US2753480A - Indirectly heated cathode structure and method of assembly - Google Patents

Description

y 1956 w. K. BATZLE ETAL INDIRECTLY HEATED CATHODE STRUCTURE AND METHOD OF ASSEMBLY INVENTGRS WILLIHM K. ba'rzm WILLIHM E. DHLEE Filed Oct. 11, 1952 Uit,

INDIRECTLY HEATED (IATHODE STRUCTURE AND METHOD Oh ASSEMBLY Application October 11, 1952, Serial No. 314,355

7 Claims. (Cl. 313-340) This invention relates to indirectly heated cathodes having a metal sleeve coated with electronically emissive material and including a heater in the form of a resistance element within the sleeve, and more particularly to a cathode construction including an improved spacer between the heater and sleeve.

The heater employed in some types of cathodes comprises a wire made of tungsten, for example, folded over upon itself to provide a plurality of strands, and having an insulating coating to electrically insulate adjacent strands from each other as well as to insulate the heater as a whole from the metal sleeve of the cathode.

Several problems have manifested themselves in connection with indirectly heated cathodes of this type. In some applications the heater and sleeve are connected to potential sources of appreciable voltage difference. Such voltage difference may give rise to several disturbing effects. One of these effects is in the form of shorts between the heater and sleeve. Such shorts are likely to occur because the usual insulating coating on the heater is sometimes inadequate to prevent voltage breakdown between the heater and sleeve. Another disturbing effect of the appreciable voltage difference between the heater and sleeve occurs as a consequence of electrostatic fields produced between the heater and sleeve, and which are objectionable.

Another problem associated with the type of cathode referred to, is that of creepage of the heater. When the cathode is heated during operation and cooled during intervals between operations, the heater tends to creep" out of the sleeve thus reducing the volume of the heater within the sleeve and reducing heat transfer to the sleeve. This may seriously affect electron emission from the oathode. It also may result in a chipping of the coating from the heater.

Several attempts have been made heretofore to meet the aforementioned problems. One expedient has taken the form of a ceramic sleeve inserted between the heater and sleeve, to supplement the insulation between these elements, to thereby prevent shorts therebetween, and also to more tightly compact the heater to reduce creepage thereof. In an attempt to eliminate objectionable electrostatic fields between the heater and cathode sleeve, a metal sleeve was disposed therebetween. Both of these attempts to meet the problems referred to proved unsatisfactory.

In each case, the sleeve interposed between the heater and cathode sleeve seriously interfered with desired heat transfer from the former to the latter of these cathode elements. The more compact disposition of the heater that the sleeve inserts provided was inadequate to prevent creepage of the heater, presumably because of the inability of the relatively smooth inner walls of the inserts to restrain such creepage. In addition, the ceramic sleeve is relatively brittle and breaks easily on insertion into the cathode sleeve.

Furthermore, each of the aforementioned expedients was intended to solve one only of the problems referred States Patent to. Thus the ceramic sleeve was inherently incapable of affecting the undesirable electrostatic fields between the heater and cathode sleeve, and the metal sleeve inherently could not serve to increase the insulation between these cathode parts. Therefore, were these inserts each capable of fully solving one of the problems aforementioned, their utility would have been limited because both the ceramic and metal inserts, could not, as a practical matter be included in the same cathode. But even though so included, they would provide only a relatively minor restraint to heater creepage.

It is therefore an object of the invention to provide an improved cathode structure of the indirectly heated type that is free from the problems aforementioned.

Another purpose of the invention is to provide an indirectly heated cathode structure characterized by improved insulation between the heater and cathode sleeve, effective neutralization of electrostatic fields therebetween, and reduced creepage of the heater with respect to the oathode sleeve.

A further aim of the invention is to provide an improved spacer between the heater and sleeve of an indirectly heated cathode comprising a body of insulation and an electrical conductor for increasing the insulation between the heater and sleeve and for screening the sleeve from electrostatic charges formed therein.

Another object is to provide a helical spacer between the heater and sleeve of an indirectly heated cathode for engaging and deforming spaced portions of the heater structure to prevent creepage of the heater from the sleeve.

