US2201731A

US2201731A - Discharge tube electrode assembly

Info

Publication number: US2201731A
Application number: US243101A
Authority: US
Inventors: Albert W Hull
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1938-11-30
Filing date: 1938-11-30
Publication date: 1940-05-21
Anticipated expiration: 1957-05-21
Also published as: GB542495A; US2201720A; FR861969A; US2246176A; BE438749A; NL55022C; FR51138E; US2201721A; GB536070A

Description

May 21, 1940. A. w. HULL I DIS CHARGE TUBE ELECTRODE ASSEMBLY Filed Nov. 30, 1938 2 Sheets-Sheet 1 Invent or: Albert \M Hull,

'3 His Attorneg.

May 21, 1940. w HULL 2,201,731

I DISCHARGE TUBE ELECTRODE ASSEMBLY Filed Nov. 30, 1938 2 Sheets-Sheei '2 In ventor:

Albert w un,

MW Himbfiorngy.

5 which are adapted to be quickly heated to an e1e-,

Patented May 21, 1940 UNITED STATES 2,201,131 DISCHARGE rune ELEc'raonE ASSEMBLY Albert w. Hull, Schenectady, N. n, assignor' to j I General Electric Company, a corporation of New York Application November-30, 1938, Serial No. 243,101

9 Claims.

The present application relates to electrical discharge devices. It is the object of my invention to provide for such devices improved thermionic cathodes, and in particular cathode constructions vated electron-emitting temperature, and will operate with improved characteristics.

Heretofore, large high-powered thermionic discharge tubes, such, for example, as rectiflers and thyratrons carrying currents of the order of 100 to 500 amperes required, preliminary to operation, a preheating period as long as about 20 to 30 minutes to bring the cathode throughout to an operating temperature. Reduction of this preheating time by increasing the heat input during preheating caused various difficulties. For example, parts immediately adjacent the cathode heater were liable to be subjected to damage by over-heating, coating material was lost by volatilization, and the life of the heater was reduced. I I

Such large cathodes, as heretofore'made, were constructed to provide heat conduction paths from the source of heat to electron-emitting members. For example, in the cathodes shown in U. S. Patent No. 1,924,318, patented to Hull and Ruggles on August 29, 1933, theelectron-emitting members are constituted by. vanes of sheet metal which are mounted upon and in good heat-conducting relation to a hollow metal cylinder.

inder is provided, which acts as a heat shield, this cylinder also being connected to the vanes at their extremities. Such a structure is thermally stable, but it has the disadvantage of heating to operaing temperature slowly from a cold state.

Even if the heat input is accelerated by increasing the cathode heater temperature above normal operating temperature, the heat conduction paths of such a structure either are inadequate to transmit heat at a sufficiently high rate to materially reduce the heating period (without fusing or otherwise injuring the base of the vanes), or, if the heat conduction paths are made larger, e. g., by making the vanes thicker, then the required amount of heat is increased which offsets the in-' crease of heat conduction so that nothing is gained.

In accordance with my present invention, I have provided improved cathode structures, one of the advantages of which is their property of being heatable at a more rapid rate than such prior structures from ambient temperature to an electron-emitting temperaturer While the novel features of this invention will be described and pointed out hereinafter in greater detail, the following features of novelty are noted: (a) the primary heater is constructed'to efi'ectively heat electron-emitting members to an operating temperature by radiation instead of by conduction; (b) the electron-emitting members are of relatively small mass and have small heat capacity. Preferably theyconsist of ribbons or reticulate material, such asmetal mesh or gauze. Their small mass and large surface permits them to be quickly heated by radiation to an electronemission temperature; (0) a surrounding heat shield is provided which is constructed to operate efliciently as a secondary radiant heater which receives heat from the primary heater whereby it may be maintained at a temperature as high as the electron-emitting members. The combined effect of the primary and secondary radiation heaters quickly'heats the otherwise substantially thermally-insulated electron-emitting members to an operating temperature. (d) The electronemitting members are so constructed and arranged that opposite emitting surfaces can effectively contribute electron emission for a space discharge supported by the cathode. These and other features of novelty are pointed out in the appended claims.

