Title:
CATHODE STRUCTURE FOR CATHODE RAY TUBE
United States Patent 3914638
Abstract:
An improved indirectly heated cathode structure is provided for use in an electron gun of a cathode ray tube. A cathode sleeve having top and bottom open ends has a terminal ledge formed at the open top thereof. A planar closure member, having a perimetric shape similar to that of the sleeve, is seated on and affixed to the ledge thereby providing a flat substrate upon which electron emissive material is subsequently disposed.
US Patent References:
Electron discharge device
Clark et al. - January 1947 - 2413689
Cathode structure
Beggs - July 1948 - 2445993
Electrode assembly for an electron discharge device made from a material having a low carbon content
Feinleib et al. - December 1966 - 3293476
Electron discharge device
Clark et al. - January 1947 - 2413689
Cathode structure
Beggs - July 1948 - 2445993
Electrode assembly for an electron discharge device made from a material having a low carbon content
Feinleib et al. - December 1966 - 3293476
Inventors:
Collins, Floyd K. (Seneca Falls, NY)
Miller, John J. (Seneca Falls, NY)
Miller, John J. (Seneca Falls, NY)
Application Number:
05/393970
Publication Date:
10/21/1975
Filing Date:
09/04/1973
View Patent Images:
Export Citation:
Assignee:
GTE Sylvania Incorporated (Stamford, CT)
Primary Class:
Other Classes:
313/337
International Classes:
H01J29/04; H01J19/48; H01J1/94
Field of Search:
313/270,337
Primary Examiner:
Lawrence, James W.
Assistant Examiner:
Chatmon Jr., Saxfield
Attorney, Agent or Firm:
O'malley, Norman Krenzer Cyril Rinn Frederick J. A. H.
Parent Case Data:
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation application of parent application Ser. No. 256,431, filed May 24, 1972, which is now abandoned and incorporates by reference the disclosure set forth in that application which is assigned to the assignee of the present application.
Claims:
What is claimed is
1. An improved two-piece indirectly heated end-emissive cathode structure having a jointure region of minimum mass for use in a cathode ray tube, said cathode structure comprising:
2. An improved cathode structure according to claim 1 wherein said planar disc-shaped closure member is affixed directly to said narrow terminal ledge by substantially continuous perimetric bonding therearound.
3. An improved cathode structure according to claim 1 wherein said planar disc-shaped closure member is affixed directly to said narrow terminal ledge at a plurality of at least three spaced-apart discrete areas of peripherally related bonding means.
4. In a cathode ray tube electron gun, an improved two-piece indirectly heated end-emissive cathode assembly having a jointure region of minimum mass, the emission area of said assembly being oriented in spaced relationship to a control grid electrode having an aperture therein smaller than the diameter of said cathode, said cathode assembly comprising:
1. An improved two-piece indirectly heated end-emissive cathode structure having a jointure region of minimum mass for use in a cathode ray tube, said cathode structure comprising:
2. An improved cathode structure according to claim 1 wherein said planar disc-shaped closure member is affixed directly to said narrow terminal ledge by substantially continuous perimetric bonding therearound.
3. An improved cathode structure according to claim 1 wherein said planar disc-shaped closure member is affixed directly to said narrow terminal ledge at a plurality of at least three spaced-apart discrete areas of peripherally related bonding means.
4. In a cathode ray tube electron gun, an improved two-piece indirectly heated end-emissive cathode assembly having a jointure region of minimum mass, the emission area of said assembly being oriented in spaced relationship to a control grid electrode having an aperture therein smaller than the diameter of said cathode, said cathode assembly comprising:
Description:
BACKGROUND OF THE INVENTION
This invention relates to cathode ray tubes and more particularly to improved indirectly heated cathodes for use in the electron gun structures thereof.
