US2080235A - Control electrode for gas-filled tubes - Google Patents

Description

May. 11, 1937. M, n- 2,080,235

CONTROL ELECTRODE FOR GAS-FILLED TUBES Filed May 2, 1935 Fig l.

23 Fig. 2.

-z5 K H3 [:1 1e 33 m Inventor:

His Attorne g- Patented my 11, 1937 2,080,235

CONTROL ELECTRODE FOR GAS-FILLED TUBES Henry M. Smith, Scotia, N. Y., assignor to General Electric Company, a corporation of New York I Application May 2, 1935, Serial No. 19,474

4 Claims. (Cl. 25027.5)

My invention relates to electron discharge detrade or grid held at some predetermined potenvlces of the type wherein a controllable discharge tial. This grid operates to diminish the velocity takes place through an ionizable medium. More of the electrons drawn from the cathode by the particularly my invention relates to improveanode potential to an extent sufiicient to prevent 5 ments in electrode structure whereby the initiionization of the s m taking p e As t l ation of an electric discharge in a gas-filled tube voltage of the control member or grid is varied, is able to be controlled accurately and dependit will be found that for a given condition of the ably. tube there is some critical voltage which will just It has previously been proposed to rovide permit the tube to start. Once current flow has means for shielding the grid member of a concommenced, however, the presence of large num- 10 .trolled discharge type of tube from deposits of bers of ions serves to neutralize the electrostatic electron emitting material and inter-electrodes effect of the g d d t make Cessation of current heat radiation and electrostatic effects. Such dependent only on areversal of anode potential. arrangements are disclosed and claimed, for ex- This characteristic pr p ty of a -fi ed e ample, in copending applications, serial Nos. gives itunique value as anelectronic valve or relay 15 704,503 and 17,765 of O. W. Livingston, assigned and has led to'its application in widespread fields.

to the same assignee as the present application. Due to the fact that many of the uses to which It is an object of the present invention to prothis type of tube is put demand exacting accuracy vide improved shielding means which, while fuland depen y of operation, i i f ex r m filling thefunction of guarding the control elecimp n ha the i l grid voltage which trode from accumulation of electron-emitting mawill cause breakdown of the tube shall not vary terial and from undesirable inter-electrode efeither with time or with conditions of operati nfects, will not unduly complicate the'plocess of Unfortunately, however, a number of factors are tube manufacture and assembly. always present which tend to make unlikely this It is a further object of my invention to prodesired uniformity of operation. Among them is 25 vide a concentric electrode arrangement which the well-known p e on of d emisSiOn. will permit an economical design of parts and consisting of the generation of electrons from the accurate and dependable predetermination of opsurface of the grid member. With a clean cold crating characteristics. More particularly it is g d this e t s f s i h importance. ut f t my object to produce a sheet metal grid type of grid becomes coated with impurities and is raised 30 gas-filled tube, in which the possibility of varito an elevated temperature by radiation from the ation from a desired standard of operation is cathode or anode, a sufficient emission may take substantially avoided by the rigid construction place to cause breakdown of the tube. This is, and favorable inter-relation of the electrode parts, of course, due to the fact that the emitted elec- Other and further objects of my invention will trons act on their way to the anode as i i 35 be apparent upon considering the following specagencies and cause the fulfillment of the condilfication in connection with the appended claims tions which render the tube conductive. and the accompanying drawing in which Fig, 1 This undesirable situation is of frequent 0cls an elevation partly in section of a tube emcurrence in gase us tubes s a present bodying my improved construction; Fig. 2 is a structed due in part to the fact that the usual 40 developed view of the sheet metal member from cathode, coated with electron emissive material, which the inner shielding electrode is formed; is apt to have its surface substances thrown ofi Fig. 3 is a developed view of the grid electrode in'considerable quantity. This maybe attributaitself, while Fig. 4 is a perspective view, partially ble either to pure heat vaporization or to-the cut away to show in detail the relative arrange familiar effect known as sputtering, caused by 45 ment of the electrode parts. ionic bombardment of the cathode. The ejected In tubes of the type containing a thermionic material tends to be collected by the grid surface cathode, an anode and a gaseous ionizing medium which, when highly heated by radiation from the at a pressure suflicient to support an arc-like discathode or anode, acts as a sort of secondary charge, the flow of a discharge current between electron source in the manner described above. 0

the cathode and anode depends on the presence Under certain conditions it is also possible for of a considerable number of ions. To prevent the grid to receive electrons emitting material the formation of such ions and the initiation of from the anode as well as the cathode. This situa discharge, it is possible to use a control elecation 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 regenerated by heat in such fashion as to be passed on to the grid. I

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 sufiicient 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 thegaseous tube, whose principal function is an initiating one, an unfore seen 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 shown a sealed envelope I of the usual type having at its base a reentrant stem 2 which terminates in a press 3 adapted to furnish a support and seal for incoming current leads. The upper section of the tube I has a reduced portion or thimble 4 which, as hereinafter explained; makes possible a greater measure of support for the electrodes contained within the tube. The envelope I is adapted to contain an ionizable medium which may consist of an inert gas such as argon, at a pressure from 1 to 500 microns or 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.

