US2445993A - Cathode structure - Google Patents

Description

July 27, 1948.

J. E. BEGGS CATHODE STRUCTURE Filed Feb. 12, 1944 Inventor:

James E 'Be gs, b 7V047' His Attorney.

Patented July 27, 1948 2,445,993 CATHODE STRUCTURE.

James E. Beggs, Scotia, N. Y., assignor to General Electric Company, a corporation of New York Application February 12, 1944, Serial No. 522,097

12 Claims. 1

My invention relates to electron discharge devices and, in particular, to a cathode structure for such devices.

It is an object of my invention to provide a new and improved cathode structure which may be employed with equal facility in receiving and transmitting tubes.

In the manufacture of electron discharge devices, particularly in electronic tubes used in radio receiving sets, one of the limitations heretofore encountered has been the necessity of using low voltages for the energizing of the heater employed for heating an electron emissive surface. One factor which has discouraged the use of higher heater voltages is that, with the rapid exhausting ordinarily given electronic tubes, it is difficult to degas a cathode employing a high heater voltage since ionization of the gas between the heater terminals occurs during the exhausting period. Another factor is the necessity of completely shielding the electron stream of the tube from the magnetic and electrostatic effects of the high alternating current voltage. Accordingly, it is an object of my invention to provide a new and improved cathode structure of an electron discharge device which permits the use of relatively high voltages of the order of the ordinary household supply voltage for energizing the heater of such a structure.

It is another object of my invention to provide a new and improved cathode structure for an electron discharge device which permits sealing of the device after the exhausting thereof has been completed.

Still another object of my invention is to provide an improved cathode construction of an electron discharge device having a low inductance.

A still further object of my invention is to provide a new and improved cathode structure for an electron discharge device which utilizes a completely shielded filament to prevent the coupling of filament supply voltages into the electron stream.

Still another object of my invention is to provide a new and improved cathode structure which is universal in nature and which may be employed in a wide variety of discharge tubes.

In a typical embodiment, the invention makes use of a hollow metal tube having an end Wall which forms a cathode for an electron discharge device and which encloses both a heater for the cathode and a getter for the discharge device. The metal tube is used in cooperation with a metallic sleeve in the enclosing envelope of an elecmolten metallic seal between the sleeve and the metallic tube so that accurate electrode spacing may be obtained after complete evacuation of the tube. The heat source of the tube is shielded and is substantially completely thermally isolated from the metal tube so that the heat from the source is directed most efficiently to the emissive cathode part.

The features of the invention desired to be protected herein are pointed out with particularity in the appended claims. The invention itself, to-

gether with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawing in which Fig. 1 is a sectional view of a composite cathode structure embodying the invention; Fig. 1a is an enlarged" cross section of the getter element of Fig. 1; Fig. 2 is a sectional view of an electron discharge device showing a modification of the getter arrangement employed in the composite cathode structure; Fig. 3 is a sectional view of a cathode employing an alternative heater structure; and Figs. 4-6 are views, partly in section, of various types of electron discharge tubes employing the universal cathode structure depicted in Figs. 1-3.

Referring particularly to Fig. 1, the composite cathode structure there illustrated includes an active or emissive part consisting of a dis [0 formed of any suitable metal, for example, nickel. This disk I0 is preferably coated on its upper surface with an activating coating, such as a thin layer of barium and strontium carbonates, and is supported by a complex structure which includes a path of low thermal conductivity comprising a sleeve ll formed from a thin metallic foil of a poorly conductive material, such as an ironnickel-cobalt alloy, the lower end of sleeve I I being welded to a hollow cylindrical metallic member l2. The metallic sleeve II is provided with flanges l3 at its upper end which are welded between the lower surface of disk III and an eyelet l4 composed of a good heat conducting material, such as nickel. A filamentary heater is is disposed within the eyelet M in close proximity to the disk l0 and is supported by means of a first lead l6 attached to one of its ends and, in turn, rigidly aifixed to the inner surface of tube 12 and asecond lead I! attached to its other terminal.

The filamentary heater l5 suitably consists of a tubular helix of tungsten or molybdenum coated with an insulating material, such as alumina, in order to prevent short-circuiting of adjacent turns of the filament.

tron discharge device and permits the use-of a The metal tube I2 is flanged inwardly atrits natively, heater I5 may be connected between leads I I and 20 with getter I9 being connected between leads I6, I'l. Leads I! and 20 pass through a vitreous seal 2| at the lower end of y tube I2, the seal 2! and disk I0 defining a substantially closed region within the tubes I I and I2. Tube I2, at its lower end, may be provided with an inwardly directed bead 22 to reenforce' the seal 2| and make it a rigid wall across the lower nor-- tion of the tube I2.

