US2825832A - Thermionic cathode structure - Google Patents

Description

March 4, 195% c. c. CUTLER 9 5 THERMIONIC CATHODE STRUCTURE Filed Dec. 3,1953

FIG. i

//v VENTOR C. C. CU TL E R A T TOR/VEY THERMIONI CATHODE STRUCTURE Cassius C. Cufler, Gillette, N. J., assignor to Bell Telephone Laboratories, Incorporated, N ew York, N. Y., a corporation of New York Application December 3, 1953, Serial No. 396,027

7 Claims. (Cl. 313-38) This invention relates to thermionic cathodes and more particularly to mounting the cathode structure in a high frequency thermionic electron discharge device.

The operating characteristics of high frequency electron discharge devices are dependent to a large extent upon the inter-electrode spacings. In order that a particular device have predetermined operating characteristics it is necessary that the inter-electrode spacings be accurately established during fabrication of the device and be maintained constant during operation of the device. This is necessary since the spacings are extremely small, and changes of small absolute magnitude therein result in relatively large variations in the operating characteristics. All this is particularly true of the cathode and its supporting structure. In addition the cathode should be thermally isolated from the other elements of the device so that the cathode may be heated to the temperature required for thermionic emission without undue power expenditure.

These several requirements have been met in prior devices by either complicated cathode supporting structures or by structures made of materials having different temperature coeflicients of expansion or both. An attempt is made in the latter structures to compensate in one portion of the supporting structure for the heat pro duced expansion in another portion. This compensation is complicated by the fact that the range of temperatures over which the device is often called upon to operate will vary with the supply voltage producing the heating. It is further complicated by the fact noted above that the component size and spacing are very small. In addition, the preparation of the supporting structure involvesfastening the supporting components together and to the cathode by some means, most often by welding, and often involves pre-forming the cathode to a particular shape in manufacture.

It is therefore an object of the present invention to mount an electron emitting cathode with respect to other elements of an electron discharge device so that the spacing between the cathode and the other elements is maintained substantially constant regardless of the temperature of the emitting cathode.

It is a further object of the invention to dispose and mount an electron emitting cathode more accurately and simply during manufacture than is possible with prior mountings.

A more specific object is to provide a mounting for a cathode in a discharge device of the type requiring the production of an accurately controlled electron beam which eliminates the necessity of special pre-forming of the cathode structure or of welding a supporting structure to the cathode, while at the same time maintaining a high degree of heat concentration at the cathode.

In accordance with one embodiment of the invention to be described, the cathode takes the form of a flattened disc having a plurality of apertures spaced near its periph' eral edge. This edge is loosely received within a cavity formed by rigid support members, which may, in the te ates atefii ice case of the cathode ray tube, comprise in part the focusing electrode itself. Thin, wire-like elements of low heat conductivity material are looped through each of the apertures to bear against the inside of the cavity. Thus, as the cathode is heated it is free to expand in a radial direction but is restrained at a substantially constant distance equal to the diameter of the wires from the supporting structure. Furthermore, since the cathode is touched at a minimum number of points, and then only by the low heat conductivity wires, little heat conduction is experienced away from the cathode. In another embodiment of the invention the loops are replaced by helical members of material similar to the wires located on either side of the cathode within the cavity.

These and other objects, the nature of the present invention, its various advantages, and its features will appear more fully upon consideration of the various specific illustrative embodiments shown in the accompanying drawings and described in the following detailed description.

In the drawings:

Fig. 1 is a cross-sectional view of a portion of an electron discharge device, in accordance with the invention, showing the mounting of the cathode and illustrating the relationship of the cathode to other elements of the device;

Fig. 2 shows in perspective view the cathode portion of the assembly of Fig. 1;

Fig. 3 is a cross-sectional viewof an alternative cathode shape in accordance with the invention;

Fig. 4 is a cross-sectional view of a second embodiment of the invention illustrating a modification of the cathode support of Fig. 1; and

Fig. 5 shows in perspective view the cathode portion of the assembly of Fig. 4.

Referring more specifically to Fig. 1, an illustrative embodiment of the invention is shown in the form of an electron gun of a beam-type tube such as a cathode ray tube or a traveling wave tube. Enclosed within portion 11 of the evacuated outer casing or bulb of the tube, and supported therein by suitable means is an anode ring 12 of conventional design having an aperture 13 in the center thereof to allow for passage of the produced electron beam to portions of the tube not shown which, however, would be located above the structure shown in Fig. l. Anode 12 is biased at a high positive potential relative to focusing electrode 14 as indicated by the conductive leads marked B+ and B.

Electrode 14 may be as illustrated, an element of conventional design comprising a ring of conductive material having a generally conical taper extending toward the center opening 15 thereof and in a direction away from anode 12. Electrode 14 is separated and supported from anode 12 by ring 16 of insulating material, such as a suitable ceramic or porcelain material.