Cathode ray tubes conventionally employ one or more electron guns having indirectly heated thermionic cathode structures therein. Such cathodes are usually end-emissive structures utilizing heat responsive electron emissive materials as sources of electron beam energy. A common type of cathode assembly is in the form of a metallic sleeve whereof one end is closed by a metallic cap on which an area of electron emitting material is exteriorly disposed. Heat is supplied to the cathode by a discretely formed heater positioned within the sleeve in a manner to promote efficient use of the heat generated.
Since the cap is fitted over the end of the sleeve, the internal dimension of the cap is of necessity greater than the exterior dimension of the sleeve to facilitate a sliding fit relationship. Manufacturing tolerances respectively inherent in the fabrication of caps and sleeves further aggravate the dimensional deviation therebetween. To affix the cap to the sleeve, it is conventional practice to dispose several spaced apart pressure welds between the skirt of the cap and the sleeve. In so doing, portions of the skirt are substantially forced in against the wall of the sleeve, a condition that slightly deforms areas of the skirt thereby aggravating and fostering the establishment of additional residual stresses in the formed cap; stresses which sometimes tend to promote a slight doming effect of the emissive surface. This tendency for doming or "oil-canning" of the end surface of the cap is an undesirable condition as it disturbs the uniformity of the emitted electron beam.
The overlapping areal contact between the skirt of the cap and the end portion of the sleeve makes up a conductive heat sink which aggravatively influences the warm-up efficiency of the cathode structure. This mass of material contiguous to the end-emissive area tends to slow the rise in temperature of the emissive area, thereby hindering rapid operational heating of the cathode.
In reduced-size electron guns, in particular, such as utilized in commercial low power applications, as for example, the 12.6 volt to 60 to 80 milliampere heater types, the diameter of the aperture in the control grid electrode approaches the diameter of the end-emissive surface of the cathode cap. For instance, a control grid aperture in a low power gun may have a diameter in the order of 0.035 of an inch, while the diameter of the cap of the cathode spatially associated therewith may be of a value in the neighborhood of 0.042 of an inch. Thus, the closeness of dimensions makes cathode-aperture alignment a critical factor. The acuteness of this alignment is further aggravated by the radius of curvature on the rolled periphery of the cap transitional to the end surface and the skirt portions thereof, which actually effects a reduction in the flat surface of the cap. This diametric reduction in conjunction with the usual feathered periphery of the emissive material disposed thereon creates an area of electron emission that closely approximates the area of the control grid aperture. Therefore, a slight misalignment of the cathode, with reference to the grid electrode aperture, produces a non-cylindrical electron beam with resultant distorted spot quality on the screen of the tube.
In view of the foregoing, it is evident that the telescoping cap-sleeve cathode structure presents several disadvantages in certain electron gun applications.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to reduce the aforementioned difficulties and to provide an improved indirectly heated end-emissive cathode structure for use in a cathode ray tube electron gun.
Another object is to provide an improved rapid-heating indirectly heated end-emissive cathode structure for use in a cathode ray tube electron gun wherein there is limited contact between the end-emissive portion and the contiguous supporting sleeve structure.
A further object is to provide a cathode ray tube indirectly heated end-emissive cathode having an improved planar emitting surface.
The foregoing objects are achieved in one aspect of the invention by the provision of an improved indirectly heated thermionic cathode structure, as utilized in a cathode ray tube electron gun, wherein a cathode sleeve of substantially passive cathode material is formed to have top and bottom open ends. At the top open end thereof, a terminal ledge is extremitally formed in a manner substantially normal to the sidewall of the sleeve. A planar closure member of a substantially active cathode material, having a perimetric shape similar to the cross-sectional shape of the sleeve, is seated on and affixed to the terminal ledge on the sleeve. There is thus provided a flat substrate upon which electron emissive material is subsequently disposed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an enlarged prior art cross-sectional view of the control grid portion of a cathode ray tube electron gun showing an indirectly heated cathode assembly positioned therein;
FIGS. 2 and 3 are enlarged sectional views illustrating two embodiments of the improved cathode structure of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following specification and appended claims in connection with the aforedescribed drawings.