In the construction of the component parts of the cathode structure, the eyelet M is made suffi'cientl'y short that it does not extend into the constricted portion I8 of tube 52 so that the cathod'ev k0 is relatively free of thermal expansion effects, the sleeve II providing a poor heat conducting pathbetween disk I I] and tube I2.

The composite cathode structure thus far describedis to be used inconjunction with a metallic sleeve 24 which is included in the enclosing envelope of any electron discharge device employing this particular type of cathode structure and which. defines an aperture in this envelope through which the cathode structure extends into the device. The method of manufacturing an electron discharge device employing the composite cathode'structure illustrated in Fig. 1 is disclosed and claimed in my copending application, Serial No. 501,791, filed September 10, 1943, which matured as Patent No. 2,428,618 on October 7, 1947, and assigned to the same assignee asthis present application. Briefly, in my copending application, there is explained the manner in whicha tube is constructed having a metallic sleeve 2 1 through which the composite cathode structure of Fig. 1 is inserted. An electron discharge device including the cathode structure is evacuated in an enclosed chamber, the cathode structure being supported separately from the remaining portions of the discharge device. After suitable conditions of vacuum are established within the device, a ring of solder 25, located between the sleeve 2 3 and tube I2 at the shoulder portion I8 of the latter, is melted, as by inductive heating, and allowed to form a molten sealbetween the sleeve 2d and tube. I2. Thereafter, the getter I9 may be flashed bycurrents supplied thereto over leads I7, 29. A. small hole or aperture 26 in the metallic sleeve II provides communication between the interior of the composite cathode structure and the region within'theremainder of the electron discharge device so that the getter I9, after evaporation, may further reduce the gas pressure within the device to a value, for example, of the order of..01 micron, creating a final vacuum condition within the device. Aperturev 26 is small enoughto prevent rapid exhaustion; of the electron tube by getter lilaso-that-if the molten seal 25 is not yet established the getter does not compete with the vacuum pump. used. to exhaust the electrontube.

The surface tension of the molten seal of solder 25: between thesleeve 24 andtube I2 maintains the difierential pressures between. the. inner and I outersurfaces. of the'electron discharge device.

After final pressure conditions are established within the device and while the metallic seal is still molten, the position of the electron emissive cathode disk it with respect to the remaining electrodes of the discharge device may be adjusted to give any desired characteristic to the electron discharge device. Ultimately, the seal formed by the solder 25 is allowed to harden, completing the manufacture of the discharge device.

By the use of the composite cathode structure illustrated in Fig. 1 in an electron discharge device, extremely good vacuum conditions may be obtained rapidly during production by the use of the exhausting technique described so that high voltages of the order of the -120 volt ordinarily available to the domestic user may be employed for energizing the filamentary heater I5 without increasing the duration of the exhausting period and without causing ionization of gas between the terminals of heater I5. Since the heater leads I'l, Iii and the heater itself are completely enclosed within the tube 52 and sleeve Ii, all coupling between the filament supply and the electron stream of the discharge device is avoided.

In Fig. 2, there is shown a conventional glass or metal type of receiving tube employing a cathode structure of the type shown in Fig. 1. The receiving tube 3!) has an enclosing envelope formed of a metal or glass portion 3! and an insulating base portion 32 which includes the cylindrical metallic sleeve 2 defining an aperture in the envelope through which the composite cathode structure extends. The electrodes of the receiving tube 3i? are of the planar type, being supported from a plurality of vitreous cylinders 33. In Fig. 2 there is also illustrated an alternative construction for the composite cathode structure cathode Iii by heater 8% and is constructed and arranged to flash when the cathode reaches the maximum temperature required in sintering the cathode so that this construction avoids the requirement of an electric current and attendant leads to flash the getter.

In Fig. 3, there is illustrated a composite cathode structure employing an alternative heater construction. The heater 35 comprises a coiled or spirally wound wire lit-of tungsten or molybdenum coated with an insulating material 3-1; such as alumina, and is in thermal contact with the lower surface of emissive disk Iii, being clamped between the said lower surface and a dish-shaped element 38. Disk Iii, element 38, and flange iii of foil I I preferably are welded together to'clamp heater 35 in position. One terminal 39 of the heater may be connected to the cathode structure and the other terminal of a wire it may be connected to a source of heater current. Getter 3% may be supported from the lower surface of element 38 and may be flashed by heating of the cathode as described above in the description of the structure of Fig. 2.

