US3154711A

US3154711A - Electron beam focusing by means of contact differences of potential

Info

Publication number: US3154711A
Application number: US118150A
Authority: US
Inventors: James E Beggs
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1961-06-19
Filing date: 1961-06-19
Publication date: 1964-10-27
Anticipated expiration: 1981-10-27

Description

Oct, 27, 1964 5 BEGGS 3,154,711

ELECTRON BEAM FOCUSING BY MEANS OF CONTACT DIFFERENCES OF POTENTIAL Filed June l9a W61 [r7 va 7? tor:

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United States Patent 3,154,711 ELEC'IRGN BEAM FQCUSING BY MEAll 0F CGN- TACT DIFFERENCES OF POTENTEAL James E. Beggs, Schenectady, N.Y., assignor to General Electric Company, a corporation of New Yorls Filed June 19, 196i. Ser. No. 118,150 7 Claims. (CL 313-82) This invention relates to electron discharge devices and more particularly to a novel structure in such devices which provides electron beam focusing. This application is a continuation-in-part of my copending application Serial No. 518,548, filed June 28, 1955, entitled Thermionic Cathode and Method of Making, now l-atent No. 3,041,209, and of my copending application Serial No. 48,752, filed August 10, 1969, and entitled Electron Beam Focusing by Means of Contact Potential Difference, now abandoned.

in electron discharge devices wherein electron beam focusing occurs, it is conventionally achieved by a grid or apertured electrode interposed in the beam path and a biasing potential applied between the cathode and focusing electrode to make the focusing electrode negative with respect to the cathode. While this provides effective electron beam focusing, it suffers the drawback that a separate electrical source and electrical connections between the source and electrode must be provided. In these cases, the focusing electrode is separately mounted and either is spaced a relatively large distance from the cathode whereby the potential gradients necessary for beam focusing are necessarily obtained by sources of relatively high potential or require elaborate and expensive techniques for mounting the focusing electrode close to the cathode.

It is accordingly a principal object of my invention to provide electron beam focusing without the use of a separate, electrical potential source and accompanying electrical lines and connections.

In accordance with my present invention, electron beam focusing is achieved in electron discharge devices employing a cathode by employing a self-biasing focusing electrode of small dimensions in relatively close proximity to the cathode which is conductively interconnected therewith. It may form an inte ral part of the cathode structure if desired. in accordance with a feature of my invention, the cathode electrode and focusing electrode have respective low and high work functions so as to establish a negative contact potential difference on the focusing electrode with respect to the cathode and in accordance with another feature, the focusing electrode is maintained clean by operating it at or near cathode temperatures to produce this contact potential difference. In accordance with one embodiment of my invention, the focusing electrode may be a structure such as a mesh in intimate contact with the cathode surface to provide a large number of focused beams emerging from the cathode. A grid or accelerating anode having apertures in alignment with the focusing electrode apertures may be operated with a positive potential because the electron beams formed and focused by the focusin electrode pass through respective aligned apertures of the grid. Thus, grid current is effectively prevented to improve the noise factor of the tube and avoid adverse factors resulting from grid current fiow. In accordance with another embodiment of my invention, the focusing electrode may comprise an annular member having a surface surrounding the emissive portion of the cathode to provide a collimating effect to the outer portion of the beam which may otherwise diverge from axial paths. The contact potential difference developed in such structures may be of the order of a few volts but the very close proximity of the focusing electrode to the emissive surface of the cathode establishes a considerable potential gradient between these electrodes, sulncient to influence and effect the focusing of the emitted electrons even under circumstances wherein a control grid or accelerating anode is employed in the device at a distance from the cathode somewhat greater that the focusing electrode and which is operated at a potential positive with respect to the cathode.

