US2842701A - Cathode heater assemblies - Google Patents

Description

July 8, 1958 l.. J. cRoNlN cATHoDE HEATER AssEMBLIEs Filed Dec. 22, 1954 //v l/f/v TOR E0 d. CPO/WN 5y Trop/v5 y lli II II IIIIIIIII United States Patent f CATHODE HEATER ASSEMBLIES Leo I. Cronin, Natick, Mass., assigner to Raytheon Manufacturing Company, Waltham, Mass., a corporation of Delaware Application December 22, 1954, Serial No. 476,877

` 8 Claims. (Cl. 313-340) This invention relates to a cathode heater assembly for electron discharge devices and, -more particularly, relates to means for rigidly supporting a heater for electron discharge device cathodes whichoperate at relative- -ly high temperatures.

Shorted turns and cathode-to-heater short circuits often occur in heater coils, particularly those made of tungsten and similar materials which operate at temperatures of the order of 2000 C. or higher, because of warpage or sag of the heater coil. Heater coil sag is especially prevalent when the heater is mounted horizontally. Coil sag may also account for uneven heating of the cathode and, consequently, an irregular electron emission pattern. The adverse ellects above mentioned are aggravated by high cathode operating temperature and mechanical shock; in addition, the difficulties are also aug-` mented as the heater coil wire becomes of smaller diameter and as the heater coil length and ratio of coil diameter to Wire size increase.

Electrically insculating supports for heaters which depend on direct mechanical contact with the heater are not feasible for use with heaters, such as tungsten, which are operated at temperatures in the vicinity of 2200 C. Suitable insulating materials, such as the alumina ceramics, shrink appreciably over a temperature range of approximately 1600 C. to 1800 C. and cracking results from this shrinkage. Furthermore, aluminum oxide melts at about 2050 C. a temperature well below the operating temperature of heaters made of materials of high melting point and low rate of evaporation, such as tungsten, molybdenum, and the like. Although some other insulating materials have somewhat higher melting temperatures than alumina ceramics, they are often unstable chemically when in contact with such heater materials as tungsten and molybdenum.

in accordance with this invention, it is possible to utilize electrical insulating materials, such as alumina ceramics, having the desired electrical, physical and mechanical properties in connection with heaters whose operating temperature exceeds that at which the insulating materials become unsatisfactory.

A heater support member made of a material capable of withstanding the high operating temperatures involved is insulatedly mounted at one end to a portion of a heater lead assembly whose temperature is below that at which the insulation deteriorates. The other end of the heater support contains a portion of enlarged external dimension which engages the inner periphery of a heater coil having one end thereof affixed to the aforesaid heater lead assembly. More than one contact may beused depending, in part, upon the length of the heater coil. The end of the heater remote from the heater lead assembly maybe `attached to a portion of the `cathode structure. Referring to the drawing:` Y

Fig. l is central longitudinal cross-sectional view `of an embodiment of a cathode heater assembly in accordance with the invention; and

CIL

. serted `freely over the latter. A

portion 2Gb of member 20 isgreatei than the inside n v12,842,701y Patented July 8, 1958 Figs. 2 4 are fragmentary views illustrating modifications of the cathode heater `assembly of Fig. l.

In Fig. l of the drawing, an indirectly heated cathode assembly is generally indicated by the reference numeral 10, and comprises a heater `subassembly 12 positioned Within a cathode subassembly 14. The heater subassembly 12 includes `a heater rod 15 consisting of an electrically-conductive heater lead 16, preferably of solid construction, and a metallic heater tube 17, into one end of which heater lead 16 is inserted, vas Ishown in Fig. l. Heater lead 16 may be welded to heater tube 17 of heater rod 15, as shown at 18.