In the accompanying drawings, Figs. 1- and 2, respectively, are vertical and horizontal sections of a cathode embodying my invention, the respective sections being taken at regionsindicated in the companion figure by lines and arrows; Fig. 3 is a fragmental sectional figure showing structure of the heat shield; Fig. 4 is a side elevation, partly in section, of a complete discharge device containing a cathode embodying my invention; Fig. 5 is a fragmental view of a modified heater; Fig. 6 is a side elevation, partly in section, of a modification in which the electron-emitter is a helical ribbon; \Fig. '7 is a top view of the same structure;

Figs. 8, 9 and 9a are side elevations, partly in section, of other modifications (part being broken away) in which the heater also serves as a source of material for enhancing electron emission; and Figs. 10 and 11 are fragmented enlarged views of wire mesh structure.

The cathode structure shown in Figs. 1 to 3 comprises a radiant heater I, which may consist of a heavy filament or .wire of tungsten, tantalum, or other suitable refractory material. Itis helically wound on a core, or support 2, of suitable refractory insulating material, such for example as beryllia or alumina. The core 2 is held in a fixed position by a heat-resistant wire I which passes through it and is embedded in the insulating supports 4 and 5. The latter may consist of alumina. In some cases the primary heat source may consist of a filamentary resistance heater, which may or may not be mounted on a support, but which is surrounded by a housing which functions as the heat-radiating member. Such a construction is shown in Fig. 5, the heater I' mounted on a support 2' being contained within a housing 8, consisting of molybdenum, tantalum, or other suitable material.

Surrounding the primary heater I are a plurality of electron-emitting structures numbered 1 to I2, each of which preferably consists of fillets or strips of wire mesh, or gauze, or metal foil, or a combination of foil and gauze. Any suitable structure of small mass and large surface area may be used, which herein will be termed generically as foil. As will be pointed out, these strips or ribbons are supported to receive radiant heat but are in poor heat-conducting relation to the heater. They may be arranged to present only their edge surface to the heater I. In this position they are nonobstructive to heat radiated directly from the heater. They are conveniently made of nickel, molybdenum, or other suitable material, and are coated with material for enhancing electron-emissivity, for example, one or more oxides of barium, strontium, or other suitable alkaline earth metal. The electron-emitting ribbons I to I2 are held between the metal supports I3 and I4. The washer-shaped upper support I3 is re-enforced by a disc I5 and is mounted upon the heater core 2 near its upper end. The lower. support I 4 is a flat ring or washer and is held by the refractory insulators I6 and I! which may consist of alumina or beryllia and serve also to support a surrounding apertured enclosure I8, which functions as a shield and secondary heater. The structure may be rigidly clamped by tightening the nuts I9, I9 upon the

terminals

20, 2|. It will be observed that only poor heat conduction paths connect the electron-emitting ribbon members with the primary and secondary heaters. The electric circuit for the heater I may be traced from a conductor 22 through the heater to the support I3, through the ribbons I to I2, and to one of the

treminals

20, 2|.

The heat shield I8 may assume various forms and even may consist of refractory nonconducting material, but it consists preferably of a plurality of closely spaced walls, or layers, which are in poor thermal contact with one another. For example, the shield may consist of layers of metal foil, such as nickel or molybdenum, through which indentations or holes are impressed so as to produce protuberances. When the layers of foil are superimposed, as by winding the foil on itself, the protuberances at the holes 23 space the adjacent layers from one another, as shown considerably magnified by Fig. 3. Heat conduction from one layer of foil to the next through the protuberances is slight. Although five layers of perforated sheet metal are shown assembled in Fig. 3, a greater or lesser number can be used as occasion may demand. The foil may have a thickness of about $4, inch. This structure provides a steep heat gradient path transversely from the interior to the exterior wall, thereby permitting the interior of the heat shield to be quickly heated and economically maintained at an elevated temperature. The-heat shield may operate with its interior wall at a temperature as high'as, or even higher than, the normal operating temperature of the ribbons I to I2, or whatever form the electron-emitting members may assume. For example, the heater I may be operated at a temperature of about 1500 to 1800 C. The heat shield I8 may be constructed and proportioned to become heated at its interior surface to a temperature of 850 C. The ribbons, when radially mounted, receive little direct heat from the heater I, are mainly heated by radiation received upon their activated surfaces from the heat shield. Their equilibrium temperature will therefore be nearly that of the heat shields. During operation, they are cooled by electronemission and their temperature may be lower than the heat shield. Egress of electrons emitted by the ribbons I to |2, or other emitting part of the cathode, occurs through a plurality of apertures 25, two of which are shown in Fig. l, which are so spaced in the cover 26 of the heat shield as to provide an opening for each of the spaces between the ribbons I to I2. The apertures may be located in any other part of the heat shield, at the sides for example.