With reference to the drawings, FIG. 1 is a prior art view illustrating an indirectly heated cathode assembly 11 positioned within the control grid electrode 13 of an electron gun of the type conventionally employed, either singly or plurally, in cathode ray tubes. The cathode assembly comprises a cathode sleeve 15 having a sidewall portion 17 with open top and bottom ends 19 and 21 respectively. Usually the cathode sleeve is fabricated of a passive cathode material, i.e., a metal or alloy that does not readily sublime or emit electrons upon heating. The top open end 19 of the sleeve is closed by a cathode cap 23 affixed thereto and made of an active cathode material, i.e., one that promotes the emission of electrons when heated. The exterior end surface of the cap 23 has disposed thereon a coating of an electron emissive material 25, such as a combination of carbonates of barium and strontium, and sometimes calcium.
Positioned within the sleeve 15 is a formed insulatively coated heater 27 which provides the operational temperature level necessary to produce electron emission from the emissive material 25. The cathode assembly 11 is suitably supported as, for example, by cathode shielding means 29 which is formed for attachment to the lower portion of the sleeve by several spaced apart welds 31. The control grid electrode 13 and the cathode assembly support means 29 are affixed to a plurality of electron gun support beads, not shown, by attachment means 33 and 35 respectively. Thus, the cathode is mounted in a manner that the electron emissive material 25 is in spaced relationship with the aperture 37 provided in the end wall 39 of the control grid electrode 13. During the operation of the cathode ray tube, the electrons emitted by the emissive material 25 pass through the grid aperture 37 as an electron beam and thence sequentially traverse the associated electron gun electrodes, not shown, to be directed to the screen of the tube.
As noted in FIG. 1, the skirt 22 of the cathode cap 23 telescopes over the top end of the sleeve 15 making extensive contact with the upper sidewall portion of the sleeve thereby effecting a comprehensive heat sink relationship. As the heater 27 warms up, during the initiation of tube operation, the heat thereof radiates to both the sleeve 15 and the mass of the skirted cap 23 thereby retarding the temperature rise of the emissive portion and lengthening the initial time-lag of tube activation.
Attachment of the cap to the sleeve is normally accomplished by a plurality of bondings, such as by welds 41. Such bonding is usually effected by pressuring the skirt 22 of the cap against the sidewall 17 of the sleeve. This procedure sometimes causes a slight doming of the flat end surface 24, as exaggeratively shown in FIG. 1, or may set up residual stresses in the cap which tends to produce doming or "oil-canning" of the end region during operational heating. Whatever the cause, any doming of the emissive surface relative to the adjacent electrode aperture 37 is detrimental to the uniformity of the subsequential electron beam and thus affects the desired operational efficiency of the tube.
There are times when the cap welding procedure produces a detrimental projection or "spike" 43 that may protrude in varying degrees from the interior surface of the sleeve 15 into the area wherein the heater 27 is oriented. During the operational life of the tube, the heater moves by temperature induced expansion and contraction and additionally may be shifted by environmental shock. Under these influences the heater is apt to come in contact with the more abrasive of such weld projections, whereupon a deleterious eroding of the heater insulation is likely to occur, with the possibility of a catastrophic heater-to-cathode short circuit developing.
As aforementioned, uniformity of the emissive area 25 relative to the size of the electrode aperture 37 are important operational considerations. As shown, the cap 23 has an external diametrical dimension "a" which is greater than the external sleeve diameter "b. " Due to the rolled periphery 45 of the cap, and the feathered periphery 47 of the emissive coating 25, the substantially uniform emission area evidences a lateral dimension "c" which substantially approximates the diameter "d" of the aperture 37. The close relationship of these dimensions, which are particularly common in low-power applications, emphasizes the criticality of alignment of the cathode and electrode aperture in such instances.
The foregoing disadvantages are overcome or minimized by the invention as shown in the separate embodiments delineated in FIGS. 2 and 3. For purposes of description, the cathode sleeves of the invention are denoted as being cylindrical, but such is not to be considered limiting as the cross-sectional shaping thereof may also be substantially elliptical or rectangular.
