerable quantity. This may be attributable either to pure heat vaporization or to the familiar effect known assputtering, caused by ionic bombardment of the cathode. The ejected material tends to be collected by the grid surface which, when highly heated by radiation from the cathode or anode, acts as a sort of secondary electron source in the manner described above. Under certain conditions it is also possible for the grid to receive electron emitting material from the anode as well as the cathode. This situation arises when a deposit of such material is first accumulated on the anode either by the processes of manufacture or during actual use, and in the further operation of the tube is regeneraated by heat in such a fashion as to be passed on to the grid.

A further objectionable circumstance which may render the gaseous tube conductive at an undesired time is the electrostatic effect of the anode field, which may be variable in extent with the potential impressed upon the anode. When the use to which the tube is to be put requires, as is frequently the case, that it be placed in a relay circuit which is subject to transient voltages, a steep wave front surge may be impressed on it at any moment. If this surge is of sufficient magnitude, the grid may be so affected by the anode field as to cease to perform its normal restraining function and the tube may fire. In tubes of the high vacuum type such an unrepeated inter-electrode disturbance would be of little significance since its effect would vanish almost immediately. In the gaseous tube, whose principal function is an initiating one, an unforeseen discharge may set in operation a series of events whose effects will be lasting and disastrous.

Referring to the device shown in Fig. 1, I have illustrated a sealed envelope l of the usual type having at the bottom thereof a reentrant stem 2 terminating in a press 3 which operates as a seal and support for the electrodes of the tube. The top 4 of the envelope I is of reduced section to make possible increased lateral support for the electrode assembly. The envelope l is adapted to contain an ionizable medium which may consist of an inert gas, such as argon, at a pressure of from 1 to 500 microns, or of caesium or mercury vapor. If mercury or caesium is the medium selected, a proper vapor pressure may be maintained by a small quantity of such medium placed in the bottom of the tube.

I have also provided a cylindrical anode member 5 which is located externally of the other electrode elements, and which by means of vertical supporting rods 6 and 9 is secured at top and bottom between mica disks 8 and I ll, of which the former bears laterally against the wall of the tube at its reduced portion 4. Current is supplied to the anode member by a lead-in conductor l2 which is connected by soldering or otherwise to an extension of the conducting supporting rod 9. I have also shown mounted on a depending portion of lead-in conductor l2 a small capsule M which is adapted to contain a chemically active material, such as barium or strontium, which may be in the form of a relatively stable alloy such as is produced by combination with aluminum. During the course of manufacture capsule I4 is heated by an external source of energy which may consist of a high frequency induction coil, and the barium or other corresponding material is vaporized into the tube where it operates as a getter to clean up undesirable occluded gases.

Concentric with the anode, which may be of molybdenum, graphite or other conventional material, and centrally disposed with reference thereto I have shown an electron emitting cathode 1 which is illustrated, comprising a helical filament member. It is, however, contemplated that this element may take other forms such, for example, as the indirectly heated type of cathode shown in Patent No. 1,924,319 to A. W. Hull, assigned to the same assignee as the present invention. The electron emitting qualities of the cathode are preferably improved by coating or otherwise treating its surface with an electronically active material such as barium oxide or some other alkaline earth metal. Heating current is supplied to the cathode by means of lead-in conductors l l which pass through the stem 2 and press 3 and one of which also serves to bear the main discharge current.

Intermediate the cathode and anode is mounted a controlling member or grid (designated as a whole by the numeral 15) which is exemplified as comprising a series of circumferentially extending rings I 8 spaced apart and supported by vertically extending conducting members l9 welded or otherwise secured to the rings at their outer peripheries as shown in Fig. 2, and attached at their extremities to the mica disks 8 and [0. As is characteristic of the tubes with which my invention is concerned, it is the function of this electrode to restrain by its electrostatic effect the initiation of a discharge between the cathode and anode. An external connection 33 is shown leading in through stem 2 and press 3 of the tube and connecting with a depending extension of one of the vertical supports l9 whereby the grid element may be kept at a predetermined level of potential.

