US2201720A

US2201720A - Thermionic cathode structure

Info

Publication number: US2201720A
Application number: US261395A
Authority: US
Inventors: Thomas A Eider
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1938-11-30
Filing date: 1939-03-11
Publication date: 1940-05-21
Anticipated expiration: 1957-05-21
Also published as: FR51138E; US2201721A; GB536070A; FR861969A; NL55022C; BE438749A; GB542495A; US2201731A; US2246176A

Description

May 21, 1940.

T. A. ELDER THERMIONIC CATHODE STRUCTURE Filed March 11, 1959 2 Sheets-She'et 1 VIII/177mm Ill/1771i Inventor Tho'm as AB! der; by W mii m May 21, 1940. T. A. ELDER THERMIONIC CATHODE STRUCTURE Filed March 11, 1939 2 Sheets-Sheet 2 Fig. 5.

Inventor". Thom as A.E|der",

- His Attorney.

Patented May 21, 1940 UNITED STATES ATENT OFFiC 2.201.120 'rnsamomc carnons smucruna New York Application March 11, 1939, Serial No. 261,395

9 Claims.

The present invention relates to electrical discharge devices and comprises, in particular, an improved cathode structure. As a consequence of my invention I have provided thermionic cathodes which have improved characteristics and, in particular, are so constructed that more efl'ective utilization will be made of electron emission from the entire emitting surface of the cathode. Y

Electronic discharge devices heretofore have been provided with ribbon-shaped, electronv emitting members which are arranged to be heated to emitting temperature by passage of current. Such cathodes, by reason of differences of potential between their terminals due to the heating current, have not been characterized by uniformity of electron emission throughout their surface. Concentration of current occurs at the negative terminal of a cathode when the go cathode is heated by passage of current. In-

equalities in resistivity leading to inequalities in temperature also are a source of non-uniformity. Unequal distribution of electron emission in a thermionic cathode leads to shortened gg-life. Difficulties also were presented in con-v structing and supporting such cathodes in order torender them resistant to deformation during use.

Separately heated thermionic cathodes having so- 'a cellular or hollow structure also are characterized by inequalities of electron emission. Electron emission is aptto occur preferentially from the more accessible parts of such cathodes, that is, near an orifice or orifices.

In accordance with my present invention, I

have provided improved ribbon-shaped filamentary cathodes which are heated by a separate heater instead of bypassage of current, and hence are free from such disadvantages.

(o Another feature of novelty is a construction which is adapted to resist deformation.

Embodiments of my invention will be shown in connection with the accompanying drawings in which, respectively, Fig; 1 is a side elevation,

partly in section, of a rectifier provided with such improved cathode; Figs. 2 and 3 are longitudinal sections of two modifications of cathodes in which the emitter is a helical ribbon 50 which is provided with a separate heater; Fig. 4 is a top view of the cathode shown in Fig. 3; Fig. 5 is aside elevation, partly in section, of a. modified electrode construction including a cathode embodyingmy invention; Fig. 6 is a sectional .view taken at the region indicated at,

H; and Figs. 7 and 8 are diagrams of electrical connections.

My invention may be embodied in a cathode comprising a ribbon, which preferably is of helical configuration. The ribbon is supported, as will be described, with adjoining turns spaced apart so as to provide an open structure. Preferably, the ribbon itself is made of mesh or other foraminous materials. The cathode preferably is coated with alkaline, earth oxide, or other suitm able material for enhancing electron emission.

In cathodes constructed in accordance with my invention the better utilization of electron emission from the entire emitting surface of the cathode, due to its open structure, makes possible 15 a' reduction in size of cathode for a given power rating. Its construction also makes possible a reduction of time required for the cathode to assume operating temperature when starting from a cold state and an increase of time required to cool to the cold state. This latter the usual base structure! which'is provided with contactplugs 3 as well understood. The envelope is-provided with acharge of ionizable gas,

ordinarily at reduced pressure. The gas content may be argon, neon, xenon, helium, nitrogen, or other of the class 'of so-called non-condensible gases, or it may consist of a vapor of a a material such as mercury, or caseium, which is condensed at ordinary temperatures, or of a mixture of such gases and vapors. A quantity of mercury is indicated at 5 which is a source of mercury vapor. Gas pressures of about one to several hundred microns ordinarily are suitable for yielding sufficient ionization to result in an arc-like discharge. For example, an otherwise evacuated ,bulb containing a. quantity of mercury at 40 C. bulb temperature will contain mercury vapor at a pressure of five or six microns. The anode 7, carried by a stem 8, leading to an external contact 9, may consist of metal, graphite, or other suitable material, and may be of any well known construction. The assembly I, consisting of the cathode and auxiliary parts, is supported by a glass stem II, which also carries the usual getter capsule l2 and exhaust tubulation.