With particular reference to FIG. 2, the indirectly heated end-emissive cathode structure 49 is oriented within the control grid 13 in a manner similar to that described for the prior art, but the upper portion of the cathode structure 49, per se, is markedly different. the substantially cylindrical cathode sleeve 51 is formed of substantially passive cathode material having open top and bottom ends, of which the top end 53 is shown. Dimensionally, the sleeve has inner and outer diameters designated as "e" amd "f" respectively. A terminal ledge 55 is extremitally formed to be circumferentially outstanding from the top end of the sleeve; such ledge 55 being formed in a manner substantially normal to the exterior surface of the sleeve 51. The diametrical span of the periphery of the outstanding ledge is referenced as "g." A substantially disc-shaped planar closure member 57, formed of a substantially active cathode material, as for example, known nickel alloys, is of a perimetric shape similar to that of the sleeve. As shown, the closure approaches the diametrical dimension g of the terminal ledge. This planar closure member is seated on and affixed to the outstanding ledge 55 in one of two manners; either by a plurality of at least three spaced bonds or welds 58, or by a substantially continuous perimetric bonding therearound. The peripheral bonding between the ledge 55 and the peripheral portion of the closure member 57 is consummated in a substantially flat surface-to-surface compression relationship wherein longitudinally directed bonding pressure fosters maintenance of the flatness of the planar member. There are no stress vectors set up of the type evidenced in the prior art cap-to-sleeve attachment. The closure member provides a flat substrate having a planar area upon which a uniform electron emissive coating 59 is disposed. In this embodiment the substrate area is greater than the cross-sectional area of the sleeve, i.e., diametrically g exceeds f. The cathode coating 59, while exhibiting a feathered periphery 61, has a uniform area of the diameter "h" which exceeds the diameter "i" of the elctrode aperture 63. Thus, the cathode structure 49 of the invention provides for greater efficiency in alignment. The required operational temperature is provided by the insulatively coated heater 27 positioned within the cathode structure as shown.
A second embodiment of the invention is referenced in FIG. 3, wherein the indirectly heated end-emissive cathode structure 65 is positioned within the control electrode 13. The upper portion of the cathode structure of this embodiment 65 differs from the described first embodiment 49, in that the top of the cathode 67 is extremitally formed to have an instanding terminal ledge 69 circumferentially thereabout. Such ledge being formed in a manner substantially normal to the interior surface of the sleeve 67. A substantially disc-shaped planar closure member 71 having a diametrical dimension "m" which does not exceed the external cross-sectional dimension f of the sleeve, is seated on and affixed to the instanding ledge 69. This affixation is in the form of at least three spaced apart bonds or welds 73 or by the application of a continuous perimetric bonding. Whichever procedure is utilized, the union is effected by a substantially even surface-to-surface compression relationship wherein the jointure pressure maintains flatness of the planar closure member 71. The flat substrate thus provided has disposed thereon a uniform area of an electron emissive coating 75 having a diameter "o" which exceeds the diamter "p" of the adjacent electrode aperture 77.
There are thus provided embodiments of an invention wherein there is limited contact between the end-emissive closure and the contiguous supporting sleeve. Since a minimum mass of layered material is involved in the jointure between the closure member and the sleeve, the heat sink aspects of that region are also minimized and the rapid heating characteristics enhanced. The flat closure substrate of the invention affords more usable cathode area for a given closure diameter than normally available on a conventional cap structure. The flat closure disc does not require forming, consequently there is a minimum of residual forming stress of the type encountered in conventional skirted-cap fabrication. Additionally, the area of bonding in the improved cathode structure is in a peripheral region remote from possible heater abrasion.
While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.
This invention relates to cathode ray tubes and more particularly to improved indirectly heated cathodes for use in the electron gun structures thereof.
