US2416661A - Dispenser type cathode electric discharge device - Google Patents

Description

Fig.2.

Inventor: El I iott J,. Lawton,

. E. J. LAWT ON Filed llay 28, 1943 -&

' raw Feb. 25,1947.

, Patented Feb; 25,1947

,- DISPENSER TYPE CATHODE ELECTRIC DISCHARGE DEVICE Elliott J. Lawton, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application May 28, 1943, Serial No. 488,825

My invention relates to electric discharge 'de- 13 Claims. (Cl. 250-275) It is a still further object of my invention to provide a new and improved electric discharge device which establishes current conduction between the anode and thecathode by virtue of an arc discharge within a short interval of time, such as a microsecond or less, and wherein the elements are arranged to permit a very rapid rate of'rise of the anode-cathode current without unduly lengthening the deionization time of the device.

It is a still further object of my invention to g provide a new and improved electric discharge device of the arc discharge type which is susceptible of transmitting between principal electrodes, such as the anode and cathode, current of relatively large value without aiiecting deleteriously the effective life of the cathode.

Briefly stated, in the illustrated embodiment of my invention I provide a new and improved construction for electric discharge devices of the type employing an ionizable medium, such as a gas or a vapor. The ionizable medium may comprise mercury at a suitable pressure preferably ranging from one or several microns to twentyfive microns of mercury for the following described range of voltages and operating frequency. A cathode construction is employed wherein a metallic member comprising a metal chosen from the class of metals such as tungsten'and molybdenum is formed to have a concave surface facing the anode and which serves as the emissive part of the cathode structure. Within the concave region formed by the metallic member there is placed, preferably in a horizontal position, a dispenser element which may be described generally as being of a stocking-like nature defined by a plurality 'of interlaced or woven fine molybdenum wires and containing a eutectic mixture capable of dispensing an acti vating material which is deposited continuously during operation of the device upon the concave surface of the metallic member, thereby rendering it electron emissive to an extent suillcient to support the arc discharge. The configuration of the metallic member is such that it extends substantially beyond the dispenser element, in at least one and preferably two or more sides, towards the anode, thereby serving to reflect a substantial part of the radiant energy incident to the operating temperature of the dispenser element which is heated electrically by the transmission of electric current therethrough.

In order to minimize the flow of heat from the dispenser element in particular and the cathode in general, the cathode is provided with a cupshaped heat shield surrounding the aforementioned metallic member and dispenser element and which extends an appreciable longitudinal distance towards the anode. A5 a further means for minimizing the heat flow from the dispenser element, I provide a plurality of spaced metallic members, which may be planar in form, above the dispenser element in a direction towards the anode.

To accelerate the initiation of the are discharge between the anode and the cathode, in

accordance with the dictates of the potential variations of a grid structure interposed between the anode and the cathode, I provide a starting member, which may be termed a grid control starting wire, preferably supported by and at the same potential as the grid and extending towards the cathode and the planar metallic members thereabove but in spaced relation with respect thereto. Upon suitable variation in the potential of the grid and the grid startingwire causing a relatively high electric field gradient to be established between the end of the grid starting wire and the upper planar member I have found that the ionization time or the time of initiation of the arc discharge between the anode and the'cathode is substantially reduced and constitute a definite improvement over the arrangements provided heretofore in devices or systems where speed of cyclic operation is desired.

l"he structure which I provide is capable of operation at a high repetition rate; that is, the

device is capable of establishing current flow, between the anode and the cathode at rates equal to or greater than 600 amperes per microsecond, establishing conduction of anode-cathode current for a predetermined interval of time, and

effecting interruption of the anode-cathode current within an overall interval of time equal to or less than one microsecond. Furthermore, the discharge device described hereinafter is capable of performing the above described operation at a rateof 400 or more times per second.

In view of the foregoing, it will be appreciated that electric'discharge devices subjected to such strenuous cycles of Operation ordinarily would impose an inordinate burden or duty cycle on cathode constructions of the conventional type. The cathode construction described hereinafter affords long cathode life, ability to establish the above described cyclic current variations, permits the use of relatively high anode voltages within the neighborhood of 20,000 to 40,000 volts.

or greater, and affords a relatively short deionization time commensurate with the above described cyclic operation. As a further matter, the discharge device affords minimization of electric field stresses due to the optimum spacing of the electrodes and of an electrostatic shield associated with the anode and the grid.