The novel features believed characteristic of my invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood with reference to the attached drawing in which:

FIG. 1 is an enlarged, cross-sectional side view of a thermionic converter device employing a focusing electrode in accordance with the present invention and showing electrode spacings exaggerated;

FIG. 2 is a partial, enlarged isometric view of the focusing electrode and control grid separated by a comminuted, ceramic material;

*lG. 3 is a cross-sectional side view of a triode electron discharge device incorporating a focusing electrode in accordance with my present invention and showing electrode spacings exaggerated;

FIG. 4 is a partial view showing a cross-sectional elevation of another embodiment of an electron discharge triode incorporating features of my invention and showing electrode spacings exaggerated;

FIG. 5 is an elevational view in section illustrating my invention embodied in a closely spaced triode type of electric discharge device; and

FIG. 6 is a simplified view, partially in cross-section and broken away, of a cathode ray tube incorporating features of my invention and showing electrode spacings exaggerated.

Referring now more particularly to FIG. 1 of the drawiugs, l designates generally an entire thermionic converter apparatus which includes a sealed, evacuated enclosure having a cathode supporting member 2, at one end, for supporting a cathode 3 and a cup-shaped collector supporting member a, at the other end, for supporting a collector 5 in spaced relationship with respect to cathode 3. A grid electrode 6 of mesh construction is disposed between cathode 3 and collector 5 and is spaced from each of these electrodes. The collector supporting member 4, grid 6 and cathode supporting member 2 are provided with respective peripheral flanges "i, d and 9. To support members 7 and 8 in insulated relationship with respect to each other an insulatinc ring it), preferably of a suitable ceramic material such as fosterite or alumina is provided between these flanges '7 and 8 and to support flanges 3 and 9 in insulated relationship with respect to each other, a ceramic ring ll and a ring 12 which may be metal, are disposed in endwise abutment with each other between these flanges. The surface be tween ring 12 and flange 9 is brazed or welded and the surfaces between ring it) and each of flanges 7 and 8, and between ring 11 and each of flange 3 and ring 12 are joined in accordance with lmown Inetal-to-ceramic sealing techniques to seal the interior of the converter 1 from ambient space. The cathode supporting member 2 is preferably provided with an annular fiexure 13 to accommodate expansion and contraction thereof under the relatively great temperature changes which it would undergo in being placed into and out of operation.

For raising cathode 3 to a temperature of electron emission, heat may be applied thereto through member 2, from a suitable external source represented by the arrows in the drawings. For effecting copious emission of electrons from the cathode 3 at elevated temperatures, its interior surface is coated with a suitable electron emission enhancing material 14 which may be barium-strontium-oxide.

In accordance with the embodiment of my invention shown in FIG. 1, a focusing electrode of mesh construction, similar to that of grid 6 and shown more clearly in FIG. 2 of the drawings, is mounted on the coated cathode surface.

In accordance with a feature of my invention, the focusing electrode is in good heat transferring relation with the cathode and is of a metal which maintains a clean surface during the operating life of the device. To this end, the focusing electrode is relatively thin, is in conductive contact with this surface and is closely spaced and insulated from grid 6 by particles of a comminuted or powdered insulating ceramic material such as shown at 16. Thus, the focusing electrode is operated at near cathode temperatures. In addition, the focusing electrode is of an active material such that, at these temperatures it is maintained clean on the surface and relatively free of contaminating impurities to which it would otherwise be vulnerable if operated at lower temperatures that exist at locations further removed from the cathode. While titanium is a preferred active metal for the high work function focusing parts of the cathode, other active metals such as zirconium, hafnium, and thorium may be used. The result of this is that the focusing electrode continues to exhibit the high work function and provide the described focusing bias voltage even after prolonged usage. The focusing of the clean or focusing portion of the cathode is enhanced if they are flush with or protrude slightly above the emitting portions.

In accordance with a feature of my invention, focusing electrode 1.5 is made of material having a work function greater than the work function of the material of cathode 3. The eflect of such a relationship between work functions of respective electrodes is to develop a contact potential diflerence between these members which makes the focusing electrode negative with respect to the cathode at elevated operating temperatures of the converter. In accordance with this construction, a potential of a few volts is developed on the focusing electrode and by reason of the small thickness of the focusing electrode a relatively high potential gradient may be established in the region of the cathode between portions of the focusing electrode and cathode. Such a potential is clearly greater at the regions near the webs of the focusing electrode and thus, electrons emitted from the surface of cathode 3 are deflected away from the webs to form a concentration of electrons at the locations more centrally of each mesh and away from the webs to form a concentration of electrons at such central locations to provide a plurality of focused beams. As the beams travel axially along the converter under the influence of the accelerating potential gradient, they readily pass through the meshes of the grid *6 since the meshes of grid 6 and focusing electrode 15 are in alignment. Accordingly, the grid collects a minimum number of electrons in the respective beams resulting in increased operating efliciency, low noise and benefits accruing therefrom.

