US2162661A - Cathode for electron discharge devices - Google Patents

Description

June 13, 1939. A. w. HULL CATHODE FOR ELECTRON DISCHARGE DEVICES Filed oct. 22, '193e Inventor:

l `v ull, 6.49'I

Albert, W. DE 5V H is,j Att. Orrw efg Patented June 13, 1939 UNITED STATES oA'rHoDE For, ELEo'rnoN DISCHARGE DEVICES Albert W. Hull, Schenectady, N. Y., assgnor to General Electric Company, a corporation of Newy York Application October 22, 1936, Serial No. 106,999

10 Claims.

The present invention relates to electrical dis.- charge devices and in particular to the cathode structure of such devices.

In tubes of the power type, it is customary to 5 employ multi-cellular thermionic cathodes which are constructed to provide a plurality of cavities open to the exterior and having an internal electron-emitting surface which is of a large area compared to the Volume of the cathode. In cath- :odes of this sort, it is also customary to employ one or more heat shields in order to reduce the thermal losses, so as to maintain the cathodes at an operating temperature with energy which is only a small fraction of the energy required, for equal electron-emitting area, with ordinary cathode structures such as are commonly employed in high vacuum tubes.

Cathodes of the prior multi-cellular type, .employed in tubes of moderate and large power size, may be divided roughly into two groups:

(l) The type having longitudinal vanes with one or more surrounding heat shields, open at one end to permit the egress of electrons. A cathode of this type is shown in Patent N o. 1,924,318 issued 25 .jointly to W. A. Ruggles and me. Y i

(2) The disk type vof cathode which is also usually surrounded by one or more cylinders but is closed at each end and has openings about the periphery of the cylinder or cylinders for the dislcharge. A typical example of such a cathode is shown in Fig. 2 of my Patenty No. 1,924,319.

While cathodes of both of these groups are satisfactory in operation, they are open to the objection, at least to some extent, that the current density throughoutV the cavities formed by the variesV or disks is no uniform. In the case of the vane type of cathode with openings at the end, i. e., nearer the anode, when the cathode and anode are mounted in coaxial alignment, it has 4U! been found that under some conditions practically all of the electron emission comes from only that portion of the cathode nearer to thev anode. Very much less electron emission is obtained from the remote or deeper parts of the i-Lcathodal cavities, presumably due to the resistance oiered to the flow of electrons by the long, narrow paths from the bottom of the cathode to the opening at the top, between the parallel walls of adjacent vanes. In the case of the disk cath- Dikode, most of the electron emission is produced Under some conditions, at the outer or peripheral portions of the disks in spite of the factthat other portions are nearer the heater and therefore are at a higher temperature and for this i reason should produce more electrons. The rea- (Cl. Z50-27.5)

son that the disk cathode. does not provide electrons evenly over its entire surface is believed to be due to the resistance of the narrow path between the parallel wallsv of adjacentdisks. These points. will be considered indetail when comparing the disk cathodeV with` the. improved vane; cathode which forms the subject of the` present invention.

Accordingly,.the main object of the present invention is to provide a cathode which isnotopen to these objections, andin particular, a structure which has a `uniform temperature over its entire active surface and a uniform. arc. current density throughout the cathodal cavities. Another object is to provide a cathode in which the electric lield tending to pull electrons from the surface is substantially uniform throughout the cathode, while still further objects are tok provide an Vindirectly heated cathode which offersVV a: relatively large electronemission surface per unit volume of structure, and which is easy to fabricate and assemble, and also maybe readily coated with elecf tron-emitting material. In carrying out these objects, ymy invention contemplates, in brief, the use of a vane type of cathode surrounded by one. or more cylinders which may serve as heat shields and in .which theV electrons leave the cathode in a radial direction, rather than from the end of the cathode nearer the anode. TheY ends of the cathodeV may be terminated by disks of solidi. e. non-apertured, material which readily lend themselves to form a frame for supporting the vanes. The invention willV be better understood when reference ismade to the following description and the accompanying drawing in which Fig. l isy an elevational view, partly broken away, showing a tube which contains a cathode improved in accordance with the'present invention; Fig. 2 is an enlarged view of a partly assembled cathode; Fig. 3 is a plan View, partly in.section,' of the cathode; Fig.` 4 is a partly exploded View of the cathode, showingthe manner of assembly; Fig. 5 s'another assembly View showing the manner in which the vanes t together; Fig. 6 is an enlarged sectional view taken along line 6-6 in Fig. 2 and looking in the direction of the arrows; and Fig. '7 is a graph depicting Vthe temperature changes over the length of the improved cathode as compared with those over atypical prior art cathode.