A further purpose is to provide an improved spacer member between the heater and sleeve of a cathode, having reduced effect on the heat transfer from the heater to the sleeve and yet characterized by the: triple advantage of augmenting the heater to sleeve insulation, shielding the sleeve from undesired electrostatic effects, and restraining the heater from creepage.

One embodiment of the invention may comprise an indirectly heated cathode having a helical spacer member interposed between the heater and sleeve of the cathode. The helical spacer member may comprise a wire coated with insulating material.

The resultant cathode structure provides increased insulation between the heater and sleeve thereof. The increase in insulation results from the addition to the single layer of insulation on the heater, of the double layer of insulation on the opposite sides of the spacer member engaging the heater and sleeve. Assuming the insulation on the spacer member is as thick as that on the heater, a three-fold increase in insulation between the heater and sleeve is realized by the structure of the invention. This increased insulation permits the cathode to be operated under conditions where a relatively high voltage difference exists between the heater and sleeve with reduced danger of shorts between these elements.

The cathode structure referred to also .isolates the sleeve from electrostatic forces normally produced between the sleeve and heater as result of a voltage difference therebetween. This isolation is best accomplished when the metal wire core of the spacer member is electrically connected to ground or some other suitable potential close to the potential of the cathode sleeve. In this situation electrostatic charges between the heater and sleeveare neu tralized by conduction through the wire or metal core of the spacer member. The harmful e lfects on cathode emission heretofore produced by the electrostatic fields referred to are therefore prevented by the structure of the invention.

In addition to the foregoing desirable results, the cathode structure described also reduces creepage of the heater from the sleeve. Such creepage is pronounced in cathodes having the folded type of heater wherein aplurality of strands extend parallel to the sleeve axis. Expansions of the strands during heating cause them to elongate and protrude from one or both ends of the cathode sleevev The relatively smooth inner wall of the sleeve provides. inadequate restraint to such elongation or creepage. However, in a cathode structure utilizingthe novel spacer member of the invention, the heater is engaged with appreciable force at spaced portions by the the heater out of parallel relation to thesleeve 21m 7 This deformation is caused by the spaced engagements between the turns of the spacer member and the heater. The

helical structure of the spacer member disposes opposite sides of the turns thereof out of registry normal to its axis. Therefore, the side of the heater structure engaged in a given plane by a portion of a turn of the spacer memher, is opposite an unengaged side thereof in said plane. The force exerted by said portion of the turn is consequently opposed solely by the heater structure which is free to bend in response thereto. The forced engagement between the plurality of turns of the spacer member and the heater structure, therefore, provide a plurality of bends in said structure. Such bends in the structure effectively prevent axial movement thereof in relation to the spacer member, and creepage of the heater is fully restrained.

. The important advantages aforementioned of the novel cathode structure of the invention are accomplished without objectionable interference with desired heat transfer from the heater to the sleeve. This is because the helical structure of the spacer member exposes the cathode sleeve to a relatively large portion of the heater area, the spaced turns of the member occupying a very small portion of the space between the heater and sleeve.

Referring now to the drawing for a more detailed consideration of the invention,

Figure 1 shows an elevation partly in section of a fragment of an electron tube mount, including a cathode structure embodying the invention;

Figure 2 is a sideview partly in section of a portion of the insulatingly coated spacer member of the invention; and

Figure 3 is an enlarged fragmentary view in cross-section of a portion of the structure shown inFigure l.

. For purposes of illustration only, and not by way of limitation, there is shown in Figure l a cathode structure in which the invention is embodied. The cathode struc- .tions14, 15, which are fixed as by welding to lead-ins 16,

17 extending through glass stem 18 and connected to a suitable potential source. The heater structure 12 comprises a wire 19, made for example of a refractory metal such as. tungsten, molybdenum or tantalum, and having a coating 20 thereon which maycomprise aluminum oxide.

The coated wire 19 is folded back upon itself severaltimes ,to provide the strands referred to.

It is conventional practice to relate the thickness of the .heater wire or core 19 to the magnitude of the voltage .intended to energize the'same. heater usually has acore 19 of six mils diameter while a 12-:voltheater usually has a core diameter of three mils.

For example, a six volt cathode sleeve.