In accordance with my invention, between the cathode 1 and the grid Hi there is mounted a cylindrical sheet metal shielding electrode 20 which is provided with circumferentially extending portions 2| and vertical connecting strips 22 formed by rectangular portions 23 punched out of the surface of the cylinder. The area of the imperforate portions of the shielding cylinder is at least as great as that of the control electrode and the arrangement of the punched-out areas with reference to the rings I 8 and supporting uprights IQ of the grid is such that solid portions of the cylinder 29 will intercept a projection of any portion of the grid upon the cathode I. In this way particles of electron emitting material thrown off from the surface of the cathode either by sputtering or otherwise are blocked before they are able to reach the grid. Furthermore, not only is the grid surface protected against becoming befouled with electronically active impurities, but whatever inherent electron emitting properties it may have are minimized by the fact that the grid elements are shielded from direct heat radiation from the cathode. An additional factor which may become of considerable importance in cases where the external circuit connections are such that the relative potential of the cathode itself is subject to variation over a large range, is that the member 23 acts as an effective electrostatic shield to prevent the grid potential from being so disturbed as to cause or permit the initiation of a discharge. A leadin conductor 34, passing through press 3, is shown as connecting to the base of cylinder 20 whereby the latter may be held at any predeterminedpotential by means of an exterior voltage source.

While the advantages contemplated by my invention may be largely realized by the use of a single shielding member such as that just described, these advantages may be enhanced and certain additional benefits obtained if a second shield is placed between the grid 15 and the anode 5. Such an additional shielding element, numbered 25, is plainly shown in Figs. 1 and 2 and is similar in form to shield 20 except that it is of a proportionally greater circumference. It is adapted to be securely mounted between disks 8 and I 0 and comprises circular elements 26 interconnected by vertical strips 21. As in the case of shielding electrode 20, the shield 25 is formed of sheet metal having rectangular portions thereof stamped out in such relation to the form of the grid H: as to leave solid sections of adequate size to intercept any direct projection of the grid upon the Wall of the anode 5. Thus, as will be seen from a consideration of Fig. 2, the ring portions I8 of the grid l5 are laterally enclosed on both sides by circular bands 2| and 26 While the vertical rods l9 are shielded on both the anode and cathode sides by vertical strips 22 and 2'! respectively. It is a beneficial result of this arrangement that electrostatic lines of force proceeding from the anode wall toward the grid are cut off by the shielding cylinder 25 so that the grid may be maintained at a substantially constant potential. In addition, any metallic impurities generated from the walls of the anode are effectively prevented from reaching the grid by the physical barrier constituted by the shielding element.

To exemplify more fully the manner in which my improved tube may be utilized, I have shown in Fig. 1 external circuit connections which, as illustrated, comprise a convention alternating current power source which includes a transformer 40, having a number of secondaries 4|, 42 and 43, respectively. Of these the secondary 43 is adapted to provide heating current for the cathode l, which is also connected by a convenient conductor to one end of the transformer 40. The anode is shown connected to the other end of the transformer 40 through a load which is conventionally illustrated as a resistance 44. The controlling function of the grid is accomplished through a suitable input circuit which includes a photoelectric cell 45 connected in a bridging arrangement with resistance member 46 connected to the cathode through a potentiometer connection 41 leading thereto, and a balancing resistor 48. Biasing potential for the grid is derived from transformer secondary 4| which is placed across resistor 46. It has been found that in a shielded tube such as the one which constitutes the subject matter of the present application it is possible to use the voltage variations obtained by the use of a photoelectric cell as a means of controlling the discharge of the tube. In the unshielded tubes of the prior art, on the other hand, the presence of inter-electrode effects made it necessary for sensitive operation to employ at least one stage of amplification between the photoelectric unit and the grid input to overcome the relatively large grid bias required to be used as a precaution against accidental discharges.