Theldevic'e shown in Fig. 1 is illustrative of the type of devices to which my invention is applicable. It comprises a sealed envelope l, preferably of a bulbous configuration; andY closed at the respective ends by the usual reentrant stems 2, 3. There is. a leading-in, conductor 4 sealed 5 in the stem L and which supports a cylindrical anode of carbon or metal 5. A cathode B of the improved type, which will be described in detail hereinafter, is arranged preferably in coaxial alignment with the anode 5 and supported from a reduced diameter portion i of the stem 3 by means of a screw clamp 8 of ordinary design. Support rods 9 are secured to the clamp at one end and welded or otherwise secured to the cathode 6 at the other end. In addition to the anode and cathode, the envelope I may contain a grid structure I formed by a mesh cylinder Which surrounds the electrodes and is held in position preferably by rods il which are welded or other- Wise secured to a screw clamp I2 embracing the stem 3. Two conductors I3 are taken from the cathode 6 to the exterior of the tube, and a lead I4 is similarly connected to the clamp I2 so that control potentials may be applied to the grid I0. A cap I5 may be secured to the cathode conductor end of the envelope, and the various conductors taken through the cap in any suitable manner.

After the envelope I has been evacuated, an inert gas or a vapor-producing material is introduced so that when proper potentials are applied to the various electrodes, an arc or glow discharge is formed between the cathode and anode. Inasmuch as the invention is directed more especially to the cathode of such a device, it is believed unnecessary to describe the tube as a whole in greater detail because devices of this character are well known in the art.

The improved cathode, as shown more clearly in Figs. 2 and 4, consists of a radiation heated member surrounded by a large number of equidistantly spaced vanes which may, if desired, be`

coated with electron-emitting material 25, as indicated in Fig. 6. A heater coil I6 of tungsten or the like is` wound on an alumina rod I 'I which has an axial bore for receiving a conductor I8 which may serve as a return wire for the heater. However, the return circuit for the heater is usually taken from the cathode so that the Wire I8 serves simply as a support. At each end of the alumina rod, there is a metal disk I9, the upper one as shown in Figs. 2 and 4, carrying a metallic cylindrical member 29 to which the adjacent ends of the conductors I9 and I3 are secured. Between the disks I9, a large number of vanes or strips 2l of metal are equidistantly spaced and secured in any suitable manner to the disks. A convenient Way of securing the vanes to the disks is to provide the vanes with V-shaped extensions or tabs 22, as seen more clearly in Fig. 4, and bending these extensions at right angles to the vane, so as to provide a flat surface for welding to the disks at the places indicated by the small cross-hatched circles 23 shown in Fig. 3.

Whereas in the case of the prior art vane type of cathode, for example, that shown in Patent No. 1,924,318 referred to hereinbefore, the inner or radial edge of the vanes was secured, as by welding, to a metal cylinder which surrounded the heater, this .separate cylinder may be entirely dispensed with and the vanes formed and assembled in such as manner that the cylinder is constituted of the vanes themselves. As shown more clearly in Figs, 4 and 5, this is accomplished by providing the vanes at their inner edges with a longitudinally extending extension 24 which, when formed and bent to the proper angle, may be arranged to overlap one another so that when completely assembled, they constitute a closed cylinder of the proper diameter efcientlvtore.-