The thickness of the insulating coating 20 is usually very nearly the same in each case. Thus the coating thickness in the six volt heater may be from three to four mils, while in the 12 volt heater it may be from three and onehalf to four mils. The minimum thickness of the coatings is therefore slightly larger in the case of the 12 volt heater than for the six volt heater.

.But while the difference in the voltage referred to would require an appreciably thicker coating than indicated, for the 12 volt heater, to provide the same degree of, insulation between the heater and sleeve as the coating on the six volt heater, a thicker coating is objectionable because it interferes with heat transfer to the cathode sleeve 1%, and therefore prolongs the time required to heat the sleeve to emission temperature. Therefore, the thickness of the coating on the 12 volt heater is sometimes inadequate to prevent shorts between the heater structure and the cathode sleeve.

in addition to incidents of shorts as aforementioned indirectly heated cathode structures of the prior art have also been characterized by objectionable electrostatic fields between the heater and sleeve. For example, when the sleeve is grounded and the heater is energized by a 12 volt potential source, a relatively strong electrostatic field is set up between the heater and sleeve. This field affects the potential on the sleeve and causes erratic electron dispersions from the, emitting surface of the sleeve. This field becomes stronger with reduced spacin between the heater and sleeve, resulting, for

example, from a relatively thin insulating coating on the heater.

Prior art indirectly heated cathodes having heater strands disposed parallel to the sleeve axis, have also suifered from movements of the heater Within the sleeve, in response to expansions of the heater and sleeve on heating. Due to the disposition aforementioned of the heater, a relatively large portion of the components of the expansive movements of the, heater are parallel to the axis of the cathode sleeve 10, and manifest themselves in an elongation of the heater structure 12. Such elongation causes one or both end portions of the heater structure to protrude beyond the ends of the sleeve 10. It is found that a subsequent cooling of the heater structure fails to restore the heater structure to its original position in the One possible explanation for this may involve the fact that the protruding portions of the heater structure expand laterally on release of the compressive end edges of the sleeve.

The relative movements referred to of the heater and sleeve may also be caused by axial expansions of the sleeve. If the sleeve is made of nickel, it has a greater coefficient of expansion than the refractory heater wire. Such expansions of the sleeve will also carry the heater with them, as a result of the electrostatic force of attraction that holds the heater forcefully against the inner wall of the sleeve. The protruding portions of the heater structure resulting from such expansions waste heat at the expense of the sleeve 10.

On embodiment of the invention that frees an indirectly heatedcathode structure from the difficulties afore" mentioned, includes a spacer member 21 comprising a helical wire 22, having a coating 23 of insulating material, as shown inFigure 2. The spacer member 21 is interposed in a snug-fit between the sleeve 10 and heater structure 12, after heating as shown in Figure 1.

The turns of the spacer member referred to should be spaced a distance at least the thickness of the coated Wire constituting-the member Thislimitation on spacing is imposed in the interests of good. heat transfer from the heater to the sleeve 10. A closer spacing of the turns than indicated, would interfere to an objectionable degree with said heat transfer. Going to the other extreme, the maximum spacing between the turns of the spacer member should be such as to provide at least two complete turns for a reason that will become apparent in the following. In Figure 1 is shown a spacer member having four and one-half turns.

The core or wire 22 of the spacer member 21 may be made of a relatively high melting point metal such as tungsten, molybdenum or tantalum and the coating 23 thereon may comprise aluminum oxide. The wire 22 may have a diameter of five mils and the coating 23 may have a thickness of about two and one-half mils.

One advantage of the spacer member referred to resides in increased insulation between the heater structure 12 and the sleeve iii. As shown in Figure 3, the insulation Zii on the heater wire 19 is supplemented by a double thickness of the insulation 23 on the spacer core 22. This double thickness comprises one thickness of coating 23 between the core 22 and the heater coating 20, and another thickness of coating 23 between the core 22 and the sleeve it]. Since as indicated before herein, the heater insulation 20 is from three and one-half to four mils thick and the spacer insulation 23 is about two and one-half mils thick, the total insulation between the heater wire 19 and the sleeve is about eight and onehalf mils thick, which is more than double the thickness of the heater insulation 20. This combined insulation is fully adequate to prevent voltage breakdown between the heater wire 19 and the sleeve 10, at a relatively high voltage difference between the heater and sleeve.