For holding the interconnected shielding electrodes 2D and 25 at a definite potential with respect to the cathode, I have shown a bridging circuit connected to transformer secondary 32. This consists of a condenser 49 which is connected between the shielding grids and the oathode and across the resistor 50 and a variable portion of resistance 5|. If it is desired to maintain the shielding grids 20 and 25 at exactly the same potential as the cathode, this may be achieved by means of a suitable direct connection between them, either inside or outside of the tube. For certain purposes it is possible, and my invention contemplates, that either or both of the shielded grids may be connected to separate external sources of potential, either alternating or direct, whereby they may be made to serve additional controlling functions without in any way impairing their normal shielding functions.

In the device shown in Figs. 3 and 4 I have shown a tube embodying an improved electrode structure illustrating the most important adaptation of which my invention is now known to be capable. This improvement is completely described and claimed in a copending application Serial No. 19,474 of H. M. Smith, assigned to the same assignee as the present application. Referring more particularly to the details or" this embodiment it will be seen that corresponding parts bear similar numbers to those employed in connection with Figs. 1 and 2. Thus, 5 is a cylindrical metal anode having concentric therewith an electron emitting cathode 1 shown as a spiral filament, these being mounted between upper and lower insulating disks 8 and Ill. Between the anode and cathode elements is provided a grid (designated as a whole by the numeral l5) which in this modification comprises a sheet metal cylinder having rectangular punched-out sections therein so as to create a series of circumferentially extending cylinders I3 interconnected by vertical supporting strips l6. Between this grid and the anode and cathode are mounted cylindrical electrodes 28 and 25, respectively, built in conformity with the principles of my invention. Thus, these comprise circumferentially extending sections and vertically connecting strips which in the case of cylinder 20 are numbered 2| and 22 respectively and, in the caseof cylinder 25 are numbered 26 and 21. Each cylinder also contains punched-out portions ('23 in the case of cylinder 20 and 24 in the case of cylinder 25) which bear such a predetermined relation to the punched-out areas of grid l5 as to yield a maximum degree of shielding against flying electron emitting particles and inter-electrode eiTects and, at the same time, to result in a minimum of interference with the normal controlling function of the grid.

In addition to the advantages which have already been described in connection with my invention, there is an additional benefit that the confinement of the arc stream to the relatively narrow areas defined by the punched-out sections of the shielding grids 20 and 25 makes possible a considerable decrease in the necessary size of the grid itself. This is due to the fact that a small equi-potential surface disposed close to a single restricted arc path has quite as much electrostatic controlling efiect as the larger surface necessary if the arc takes place over a widely distributed area. Cutting down of the effective grid surface necessarily results in an almost proportional decrease in the amount of electron emission which may be expected from the grid and in the undesirable grid which is apt to flow during periods of active current discharge. This is an effect entirely independent of the other benefits derived from the use of shielding electrodes, and is made possible by the special inter-relation of parts which is characteristic of my invention.

From a practical standpoint, it is of considerable importance that the metallic shielding cylinders may be formed from sheet metal blanks which can be placed in high speed machinery and punched, rolled and welded with a high degree of accuracy and at extremely low cost. Furthermore, due to the strength and relative symmetry of the individual electrodes the handling and mounting of parts can be carried out with great facility and in a manner eminently fitted for mass-production methods.

It will thus be seen that I have provided a fiveelement electrode assembly which is capable of 75 mounting within a relatively small space and, at the same time, makes possible the utilization of all the shielding functions which it is desired to accomplish by the use of intermediate grids. Furthermore, in spite of the multiplicity of functions which are necessarily required of each of the shielding elements, I have accomplished their incorporation in an electrode assembly of the concentric type without sacrificing either rigidity or simplicity of construction. On the contrary, tubes built in accordance with my invention have been found to be characterized by an extremely high degree of mechanical strength and freedom from variations in operating characteristics due to rough treatment.

It is not necessary for the purpose of my invention that the various grid and anode electrodes should be exactly cylindrical and, in using this term in the appended claims, it is intended to cover equivalent constructions in which the electrodes shall be of a laterally enclosed type. Furthermore, for many purposes some departure from an absolute concentricity of parts will prove permissible, and the use of the term concentric in the claims is understood to include all such equivalent variations.