Cathode ray tubes conventionally employ one or more electron guns having indirectly heated thermionic cathode structures therein. Such cathodes are usually end-emissive structures utilizing heat responsive electron emissive materials as sources of electron beam energy. A common type of cathode assembly is in the form of a metallic sleeve whereof one end is closed by a metallic cap on which an area of electron emitting material is exteriorly disposed. Heat is supplied to the cathode by a discretely formed heater positioned within the sleeve in a manner to promote efficient use of the heat generated.
Since the cap is fitted over the end of the sleeve, the internal dimension of the cap is of necessity greater than the exterior dimension of the sleeve to facilitate a sliding fit relationship. Manufacturing tolerances respectively inherent in the fabrication of caps and sleeves further aggravate the dimensional deviation therebetween. To affix the cap to the sleeve, it is conventional practice to dispose several spaced apart pressure welds between the skirt of the cap and the sleeve. In so doing, portions of the skirt are substantially forced in against the wall of the sleeve, a condition that slightly deforms areas of the skirt thereby aggravating and fostering the establishment of additional residual stresses in the formed cap; stresses which sometimes tend to promote a slight doming effect of the emissive surface. This tendency for doming or "oil-canning" of the end surface of the cap is an undesirable condition as it disturbs the uniformity of the emitted electron beam.
The overlapping areal contact between the skirt of the cap and the end portion of the sleeve makes up a conductive heat sink which aggravatively influences the warm-up efficiency of the cathode structure. This mass of material contiguous to the end-emissive area tends to slow the rise in temperature of the emissive area, thereby hindering rapid operational heating of the cathode.
In reduced-size electron guns, in particular, such as utilized in commercial low power applications, as for example, the 12.6 volt to 60 to 80 milliampere heater types, the diameter of the aperture in the control grid electrode approaches the diameter of the end-emissive surface of the cathode cap. For instance, a control grid aperture in a low power gun may have a diameter in the order of 0.035 of an inch, while the diameter of the cap of the cathode spatially associated therewith may be of a value in the neighborhood of 0.042 of an inch. Thus, the closeness of dimensions makes cathode-aperture alignment a critical factor. The acuteness of this alignment is further aggravated by the radius of curvature on the rolled periphery of the cap transitional to the end surface and the skirt portions thereof, which actually effects a reduction in the flat surface of the cap. This diametric reduction in conjunction with the usual feathered periphery of the emissive material disposed thereon creates an area of electron emission that closely approximates the area of the control grid aperture. Therefore, a slight misalignment of the cathode, with reference to the grid electrode aperture, produces a non-cylindrical electron beam with resultant distorted spot quality on the screen of the tube.
In view of the foregoing, it is evident that the telescoping cap-sleeve cathode structure presents several disadvantages in certain electron gun applications.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the invention to reduce the aforementioned difficulties and to provide an improved indirectly heated end-emissive cathode structure for use in a cathode ray tube electron gun.
Another object is to provide an improved rapid-heating indirectly heated end-emissive cathode structure for use in a cathode ray tube electron gun wherein there is limited contact between the end-emissive portion and the contiguous supporting sleeve structure.
A further object is to provide a cathode ray tube indirectly heated end-emissive cathode having an improved planar emitting surface.
The foregoing objects are achieved in one aspect of the invention by the provision of an improved indirectly heated thermionic cathode structure, as utilized in a cathode ray tube electron gun, wherein a cathode sleeve of substantially passive cathode material is formed to have top and bottom open ends. At the top open end thereof, a terminal ledge is extremitally formed in a manner substantially normal to the sidewall of the sleeve. A planar closure member of a substantially active cathode material, having a perimetric shape similar to the cross-sectional shape of the sleeve, is seated on and affixed to the terminal ledge on the sleeve. There is thus provided a flat substrate upon which electron emissive material is subsequently disposed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an enlarged prior art cross-sectional view of the control grid portion of a cathode ray tube electron gun showing an indirectly heated cathode assembly positioned therein;
FIGS. 2 and 3 are enlarged sectional views illustrating two embodiments of the improved cathode structure of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following specification and appended claims in connection with the aforedescribed drawings.
