US2558603A - Protective means for rectifier cathodes - Google Patents

Description

June 26, 195] z. J. ATLEE PROTECTIVE MEANS FOR RECTIFIER CATHODES Filed March 4, 1949 INVEN TOR. Zed r]. Af/ee A/MM 5 rlrlll t V Patented June 26, 1951 PROTECTIVE MEANS FOR RECTIFIER CATHODES Zed J. Atlee, Des Plaines, Ill., assignor to Dunlee Corporation, Chicago, 111., a corporation of Illinois Application March 4, 1949, Serial N 0. 79,683

7 Claims. (Cl. 313-240) tary cathode along the axis of the anode cylinder in order to shield the envelope from' electron bombardment and minimize accumulation of charges thereby which might adversely affect the operation of the device. Precautionary measures, however, must be taken in order to protect the cathode thus arranged from various destructive phenomena. For example, electrostatic forces resulting from the high potential diiference between the anode and the cathode tend to pull the cathode filament laterally from the axial position toward the anode. It will be understood that, if the cathodewere mounted in an accurately centered relation upon the axis of the anode, the electrostatic forces would be balanced upon all sides of the cathode and no ill effects would occur. Obviously, such precise mounting of the cathode is impractical, if not impossible, in ordinary factory assembly procedures, with the result that the filament is always more or less eccentric with relation to the anode axis. Any eccentricity of the cathode will result in an unbalance of the electrostatic forces acting upon the cathode and, because of the fact that the cathode when heated to incandescence is inherently relatively weak, these forces may result in the destruction of the filament.

One common method of protecting the cathode from rupture due to electrostatic forces has been to form the lead-in wire supporting the outer end of the cathode into the shape of a spiral consisting of one or more convolutions surrounding the cathode in spaced relation therewith. Since the spiral is of the same potential as the filament, the latter is thereby surrounded with a protective electrostatic field and most of the voltage gradient is taken off of the weak cathode.

One disadvantage resulting from the enclosure of the filament within such a spiral protective.

shield is that the space charge created thereby seriously impedes electron fiow from the cathode to the anode so that a high voltage drop, frequently of the order of 2 to 5 per cent, occurs within the tube. In order to reduce the internal impedance of the tube, the number of convolutions of the spiral are sometimes reduced to a minimum of one or two turns; and, while this had been effective for reducing the internal voltage drop of the tube, the end portions of the filament are thereby exposed to other destructive phenomena. For example, numerous cathode failures have been observed which are believed to have resulted from inverse bombardment. Inverse ionic bombardment may occur due to the fact that there are always traces of gas present within the envelope and, as a gaseous molecule is struck by an electron, other electrons are driven from such molecule and pass to the anode while the remaining positively charged ion is attracted toward the cathode. Because of the mass of the relatively, heavy, positively charged ion and because of the speed imparted. thereto by the high potential difference existing between the anode and the cathode, the ion will not, necessarily, be deflected from its path of travel toward the filament by the spiral electrostatic shield and the ion, upon striking the filament, causes a burn-out failure thereof. Such failures of the filament have been observed to occure particularly adjacent the opposite ends thereof where, due to the spiral configuration of the electrostatic shield, the cathode is exposed over arelatively large portion on one side thereof.

Failure of the filament may also occur as a result of the inverse electron bombardment due to electron flow from the anode to the cathode and which may be caused by overheating of some minute projection on the anode surface which, upon reaching incandescence, will, in and of it,- self, form an electron emitter and from which electrons may flow to the cathode at some relatively unprotected point during the inverse half cycle. The impingement of such inverse electron flow will result in excessive heating of the filament at that particular point giving rise to increased electron emission at that spot which, in turn, will result in increased current flow back to the anode, thus further heating the anode emitter. spot causing still further increase in inverse electron flow which condition will progress with extreme rapidity and until terminated by a burn-out of the filament. Whatever the cause for the failures may have been, I have discovered that, with a protective shield formed and arranged in accordance with the present invention, failures of this type have been prevented.

It is a general object of the present invention, therefore, to provide a new and improved shielding arrangement for filamentary cathodes of high voltage rectifier tubes whereby the filament is adequately protected against deleterious effects of electrostatic forces as well as against inverse bombardment while at the same time maintaining interelectrode impedance at a minimum.