ceive heat from the heater coil I6. While various Ways will occur to those skilled in the art for assembling a cathode of this character, it has been found that the most satisfactory way involves the use of a demountabe mandrel (not shown) which holds the two disks I9 in position While the vanes are being assembled and welded to the disks. Wooden Wedges (not shown) may be employed to space the vanes apart and to maintain their proper position during the welding operation. After the vanes have been secured to the disks, the mandrel is taken apart. The feature of providing the vanes with the arcuate extensions 24 and the flat tabs 22, also the feature of using a collapsible mandrel for assembling and mounting the vanes, as described hereinbefore, are disclosed and claimed in U. S. Patent 2,154,298, issued April 1l, 1939, in the name of Raymond B. Ayer, and entitled Cathodes for electron discharge devices and methods of fabrication. It is apparent that when the vanes are constructed and assembled in the manner described, they may be made on a quantity production basis, rolled and cut to size and formed from strip material. The vanes may be coated with electron-emitting material 25 (see Fig. 6) before being assembled, as the subsequent welding operation is not such as detrimentally to affect the coating, or if desired, may be coated after assembly.

The lower `disk I9 has three equidistantly spaced apertures which receive the support rods 9 referred to hereinbefore. These rods may have their upper ends bent at right angles in order more securely to be attached to the disk.

The operation of an improved cathode of this sort is well known in the art and little or no explanation appears necessary. The heater coil radiates heat to the cylinder formed by the overlappingvane extensions, and this heat is conducted to the vanes due to the integral metal connection and when the vanes reach a suitable temperature, electrons are emitted and flow toward the anode 5, after being electrostatically controlled by the grid I0. In the case of a gas or vapor device and when the applied potentials exceed the ionization voltages, a glow or arc discharge is produced in the tube, causing the production of positive ions which travel toward the cathode. It will be noted that the direction of travel of the electrons and positive ions is radial with respect to the cathode cylinder. There is no end emission because the disks I9 contain no discharge openings.

-In order to reduce the thermal losses of the cathode, thereby minimizing the energy required Y to maintain the electron-emitting surfaces at the proper operating temperature, one or more heat shields 21 may be provided, th-e innermost shield being secured to the disks I9 or to` the vanes 2l or both in such a manner as to reduce as far as possible heat conduction at the joints. These heat shields, as shown more clearly in Fig. 1, have a number of apertures 28, arranged in either a staggered or linear relation, but extending only over the periphery of the heat shields. An outer cap 29 may also be provided at each end of the outermost heat shield to add to the rigidity of the structure and prevent radiation of heat at the ends of the cathode. It will be noted that the apertures 28 are in a position substantially coextensive with, or comparable to, the distance over which the vanes are presented to the interior surface of the heat shield or shields so as to permit egress .of the electrons produced by the active surfaces of the vanes 2|. The electrons, and in the case of a gas-or vapor tube, also the positive yions leave or enter the cathode in a radial direction.

A vane cathode of the type described and having radial or side electron emission offers certain advantages overy prior vane cathodes which have end emission, such as for example that shown inA Patent No. 1,924,318 referred to hereinbefore. In the case of the radialj emitter, which forms the -subject of the present invention, the depth of the tively reach into only those cavities which are of shallow depth, such as are provided by the radial emitter vane cathode. drop which causes the electrons togleave the surfaces of these innerrecesses is greater than in Ythe case of the end emitter vane cathode.

The radial emitter vane cathode, which forms the subject of the present invention, also offers many advantages over the radial emitter disk cathode. In the rst place, vthe vanes permit Y*ready conduction of heat throughout the length of the cathode, thus causing the various surfaces to operate at a uniform temperature. In the disk cathode. there is'little or no opportunity for the heat to flow from one disk to another except through the inner cylinder to` which the disks are secured. The transfer of." heat from the outer portion of one disk to thatofthe next is practically limited to` radiation.. Consequently, the temperature of all parts of `the diskcathodey is not uniform but is subject to considerable variation, Which condition is not conduciveto high efilthe various openings in thev heat shield.

ciency of operation. Moreovenin,theimproved` vane cathode the resistance to heat flow from `the center to the periphery is much smaller than in. ythe disk type, so that there is kless difference in temperature between portions near thefcenter and at the periphery. v