Coaxially arranged with aperture 13 of anode 12 and aperture 15 of electrode 1 1 is the cathode assembly in accordance with the invention. This assembly comprises an emissive layer 17, such as an oxide of one of the rare earths, for example, barium or strontium. Layer 17 is carried by a flattened disc or button-like plate 18 of a high melting point, high heat conductivity, electrically conductive material such as nickel or iron. integrally connected to plate 18 and coaxially arranged therewith is a tubular heat shield 19 of material similar to plate 18. Shield 19 surrounds a conventional tungsten heating element 20.

Located around the periphery of plate 18 and displaced toward the center from the edge thereof by a convenient distance, are a plurality of apertures 21 extending through the thickness of plate 13. This is most 'clearly' seen in the perspective view of Fig. 3. If, for

. t aeeasee e example, the diameterofplate; 18 is in the order of'.150

' shown in Fig.' 5. A plurality of helical members 31 inch a convenient distance from the edge th ereof. for

Before assembling :the cathode structure.aashortt length'g.

of wire-like element 22 of material having: a low; -co-. eificient of; heateconduetivity and a high melting .point .:is

looped through each;,of ;ap,ertu res .21 Suitabletmatcrials for wires 22-may be. tantalum,,stainless stealer-other. lowheat conductivity material; However: nonmetallic materials, may also bQMSEd-ZQSWllltbB. described herein after, Preferably, three such. aperture-wire. combinations I may-be employed, butmoretmay; satisfactorily be.pro

videdifdesired After wires; 22i.are1 inserted as=described, plate 18. is located elative to aperture of; electrode 14 asdefined above so that wires-.22.:zbear against the lower. surface:

of -electrode:.14,; as shown. Thus,:wires- 22 provide a separation between electrode. 14.and plate 18 substantiallyequal to the dimeter ofwires22 and this dimension-- may. be critically selected by properchoiceot the wirediameter, Upon this, assemblyis placed a clamping ringample, smallmachine bolts. extending into tapped holes.

in electrode 14. Thus, recess 24 together with the-lower face. of electrode14 forms a cavity =within-whieh wires 22 are pressed against the inside surfacethereof; The

slight resilience of wires 22 will hold plate 18 securely at a substantially constant distance from-focusing electrode 14.. As.thecathode is heated, however, plate 18 is free to expand laterally in a radial direction. In order to accommodate this expansion the diameter of recess 24 should be substantially equal to the heated diameter of I plate 18 plus twice the diameter of wires 22. Therefore, upon heating the cathode thev emissive layer 17 thereof on plate 18 will be substantially centeredwith respect to aperture 15. Since the only physical contact between plate 18' and its supporting structure comprising ring 23 and electrode 14 is through the point-like contact of the several low heat conductivity Wires 22, little heat'transfer is experienced away from the cathode structure 1718--19.

In the embodiment illustrated cathode 17 1819 is operated at the potentialof electrode 14 as is common in cathode ray-type tubes. However, if electrical'isolatiou is desired, wires 22 may be made of drawn quartz strings or other insulating material having a low heat conductivity and a suitable amount of resilientfiexibility, Alternatively, either ring 23- or a portionof electrode 14 or both may be of insulating material, such as a suitable ceramic material. Also, the present cathode Support is equallysuitable for use in an electronv discharge device employing a control grid. In this application a conven-- tional support of the control grid replaces electrode 14 in. the structure illustrated. Furthermore, the cathode may be heated by electron bombardment in which case. the principles described in United States Patent 2,509,053, granted May 23, 1950, to 0 Caloick may be employed.

The plane shape of cathode, plate lSismerely shown by way of example. The form thereof is susceptible to variation as shown in Fig. 3, in which a plate 26.having apertures 28 isprovidedwtih a. raised portion in the cen-.. ter thereof which carries the electron emissive coating 27.

In Fig 4 an alternative embodiment of the invention is shown as a modification of the structure of Fig. 1 Withcorresponding reference numerals. employed to desige nate similar components. Modification will; be seen to reside in the cross-sectional shape. of cathode plate 29 which is now provided wtih a tongue-type eutensiongl) from its periphery. A perspective detail'of plate-29 is each composed of several .turns. of fine -.-.wire-0f--materialsimilar to the wires 22 of Fig. 1 are disposed at spaced locations around the circumference of plate 29. Thus members 31 bear upon the inside surface of cavity 24 formed by the recess between tongue 30, plate 29 and the corner between clamp 2 3 -,-and;electrode 14. A similar plurality of helical members 32 are placed on the other side of tongue..30. withinthewcavity 33- formed by the cessinla nu i. e-fi lsandrp ta 9- H i al members 31 and L er ea sim lae uns anl.w hs mflaradvantages, to wire elements 22 of,Fig..l.'