While various different materials may be utilized for the surfaces of cathode 3 and the focusing electrode 1.5, a thermionic converter of the construction shown in FIG. 1 and having a platinum cathode 3 with the surface thereof coated with a suitable oxide such as bariumstrontium-oxide for enhancing electron emission and lowering its work function and a titanium focusing electrode surface has been constructed and operated with admirable success. Utilizing these materials a potential of from 1.8 to 2.4 volts is developed on the focusing electrode in a temperature range of from 850 C. to 700 C. While this is a typical range of operating temperatures, temperatures as high as 1000 C. and as low as 350 C. may be utilized to produce useful potentials from approximately 1.6 to 3.0 volts. With a typical focusing electrode thickness of .002 centimeter, a maximum potential gradient of the order of 1000 volts per centimeter is achieved. The grid electrode may be spaced at a distance of the order of .005 centimeter and the grid may be operated positively with respect to the cathode at potentials to approximately 510 volts without interfering with focusing action of the potential developed by the focusing electrode. In general, for the focusing electrode surface active materials having a higher work function than that of the cathode surface are also satisfactory. Zirconium and thorium may be utilized as a focusing electrode surface but require somewhat higher operating temperatures than titanium. While oxide coated platinum is deemed a preferable cathode construction for the purposes of my invention, oxide coated nickel or tungsten may also be used with good results.

In accordance with the foregoing description, it is seen that electron beam focusing may be achieved by a sim-' plified structure that automatically produces a focusing potential Without the necessity of auxiliary components and external connections. While member 15 has been described as a focusing electrode, it is apparent that it is electrically and thermally a part of the cathode structure and the structure may be aptly termed a bipotential cathode.

In accordance with another embodiment of my invention, focusing may be provided in a heater-type tube as shown in FIG. 3 of the drawings. In this embodiment of my invention, a cathode 17, a grid 18 and an anode 19 are mounted in spaced, insulated relationship with respect to each other by a pair of annular, ceramic insulators 2i) and 21 disposed between separating peripheral flanges of the cathode and grid and the grid and anode. The anode 19 extends across and is effective for closing off one end of the tube and the other end of the tube is closed off by a conductive member 22 which is spaced from the peripheral flange of cathode 17 by an annular insulating member 23. Each of the metallic members which is in abutment with a ceramic insulating member is sealed thereto in accordance with any of the suitable and well-known metal-to-ceramic sealing techniques.

For raising the cathode to temperature of copious electron emission, any suitable heater such as resistance heater 24 is disposed in proximity to the cathode 17. Heater 24 has one end thereof connected to closure member 22 and the other end connected to an annular heat confining flange 25 depending from the cathode. Heater current may be provided from an electrical source, not shown, through exterior contact with cathode 17 and member 22.

"In accordance with a main feature of my invention, a focusing electrode'26 is mounted on the surface of cathode 17 in good electrical contact and also in good heat transferring relation therewith. The construction of this focusing electrode is of a mesh similar to that of focusing electrode 15 shown in FIGS. 1 and 2. Similarly, grid electrode 18 is of a mesh construction similar to that of focusing electrode 26 and the apertures of these electrodes are in axial alignment with each other for accornmodating electron flow from the cathode to the anode. For enhancing electron emission from cathode 17, the portions of the cathode not covered by the focusing electrode 26 are coated with a suitable electron emission enhancing material such as barium-strontium-oxide as shown at 27 in this figure of the drawings.

In this embodiment of invention, the cathode 17 may be oxide coated platinum having a work function of approximately 1.8 volts at an operating temperature of approximately 750 C. The focusing electrode may be titanium having a work function of 3.9 volts at this same temperature to produce a contact potential difference of approximately 2.1 volts.