The non-uniformityof temperature throughout the disk cathode is illustratively exemplified in Fig. 7 of the drawing. which graphicallyA depicts the temperature variations throughout the length of a. disk cathode compared to-thoseofthe improved vane cathode. In this figure, the ordinates are in degrees centigrade temperatureand the abscissae are divided into four parts to indicate the condition at certain' distances paral.-

lel to the axis ofthe respectivel cathodes Whiclci-inY thecase of a cathode having( a` heat shield with ve apertures,.would.represent thefcondition at The full lines represent the temperature variationso-f the improved vane cathode, while-the broken lines represent similar variationsA of the disk cathode. Taking a few values, for" example, theV middle set of curves which haveI been designated. X

and Y respectively and represent normal operating values, it will be noted that the curve X is almost a straight line and nearlyv flat except for a slight dip between the last opening and the bottom of the cathode.

vane cathode, the temperature was approximately 875 C. and at the second opening in the heat shield, or one-fourth of the distance down the cathode, the temperature had dropped only a few Consequently, thevoltage This particular: curve: shows that at th-e top or first. openingA of: thek degrees, and at the third.A and fourth openings which would represent one-halfway downy and three-fourths Way down the cathode respectively, the temperature was approximately the same as at the second opening in the heat` shield. The

' most extreme difference in temperature appeared atthe bottom of! the cathode andvat this point, the temperature was approximately 860 C., leava temperature differ-ence of only C. from top to bottom. Examining the broken line curve Y, which depicts the temperature conditions throughout the length of a disk cathode, We find that the top'of the cathode, at the rst opening, is approximately 835 C. and at the second opening, the temperature has risen to 880 C.; While one-half way down, the temperature had gone still' higher-to 910 C. and had then dropped very abruptly so that at the fourth opening,v the temperature was approximately 875 C., while the bottom, at the last opening of the cathode, was considerably less than 800 C. Consequently, from the top to the middle of the cathode there was an increase in temperature of approximately 75 C. and between the middle of the cathode and the bottom there was a drop of mor-e than 100 C. It will also be noted in Fig. 7 that these extreme variations of temperature are present in the disk cathode even when the temperature of the cathode is raised or lowered. In all cases, the temperature curves for the vane cathode are much more nearly straight than the disk cathode, indicating that the temperature throughout the vane cathode is more nearly uniform.

In addition to providing an advantage by way of greater uniformity of temperature, and therefore uniformity of electron emission, the vane cathode having radial or side emission offers, in the case of a gas or vapor tube, the distinct advantage of providing a more uniform arc current density throughout the various recesses or cavities, due to the fact that the width of the cavities increases uniformly in going from the center to the outside. Consequently, in this type of cathode, all parts of theA entire surface contribute electrons evenly to the arc current.

The advantage of the radial or side emission vane cathodev over the disk cathode in regard to greater uniformity of arc current density may be expressed mathematically as follows: Taking up first the prior art disk cathode, let 1' equal the radius of each disk and h the distance between disks. If I is the electron current per unit area from the disks, the total electron current produced by the opposed sides of two adjacent disks is the area ofl each disk times I which is equal to 21rr2I. This electron current has to leave the outer periphery of the two disks through an area of cylindrical configuration and having a length equal to the diameter of the disks and a width equal to the distance between disks. This area is equal to 21m-h. The current density atI the periphery of the disks is therefore equal to ItWillrbe noted that the current-density factor includes the symbol r, showing. that in the case of the disk cathode, thev current density varies with the radius or diameter of the disks.

On the other hand, in-the case ofthe vane type of cathode, it will be shown that the current density does not dependv on the diametral size of the cathode, and hence, is substantially uniform at any diametral position in the cathode. Let r equal the radius of the cathode, i. e., the width of the entire area of the vane, the current density each vane, and I-I the distance between disks I9 at the top and bottom of the cathode, which is equivalent to the length of each vane. Now let the angle between adjacent vanes be 9, and I the electron current per unit area of the cathode surface.

The total electron current produced by the opposed surfaces o two adjacent vanes, i. e., a segment of the cathode, is ZTI-II. This current has to flow through an area at the boundary of the cathode which is equal to 2r sin1/29H. The current density is therefore,

zrHI I ZtXsin %6H sin ym? It will be noted that the current density expressed immediately above contains no factor relating to 1' because the r in the numerator of the fraction is canceled by the r in the denominator, Consequently, assumingr uniform emission from is not related at all to the diameter of the vane cathode but is constant at any and all diametral positions in the cathode. As the vane emitting area is increased by going farther from the center,

the cross-sectional area increases at a proportional rate and the current density remains conetant. Inasmuch as the current density remains constant, the electric eld tending to draw electrons from the electron-emitting surfaces also tends to remain constant, so that the outer portions of the vanes do not contribute any more than their equal shareof electrons to the arc or electron current. On account of this uniformity of arc density, it follows that a vane cathode of this type has a longer operating life than the disk cathode.