With reference to the drawings, FIG. 1 is a prior art view illustrating an indirectly heated cathode assembly 11 positioned within the control grid electrode 13 of an electron gun of the type conventionally employed, either singly or plurally, in cathode ray tubes. The cathode assembly comprises a cathode sleeve 15 having a sidewall portion 17 with open top and bottom ends 19 and 21 respectively. Usually the cathode sleeve is fabricated of a passive cathode material, i.e., a metal or alloy that does not readily sublime or emit electrons upon heating. The top open end 19 of the sleeve is closed by a cathode cap 23 affixed thereto and made of an active cathode material, i.e., one that promotes the emission of electrons when heated. The exterior end surface of the cap 23 has disposed thereon a coating of an electron emissive material 25, such as a combination of carbonates of barium and strontium, and sometimes calcium.
Positioned within the sleeve 15 is a formed insulatively coated heater 27 which provides the operational temperature level necessary to produce electron emission from the emissive material 25. The cathode assembly 11 is suitably supported as, for example, by cathode shielding means 29 which is formed for attachment to the lower portion of the sleeve by several spaced apart welds 31. The control grid electrode 13 and the cathode assembly support means 29 are affixed to a plurality of electron gun support beads, not shown, by attachment means 33 and 35 respectively. Thus, the cathode is mounted in a manner that the electron emissive material 25 is in spaced relationship with the aperture 37 provided in the end wall 39 of the control grid electrode 13. During the operation of the cathode ray tube, the electrons emitted by the emissive material 25 pass through the grid aperture 37 as an electron beam and thence sequentially traverse the associated electron gun electrodes, not shown, to be directed to the screen of the tube.
As noted in FIG. 1, the skirt 22 of the cathode cap 23 telescopes over the top end of the sleeve 15 making extensive contact with the upper sidewall portion of the sleeve thereby effecting a comprehensive heat sink relationship. As the heater 27 warms up, during the initiation of tube operation, the heat thereof radiates to both the sleeve 15 and the mass of the skirted cap 23 thereby retarding the temperature rise of the emissive portion and lengthening the initial time-lag of tube activation.
Attachment of the cap to the sleeve is normally accomplished by a plurality of bondings, such as by welds 41. Such bonding is usually effected by pressuring the skirt 22 of the cap against the sidewall 17 of the sleeve. This procedure sometimes causes a slight doming of the flat end surface 24, as exaggeratively shown in FIG. 1, or may set up residual stresses in the cap which tends to produce doming or "oil-canning" of the end region during operational heating. Whatever the cause, any doming of the emissive surface relative to the adjacent electrode aperture 37 is detrimental to the uniformity of the subsequential electron beam and thus affects the desired operational efficiency of the tube.
There are times when the cap welding procedure produces a detrimental projection or "spike" 43 that may protrude in varying degrees from the interior surface of the sleeve 15 into the area wherein the heater 27 is oriented. During the operational life of the tube, the heater moves by temperature induced expansion and contraction and additionally may be shifted by environmental shock. Under these influences the heater is apt to come in contact with the more abrasive of such weld projections, whereupon a deleterious eroding of the heater insulation is likely to occur, with the possibility of a catastrophic heater-to-cathode short circuit developing.
As aforementioned, uniformity of the emissive area 25 relative to the size of the electrode aperture 37 are important operational considerations. As shown, the cap 23 has an external diametrical dimension "a" which is greater than the external sleeve diameter "b. " Due to the rolled periphery 45 of the cap, and the feathered periphery 47 of the emissive coating 25, the substantially uniform emission area evidences a lateral dimension "c" which substantially approximates the diameter "d" of the aperture 37. The close relationship of these dimensions, which are particularly common in low-power applications, emphasizes the criticality of alignment of the cathode and electrode aperture in such instances.