While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since many modifications in the structure may be made, and I contemplate by the appended claims to cover all such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:-

1. In an electric discharge tube containing an ionizing medium, a substantially cylindrical discharge receiving electrode, a second substantially cylindrical electrode electrostatically effective to control initiation of a discharge to said discharge receiving electrode, one of said electrodes surrounding the other, and a sheet metal cylinder between said electrodes having punchedout sections therein, the imperforate portions of said cylinder being of greater area than said second electrode and positioned to intercept a projection of the same on said discharge receiving electrode.

2. In an electric discharge tube containing an ionizing medium, a control electrode electrostatically effective to control initiation of a discharge through said ionizing medium, a discharge receiving electrode producing electrostatic and heat radiation effects, one of said electrodes surrounding the other, a sheet metal cylinder be-. tween said discharge receiving and said control electrodes and having punched-out sections therein, the imperforate portions of said sheet metal cylinder being of at least as great area as said control electrode and positioned to insure the effective shielding thereof from said elec-,

trostatic and heat radiation effects.

3. In an electric discharge tube containing an ionizing medium, a substantially cylindrical discharge receiving electrode having an electrostatic field extending therefrom, a control electrode electrostatically controlling initiation of a discharge to said cylindrical electrode and enclosed thereby, and an apertured sheet metal cylinder between said electrodes having solid portions of at least as great area as said control electrode and positioned to intercept a projection thereof on said discharge receiving electrode.

4'. In an, electric, discharge tube containing an ionizing medium, a cathode, ananode laterally surrounding said cathode and subject to having transient potentials impressed thereon, said anode having an electrostatic field variable with said impressed potential and extending toward said cathode, an electrostatic control electrode disposed between said cathode and said anode and adapted to initiate a discharge therebetween, a sheet metal cylinder between said anode and said control electrode having punched-out sections therein, the imperforate portions of said sheet metal cylinder having an area at least as great as that of said control electrode and being positioned to shield the same from said anode electrostatic field whereby initiation of said discharge is rendered substantially independent of said transient potentials.

5. In an electric discharge tube containing an ionizing medium, a cathode effective to produce heat radiation, a substantially cylindrical anode surrounding said cathode and having an electrostatic field extending therefrom, a control electrode between said cathode and said anode, and sheet metal cylinders between said control electrode and said anode and cathode respectively having punched-out sections therein, the imperforate portions of said sheet metal cylinders being so related to said control electrode as to shield the same from said heat radiation and said electro-static field.

6. In an electric discharge tube, an ionizing medium, substantially concentric cathode and anode members, one of said members laterally enclosing the other, a control electrode disposed between said cathode and said anode and subjected to radiation and electrostatic effects therefrom, apertured shielding members intermediate said control electrode and said anode and cathode respectively, said shielding members having solid portions of greater area than said control electrode and being positioned to intercept projections of the surface of said control electrode on said anode and cathode.

'7. In an electric discharge device containing an ionizing medium, a cathode having a surface containing an electronically active material, a substantially cylindrical anode surrounding said cathode, a control electrode between said cathode and said anode operable to control initiation of a discharge through said ionizing medium, a sheet metal cylinder between said cathode and said control electrode and having punched-out sections therein, the imperforate sections of said sheet metal cylinder being of at least as great area as said control. electrode and positioned to shield the same from material thrown off by said cathode surface.

8. In an electric discharge device containing an ionizing medium, a discharge receiving electrode having a surface containing an electronically active material subject to sputtering, a control electrode presenting a substantially concentric lateral surface to said discharge receiving electrode, and a sheet metal member between said discharge receiving and control electrodes and having punched out sections therein, the imperforate sections of said sheet metal member being so related to said control electrode as to shield the same from material sputtered from said surface of said discharge bearing. electrode.

ORRIN W. LIVINGSTON.

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