The vane cathode also offers many practical advantages over the disk cathode by way of manufacture and assembly. For example, the vanes may be readily cut from strip material and formed with the least amount of waste and due to their general rectangular conguration, may be readily coated with electron-emitting material. Indeed, if desired, a mesh may be welded to each side of the vane in order to retain heavier coatings of active material.

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

l. An electron-emitting electrode for an electron discharge device comprising an elongate enclosure containing a heater, longitudinal vanes secured to said enclosure forming side and end openings, and means including a closure for the end openings for preventing electrons from leaving th-e ends of said vanes whereby electrons are permitted to leave the electrode only through the side openings.

2. An electrode for an electron discharge device comprising an elongate enclosure containing a` heater, longitudinal vanes secured to said enclosure and a second enclosure surrounding said vanes and having peripheral discharge openings which extend lengthwise of said enclosure over a distance comparable to the length of said vanes.

3. A cathode for an electron discharge device comprising a cylinder containing a heater, longitudinally extending vanes secur-ed to said cyln inder, and a cylindrical enclosure surrounding said vanes, said enclosure being provided with radial discharge openings presented substantially to the entire length of the vanes.

4. A cathode for any electron discharge device comprising a cylinder, a heater therein, vanes mounted externally on said cylinder, a coating materialhaving high electron emissivity on said vanes, and a heat conserving cylindrical enclosure surrounding said vanes and having radial openings for the egress of electrons emitted by said material, said openings extending lengthwise of said enclosure over a distance comparable to the length of said vanes.

5. A cathode structure adapted for use with an electrical discharge device comprising a cylinder, a member of refractory insulating material in said cylinder, a heater supported by said member, longitudinal vanes mounted upon the external surface of said cylinder and a second cylinder surrounding said vanes, said second cylinder being provided throughout substantially its entire peripheral surface with discharge openings.

6. An electron discharge device comprising an envelope containing a plurality of electrodes including an anode presented entirely to the end of a cylindrical cathode, said cathode being provided with longitudinal vanes and being com pletely enclosed except for radial discharge openings, which extend lengthwise of the cathode a distance comparable to the length of said vanes.

7. An electron discharge device comprising an envelope containing a plurality of electrodes including an anode-presented entirely to the end of a cylindrical cathode, said cathode being provided with longitudinal vanes and having openings except at the end adjacent said anode, said openings extending substantially over the entire length of the cathode.

8. A` cathode structure for an electron discharge device comprising a cylinder containing a heater, longitudinally extending vanes secured to said cylinder and adapted to emit electrons, a cylindrical enclosure surrounding said vanes, said enclosure being completely closed except for peripheral discharge openings which extend a distance along the enclosure comparable to the distance over which said Van-es are presented to the interior surface of the enclosure whereby the electrons emitted by the entire length of each vane are permitted to pass through the enclosure in radial directions.

9. An electron discharge device comprising an envelope, a charge of gas therein, a-piurality of electrodes including a cathode, said cathode comprising an elongatedheater Vand a plurality of elongated strips extending longitudinally in said cathode outwardly from said heater, the elongated spaces between the outer edges of said strips being open, a material of high electronemissivity coating said strips, and a shield surrounding said strips, said shield having openings which are spaced over substantially the length of the spaces between the` outer edges of said strips. 10. A thermionic cathode structure adapted for use in electrical discharge devices comprising an elongated resistance heater, a metal housing closely surrounding vsaid heater, a plurality of elongated metal strips which are arranged with one longitudinal edge adjacent said heater, the spaces between the opposite edges of said strips being open, a material on said strips for enhancing electron emission, plates closing spaces between the ends of said strips and a surrounding heat shield having longitudinally arranged openings which are substantially co-extensive with Vthe spaces between the outer edges of said strips.

ALBERT W. HULL.

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