As shown in Figs. 2 and 3, the assembly oomprises a helical ribbon cathode l4, a heater l5, and a surrounding heat shield I6. The cathode [4, shown in Fig. 2, consists of crimped wire mesh material. The crimping increases the surface available for electron emission, but uncrimped structures also may be used as shown in Fig. 3. The cathode l4, which may consist of nickel, molybdenum, tungsten, or other suitable metal, is coated with electron-emitting materials as, for example, a mixture of barium and strontium oxides in accordance with well known practice.

The cathode I4 is supported by a frame which may consist of two longitudinal metal rods l8, IQ of nickel, or other suitable metal, which are insulated from the shield l6 by the

insulators

20, 2|. A ring-shaped plate 22 may be provided as a. steadying support at the bottom of the cathode structure, tension being applied against this plate by the

external nuts

23, 24. An insulator I3 is interposed between the plate 22 and the heat shield, thus insulating the cathode therefrom. The cathode preferably should be welded to the rods l8, [9 at the regions of contact, the rods acting as current collectors for the respective sections of the cathode.

The heater l5, which may consist of a suitable refractory metal, for example, tungsten, is connected at its upper end to a metal cross rod 25 which is connected to the support rods l8, I9 whereby current may be conveyed, thus making the several rods and helical cathode a unitary structure. Contact to the opposite end of the heater may be made by a terminal wire 26 which, as will presently be described, leads through the intermediary of one of the contacts 3 to an external source of heating current.

The heat shield I6 is made up of a plurality of spaced layers which may consist of metal. Relatively rigid inner and outer walls may be provided, as shown, between which is supported metal foil in loosely spaced relation, as described in British Patent 383,645 of November 24, 1932. A cover 28, which also has a plurality of spaced metal walls, is secured to the cylindrical part of -the shield by a reenforcement 29. An orifice 30 is provided in the cover for the passage of electron current emitted by the cathode. Other heat shield structures may be employed for the cathode as well understood.

The construction of the cathode shown in Figs. 3 and 4 is similar in most respects to the cathode shown in Fig. 2. The helical cathode l4 in this modification is not crimped. It is supported by the rods [8 and I9 and rests at its lower end directly on the insulators 32, 33. An insulator 34 is provided for the heater terminal 26. The cathode l4 also is coated with suitable material for enhancing electron emission. The orifice 35 (Fig. 4) is annular. The ribbon l4 also is indicated in the drawings as consisting of mesh material, but it may consist of non-foraminous metal. In both structures shown in Figs. 2 and 3, the ribbon is arranged flatwise or crosswise with respect to heat radiations emitted by the elongated filamentary heater, thereby increasing the direct heating of the cathode. Heat also is received by radiation from the interior of the heat shield which is highly heated.

In the modification illustrated by Fig. 5, a plurality of orifices are indicated at 31, 38, 39 and 40

ports

43, 44 terminating in a clamping ring 45 which is mounted on the stem II. The shield 42 may function as a grid, and when so used preferably is provided with an inner ring 48 provided with an orifice 41. In a rectifier the ring 46 may be omitted.

In this modification the anode 48 consists of a shallow cup, which may consist of graphite, as indicated, or of metal or other suitable material. abutting on and nearly closing the upper end of the outer shield 42. A plate 49 nearly closes the lower end of the shield 42. The surrounding envelope has been omitted in the drawings. In contains a gas or vapor at low pressure, as described in connection with Fig. 1.

Two forms of electrical connections are shown in Figs. 7 and 8. Referring to Fig. 7, the heater I is connected by the conductors 50, 5| to a. source of energy, as represented by the secondary winding of a transformer 52. The cathode supports I8, l9 and the cross rod 25 are included in this circuit. The cathode l4 and the anode l are respectively connected to a load circuit, as represented by the

conductors

53, 54, the latter containing a resistor 55 as representative either of a load, or of a limiting resistance for a load which is not shown. Fig. 7 shows a high resistance element 56 (e. g. 10,000 ohms or more) in a circuit 5'! connected between the cathode l4 and the shield IE, but this circuit and resistance may be omitted, the heat shield being electrically insulated from the cathode. A suitable source of electric energy is assumed to be connected to the terminals 58 during operation. Two of the con tacts 3 of Fig. 1 ordinarily would be included in the circuit conductors 50, Si, a third completing the load circuit 53 and a fourth connecting the resistance 56 in circuit if it is located external to the tube.