This object is accomplished by the present invention by providing a metal ring around the of the invention, I prefer to utilize a thoriated-r and carbonized tungsten filament whereby adequate electron emission may be obtained witlri a cathode filament of relatively short length. in accordance with common practice; the filament is wound in a compact helical :ienm :in orde further to maintain its overall longitudinal dimensions at a minimum. Upon .turningthe tube.

on and off, inductive forces acting uponthefilament turns impose a rather severe mechanical tensioning spring .means in the .cathodesupport- 1 .ing structure tor compensation .of strains .thus 'imPQSed .upon .the -,cathode.he1ix.

n ,fnrther .object cf .the present invention, therefore, is ..to provide anew andiimproved supportingarrangement for-.cathodesrofelectric .dis-

.charge devices .cf the .type referred to including ;a resilient .elementlf or absorbing. excessivetensile strains imposed upon th catho e.

Additional objects and advantages f the presentjnvention will become apnarent as the follow- ,description proceeds and taken in connection with the accompanying drawing forming a ,part of ,thepresent idisclosure.

In the drawing, Fig. 1 is .anelevational viewof ;a ,high .voltagerectifier constructedin accordance With-.0ne .iorm of the present invention; Fig. 2 is a .sectional view taken longitudinally of the anode ,to .show the relative positions pf the filamerit and .p ptective arrangement; .and Fig. .3

is a view taken alon :the line =3- -T3 of Fig. 2;

,Eig. ,4 .is .a modificaticn of .Fig. 2, showing an alternative arrangement ;of thetensioning spring.

Referring to Fig. .11 .of the drawing, ahigh voltag rectifier -.isshown comprising .an envelope 1.0 haying .a cylindrical anode. structure J4 "supported inv one end and a cathode structure supported in the opposite end thereof. The .tube illustrated in this instance ,is particularly suitable for X-ray use, is highlyeyacuated, anddesigned for operation a't 1. .0.600 volts. The anode consists ofa cylindri al cup arran e .ceaxiaily w th the envelope .lll .and which .is supported .upon a reentrant sleeve portion .of the envelope .and through which lead-in connection is made to the an de.

'Ihehathodestructure includes a metal bonnet l9 supported in any suitable manner upon the reentrant .pertion .l] of the envelope and about which the low r end of t e bonnet skirt extends. Extending upwardly from the metal bonnet 19 a first heavy gauge lead in wire .29 arranged parallel with the axis .of the .anode cup J1 and. disposed at one .side thereof, .the upper end of the lead-in wire being doubled back .upon itself as indicated at .2 L. .The second .leadein wire .22

Changes in temperature of extends through the opening l8 in the bonnet l9 substantially along the axis of th cup H. Secured to the upper end of the lead-in wire 22 is a helical coil element, or spring 23 of relatively heavy gauge resilient metal while the cathode 24 is connected between the adjacent ends of the .lQad-in vne portion 2| and the resilient spring element 23 and-extendssubstantially ,along the axis of the anode cup.

:It is preferred that the cathode 24 be wound in a compact helix and be formed of carbonized ithoriated tungsten, such as described, for example in U. S. Patent No. 2,332,428 to Atlee et a1.

.-.$.uch. .filaments have-the advantage of high rate znf ielectrnnemission at relatively low temperature whereby for any :given output the overall dimen- ,s i o.ns. of thefilament may be reduced considerably as compared with pure tungsten. Surrounding theacathode 24 is a metal ring til and which is supported upon the upper end of a length of wire e1 .secure d.at its lower-end to the bonnet ll! through which connection is made to the'leadein wire 20. The ring 30 extends inaplane at right :anglesto the .axisof the anode .H and substantially ,at the midpoint of the cathode 2d. The diameter '01 the ring 30 isnot less than one-half the length of i the cathode .24 while the relative diameters for the ring 35 and the vcup H are 'determinedlby the operating voltageof the-device; Generallythe diameterof the'ring 30-withrespect tothe inner idiameter of the ia-node cup H willibe substantiallyqin-the ratio of 1/8, e :being the-base of naturalilogarithms.

With the :ring .39 surrounding the cathode .24 substantially at the :midpoint thereof and con .nected to :thelead-in :Wile 20, the :space charge created thereby :is' insufilcient :to interfere :substantially-withthe electron fiowbetweenithe cathode 2ll-and the anode I I. It-will'be observed'that the .ring 3fl extends 'around the leadin wire 20, and further, that the ring is substantially concentric with the cathode as Well as the anode cup;

In tubes which have been actually manufactured according to the present invention and used, the normal voltage drop has been observed to be less than 1 per cent. Because of the fact that the ring v3i) surrounds 'thecathode 24 at the midpoint thereof, the deleterious effects of the electrostatic field created by the potential differences 'between th'e cathode and anode are e'f fectivelynullified. While .the exposed end portions of "the cathode 24 are actually exposed .tO. electrostaticpull. these portions areso shorttha't -the electrostatic iorces have no appreciable influencethereu'pon. The practical effect .of the.

ring is to provide a support ,for the midpoint ofithe cathode as regards the electrostaticforces acting upon the cathode .on opposite sideso'fthe ring.