For a bette understanding of my invention, reference may be had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims. Fig. 1 is a cross-sectional longitudinal view of an electric discharge device built in accordance with my invention, and Fig.

2 is a partial cross-sectional view taken at a plane displaced 90 geometrical degrees with respect to that shown in Fig. 1. Fig. 3 is a cross-sectional view showing the grid structure and the grid starting wire, and Fig. 4 is a plan view, shown partly-broken away, -of the cathode construction including the dispenser element and the planar heat shields positioned thereabove.

Referring now to Fig. l of the accompanying drawing, I have there illustrated my invention as applied to an electric discharge device including a, tubular metallic member i which defines a part of the enclosing structure or envelope for the elements of the device, and which is provided at its ends with flanged metallic end pieces 2 and 3 which are preferably welded or soldered to the inner surface of tubular member I near the ends thereof. The anode 4 is preferably of elongated nature as illustrated, being supported from the top of the enclosing and supporting structure by means of a threaded adaptor cap 5, a metallic end part 6 and a tubular vitreous insulator I, all of which constitute part of the enclosure structure or envelope of the discharge device. upper part 8 of the anode 4 may be of large diameter than the lower part in order to establish the optimum distances between the anode and the enclosing structure, particularly the inner surface of the vitreous insulator I. The anode may be constructed of any suitable metal; I have found that iron may be employed fo this purpose. If desired, the exterior surface of the anode maybe finished or ground as smooth as possible in order to minimize electric field gradients. The vitreous insulator I may be supported in turn by means of a metallic collar 9 which is sealed to the lower end of the insulator and which is also welded to the end piece 2. 3

In spaced relation between th restricted lower part of the anode and the inner surface of the enclosing envelope, I provide an appropriately spaced electrostatic shield I0 constructed of metal and having a tubular configuration provided with The rality of'wires defining spaces the areas of which and the aggregate effect thereof being determined by the requirements imposed by the arc ignition and the arc deionization times. In order to obtain the desired rate of. rise of anode-cathode current in accordance with the grid potential, I have found that a relatively coarse grid. structure should be employed. For example, one form of grid structure which I have found to operate successfully is one defining twelve substantially square openings per linear inch formed by 15 mil molybdenum wire.

The grid i3 is supported by a metallic grid supporting member I4 and which may be provided on the inner surface with a pair of adjoining flanged supporting members l5 and IS, the latter two members serving to hold the grid l3 in the desired spaced relation with respect to the anode, particularly the cooperating surface l2 thereof. I

The cathode construction which I provide and which is described in detail hereinafter is an improvement of the structures disclosed and claimed in U. S. Letters Patents No. 2,201,731 and No. 2,246,176, granted May 21, 1940, and June 17, 1941, espectively, upon applications of Albert W. Hull,

andwhich are assigned to the assignee of the present application. The cathode which I provide may be generally described as being of the dispenser type which comprises a stocking-like member including a eutectic mixture which continually produces or distils an activating material which is deposited on the cooperating surface of the cathode structure: thereby endering it electron emissive, and which may be considered in conjunction with the dispenser surface as the active surface of the cathode. The evolution of the activating material proceeds continuously during intended operation Olf the discharge device, and is effected by the transmission of, an appreciable value of current through the stocking or the dispenser element sufiicient to raise th temperature thereof to a value which distils or dispenses the activating material.

Referring more particularly tothe drawing, I provide an improvement which comprises a metallic member ll, preferably chosen from that class of metals consisting of tungsten and molybdenum, and which is arranged to have a form or configuration providing a concave surface 18 facing the anode of the discharge device. The

i member-ll may be corrugated as illustrated in Figs. 2 and 4 and may be generally described as having a pan or dish shape. I contemplate the provision of such a member which not only affords a relatively large projected area, viewed from the anode position, but which also has a suflicient longitudinal dimension along and beyond the dispenser element to be described presently and which effects the reflection of a substantial amount Oif the radiant energy due to the operating temperature of .the dispenser element.