In all cases, itis understood that the above described arrangementsare simply illustrative of the small number of the many possible;specifie embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can readily be devised in accordance with these principles by. those. skilled inlthe art withoutdeparting from the spirit andscopeof theinvent'ion;

What is claimeddst 1 11A cathodestructure for electron dischargejdeyices comprisinga flattened cathode member having anelec: tron emiss'i-v'esurface-extending-in a transverse direc' tion at-least two-rigid-supporf members with means}. for-constraining said members relative to, one another in} a longitudinal direction, said m'embers fo'rming .acavityj to loosely-receive said cathode around the peripheral edge, of said-cathode, anda plurality of thin wire, element's interposed longi-mdinallyand transversely betweensaid support members and said 'cathode, saidelements' beaningat leas't in; a-longitudinaldirection against the insideof said-cavity.

2; The-combination of claim- 1 wherein said cathode memberhas' a plurality of apertures extending through the thickness thereof, and wherein each ofl sai'd elementlsgl comprises a loop of low heat conductivity material passing through each of said aperturesg 3. The combinati'on of clainijl wherein each .of ,said elements comprises a helix of low he'at co'nd'uctivit'y ma terial a portion of saidf elementsbeing located wi'thin, 7 said cavity on one side of said cathode and the; remainder of said elements being" located within said} cavity on .the... other side-of said'cath ode'.

4. In an electron discharge device ofthetype' haying; a rigidelectrode withreg'ard to which .a flattened vtherrnic thvdei to PP td an-d. sp c :1 embe having. an annular recess deeperj than the. thickness. -of ai hod andwid'e n. e ne m n ennfisai da d memb r ssu o t o s sa d. electrode; with said cathode disposed .withinsaid, recess; said. atho e; vi .,-a' l l ty f ap r e -e amine, through the thickness thereof, anda wire-lilgeloop of low r. heat conductivity. material ,passingthrough each: of said; apertures to bear against said .electrodepn oner side of said" cathode and against, said; member on 'the othen-side -n f idca h de- 5. In. an. electron discharge .,,device, a fiattened dise;v of high heat conductivity material. havinga-coatingmf-g thermionic material a tubular shield of highgheat con-; ductivity. material having one end,.:coaxially: connected;:.-- to said. disc, a thermionic heating! element located-with. in said' shield to raise the temperature of said coatingltoi. the temperature of thermionic emission, rigid support-- members. forming auannular cavity to loosely receive said discj around. the peripheral edge of said-disc, said-disc having. a: plurality of,aperturesgextending through; the. thickness thereof. and spaced; around the-peripheral edge thereof, and athin wire; element of, low heat. conductivity. materialllooped around ,tl e,of said discz and-through; each ofg'said apertures; disc ying ;a -heated;diameter-. substantiallvequahto sof- SaldfCaV lCY-"ICSS se-fl edie e efi a d a v s naa ness, substantiall egu widthiofgsaidcavity-less ces es iensl a te e inherit.

6. In an electron discharge device, a flattened disc of high heat conductivity material having a coating of thermionic material extending in a transverse direction, a tubular shield of high heat conductivity material having one end coaxially connected to said disc, a thermionic heating element located within said shield to raise the temperature of said coating to the temperature of thermionic emission, said disc having a tongue-like extension extending out from its periphery, at least two rigid support members with means for constraining said members relative to one another in a longitudinal direction forming an annular cavity to loosely receive said tongue, and a plurality of helical members of low heat conductivity material located on both sides of said tongue, said disc excluding said tongue having a diameter substantially equal to the diameter of said cavity less twice the diameter of said helix, said tongue having a thickness substantially equal to the width of said recess less twice the diameter of said helix so that said helical members bear at least in a longitudinal direction against the inside of said annular cavity.

7. A cathode structure for electron discharge devices comprising a flattened cathode member having an electron emissive surface extending in a transverse direction, a plurality of thin wire elements each of said elements having transversely extending portions in contact with said cathode, and having one longitudinally extending portion spaced from said cathode, at least two rigid support members forming a cavity to loosely receive said cathode around the peripheral edge of said cathode, means for constraining said members relative to each other in a longitudinal direction with said members hearing upon each of said wire elements in a longitudinal direction and otherwise forcing said one longitudinally extending portion of each of said elements to substantially conform transversely to the inside of said cavity.

References Cited in the file of this patent UNITED STATES PATENTS 2,154,275 Linn Apr. 11, 1939 2,310,811 Schantl Feb, 9, 1943 2,413,689 Clark Jan. 7, 1947 2,421,767 Varian June 10, 1947

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