In the arrangement of Fig. 8, the circuit from the transformer to the heater I5 is completed by the conductors 59, 60, and this circuit includes the heat shield 16, but not the supports or other part of the cathode l4. As in the case of the system of Fig. 7, a resistor 56 of high ohmic value may be connected between the heat shield and the cathode, or, if desired, this connection may be omitted, the shield then being electrically leftfloating, that is, insulated from the cathode.

Electrical connections also have been indicated in Fig. 3, the secondary of the transformer 52 being connected between the heater terminal'26 and the shield terminal 6|, the cathode and the shield being connected through a high resistance 56 (which, as stated, may be omitted) to the cathode. The cathode conductor 53 and the anode conductor 54 terminate in contacts 58 to which an appropriate source of power is connected during operation of the device.

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

l. A structure for use in electric discharge devices comprising a radiation heater, a substantially helical, ribbon-shaped cathode having turns spaced apart with main heat-intercepting surfaces arranged crosswise with respect to the path of heat radiations from said heater in position to be heated to electron-emitting temperature by said heater, a load conductor connected to said cathode and a circuit independent of said load conductor containing a source of current for energizing said heater.

2. An electric discharge tube structure comprising the combination of an elongated radiation heater, an elongated electron-emitting member sinuously arranged in heat-intercepting relation to said heater, one or more current-conveying conductors supporting and electrically connected to said member at a plurality of spaced regions, and means for heat-shielding said member.

3. In an electric discharge tube the combination of an enclosure having an opening, a' heater located therein but spacially separated therefrom, a helical ribbon having turns spaced apart and surrounding and spaced away from said heater, a material capable of enhancing electron-emission coating the surface of said ribbon, conductive supports electrically connected to the several turns of said ribbon, and means for electrically insulating said supports from said enclosure, and separate current-conveying conductors respectively for said heater and said ribbon.

4. In an electric discharge tube the combination of a heat shield having an opening, an elongated incandescent filament therein, an oxidecoated, ribbon-shaped cathode of mesh material, means for supporting said cathode in spaced relation about said heater with the broad surface of said ribbon crosswise the path of heat radiations from said filament and separate currentconveying conductors for said heater and said ribbon.

5. A discharge tube structure comprising the combination of a heat-insulating enclosure, a radiation heater located therein, an electron emitter surrounding said heater in heat-intercepting relation, said emitter consisting of a ribbon helically wound with turns spaced apart from one another and being arranged fiatwise with respect to heat radiation from said heater, means electrically insulated from said enclosure for supporting said ribbon, a material enhancing electron-emission coating said ribbon and conductors which are respectively connected to said heater to convey heating current and to said emitter to convey load current.

6. A discharge tube structure comprising the combination of an elongated enclosure having an aperture, a filamentary heater longitudinally supported at the axis of said enclosure, a ribbon having sinuous turns spaced apart and arranged in heat-intercepting relation with respect to said heater, electronically activating material associated with said ribbon, a plurality of support wires welded to said ribbon at a plurality of regions intermediate the ends of said ribbon, means for insulating said wires from said enclosure, and electrical conductors leading respectively to said ribbon and'said heater.

7. An electrical discharge device comprising a sealed enclosure, a charge of gas therein at sumcient pressure to permit of the operation of an arc-like discharge therein, cooperating electrodes including an anode and a cathode, said cathode consisting of a ribbon helix, an elongated radiation heater which is axially positioned within said helix, means for heat-insulating said cathode, and means for separately conducting current to said heater and to said cathode.

8. An electrical discharge device comprising the combination of a sealed envelope, a charge of gas therein at a pressure within the limits of about one to several hundred microns of mercury, a ribbon-shaped cathode, a filamentary heater for said cathode, the turns of said cathode being spaced apart to permit of the passage of a discharge therebetween and being arranged crosswise with respect to said heater to intercept efliciently heat radiations from said heater, an apertured, electrically insulated heat shield sur-- rounding said cathode, a current supply circuit including said cathode for energizing said heater, and a separate load circuit connected to said anode and said cathode.

9. In an electrical discharge device, the combination of a filamentary heater, a ribbon-shaped cathode surrounding said heater and having turns spaced apart from one another and arranged in heat-intercepting relation to said heater, an apertured, electrically insulated heat shield surrounding said cathode, and connected to one terminal of said heater, a load conductor connected to said cathode and independent conductors including said heat shield for conveying heating current to said heater.

THOMAS A. ELDER.