Within the concave region defined by member I1, I provide a dispenser element l9 shown in longitudinal and planar views in Figs. 2 and 4, respectively. The dispenser element l9 comprises a stocking-like fabric defined by closely woven molybdenum wires and is preferably horizontally disposed near the bottom of member ll; The eutectic mixture within the. stocking fabric may comprise barium oxide and aluminum oxide in granular form which upon raising the temperature thereof to values within the vicinity of 1250 C. causes the dispensation of barium oxide from the stocking to the surrounding molybdenum- 5 member i1, rendering the latter electron emissive.

The metallic member I! and the dispenser element I 3 are surrounded by a heat shield 20 which may have a cup-shaped configuration as illustrated, and'may comprise a plurality of laminations of. relatively thin prick-punched metal. The heat shield, it will be noted, extends appreciably in a longitudinal direction beyond the aioredescribed cathode elements and may be provided with a metallic face plate 2|.

The dispenser element I9 is supported by and maintained in insulated relation with the bottom surface of the metallic member I! by means of refractory insulating supports 22 and 23, each of which may comprise a plurality oi appropriately formed segments held together by means of threading, supporting and retaining wires (not shown).

Electric current for raising the temperature of the dispenser element is to the desired operating value may befurnished to this element by means of a pair of conductors 24 and 25, the former of which is electrically connected to the tubular member I through a strip conductor 26 and the latter of which is insulated from the cathode structure parts l1 and 20 by means of a tubular insulator 21, and which is in turn connected to an externally accessible terminal 28 through a strip conductor 29 and conductor 30. Conductor 30 is sealed to the enclosing envelope by means of a sealing structure 3| which may comprise a pair of metallic collars 32 and 33 and an intervening vitreous tube 34. The sealing structure Il may employ metals, such as alloys of iron, nickel and cobalt or variations thereof.

In order to further minimize the flow of heat from the dispenser element [9, I provide means positioned above the dispenser element for reflecting the radiant energy incident to the operdischarge device which I provide in that they offer a point or a'plane with which an arc accelerating means, such as grid starting wire 31, cooperates to establish a relatively high electric field gradient in accordance with the dictates of the grid potential. The grid starting wire 31 is preferably electrically connected to the grid supporting structure, principally member I 4 or member l6 as. shown, and the potential thereof consequently varies in accordance with the potential of the grid- I have found that in one type of electric discharge device which operates successfully that the end of the grid starting wire 31 may extend towards the plane of member 35 to establish'a distance of approximately one-quarter of an inch therebetween. The plan view of Fig. 3 shows the manner in which the grid starting wire 31 may be supported to the grid structure, principally the grid supporting member i6.

Electrical connection to the grid structure including the meshgrid l3 and the grid starting wire 31 may be accomplished by, means of a metallic strip conductor 38 which is supported between a pair ofinsulating washers 39 and I fastened to the cathode structure, and which is ultimately connected to an externally accessible terminal through another conductor 4|, and a lead-in conductor 42 (Fig. 2), the latter of which may be provided with a sealing structure 43 6 similar to the sealing structure 3i described hereinbefore.

The grid-structure may be mechanically supported in the position illustrated by additional annularly spaced members such as: strips of metal which are welded at their upper-extremities;to

the member l4 and are supported at theirturnedin lower extremities by means of insulators ll means of a suitable mechanical expedient which may comprise a bolt 41 and nuts II. The entire above described cathode, structure which insturn supportsthe grid structure, may be held ini-the of a' plurality of circumferentlally spaced mechanical supports only two of which, 49 and" 33,

are illustrated. These mechanical supports may consist of strips of metal-such asiron or steel and which are welded tothe inner surface of end piece 3. Desired radial orv horizontal spacing may be obtained by attaching to each of the latter members a flanged strip it which engages the inner surface of tubular member I, thereby accurately centering the entire structure particularly the cathode and grid elements. V 1 A vitreous tubulation 52 maybe employed .for

evacuation purposes and :may be sealed to the end piece 3. Where an. ionizable mediumisuch as mercury is employed,.the mercury to a certain extenttends to collect in "this tubulation -or appendix. ,The temperature of the mercury may be controlled by providing anopen-ended hollow tube 53 which may beof metal surrounding the tubulation 52 and mayalso beafilxed to prising the transmission of a series of distinct pulses of current between theanode and'cathode, so an ordinary oxide coated cathode is subjected to inordinate operating burdens; Th'e abovedescribed typ of construction forthecathcide arfords a vsuiliciently long'life'foruthecatl' de in particular, and the. discharge: deviceiin general, A I