Cathodes constructed "according to Fig. l (the cathode and heat shield assembly being indicated as a whole by the numeral 29) may be quickly heated to an electron-emitting temperature by operating the heater during the starting, or warming-up period at a temperature above the normal operating temperature, and as high as about 2300 C., and then reducing the temperature to a desired operating value. In this way the starting period may be as short as 30 seconds.

A cathode constructed as herein shown and described may be mounted in any suitable electron discharge device of which many forms are known. For illustrative purposes, there has been shown in Fig. 4 one such device comprising an elongated glass envelope, or container, 28 in which is mounted the assembly 29 (such as herein shown), an anode 30, and a grid 3|. The envelope contains also a charge of gas, such, for example, as argon, or other rare gas, which may be at a pressure within the limits of several microns to several millimeters; or an easily vaporizable metal, such as mercury or caesium, as indicated at 32, or some combination of these. The anode may consist of graphite or metal. It is mounted on and connected electrically to a conductor 33 which is sealed in a press 33. The grid 3| may have the form of a perforated plate and may consist of graphite. It is attached by the screws 34 to a tubular shield 35 which is supported at the anode end upon a glass stem by a clamp 36 and at the cathode end by wires, two of which are shown in Fig. 4 as 31, 38. These support wires are spaced from the cathode by insulators (Fig. 2), two of which, 39 and 40, are shown in Fig. 4. This structure is mounted, as indicated, upon a stem 4| by the clamp 42. External circuit connection to the grid is provided by a sealed-in conductor 43. The shield 35 extends in opposite directions from the grid, overlapping both the anode and the cathode. A capsule 44 may be provided for supplying getter material as well understood.

The

cathode wires

20, 2|, and 22 are sealed into the stern 4| and lead to external contact devices (not shown). Current for operating the heater I may be supplied by the

conductors

20 and 22. Connection to a circuit (not shown) for supplying space current may be made by the conductor 2|. If desired, the interior surface of the heat shield may be coated with oxide, or

other activating material, for example, barium or strontium oxide, or both, for replenishing the coating on the electron-emitting ribbons.

Electron emission does not occur to an appreciable degree from the heat shield which is connected to the cathode by a circuit containing an impedance 48. This impedance may take the form of a resistor having a resistance within a range of one thousand to one million ohms. For example, when the impedance 48 has a resistance value of 10,000 ohms, the heat shield space current to the anode is only .001 or one per cent of the normal cathode current. This feature is described and claimed in my copending application, Serial No. 213,228, filed June 11, 1938. In Fig. 4, a high resistance element connected between the heat shield and the active emitting part of the cathode is indicated diagrammatically by the rectangle 48.

Of course, if desired, the impedance (resistor) 48 may be omitted, the heat shield being connected directly to the coated ribbons and may take part in the emission of electrons.

The configuration and arrangement of the electron-emitting members may be varied without departing from my invention. For example. in the modification shown in Figs. 6 and '7, the electron-emitter 48, which may consist of wire gauze, as described in connection with other figures, has the form of a helically-wound ribbon, the turns of which are spaced apart. The emitter 48 surrounds in spaced relation a heater 48 whereby it is heated to an electron-emitting temperature, the heat being radiated by the heater and reradiated by the heat shield 58. The ribbon 48 is supported by longitudinally extending spaced support wires, two being indicated at 58 and 5|. The wires 50, 5| pass through insulating bushings 52 and 58, and serve also as current-conveying conductors. One end of the heater 49, which may consist of tungsten, or other suitable refractory material, is connected to a conductor 54 passing through an insulating bushing 55. The other end is connected to a plate 56 and a metal yoke 51 (see Fig. 7). These members in turn are carried by and connected to the heat shield 58. The latter may consist of multiple layers of thin sheet metal as described in connection with Figs. 1 3. The ribbon 48 may be coated .with emission-enhancing material, such as alkaline-earth oxide.