In Fig. 4, there is illustrated an electric discharge device 40 for ultra high frequencywapplications and which may be inserted in a concern tric type of transmission line.

constitutes the anode of the device and the composite cathode structure 42 formed as illustrated in Fig. 1. The grid or control electrode 43 is supported from and connected to two metal sleeves 44 and 45 which concentrically surround, respectively, the lower portion of anode 4| and the upper portion of the cathode structure 42. The cathode structure 42 is surrounded by sleeve 24 which is supported from sleeve 45 through a vitreous seal 46. A similar seal 41 supports anode 4| from sleeve 44. When inserted in a concentric transmission line, a tube of the type of discharge device 40 has its anode 4| and cathode 42 connected to the inner conductor of the transmission line and the metallic sleeves 44 and 45 connected to the outer conductor. Tubes of this structure are disclosed and claimed in my copending application, Serial No. 501,790, filed September 10, 1943, and assigned to the same assignee as the present invention.

In Fig. there is shown in vertical section another form of ultra high frequency electric discharge device of the so-called light house type which employs the cathode construction shown in Fig. 1. In the electric discharge device 56 shown in Fig. 5, three spaced concentric disks 5|, 52, 53 are supported in mutually spaced relation by means of vitreous cylinders 54, 55. The metallic sleeve 24, which forms a part of the enclosing envelope of the device has a flanged portion 56 which is connected to the disk 53 for alternating currents, being insulated therefrom for unidirectional currents by means of an insulating spacer 56' and sealed thereto by means of a vitreous seal 51. Tubes of this structure are disclosed and claimed in my copending application, Serial No. 436,633, filed March 28, 1942, which matured as Patent No. 2,416,565 on February 25, 1947, and assigned to the same assignee as the present invention.

In Fig. 6 there is illustrated another form of ultra high frequency tube suitable for use in axially aligned and adjacent space resonant cavities of the concentric transmission line type. The electric discharge device 69 there illustrated comprises a cylindrical anode 6| and a composite cathode structure of the form illustrated in Fig. 1 and encircled by a metallic sleeve 24. A metal disk 62 supports a grid 63 in spaced relation between the anode 6| and the electron emitting cathode. The disk 62 is maintained in spaced relation between the anode 6| and the cathode by means of a pair of insulating cylinders 64, 65.

In all of the illustrated embodiments of my invention, it is seen that the composite cathode unit may be inserted easily and precisely to obtain a desired spacing between the electron emissive surface of the cathode and a control grid or an anode. The molten metallic seal between the sleeve 24 and the tube l2 provides a means whereby a very fine adjustment of such spacing may be obtained. That is, the adjustment of spacing may be made by heating the seal to its molten condition either inductively or by excessive heating of the cathode. In this way tubes having more uniform characteristics may be manufactured. Moreover, the cathode structure, due to the use of cylinders l2 and H, has an extremely low value of inductance which permits use in tubes operating over a very wide range of frequencies from the very low frequencies encountered in ordinary amplifying circuits to the extremely high frequencies used in ultra high frequency work. Since the inductance of the cathode structure is very low, it is apparent that tubes having very low capacitances may be designed, thus permitting operation at very high frequencies.

While the invention has been described by reference to a particular embodiment thereof, it will be understood that numerous modifications may be made by those skilled in the art without d parting from the invention. I therefore aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electron discharge device having an enclosing envelope, a cathode extending into said envelope, said cathode comprising a hollow tube hermetically sealed to said envelope, said tube having an emissive end wall lying within said envelope, a heater supported within said tube in thermal relation with said end wall, gettering material disposed within said tube, means within said tube whereby said gettering material may be flashed, and means within said tube for limiting the rate of removing residual gas from said device by said material.

2. A cathode comprising a hollow metallic structure, a metal disk sealed across one end of said structure, said disk having an electron emissive coating on its outer surface, insulating means sealed across said structure at a point spaced from said disk, said disk and said insulating means forming with said structure a closed chamber, a heating element supported Within said chamber in close proximity to said disk, and a getter material disposed within said chamber, said structure having a small aperture therein providing communication with said chamber.

3. An electron discharge device having an enclosing envelope, a cathode extending into said envelope, said cathode comprising a hollow metallic structure hermetically sealed to said envelope, said metallic structure having an emissive end wall lying within said envelope, an air-tight seal across said metallic structure at a point spaced from said end wall and formin therewith a substantially confined chamber, a heater supported within said chamber in thermal relation with said end Wall, gettering material disposed within said chamber, said metallic structure having an aperture providing communication with the space within said envelope, said aperture being sufiiciently small to limit the rate of removing residual gas from said space by said ma terial.

4. A cathode structure comprising a metal disk having an electron emissive coating on one of its surfaces, a heating element disposed in thermal relation with the other surface of said disk, and means connected to said element whereby heating currents may be supplied to said element, and a, getter material supported in close thermal proximity to said other surface and said element whereby said getter may be flashed by heat from said element.