A centrally disposed heater support member 20 constructed, for example, of tungsten, molybdenum, or any material having a high melting point and a low vapor pressure and capable of withstanding temperatures in excess of 2000 C. comprises a-reduced portion 20a which is adapted to iit within one end of heater tube 17. Central support member 2li is electrically insulated from metallic heater tube 17 by means of a plurality of electrically insulating sleeves 22 constructed, by way of example, of alumina ceramic; these sleeves are inserted over portion 20a of support member 20. A single elongated sleeve, such as shown in Fig. 2, may be used in lieu of the several short sleeves shown in Fig. l; it is often preferable, however, to use several short sleeves rather than one long sleeve, since the ceramic sleeves must be tired at high temperatures to prevent shrinking when in actual use, and the dimensional stability of short sleeves is considerably better than that of a longer sleeve. Although the individual sleeves 22 of Fig. l are adapted to slide freely on member 4Ztl and are somewhat smaller than the bore of heater Vtube 17, any tendency for the member 20 to wabble or shift position laterally within heater tube 17 is prevented by using several sleeves. in order to insure adequate insulation of heater support member Ztl, the compound insulating sleeve 22 of Fig. l and the single insulating sleeve of Fig. 2 is permitted to extend somewhat beyond the end of the support member.

Longitudinal movement of support member Ztl within heater tube 17 in the direction of heater lead 16 is prevented by means of an inwardly projecting tab 23 forming part of tube 17.

Instead of heater rod 15 includuing a solid heater lead 16 and a tubular portion 17, heater rod 15 may `be entirely of `solid construction, as shown in Fig. 2, and the end of the solid heater rod 15 may be drilled out to form a bore 25 in which the central support member 20 `is inserted. ln this case, an electrically Vinsulating disk 27 which, like sleeves 22, may be made of ceramic is positioned at the bottom of the bore.

An enlarged insulator 28, whose purpose will be described later, surrounds the heater tube 17 and is maintained against-longitudinal movement in one direction by a ta'b 29 of heater tube 7 which is extended slightly beyond the outside diameter of tube 17.

As shown in Fig. l, a slot or groove 31 is provided between the main portion 20b' and portion 20c of central heater support member 20. A heater support contact 30, which may consist of a small coil of one or more turns of wire, is inserted over portion 20c of heater support' member 20 and seats on the shoulder 21 of member 20 formed by slotl 31. The inside diameter of heater support contact coil 3) is greater than the diameter of portion 20c of member 20 so that it may be in- The voutside diameter of diameterV of coil 30 so that coil-30 is restrained from moving past shoulder 21. With the heater support contact coil 30 in position against shoulder 21, heatercoil 35, consisting of several turns of electrically-conductive wire ofhigh melting point and low rate of evaporation and capable of withstanding elevated temperatures, such as tungsten, molybdenum, tantalum, is inserted, as by rotating, over heater support contact coil 30 and one end of heatertube 17. Contact `coil 30 is preferably made of the same material as heater coil 35. A sufficient number of shorted turns at this end of the heater coil was necessary to provide adequate mechanical support. In order to provide for sufficient temperature drop in the region of the electrically insulated sleeves 22, sufficient space should be maintained between the end of coil 35 and the end of sleeve 22 adjacent said coil. After heater coil 35 has been attached to heater tube 17, the tab 37 adjacent the end of tube 17 is bent outward to retain the heater coil 35 in a fixed position. It is possible, of course, to attach heater coil 35 to tube 17 by soldering or welding.

The diameter of the support contact coil 30 and the diameter of the wire comprising the coil are such that adequate contact is made between heater coil 35 and support member 20. It should be noted that the heater support contact lcoil 30, which is in contact with central heater support member 20 and heater coil 35, provides lateral support for the heater coil while requiring a minimum number of shorted heater coil turns. Moreover, the electrically insulating spacers 22 are disposed in the region of the heater which is sufficiently cool to prevent deterioration of the insulation. The heater coil support contact need not take the form of coil 30 of Fig. l, but may be of any configuration such as exemplified by the cup-shaped contact 60 of Fig. 2. The contact, furthermore, may be integral with either the support member 20 or may be a separate member attached to support member 20 adjacent one end thereof.

The other end of heater coil 35 may be attached to a tapered adapter 38, which may be m-ade of molybdenum or any other materials having the characteristics required of the other electrically-conductive portions of the heater. Adapter 38 is contained within cathode sleeve 40 adjacent one end. Also adjacent said one end of cathode sleeve 40 and surrounding the same is a first end shield 42. Adapter 38 and end shield 42 are welded to cathode sleeve 40 through apertures 43 in end shield 42. A second end shield 44 spaced from end shield 42 is similarly welded to cathode sleeve 40 through apertures 45 in end shield 44. The longitudinal position of end shield 44 prior to welding may be determined by upwardly projecting tab 47 in cathode sleeve 40. As is well known, the purpose of these end shields is to prevent longitudinal emission of electrons into the end spaces of tubes of magnetron type. An electron emissive `coating 50 is applied to the region of the cathode sleeve lying between end shields 42 and 44.