55 even whensubjected to this typeof operation; I

Furthermore, in the transmission: of a'pulseoi current having a duration of aone tenthtotwo tenths of a microsecond, the potentialoftheg'rid is caused to become positive temporarily,the so the magnitude of the positive variationsof grid potentialgrising to the orderx of -20-to'50*volts. As is well appreciated bylthoseskilled-intheart,

if the potential ofi any .of :the discharge ;de'- vice efi'ectively cooperating: in the I are discharge as phenomenon rises above anparticular valuefor the specificionizing mediumiemployed, theposi-v tive ions of the medium are accelerated towards the cathode structure thereby dislodging thej ac tivating material of the cathode and causing 7o cathode sputtering, irregulanbperation and re-- duction in the effectivelife o'fthedevicen However, in the above described device' 'employ'ing the improved form of "dispenser cathode'in-whlch the emitting surface ofiers alargeprojected area.

facing the anode and:whichissbeing' continuand 43 which are held in closespaced relationship, thereby securing the supporting members by desired axial or longitudinal positions by-means end piece-,3. A chimney: effect may be obtained actual life of the discharge device will be determined by the amount of the eutectic mixture which can be packed into the molybdenum stocking which, in turn, is controlled principally by the physical dimensions by which the designer is limited.

By the optimum choice of the grid structure, particularly the size of the wires and theopenings defined therebetween, the amount of grid emission-may be kept under control. In addition, theplanar heat shielding elements or members 35 and 36 in spaced relation between the dispenser element i9 and the grid I3 serve to limit the amount of radiant energy which is transmitted to the grid l3, thereby maintaining the temperature below that value which would tend to cause appreciable grid emission.

The spacing between the anode 4, the grld structure and the anode shield I is critically established in accordance with Paschens law in order that the device may be capable of withstanding large forward voltages and 1arge inverse voltages. Paschens law refers generally to the relationship between. the sparking or break-down voltage and the product of pressure of the medium and distance between surfaces of the elements. The spacing of the aforementioned elements of the discharge device is such that the maximum spacing or the distance between the remotest parts is such as to assure'a large breakdown voltage or forward-voltage rating. It will 5' be noted, by referring to the drawing, that substantially the same distance is maintained be- In accordance with one aspect of my invention,.the spacing between the electrode structure. and the enclosing envelope is maintained at a relatively small value thereby reducing the tendency to establish ionization between the electrode structure and the envelope. It will be noted that, for example, the spacing between the enlarged .part 8 of the anode and the vitreous insulator I and the metallic member 6 is of a relatively'small value compared with the total volume of the discharge device, or the transverse dimension. thereof. If large spacings were employed, the volume becomes important and may cause deleteriousefiects incident to the production of ionization by stray electrons reaching into the vicinityof the glass-to-metal seals between-parts 6. and 1-, and I and 9. Such stray electrons in the absence of the optimum design which I provide would be instrumental in charging the glass within the region'of the glass-to-metal seals. As an example, if the charge which is produced by such stray electrons establishes a potential difference of only 1 volt. in close proximity to the. negative metal portion of the seal there may well be established avoltage gradient as large as 10 volts per centimeter,

thereby causing field emission electrons to leave the metal surface resulting in undesired voltage breakdown. Accordin ly, it will be appreciated that by reducing or minimizing the volume between the electrode parts and the envelope, particularly near the glass-to-metal seals, the tendency to produce ionization by the stray electrons within the region is substantially reduced or eliminated. 1

While I have disclosed and illustrated my inventio as comprising an electric discharge device including elements having particular configuration, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention, and I, therefore, aim in the appended claims to cover all such changes and modifications as fall 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. In' an electric discharge device comprising I a plurality of enclosed electrodes including an anode, a cathode construction comprising. a member concave towards the anode for providing an electron emissive surface, and a dispensertype element lying withinthe concave region of said member for emitting activating material which is deposited on the surface of said mem ber.

2. In an electric discharge device comprising a plurality of enclosed electrodes includin an anode a cathode construction which comprises a metallic member of the group including molyb- 'denum and tungsten and having a configuration to provide a concave surface towards said anode and a dispenser-type'element for continuously genera-ting an activating materialyihich is deposited on the concave surface ofsaid member thereby rendering it electron emissive.