The foregoing disadvantages are overcome or minimized by the invention as shown in the separate embodiments delineated in FIGS. 2 and 3. For purposes of description, the cathode sleeves of the invention are denoted as being cylindrical, but such is not to be considered limiting as the cross-sectional shaping thereof may also be substantially elliptical or rectangular.
With particular reference to FIG. 2, the indirectly heated end-emissive cathode structure 49 is oriented within the control grid 13 in a manner similar to that described for the prior art, but the upper portion of the cathode structure 49, per se, is markedly different. the substantially cylindrical cathode sleeve 51 is formed of substantially passive cathode material having open top and bottom ends, of which the top end 53 is shown. Dimensionally, the sleeve has inner and outer diameters designated as "e" amd "f" respectively. A terminal ledge 55 is extremitally formed to be circumferentially outstanding from the top end of the sleeve; such ledge 55 being formed in a manner substantially normal to the exterior surface of the sleeve 51. The diametrical span of the periphery of the outstanding ledge is referenced as "g." A substantially disc-shaped planar closure member 57, formed of a substantially active cathode material, as for example, known nickel alloys, is of a perimetric shape similar to that of the sleeve. As shown, the closure approaches the diametrical dimension g of the terminal ledge. This planar closure member is seated on and affixed to the outstanding ledge 55 in one of two manners; either by a plurality of at least three spaced bonds or welds 58, or by a substantially continuous perimetric bonding therearound. The peripheral bonding between the ledge 55 and the peripheral portion of the closure member 57 is consummated in a substantially flat surface-to-surface compression relationship wherein longitudinally directed bonding pressure fosters maintenance of the flatness of the planar member. There are no stress vectors set up of the type evidenced in the prior art cap-to-sleeve attachment. The closure member provides a flat substrate having a planar area upon which a uniform electron emissive coating 59 is disposed. In this embodiment the substrate area is greater than the cross-sectional area of the sleeve, i.e., diametrically g exceeds f. The cathode coating 59, while exhibiting a feathered periphery 61, has a uniform area of the diameter "h" which exceeds the diameter "i" of the elctrode aperture 63. Thus, the cathode structure 49 of the invention provides for greater efficiency in alignment. The required operational temperature is provided by the insulatively coated heater 27 positioned within the cathode structure as shown.
A second embodiment of the invention is referenced in FIG. 3, wherein the indirectly heated end-emissive cathode structure 65 is positioned within the control electrode 13. The upper portion of the cathode structure of this embodiment 65 differs from the described first embodiment 49, in that the top of the cathode 67 is extremitally formed to have an instanding terminal ledge 69 circumferentially thereabout. Such ledge being formed in a manner substantially normal to the interior surface of the sleeve 67. A substantially disc-shaped planar closure member 71 having a diametrical dimension "m" which does not exceed the external cross-sectional dimension f of the sleeve, is seated on and affixed to the instanding ledge 69. This affixation is in the form of at least three spaced apart bonds or welds 73 or by the application of a continuous perimetric bonding. Whichever procedure is utilized, the union is effected by a substantially even surface-to-surface compression relationship wherein the jointure pressure maintains flatness of the planar closure member 71. The flat substrate thus provided has disposed thereon a uniform area of an electron emissive coating 75 having a diameter "o" which exceeds the diamter "p" of the adjacent electrode aperture 77.
There are thus provided embodiments of an invention wherein there is limited contact between the end-emissive closure and the contiguous supporting sleeve. Since a minimum mass of layered material is involved in the jointure between the closure member and the sleeve, the heat sink aspects of that region are also minimized and the rapid heating characteristics enhanced. The flat closure substrate of the invention affords more usable cathode area for a given closure diameter than normally available on a conventional cap structure. The flat closure disc does not require forming, consequently there is a minimum of residual forming stress of the type encountered in conventional skirted-cap fabrication. Additionally, the area of bonding in the improved cathode structure is in a peripheral region remote from possible heater abrasion.
While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.