5. A cathode comprising an emissive part, a hollow metallic supporting structure therefor sealed to said part, said structure including a metallic path of low thermal conductivity adjacent said part, insulating means sealed across said hollow structure at a point spaced from said part, said insulating means and said part defining with said structure a closed region, a heater supported within said region in proximity to said part, a thermally conductive body surrounding said heater and interposed between said heater and said path, said body having a good thermal connec- 7 tion with said emissive part for conducting heat to said part and for preventing radiation of heat to said structure, and gettering means supported within said region, said metallic path having an aperture therein providing communication with said region.

6. An electron discharge device having an enclosing envelope and a cathode comprising an emissive part, a hollow metallic supporting structure therefor sealed to said part, said structure including a metallic support of low thermal conductivity adjacent said part, insulating means sealed across said hollow structure at a point spaced from said part, said insulating means forming a portion of said envelope and said insulating means and said part defining with said hollow structure a closed region, a heater supported within said region in proximity to said part, a thermally conductive body surrounding said heater and interposed between said heater and said support, said body having a good thermal connection with said emissive part for conducting heat to said part and for preventing radiation of heat to said structure, and gettering means supported within said region, said metallic support having an aperture therein providing communication between said region and the space within said device to facilitate evacuation of said space by said gettering means.

7. An electric discharge device having an enclosing envelope, a metallic sleeve extending through an aperture in said envelope, and a cathode extending into said envelope through said sleeve, said cathode comprising a hollow metallic structure hermetically sealed within said sleeve, said metallic structure having an end wall lying within said envelope, a vitreous seal across said metallic structure at a point spaced from said end wall, said seal and said metallic structure constituting a sealed closure for said aperture in said envelope, and a heater supported within said metallic structure between said end wall and said seal, said end wall having an electron emissive surface heated by said heater.

8. An electric discharge device having an enclosing envelope, a metallic sleeve extending through an aperture in said envelope, and a cathode extending into said envelope through said sleeve, said cathode comprising a hollow metallic structure hermetically sealed within said sleeve, said metallic structure having an end wall lying within said envelope, a vitreous seal across said metallic structure at a point spaced from said end wall, said seal and said metallic structureconstituting a sealed closure for said aperture in said envelope, a heater supported within said metallic structure between said end wall and said seal, said end wall having an electron emissive surface heated by said heater, and a getter material disposed within said metallic structure between said end wall and. said seal, said metallic structure having a small aperture providing communication With the space within said envelope.

9. A cathode structure comprising a metallic disk member, a dish-shaped member in closely spaced relation with said disk member, said disk member having an electron emissive coating on its surface remote from said dish-shaped member, a heater disposed between said members and in thermal contact therewith, and a getter mate rial supported from said dish-shaped member on its side opposite said heater and adapted to be flashed by heat irom said heater when said disk member is heated by said heater for sintering purposes.

10. A cathode structure comprising a hollow metal tube, a metal disk sealed across one end of said tube, said disk having an electron emissive coating on its outer surface, insulating means sealed across said tube at a point spaced from said disk, said disk and insulating means forming with said tube a closed chamber, a dish-shaped metallic member supported within said tube in closely spaced relation with said disk, a heating element disposed between said disk and said dishshaped element and in thermal contact therewith, and a getter material supported from said dishshaped element to be flashed by said heater element.

11. A cathode comprising .an emissive part, a hollow metallic supporting structure therefor sealed to said part, said structure including a metallic support of low thermal conductivity adjacent said part, insulating means sealed across said hollow structure at a point spaced from said part, said insulating means and said part defining with said hollow structure a closed region, a dish shaped metallic member disposed within said region and in thermal contact with said part, a heater interposed between said part and said dishshaped member and shielded from said support by said dish-shaped member, said dish-shaped member being adapted to conduct heat to said part and to prevent radiation of heat to said structure, a getter material in thermal contact with said dish-shaped member on its side remote from said part, said metallic support having an aperture therein providing communicaton with said region.

12. A cathode comprising a hollow metallic structure, a metal disk sealed across one end of said structure, said disk having an electron emissive coating on its outer surface, sealing means connected across said structure at a point spaced from said disk, a heater supported within said structure between said disk and said sealing means, and a getter material disposed within said structure in close thermal proximity to the said heater and whereby said getter may be flashed by heat from said heater when said disk is heated by said heater.

' JAMES E. BEGGS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,797,990 Lucian Mar. 24, 1931 2,032,179 Lowry Feb. 25, 1936 2,078,892 McCullough Apr. 2'7, 1937 2,146,365 Batchelor Feb. 7, 1939 2,244,358 Eiwald June 3, 1941 2,341,941 Mouromtseff et al. Feb. 15, 1944 2,353,743 McArthur July 18, 1944

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