With the diameter of the ring .30 being relatively large as compared with the .overalllengt'h of the cathode,.substantially complete protection is provided .for the cathode against inverse .bombardment becauseof the shorter distanceexistin .between jthe .anodccup H and the ring 30, :as compared withthe distance between the anode and any pointof the-cathode 24, ions of .gasrmole- .culesfohmed Withinthe anodespace areattracted to the ring .and, because of its relatively indestructible nature, the impact force of-such ions is readily absorbed thereby. Also as regards inverse electron fiow,-any electrons emitted from; the anode, such :as fromsurfacyelectricity, will; be attracted by the ring 30 by :reason of the .fact

that'it is 'closer'to the anode than any point of the cathode. i I .1

Due to the resiliency of the spring 23 arranged between the upper end of the lead-in wire 22 and the lower end" of the cathode, mechanical tensile strains imposed upon the cathode due to voltage surges or otherwise are readily absorbed with complete protection provided for the fragile cathode helix.

Irrespective of theory, it has been definitely established that the cathode protective arrangement of the present invention is far superior to shown in Fig. 2, it may be desirable, under certain circumstances, to provide a tensioning spring in the bonnet I9 as illustrated in Fig. 4. 'In this instance the lead-in wire 35 is longitudinally movable through a cooperating bore in the .top of the bonnet 36 and is connected by a flexible section 3! to the lead-in wire portion 38. A stop collar 39 is secured to the wire. 35 while a spring 4| is arranged between the collar 39 and a shoul-. der formed at the lower end of the cooperating bore. The spring 4! under compression holds the cathode under continuous tension and compensates .for excessive strains imposed upon the cathode. the spring may be arranged about the second lead-in wire, 42 in the opening l8 with a suitable insulation bushing providing insulation between the lead-in wire 42 and the bonnet. Having described the invention in what I consider to be a preferred embodiment thereof, it is desired that it be understood that the specific details shown are merely illustrative and that the invention may be carried out according to .vari-' ous other embodiments, all of which are intended to be included within the scope of the following claims.

I claim:

1. In an electric discharge device including an envelope containing a cylindrical anode and an elongated filamentary cathode positioned along the axis of said anode, means supporting said anode at one end of said envelope, a pair of leadin wires for the cathode mounted in the opposite end of said envelope, a first of said lead-in wires extending parallel with and spaced from said cathode, the upper end of said first lead-in wire being doubled back upon itself and having the outer end of said cathode connected thereto, the second of said lead-in wires being connected to the opposite end of said cathode, the invention consisting of a metal ring surrounding said cathode substantially at the midpoint thereof and lying in a plane extending at right angles'to said axis, said ring being electrically connected to said first lead-in wire and having an outer diameter corresponding to at least one half the length of said cathode.

2. In an electric discharge device including an envelope containing a cylindrical anode and an elongated filamentary cathode positioned along the axis of said anode, means supporting said It will be obvious that, alternatively;;

6 anode at one end of said envelope, a pair of leadin wires for the cathode mounted in the opposite end of said envelope, a first of said lead-in wires extending parallel with and spaced from said cathode, the upper end of said first lead-in wire being doubled back upon itself and having the outer end of said cathode connected thereto, the second of said lead-in wires being connected-to the opposite end of said cathode, the invention consisting of a metal ring surrounding said cathode substantially at the midpoint thereof and lying in a plane extending at right angles to said axis, said ring being electrically connected to said first lead-in wire, the diameter of said ring being at least one-half the length of said cathode and the diameter of said ring with respect to the internal diameter of said anode being substantially in theratio of 1/6, where e is the base of natural logarithms.