The spacer member 21 terminates in a bare portion 24 which may be connected through lead-in 25 to a suitable potential source such as ground for example. The cathode sleeve 10 may also be connected to lead-in 25 by a connector 26. When so connected, the spacer member effectively neutralizes any electrostatic charges between the heater wire 19 and the sleeve 10. The heater and sleeve referred to may therefore be operated at an appreciable voltage difference without causing objeo tionable effects such as fluctuation, in the emission from the cathode, or serious cathode-heater leakage.

The helical configuration of the spacer member 21 disposes only one portion of a turn thereof in a given plane transversely of the sleeve it). The portion of the heater structure 12 in said plane is therefore engaged on one side thereof only, by the spacer member. The heater structure and spacer member are preferably so proportioned as to provide a tight fit therebetween when mounted in a cathode. This causes the spacer member to engage the heater with appreciable force. Since this force is applied at each turn of the spacer member to one side only of the heater structure, as aforementioned, and since the opposite side of the heater in the plane of the heaterspacer engagement in unsupported, the heater structure responds to said force in a deformation in said plane and outwardly from said opposite side thereof. The spacer member engages the heater structure in this manner in a number of parallel planes normal to the heater structure. Therefore, a plurality of such deformations in the heater structure are produced. Such deformations impede axial elongation of the heater structure. But even though some elongation should occur, it would tension the coiled spacer member 21, and thus produce a force urging the heater structure back to its original position. Furthermore, the spacer member spaces the heater structure transversely from the end edges of the sleeve 10, so that said edges provide no restraint to a contractive movement of the heater structure on cooling. The usual progressive enlargements of the protruding portions of the heater structure involved in heater creepage of the prior art, are therefore effectively prevented by the cathode structure of the invention.

In untensioned state and prior to insertion in the sleeve, the coil form of the spacer member 21, preferably has a diameter for easy entrance into the sleeve 10. Suitable means, such as the fixing of the member to lead-in 25 or a dielectric cement 27, may be used for supporting the spacer member in the sleeve. A daub of similar cement 28 may be used to lock the spacer member to the heater structure.

In assembling the cathode structure of the invention, the spacer member 21, after some lateral compression, as aforementioned, is inserted into the sleeve 10. Thereafter, the heater structure 12 including the strands 13, may be forcefully threaded into the space defined by the spacer member. Alternately, the heater may first be snugly inserted into the spacer member, and the latter may then be compressed laterally, not only to enable it to enter the sleeve 10, but also to firmly engage the heater structure 12. This produces a firmer engagement between the spacer member and heater structure than the first described assembly method.

It will be apparent from the foregoing that there is provided according to the invention an improved cathode structure having the advantages of increased freedom from heater-to-sleeve shorts, effective neutralization of objectionable electrostatic fields between the heater and sleeve, and reduced creepage of the heater in the heater. These advantages are obtained without sacrificing desired heat transfer from the heater to the sleeve of an indirectly heated cathode. The cathode structure of the invention therefore contributes to improved operation of an electron tube in which it is used.

We claim:

1.,An indirectly heated cathode comprising a sleeve having a thermionically emissive coating thereon, a heater having a coating of insulating material thereon within said sleeve, said sleeve and said heater having facing surfaces for heat transfer, and an electrically conducting member having a coating of insulating material on all sides thereof between said sleeve and heater, said coatings of insulating material on said heater and said conducting member constituting the only insulating material between said heater and said sleeve, whereby said insulating material on said conducting member is adapted to contribute to insulation between said heater and sleeve and to isolate said sleeve from objectionable effects from said heater, said conducting member extending between a relatively small portion only, of said facing surfaces for preserving said heat transfer.

2. An indirectly heated cathode comprising a sleeve having a thermionically emissive coating thereon and inner opposite walls, a heater within said sleeve, and a spacer member under outwardly directed lateral stress between said heater and sleeve, whereby said member forcefully engages said inner opposite walls of said sleeve and is restrained from movement with respect thereto, said member engaging said heater with appreciable force at one side only of portions thereof spaced axially of said sleeve, the opposite side of said portions being unsupported, the engaged sides of adjacent portions being :angularly spaced, whereby said appreciable force causes deformation of said heater outwardly from said opposite side of said portions and said deformation impedes axial relative move ment between said member and said heater.