A substantially cylindrical anode 5 is provided, which may be of any suitable material such as molybdenum, graphite or carbonized nickel, and which in cooperation with the cathode member 1 is adapted to bear the main discharge of the tube. The cathode 1 may consist of a directly heated spiral filament, but may alternatively comprise a straight filament or one of the various indirectly heated cathode forms now in general use. It is coated or otherwise suitably treated with an electron-emissive substance such as barium oxide which when heated is able to afford a sufiicient supply of electrons to ionize the gaseous medium in the tube; Both the cathode and anode are supported at top and bottom by insulating mica disks 8 and), respectively, which also serve to space and support the remaining electrode members in axial and transverse alinement. The upper disk 8 is of such size as to bear laterally upon the wall of the envelope l at its reduced portion 4, thus affording the electrodes considerable protection against lateral displacement.

Heating current from a suitable source (not shown) is delivered to thecathode filament by means of lead-in wires H which pass through the press 3, and one of which also serves to bear the main discharge current. A return path for the discharge current is provided through a vertical rod 9 whose lower portion I6 is connected by welding or otherwise to a horizontal extension of lead-in wire I2. The support 9, along with a corresponding supporting member 6, is also used to secure the anode to the insulating disks 8 and "portions of the grid electrode.

I 0. Upon 8. depending extension l3 of the leadin wire I2 there is mounted a small capsule I, having an orifice therein (not shown) and 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 the 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.

Intermediate the cathode and anode and presenting its lateral surfaces to them is mounted a cylindrical grid member which, as is more adequately shown by Figs. 3 and 4, consists of circumferentially extending imperforate sections l8 spaced by punched-out sections l9 and vertically connected by strips 28. This grid electrode, which may be of any suitable conducting material, is supported at the top and bottom by the mica disks 8 and i0 and is connected to a lead-in conductor 3| which passes out through press 3 and stem 2 of the tube. By means of this connection the grid can be maintained at a desired constant or variable potential to restrain or permit the initiation of an ionizing discharge between the cathode l and the anode 5 in a manner well understood in the art.

In addition to the discharge bearing electrodes 5 and l and the grid l5, which constitute the usual electrodes employed in gaseous tubes, a cylindrical shielding electrode member 'is placed surrounding the cathode l and between it and the grid l5. This may be of metal or other conducting material and is similar in shape and appearance to the grid itself, but, as shown in Fig. 2, has punched-out portions 22 which are of appreciably smaller area than the punched-out Consequently, both the circumferentially extending sections 2| and the vertical connecting sections 25 present a greater surface to the cathode than do the corresponding portions of electrode 15. Otherwise expressed, the imperforate portions of electrode 20 are of at least as great area as the corresponding portions of control electrode I 5 and are positioned to intercept a direct projection of the surface of the latter on the cathode member. It is, therefore, apparent that any particles of solid material vaporized or sputtered from the surface of the cathode and traveling in straight lines will be prevented from reaching the solid portions of the cylinder l5. For this reason there will be little or no tendency for the control element to become coated with electron emissive substances which might seriously heighten the effect of grid emission and lead to a change in the control characteristic of the tube. In addition, the control electrode is adequately hidden from heat radiations from the cathode 1 so that any inherent electron emitting properties which may exist in the control electrode will be reduced to a minimum. With certain special circuit arrangements in which the cathode potential is apt'to be varied over such a range that its electrostatic field might adversely affect the grid member, my improved shielding means has the additional virtue of blocking all lines of electrostatic force which proceed normally from the cathode surface.

A lead-in conductor is brought through press 3 and connected in any desired manner to the shielding cylinder 20, whereby it may be held at any desired potential. While it is true that" for most purposes it will be found convenient to maintain the shield at a constant voltage, it is also possible and is included within the scope of my invention to use a variable voltage'which may impart an additional controlling function to cylinder 20 without impairing its effectiveness as a shield.