The ceramic sleeve 28 previously referred to and positioned between leader tube 17 and cathode sleeve 40 serves to electrically insulate said heater coil 35 from cathode sleeve 40 and provides lateral support for heater rod 15, maintaining the latter concentric with sleeve 40.

Heater rod 15 is rigidly Vsupported by means including an electrically-conductive cup 52 disposed at vthe other end of cathode heater assembly and soldered to heater lead 16 by means of solder ring 5,3. Cup 52 is Vattached to a glass or ceramic seal 55 to which also is attached one end of a tubular element 56 Aarranged coaxially with heater rod 15. The other end of tubular element 56 is adapted to fit over the end of cathode sleeve, 40 remote from the emissive surface 50 and is welded to cathode sleeve 40 through apertures 57 therein.

Although the heater support has been shown as singleended in Figs. vl and 2, it should be understood that the heater support may also be *double-ended, as shown in Fig. 3. A

In the embodiment illustrated in Fig. 3, in whichcertain details of the cathode assembly 14 have been omitted for thesake of clarity, a first heater support member 20, similar to that of Fig. l, is insulatedly mountedwithin heater tube 17. A second heater support 20' is spaced from support member 20 and is attached to a sleeve 65 of a material such as ceramic. Sleeve 65 is mounted within adapter 38 which, in turn, is affixed within cathode sleeve 40, as shown in Fig. l. The heater support contact 60 is affixed to a heater support member 20 adjacent one end thereof, as clearly shown in Fig. 3, and is adapted to engage a limited portion of a heater coil 35, as in the case of heater support contact 60 of Fig. 2. Similarly, heater support contact 60 is positioned adjacent the free end of heater support member 20'. The spacing between heater support members 20 and 20' prevents electrical short circuits from occurring in the cathode heater assembly 10. The arrangement shown in Fig. 3 is especially adapted for use with long heater coils since heater coil 35 may be supported at two points rather than one.

A further modification of a cathode heater assembly is shown in Fig. 4, wherein a single heater support member 20 is used which is attached at both ends instead of at one end only, as in the case of Figs. 1 and 2. Heater support member 20 is attached at one end to heater tube 17 in the manner shown in Fig. l and is attached at the other end in the manner indicated in Fig. 3. A single heater support contact 60 is mounted approximately midway between the ends of heater coil 35.

In the modifications of Figs. 3 and 4, wherein support is provided at both ends of the heater support member, the stability of the cathode heater assembly 10 is increased.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

l. In combination, a cathode sleeve having a portion thereof whose outer surface contains an electron emissive material, an elongated current-carrying member, a heater element attached at one end thereof to said member, an elongated electrically-conductive supporting element having a first portion mounted to said member, an electrical insulator for electrically insulating said supporting element from said member and positioned at a region of said member whose temperature is sufficiently low to prevent deterioration of said insulator, said supporting element including an electrically conductive second portion of enlarged cross section engaging said heater element over a limited region thereof spaced from said one end.

2. In combination, a cathode sleeve having a portion thereof whose outer surface contains an electron emissive material, an elongated current-carrying member positioned within said sleeve, a heater element attached at one end thereof to said member, an elongated electricallyconductive supporting element having a first portion mounted to said member, the portion of said heater element adjacent said electron emissive portion of said cathode being capable of operation at a temperature of the order of 2000 C., an electrical insulator for electrically insulating said supporting element from said member and positioned at a region of said member whose temperature is sufiiciently low to prevent deterioration of said insulator, said supporting element including an electrically conductive second portion of enlarged cross section engaging said heater element over a limited region thereof spaced from said one end.

3. In combination, a cathode sleeve having a portion thereof whose outer periphery contains an electronemissive material, a heater centrally positioned within said sleeve, said heater including an elongated currentcarrying member and a heater element attached at one end thereof to said member, an elongatedelectricallyconductive supporting element having a first `portion attache'cl to and electrically insulated from said member and an electrically conductive second portion of enlarged cross section engaging said heater element over a limited region thereof spaced from said one end, and for maintaining the position of said heater element substantially fixed means for electrically insulating said cathode sleeve from said member.