In the operation of the tube, suitable potentials are applied to the cathode, grid and anode electrodes so as to make the anode highly positive with respect to the grid and cathode and the grid 18 may be biased at a potential either somewhat negative or positive with respect to the cathode 17. In a manner described hereinabove with respect to the embodiment of my invention shown in FIGS. 1 and 2, the focusing electrode 26 is made of a material having a work function higher than the work function of cathode 17 whereby a contact potential difference between these electrodes is produced. This contact potential difference makes the focusing electrode negative with respect to the cathode by a few volts. Since the focusing electrode 26 is of very small axial dimension, the potential gradient is relatively high and, therefore, the influence of this potential is relatively great in focusing the electrons emitted from the cathode along axial paths. The potential gradient produced by the focusing electrode and influencing the flow of electrons emitted from the cathode is effective in collimating electron beams as potential gradient produced by the grid electrode 18 accelerates them along the tube. Thus, even when the grid 18 is operated somewhat positively with respect to the cathode, the influence of the focusing electrode causes the electrons to follow beam paths between the grid mesh and the grid does not conduct current.

It is again to be observed according to FIG. 3, that electron beam focusing may be achieved by a structure which is greatly simplified over those of the prior art and which does not necessitate the use of an auxiliary source of potential and external connections.

It is again to be noted that it is within the contemplation of my invention to use difierent material for both focusing and cathode electrodes in FIG. 3 as specified with respect to the embodiment of invention shown in FIG. 1.

In accordance wih another embodiment of my invention as shown broken away in FIG. 4 of the drawings, electron beam focusing may be achieved in a tube represented generally at 23. In accordance with a feature of this apparatus, it includes a cathode supporting member 29 which may be of a suitable material such a titanium and having a fiat interior surface with an interior recess 3t) producing a peripheral, interior, annular surface 31. The recess 38 is filled with a member 32 of suitable material such as platinum or other passive base metal such as nickel, coated with an emission enhancing oxide coating 33 to provide it with a work function lower than the work function of member 29. The exposed surface of the member 32 is coplanar with surface 31. A grid electrode 34 is mounted in spaced relationship with respect to the cathode by an annular insulating member 35 disposed between peripheral flanges of the grid and cathode. As in the aforedescribed embodiments of the invention the grid electrode comprises a screen or mesh to accommodate the passage of electrons emitted by the cathode.

In accordance with this embodiment of my invention, a contact potential difference produced by the relative high and low work functions of the cathode supporting member 29 and cathode 32, establishes a potential on the surface 31 which is negative with respect to the surface of cathode 32. With a titanium cathode supporting member 29 and an oxide coated platinum cathode 32, a contact potential difference of approximately 2.0 volts may be developed at the surface 31. The effect of this potential which is uniform about the continuous surface 31, is to collimate or focus the electron beam emitted from the cathode 32, particularly electrons near the edges of the beam. This embodiment of my invention differs from those previously described in that only a single electron beam rather than a large number of beams is focused. Each of the embodiments of the invention shown in FIGS. 1, 2 and 3 has been described as incorporating a inesh construction of focusing electrode. It is to be understood, however, that it is within the purview of my invention to provide a focusing electrode of different construction, iobeing only necessary to establish conductive contact to the cathode with the relatively high work function focusing electrode and to provide apertures or spaces between focusing electrode surfaces to accommodate the passage of electrons. Examples of some other constructions of focusing electrodes are parallel spaced wires and perforated plates.

In FIG. 5 I have illustrated my invention embodied in a triode electric discharge device in which a cathode produces a single electron beam focused by the contact potential of an emitting area and a surrounding or guard ring of clean titanium. This construction is eifective to maintain the beam in the device with closely spaced electrodes in a well defined cross-section so that essentially all the electrons are collected on an anode surface of dimensions similar to those of the emitting area and as a result all of the electrons have essentially the same transit time. This enhances the high frequency performance of the device. As illustrated in FIG. 5, the device is provided with a cathode assembly embodying the present invention and including a cylindrical hollow support 36, preferably of titanium, mounted in and hermetically sealed to an annular supporting insulating washer 37. The cylinder 36 preferably terminates flush with the upper surface of the member 37 and includes a disk 38 of an active metal, preferably titanium, which is provided with a central recess providing the emitting cathode area. The recess as illustrated is partially filled with a porous material such as a body of sintered tungsten 39 or other refractory metal which is impregnated and coated with an overlayer 44) of an emission enhancing material such as the alkaline earth oxides. The disk 38 provides a circular surrounding surface 41 which is essentially flush with or a little above the emitting area and which is, as previously stated, of active metal which maintains the surface thereof free of impurities, particularly at the temperatures at which it functions. The cathode assembly is completed by a cathode heater element illustrated at 42 having one terminal connected with the conducting cylinder 36 and the other terminal terminating in a disk-like terminal 43, bonded to a ceramic disk 44 through which the heater lead passes and to which both the heater lead and and the cylinder 36 are bonded.