3. A cathode comprising a metal sleeve having a thermionically emissive material on its outer surface, a plurality of connected elongated parallel strands of insulated resistance wire forming a heater structure and disposed within and generally in axial parallel relation with said sleeve, said heater structure having a spiral depression in the sides thereof and extending from one end of the structure to the other, and an elongated spacer member under outward lateral stress between said heater structure and said sleeve, and engaging said member and said depression in said structure, whereby said structure is restrained from axial movement with respect to said spacer member, said outward lateral stress of said spacer member providing frictional restraint to relative movement between said member and said sleeve.

4. An indirectly heated cathode comprising a metal sleeve, a heater within said sleeve, and a spacer member between-said sleeveand heater and surrounding saidheater,

said spacerlmembertcomprising a helical structure made of a refractory metal and having thereon a coating of insulatingtmaterial, said coating being disposed between said refractory metal and .said heater-and between said refractory metal and said sleeve, whereby said refractory metal is insulated from said heater and sleeve, said heater having a coating thereon ofinsulating material, whereby said insulating material on saidrspacer member provides insulation between .said heater and sleeve, additional to that of the. insulating material on said heater.

5. An indirectly heated cathode according to claim 4, and. wherein adjacent turns of said helical structure are spaced a distance atleast as great as the thickness of the material of said structure fordesired heat transfer from said heater to said sleeve.

6. Anindirectlyheated cathode according to claim 4, andwherein saidchelical-structure includes at least two complete turns tsaidttwo turns comprising turn portions spaced ,axiallyof said structure, each of said turn portions engaging aside portion of the inner wall or" said sleeve, said inner wall having airee wall portion opposite said side portion, said heater havingla larger transverse extent than thespace; defined by saidhelical structure, whereby a portion of .tsaidlheaterr between said two turns isdeformed to extend towardsaid free :wall portion, and relative movement between said heater and helical structure is restrained, ,said. helical structure being stressed outwardly and frictionally engaging said inner wall of t the sleeve, whereby relative movement between said sleeve and helical structure is restrained.

taneously laterally displacing portions of said heater between adjacent turns of said helical spacer and contracting said spacer radially to a diameter smaller than said predetermined diameter, and inserting said helical spacer .member with said heater into said sleeve, whereby said member expands to a magnitude to firmly engage said sleeve when heated while preserving a portion of the 1 lateral displacement of said heater portions.

ReferencesCited in the file of this patent UNITED STATES PATENTS 1,599,180 McIlvaine Sept. 7, 1926 1,870,968 a Sinden Aug. 9,1932 2,158,665 ONeill May 16, 1939 2,164,913 Goodchild July 4, 1939 2,227,046 1 Waldschmidt Dec. 31, 1940 2,436,907 :Trimble Mar. 2,1948

Claims (1)

1. AN INDIRECTLY HEATED CATHODE COMPRISING A SLEEVE HAVING A THERMIONICALLY EMISSIVE COATING THEREON, A HEATER HAVING A COATING OF INSULATING MATERIAL THEREON WITHIN SAID SLEEVE, SAID SLEEVE AND SAID HEATER HAVING FACING SURFACES FOR HEAT TRANSFER, AND AN ELECTRICALLY CONDUCTING MEMBER HAVING A COATING OF INSULATING MATERIAL ON ALL SIDES THEREOF BETWEEN SAID SLEEVE AND HEATER, SAID COATINGS OF INSULATING MATERIAL ON SAID HEATER AND SAID CONDUCTING MEMBER CONSTITUTING THE ONLY INSULATING MATERIAL BETWEEN SAID HEATER AND SAID SLEEVE, WHEREBY SAID INSULATINT MATERIAL ON SAID CONDUCTING MEMBER IS ADAPTED TO CONTRIBUTE TO INSULATION BETWEEN SAID HEATER AND SLEEVE AND TO ISOLATE SAID SLEEVE FROM OBJECTIONABLE EFFECTS FROM SAID HEATER, SAID CONDUCTING MEMBER EXTENDING BETWEEN A RELATIVELY SMALL PORTION ONLY, OF SAID FACING SURFACES FOR PRESERVING SAID HEAT TRANSFER.

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