The emission of electrons from the heated ribbon 48' to the anode or anodes (not here shown, see Fig. 4) occurs through a circular aperture 59, although apertures may be provided elsewhere, as shown for example in the modification illustrated by Fig. 8. Electric energy for the heater may be derived from any suitable source, as here represented by a transformer 60. A high resistance connection between the electron emitter and the heat shield is provided at 8|. Electric connections to the electron emitter 48 and to the anode (not shown) from a source of current (represented by the terminals 62) are indicated at 63 and 63', a resistance 84 being included. The ribbon 48 being supported circumferentially intercepts a greater proportion of heat radiated from the heater than the radially supported ribbons of Figs. 1 and 2, and hence is heated even more rapidly to an electron-emitting temperature. In a copending application, Serial No. 261,395, filed March 11, 1939, by Thomas A. Elder, specific claims are made for the structure of Figs. 6 and 7.

In the structure shown in Fig. 8, the heater 85 consists of a closely woven wire-mesh structure which dispenses a supply of barium or some compound of barium, or other material for enhancing the electron emission of the electron-emitting members 88 and 81. Such a structure is described in my prior Patent No. 2,107,945, issued February 8, 1938, but instead of as described in this patent, the heated structure being itself an electron emitter, it here dispenses by vaporization a supply of electron-emissive material for detached uncoated electron emitters, only two of which are shown at 86 and 61. A greater number may be used.

The dispenser 65 may consist of a sheath 68 of closely woven wire, for example, molybdenum, and contains a supply of alkaline-earth oxide, or a mixture or compound of an alkaline earth oxide with a refractory oxide, such as alumina (with or without an associated reducing agent). The oxide (and apparently also alkaline earth metal which is formed by reduction during operation) slowly migrates to the surface of the dispenser and evaporates, thus maintaining an active coating on'the electron emitters. In the structure illustrated by Figi 8, the dispenser 65 is provided with a coreconsisting of a wire 69 surrounded by a coating 10 of alumina, ceria, or other suitable refractory material around which is packed the source composition ll from which the activating materialis derived. The coating 10 may be applied as a paste or may be preformed. The purpose of the coating 10 is to insulate the wire 68 from the source material in case this wire is used as a heater to supply part of the heat. In case the wire is merely a mechanical support, or a return lead for the heating current, the coating "still is desirable but is not necessary. The source composition H here is shown as granular, although it may assume other forms.

The dispenser may be heated by current passing through the core wire 88 and the. sheath 68 which are connected in series with one another by the metal plug 12. The core wire is supported by a plate 13 which rests on theinsulators 14 which in turn are supported by the heatshield 15. The heat shield 15 is provided with apertures 16 for the egress of the discharge during operation.

Assuming the composition H to consist of barium oxide, with or without a reducing agent such as molybdenum, its operation at an elevated temperature, such as 1150 0., results in the migration from the interior to the exterior of the closely woven mesh 68 of material for enhancing electron emission to the surface of the dispenser where it evaporates and is condensed on the electron emitters 66 and 61 which are heated to a materially lower temperature, say to about 800 C. These ribbon or plate structures 66 and 61, which as indicated, may consist of wire gauze, are initially uncoated with oxide, the metal being clean and bright and yet becoming thermionically active without being visibly coated, indicating that the deposited activating material need be present only as an exceedingly thin and apparently monomolecular layer. Any appreciable coating which may accumulate slowly on these structures during life apparently is not necessary for the electron emission. Only a minute amount of material is vaporized from the dispenser. Hence, the dispenser has a long life.

In my application Serial No. 266,803, filed Ap i 8, 1939, claims are made on discharge tubes having dispenser cathodes. They are described herein because they embody also the present invention.

As shown by Figs. 9 and 9a, the dispenser when of small diameter may be unprovided with a metal core. It is shown as being wound on an insulating support 19. The dispenser 80 of the structure shown in these figures consists of a woven mesh of molyibdenum or the like which is filled with barium oxide or other suitable source material. The sheath 8i and the filling 82 are shown in Fig. 9a, in which the sheath is shown partly cut away. The required heat is produced by current traversing the sheath from a source which has not been shown in order to simplify the drawings.

As shown by Figs. 10 and 11, the electronemitting structures may consist of thin convolute sheet metal 83 on which is applied a woven wire structure 84 for increasing their area. The structure 84 assists in holding in place the oxide or: other coating materials in those structures in which an oxide coating is applied to the emitting members. It serves to increase the electronemitting area in cathodes of the dispenser type in which no coating is applied to these members.