The device is completed by an annular grid supporting ring and terminal 45 bonded to the insulator 37 and an anode supporting insulator 46 bonded to the terminal 45 and to a cylindrical anode member 47 which extends within the device in closely spaced and aligned relation with the emitting area of the cathode. The grid is illustrated as including a plurality of spaced conductors 48 extending across the opening in the grid terminal 45. As illustrated the anode has a diameter essentially the same as the diameter of the emitting area of the cathode. With the present invention which provides a focusing action due to the difference in work function between the clean titanium ring surface 41 and the emitting area the anode is effective to collect essentially all of the electrons emitted and since these electrons travel the same distance, the transit time will tend to be more uniform.

The porous sintered tungsten cathode may be manufactured in accordance with the process described and claimed in my aforementioned copending application Serial No. 518,548, filed June 28, 1955, of which this application is a continuation-in-part. As will be readily understood by those skilled in the art, the porous underlayer of refractory material may be utilized in the other illustrated embodiments of the invention described in the present application.

In accordance With still another embodiment of my invention shown partially schematically in FIG. 6 of the drawings, focusing of the electron beam in a cathode ray tube shown generally at 48 may be accomplished. The cathode ray tube 48 includes a sealed enclosure 4? containing conventional parts such as horizontal and vertical deflecting electrodes and a phosphor screen, all of which 2 form no critical part of the invention and therefore are not shown in the drawing.

An elongated neck portion 49 of the tube 48 contains an electron gun shown generally at 58 and which includes a cathode 51 with a cylindrical shell 52 and a planar face 53. A suitable cathode heater is provided and, as shown, the side of the cathode 51 remote from face 53 may be recessed at 54 to accommodate a resistance heater 55 having one end conductively connected to the cathode. Electrical connections to the cathode and to respective ends of the heater from an external potential source are enabled by conductive leads -6 and 57 extending through the end of the envelope 4?. A suitable potential source represented by a battery 58 may be provided for supplying heating current through the heater 55. For providing high electron emission from the cathode 51 at elevated temperatures, the face 53 thereofis coated with a suitable electron emission enhancing material 59 which may be barium-strontiumoxide.

In accordance with a feature of this embodiment of my invention, a focusing electrode 60 which is self-biasing is interposed in the electron beam path. This electorde is of a cup shape with an aperture 61 centrally disposed in the bottom 62 thereof and is telescopically fitted over the cathode 51 making good conductive contact between the side d3 of the focusing electrode and the cathode periphery 52. For establishing a biasing contact potential difference between the focusing electrode 69 and the cathode 51, the cathode is of a material having a lower work function than that of the focusing electrode 69. Typical materials which may be utilized for achieving this result are oxide coated platinum used for the cathode 51 and clean titanium for the focusing electrode 60. At operating temperatures of the cathode and focusing electrode, the focusing electrode remains clean to establish a contact potential difference of approximately 2 volts between this electrode and the emitting cathode surface and with a spacing of .002 centimeter between the bottom 62 of the focusing electrode and the face 53 of cathode 51, a potential gradient of approximately 500 volts per centimeter is established. The electrons emitted from the face 53 of cathode 51 are influenced by this potential gradient to pass centrally through the aperture 61, axially along the interior of tube '48. For imparting sufficient energy to the electron beam for exciting the phosphor materials utilized for the cathode ray tube screen, an apertured accelerating electrode 64 is disposed between the electron gun and other parts of the tube and a high positive potential is applied to this electrode with respect to cathode 51 by a source represented by battery'65. It is to be noted that. with a spacing of approximately 0.25 centimeter between the accelerating anode 64 and cathode 51 and an accelerating potential of approximately 200 volts supplied by battery 65, the influence of focusing electrode 61 is such as to collimate the :beam as it is accelerated by the accelerating'anode from the'face S3 of the cathode.