4. In combination, a cathode sleeve having a portion thereof whose outer periphery contains an electronemissive material, a heater centrally positioned Within said sleeve, said heater including an elongated currentcarrying member having at least a portion thereof of tubular configuration and a heater coil attached at one end thereof to said member, a tubular electrical insulator inserted within said tubular portion of said member, an elongated electrically-conductive supporting element having a iirst portion inserted within said tubular insulator and electrically insulated from said tubular member and an electrically conductive second portion of enlarged cross section engaging said heater element over a limited region thereof spaced from said one end for maintaining the position of said heater element substantially fixed, and means for electrically insulating said cathode sleeve from said member.

5. An indirectly heated cathode comprising a cathode sleeve having a portion thereof Whose surface contains an electron-emissive material, a heater positioned within said sleeve, said heater including a current-carrying member insulated from said cathode sleeve and a heater element attached at one end thereof to said member and at the other end thereof to a portion of said sleeve, an elongated electrically-conductive supporting element including a first portion and a second portion of cross se-ction greater than that of said first portion, an electrical insulator spaced between said first portion and said member for insulatedly mounting said supporting element within said member over a region of said element, said second portion of said supporting element engaging said heater element over a limited region thereof spaced from said one end.

6. An indirectly heated cathode comprising a cathode sleeve having a portion thereof whose surface contains an electron-emissive material, a heater positioned within said sleeve, said heater including a current-carrying member insulated from said cathode sleeve and a heater element attached at one end thereof to said member and at the other end thereof to a portion of said sleeve, an elongated electrically-conductive supporting element including a first portion and a second portion of cross section greater than that of said first portion, an electrical insulator spaced between said first portion and said member for insulatedly mounting said supporting element Within said supporting member over a region lying beyond the confines of said heater element, said second portion of said supporting element engaging said heater element over a limited region thereof spaced from said one end.

7. In combination, a cathode sleeve having a portion thereof whose outer periphery contains an electron-emissive material, a heater centrally positioned within said sleeve, said heater including an elongated current-carrying member and a heater element attached at one end thereof to said member, a first elongated electricallyconductive supporting element having a first portion adjacent one end of said member, an electrical insulator spaced between said first portion and said member for mounting said lirst portion in electrically insulated relation with said member, said first supporting element further including an electrically-conductive second portion of enlarged cross section engaging said heater element over a first limited region thereof spaced from said one end, a second elongated electrically-conductive supporting element, an insulator spaced between said cathode sleeve and one end of said second supporting element for mounting said second supporting element to said cathode sleeve, said second supporting element further including an electrically-conductive second portion of enlarged cross section engaging said heater element over a second limited region thereof spaced from said one end and from said'first region, and means for electrically insulating said cathode sleeve from said member.

8. In combination, a cathode sleeve having a portion thereof whose outer periphery contains an electron-emissive material, a heater centrally positioned within said sleeve, said heater including an elongated current-carrying member and a heater element attached at one en'd thereof to said member, a first elongated electrically-conductive supporting element having a first portion adjacent one end of said member, a first electrical insulator spaced between said first portion and said member for mounting said first portion in electrically-insulated relation with said member, said supporting element further including an electrically-conductive second portion of enlarged cross section engaging said heater element over a first limited region intermediate the ends of said element, a second elongated electrically-conductive supporting element including an electrically-conductive portion of enlarged cross section engaging said heater element over a second limited region intermediate the ends of said element, and a second electrical insulator spaced between said cathode sleeve and said current-carrying member adjacent the other end of said member for further mounting said supporting element.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES P ATEN'I OFFICE CERTIFICATE OF CORBECTIN Patent Noa 2,82y'701 July 8p 1958 Leo J Cronin It is hereby certified 'that error appears in the printed specification of the' above numbered patent requiring correction and that. the seid Letters Patent should read as corrected below.

Column 5, line 4, strike ourJ end"; line 6, after "fixed" insert Signed and sealed this ll'bh day of November 1958.,

SEAL) Attest:

KARL He .AXLINE Attesting Ocer ROBERT C. WATSON Commissioner of Patents

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