3. In an electric discharge device including a plurality of enclosed electrodes comprising an anode, a cathode construction including a metallic member formed to provide a concave surface facing said anode and comprising a metal 4 chosen from the group consisting of tungsten and molybdenum, a horizontally positioned dispenser element lying within the concave region of said member, and means for supplying heat to said dispenser element to cause the dispensa- 5 tion of activating material therefrom which is deposited on the concave surface of said metallic member thereby rendering it electron emissive.

4; A cathode construction for an electric dis- 55 charge device including a plurality of electrodes comprising an anode and a cathode which includes a metallic member concave towards the anode and having therein a horizontally positioned dispenser element for producing an ac- '60 tivating material which is deposited on the concave surface of said metallic member thereby rendering it electron emissive, said metallic member having a concave configuration such that a substantial portion of the radiant energy due to the heating of the dispenser element is reflected toward said dispenser element.

5. A cathode construction for an electric discharge device including an anode and a cathode- 6. A cathode construction for an electric discharge device including a plurality of enclosed electrodes including an anode and a cathode which comprises a corrugated molybdenum panshaped metallic member concave towards said anode, and a dispenser element within the concave region of said member comprising a eutectic mixture for evolving an activating material which is deposited on the concave surface of said member thereby rendering it electron emissive.

7. A cathode construction for an electric discharge device including a plurality of enclosed electrodes including an anode and a cathode which comprises a corrugated molybdenum panshaped metallic member concave towards said anode, a dispenser element within the concave region of said member comprising a eutectic mixture for evolving an activating material which is deposited on the concave surface of said member thereby rendering it electron emissive, and electric insulating means for supporting said dispenser element in position.

8. A cathode construction for an electric discharge device including a plurality 01' electrodes comprising an anode and a cathode which comprises a metallic member having a configuration concave towards said anode, a dispenser element lying within the concave region of said member for producing an activating material which is deposited on the concave surface of said member: thereby rendering it electron emissive, ancl ia heat shield surrounding said member and said element and extending an appreciable longitudinal distance towards said anode.

9. A cathode construction for an electric discharge device including a plurality of electrodes comprising an anode and a cathode which includes a metallic member concave towards said anode and having therein a dispenser element for producing: an activating material which is deposited on the concave surface of said member thereby rendering it electron emissive, and a metallic member positioned above said dispenser element in spaced relation between said anode and said dispenser element for limiting the heat flow from said dispenser element.

10. A cathode construction for an electric discharge device including a plurality of enclosed electrodes comprising an anode and a cathode which comprises a metallic member concave towards said anode, a horizontally positioned dispenser element within the concave region of said member for dispensing an activating material which is deposited on the concave surface of said member thereby rendering it electron emissive, and a planar metallic member positioned above said dispenser element for limiting the heat flow from said dispenser element.

, 1o 11. A cathode construction for an electric discharge device including a plurality of enclosed electrodes comprising an anode and a cathode which comprises a metallic member concave towards said anode, a horizontally positioned dispenser element within the concave region of said member for dispensing an activating material which is deposited on the concave surface of said member thereby rendering it electron emissive, and a pair of longitudinally spaced planar metallic members positioned above said dispenser element for minimizing the heat flow from said dispenser element.

12. In an electric discharge device including a plurality of enclosed electrodes, the combination comprising an anode, a grid in spaced relation with said anode, and a cathode including a metallic member concave towards said anode and having within the concave region formed thereby a dispenser element for producing an activating material which is deposited on the concave surface of said member' thereby rendering it electron emissive, and means for accelerating the initiation of an electrical discharge between said anode and said cathode comprising a starting wire supported by said grid and extending towards said cathode.

13. In an electric discharge device of the type employing an ionizable medium and including a plurality of enclosed electrodes, the combination comprising an anode, a grid in spaced relation with respect to said anode, and a cathode comprising a metallic member concave towards said anode having positioned within the concave region a, dispenser element which produces an activating material which is deposited on the concave surface of said member, a planar metallic memmember positioned above said dispenser element, and means for accelerating the initiation of an arc discharge between said anode and said cathode in accordance with the potential variations of said grid comprising a conductive member supported by the grid structure and extending towards said planar metallic member.

ELLIOTT J. LAWTON.

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

UNITED STATES PATENTS

Images