. 3. In an electric discharge device including an envelope containing a cylindrical anode and an elongated filamentary cathode positioned along the axis of said anode, means supporting said anode at one end of said envelope, a pair of leadin wires for the cathode mounted in the opposite end of said envelope, a first of said lead-in wires extending parallel with and spaced from said cathode, the upper end of said first lead-in wire being doubled back upon itself and having the outer end of said cathode connected thereto, the second of said lead-in wires being connectedto the opposite end of said cathode, the invention consisting of means for protecting said cathode from destructive electrostatic forces and inverse bombardment, said means including a metal ring surrounding said cathode substantially at the midpoint thereof and lying in a plane extending at right angles to said axis, said ring also extending around said first lead-in wire and electrically connected thereto, said ring being substantially concentric with said cathode and said anodeand' having an outer diameter corresponding to at least. one half the length of said cathode.

4. In an electric discharge device including an envelope containing a cylindrical anode and'an elongated filamentary cathode positioned along the axis of said anode, means supporting said anode at one end of said envelope, a pair of lead-in wires for the cathode mounted in the opposite end of said envelope, an annular metal member surrounding said cathode in spaced relation therewith substantially at the midpoint thereof, means electrically connecting said memher to one of said pair of lead-in wires, the outer diameter of said member being at least as great as one-half the length of said cathode and the ratio of the outer diameter of said member with respect to the inner diameter of said anode being substantially in the ratio of l/e, where e is the base of natural logarithms, whereby the distance between any point on said anode and any point on said cathode is greater than the distance between said point on said cathode and a point on either said member or on one of said lead-in wires, said member providing complete protection to said filament as regards both destructive electrostatic forces and inverse bombardment.

5. In an electric discharge device including an envelope containing a cylindrical anode and an elongated filamentary cathode positioned along the axis of said anode, means supporting said anode at one end of said envelope, a pair of lead-in Wires for the cathode mounted in the opposite end of said envelope, the invention consisting of means for protecting said cathode against destructiveelectrostatic forces and insverse bombardment-said means consisting of a metal loop extending around said cathode and connected to one of said lead-in wires, the outer diameter of said-member being atv least as great as one-half the length er said cathode and the ratio of the outer diameterof said member with respect to the inner diameter of said anode being substantially in the ratio-oi l/e, where 'e is the base of natural logarithms, whereby the distance between any point on said anode'and any point on said cathode is greater than the distance between said point on said cathode and-a point on either said member or on one of said lead-in wires.

6. In an electric discharge device including an envelope containin a cylindrical anode and an elongated filamentary cathode positioned along the axis of said anode, means supporting'said' anode at one end of said envelope, a pair of leadin wires for the cathode mounted in the opposite end'of said envelope, a-first of said lead-in Wires extending parallel with and spaced from said cathode, the upper end'of said'first lead-in wire being doubled back upon itself and having the outer end of said cathode connected thereto, connection means connecting the second of said leadin Wires to the opposite end of said cathode, the

invention consisting of a metal ring surrounding said cathode substantially at the midpoint thereof and lying in a plane extending at right angles, to said axis, a wire supporting said ring extending parallel with and electrically connected to said first lead-in wire, said connection means, consisting of a member relatively resilient in the directionof said axis, the outer diameter of said member being at least as great as onehalf the length of said cathode and the ratio of the outer diameter oflsaid member with'respect. tothe inner diameter of said anode being substantially in the ratio of l/e, Where e is the base of natural logarithms, whereby the distance between any point on said anodeand any point on said cathode is greater than the distance between said point on said cathode and a point on either said member or on one of said lead-in wires.v

7. In an electric discharge device including an envelope containinga-cylindrical 'anOde-atidan' elongated filamentary cathode positioned along the axis -of said anode, meanssupportingsaid anode at one end of'said envelope, a pair of leadin wires for the cathode mounted-in the optic-- site end of said envelope, a first of. said lead-in wiresextending parallel with and spaced from said cathode, the upper end of said first lead in' wire being doubled back upon itself and having the outer end of said cathode-connected thereto, the 'secondof saidglead-in wires being connected to the opposite end of said cathode, theinvention consisting of means 'for protecting said cathodefrom destructiveelectrostatic forces and inverse bombardment, said "means including -*'a metal ring. surrounding said-cathmie substantiallyat-themidpoint thereof and lying in a planeex tending-at right angles to said axis, a wire-supporting' said ring and electrically connected to said first lead-in wire, tension spring means o erativelyassociatedwith one of said lead-in wires REFERENCES CITED The following-references are of record in'the file of this patent:

UNITED STATES PATENTS Number Name Date 1,287,265 Dushman Dec. 10, 1918 1,860,152 Mc'Arthur May 24, 1932 1,959,195 Charlton et'al May 15, 1934 FOREIGN PATENTS Number Country Date 326,311 Germany 'Sept. '27, 1920

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