One of the advantages of radiation-heated electron-emitting members (as contrasted with filamentary structures which are heated by passage of current) is the uniformity of electron emission over their surface, the emission not being afiected by inequality of electric potential. While my invention has been indicated as being particularly applicable to high-powered devices of large current loading, the advantagesresulting from my invention are obtainable over a wide range of current loadings.

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

1. In an electric discharge device, the combination of an apertured heat-confining enclosure, an unshielded heater therein, one or more ribbon-shaped electron-emitting members located in said enclosure in a position to receive directly heat radiated by said heater and support means for said electron-emitting members, said means providing substantial heat insulation from contiguous heated parts.

2. In an electric discharge device, the combination of an elongated radiation heater, an electron-emitting ribbon consisting of thin material of low heat conductivity having provided thereon a material of high electron-emissivity, means for supporting said ribbon in unobstructed radiation-receiving relation to said heater, said supporting means being substantially thermally insulated from said heater, and a surrounding heat shield arranged and proportioned to effectively reflect and reradiate upon said ribbon heat received from said heater, whereby said ribbon may be uniformly heated to electron-emitting temperature.

3. In an electric discharge device, the combination of an elongated unshielded resistance heater, one or more electron-emitting ribbons of foil material arranged in unobstructed radiationreceiving relation to said heater, means having poor heat conductivity for supporting said ribbons, and means for reflecting heat radiation emanating from said heater upon said ribbons.

4. An electrical discharge device structure comprising the combination of an elongated resistance heater, a plurality of ribbons each having electrically activated surfaces of extensive area and edge surfaces of negligible area, means for supporting said ribbons about said heater in thermally non-conducting relation therewith, the ribbons being arranged to present only their said edge surfaces to the heater whereby they are substantially nonobstructive to heat radiated directly therefrom, and a housing enclosing the ribbons and constructed to reflect heat emanating from the heater upon the said activated surfaces.

5. In a gas ionization discharge device, the combination of a rod of refractory insulating material, an elongated radiation heater mounted thereon, a plurality of oxide-coated wire gauze members positioned radially about said heater. supports for said members mounted near the extremities of said heater, a thermally insulated enclosure having one or more apertures surrounding said members and being supported in unobstructed heat-receiving relation to said heater, said enclosure consisting of a plurality of laminae assembled in substantially heat-insulated relation to one another, and being proportioned to eiiectively reradiate heat received from said radiation heater, whereby said members are heated throughout to an electron-emitting temperaturaag' and means for supplying energy to said heater.

6. An assembly for an electric discharge device comprising the combination of a radiation heater, a spaced heat-conserving enclosure therefor having an aperture for the passage of a discharge. one or more electron-emitting members located therein, said enclosure and members both being positioned to receive heat by radiation directly from said heater, and support means for said members providing substantial insulation for heat conduction thereto, said emitting members being so constructed and so arranged within said enclosure that opposite surfaces can eflectively contribute electron emission for maintaining an electrical discharge emanating therefrom.

7. An assembly for an electrical discharge device comprising the combination of an apertured enclosure constructed to provide a steep, transverse heat gradient through the wall thereof, an unshielded filamentary radiation heater supported therein, conductors for supplying an energizing current to said heater, a plurality of electrically connected foil strips arranged about said heater in said enclosure and spaced from one another in position to be heated by direct heat radiations from said heater but not wholly shielding said enclosure from such radiation, said strips being provided with activating material for enhancing electron emission, support means for said strips arranged in non-heat-conveying relation to said heater, and a load circuit conductor connected to said strips.

8. An electrical discharge device comprising the combination of an envelope, a charge of attenuated gas therein, an anode, a cathode consisting of one or more electron-emitting ribbons, a separate heater therefor, said ribbons being coated with a material for enhancing thermionic electron emission, means aifording negligible heat conductivity supporting said cathode, a substantially insulated,

electric connection for conducting an energizing current to said heater'and a separate connection for conducting a load current to said cathode.

9. In a gas ionization device, the combination of an apertured heat-confining enclosure, an unshielded resistance heater of refractory metal located in said enclosure, a plurality of ribbon cathode elements arranged about said heater in said enclosure to present substantially only their edge surfaces to said heater, a material for enhancing thermionic electron emission of said ribbons, means for electrically connecting said elements to one another and support means for said cathode elements afiording negligible heat conductivity between said heater and said elements.

ALBERT W. HULL.