While the advantageous results contemplated by my invention may be largely obtained by the use of a single shielding electrode such as that just described, further and additional advantages may accrue from the use of' a second shielding cylinder interposed between the control electrode l5 and the anode 5. Such a second shielding electrode, numbered 23, is shown in Figs. 1 and 4 and is of a constructionsimilar to that of the shielding member 20 butyof course, is proportionately increased in circumference. As in the case of the electrode 20, electrode 23 consists of solid circumferential portions 24 spaced by punched-out areas 26 whichare radially alined with the similar perforations of the'control electrode 20 and supported by intermediate connecting portions 32 wider than the connecting portions 25 of the control electrode. Since, as is clearly shown in Fig. 4, this second shielding element is also of such size as to intercept a projection of the control member IS on the surface of the anode 5, it will serve to cut off heat radiations proceeding from the anode. In the same way it will intercept or materially reduce the anode electrostatic field which tendsto reach into the control electrode space and interfere with the normal operationof the tube. In this way those grid emission effects which are blamable on the anode as well as accidental tube discharges due to steep wave front transient surges are effectively prevented. Electrode 23 is shown as being electrically connected to electrode 20 by a bridging connection. but it may alternatively be provided with a special lead-in wire by means of which it can be connected to an external source of potential either constant or variable.

Fig. 2 shows in developed form the sheet metal member which is used in the formation of the shielding electrode 20. It will be readily apparent how it is possible to take a solid sheet of metal and in a single operation punch out or otherwise remove rectangular sections 22 of any predetermined size. After the punching operation, the sheet may be rolled into a cylinder 20 by suitable machinery, and its edges 33 and 34 lap welded or otherwise secured together. In this manner an exceptionably durable and accurately propor- ...tioned electrode may be made.

Fig. 3 shows a similar developed "view of the main control electrode itself. This can be punched, rolled and welded into a cylinder, as

explained above, and placed concentrically with the cylinder 20. It is an extremely important advantage of this type of construction that the relative areas and positions offthe punched sections can be predetermined with any desired amount of accuracy. Consequently, the dimensions of the various parts may be specified with a degree of fineness sufficient to guarantee the optimum amount of shielding with a reasonable certainty that variations in assembly will not be great enough to impair the control constants of the tube. Furthermore, due tothe inherent rigidity ofthe finished electrodesand the secure shall be of the laterally enclosed type. Fur-- thermore, for many purposes some departure from an absolute concentricity of parts will prove permissible and the use in the claims of the word concentric is to be understood as including all such 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 isi 1. In an electric discharge device containing an ionizing medium, a discharge-receiving electrode producing electrostatic and heat radiation effects, a cylindrical sheet metal control electrode concentric with said discharge receiving electrode, a cylindrical sheet metal shielding electrode between said discharge-receiving and said control electrode, and means rigidly supporting said cylindrical electrodes in coaxial and transverse alinement, said cylindrical electrodes being provided with radially alined perforations of predetermined size relation, the imperforate portions of said shielding electrode being of at least as great area as the imperforate portions of said control electrode and positioned to insure effective shielding of the control electrode from said electrostatic and heat radiation effects.

2. In an electric discharge device containing an ionizing medium, a cathode member, a cylindrical anode member surrounding said cathode member, a metal control electrode cylinder disposed between said members, a metal shielding cylinder between said control electrode and one of said members, and a pair of spaced planar elements respectively abutting the opposite ends of said anode member and both of said cylinders for maintaining the same in transverse and axial alinement, said cylinders being provided with alined perforations of predetermined size relation, the imperforate portions of said shielding cylinder being of greater area than the imperforate portions of said control cylinder and so positioned as to intercept projections thereof on the surface of said one of said members.

3. In an electric discharge device containing an ionizingmedium, a cathode, an anode mount- .ed concentrically with and surrounding said cathode, a shielded control electrode structure comprising three concentric sheet metal cylinders surrounding said cathode and disposed between the cathode and anode, and means rigidly supporting said cylinders in coaxial alinement, said cylinders being provided with radially alined perforations of accurately predetermined size relation, the imperforate portions of the inner and outer ones of said cylinders being of such size with respect to the imperforate portions of the center cylinder as to intercept projections thereof on said anode and cathode respectively.

4. In an electric discharge device containing an ionizing medium, a cathode, a substantially cylindrical anode surrounding said cathode, a shielded control electrode structure comprising three concentric sheet metal cylinders surroundi ng said cathode and disposed'between the oath- -ode and anode, and a pair of plates of insulating material respectively abutting the opposite ends of said anode and each of said cylinders for main- 5 taining the same in axial and transverse alinement, said cylinders being provided with radially alined slots of similar configuration and accu rately predetermined size relation, the solid portions of the inner and outer ones of said cylinders being of such size and so positioned with respect to the solid portions of the center cylinder as to intercept projections thereof on said anode and cathode respectively.

HENRY M. SMITH.

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