It is again noted that in accordance with my invention the cathode and focusing electrode in FIG. 6 may be made of other materials as specified hereinabove with respect to FIG. 1. It is important that the focusing electrode be of a metal which maintains itself clean during operation so that the high work function is maintained.

In accordance with the foregoing, it is to be observed that focusing of an electron beam in a cathode ray tube is achieved in accordance with a simplified and effective structure totally devoid of external connections and potential sources for a focusing electrode. While the present invention has'been described by reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

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

1. An apparatus comprising a cathode electrode including a metal support and an electron emissive coating thereon responsive to the application of heat for emitting elec- -trons,'a focusing electrode of active metal selected from the group consisting of titanium, zirconium and thorium having a plurality of apertures and having one side in electrical conductive contact and in good heat transfer relation with said cathode so that the surface of said focusing electrode remains clean during operation and the .work function of said focusing electrode is greater than the work function of the coating of said cathode at the operating temperatures of said apparatus whereby a contact potential difference is established between said focusing electrode and said cathode which is negative at said focusing electrode with respect to said cathode to focus electrons emitted from said cathode and passing through said apertures.

2. An electron tube apparatus comprising a cathode including an electron emissive coating, an anode and a grid electrode disposed therebetween, a focusing electrode of an active metal selected from the group consisting of titanium, zirconium and thorium having a plurality of apertures therein and having one side in conductive contact and in good heat transfer relation with the coated side of said cathode so that said focusing electrode remains clean during operation of the tube apparatus, said grid electrode having a plurality of apertures each being in alignment with an aperture of said focusing electrode, the work function of the active material of said focusing electrode when clean being sufficiently greater than the work function of the coating of said cathode to establish a negative contact potential difference on said focusing electrode with respect to said emissive coating of said cathode to focus a plurality of beams of electrons emitted from said cathode and passing through said grid to said anode.

3. An apparatus comprising a cathode electrode having an oxide coated surface on a passive metal support and being capable of electron emission at elevated temperatures, a focusing electrode having a titanium surface and a plurality of apertures therein, said focusing electrode having one side thereof in conductive contact with the surface of said cathode whereby a contact potential difference is established between said'focusing electrode and said coated cathode at elevated operating temperatures thereof to produce a negative biasing potential on said focusing electrode.

4. An apparatus comprising a cathode electrode having a recess in one side thereof, an uncoated area of active metal surrounding said recess having a predetermined work function and selected from the group consisting of titanium, zirconium and thorium, said recess being filled with a material having a work function lower than said predetermined work function whereby a contact potential difference, produced between the uncoated area of said cathode surrounding said recess and the material disposed in said recess is cfiective in focusing a beam of electrons formed by emission from said cathode, said active metal maintaining a clean surface during operation to the cathode to maintain the contact potential difference and focusing action of the cathode.

and said cathode, the active metal of said focusing electrode maintaining itself clean during operation of the gun and having a work function greater than the work function of the coating of said cathode whereby a negative contact potential difference is established on said focusing electrode with respect to said cathode to focus electrons emitted from said cathode through said apertured focusing electrode.

6. A composite cathode comprising an annular planar surface of titanium surrounding a recessed base portion, a layer of porous refractory metal in said recess and an outer layer of electron emissive coating filling the pores of said porous refractory metal layer and forming a layer thereon, the depth of the recess being substantially equal to the thickness of the porous refractory metal layer and emissive coating so that the emitting surface is substantially level with said surrounding surface of titanium, said titanium maintaining itself clean during operation of the cathode so that the Work function thereof remains high as compared with the work function of the emissive coating and the resultant contact potential of the titanium with respect to the emissive coating provides a focusing action on the electrons emitted by said coating.

7. A composite cathode structure for exerting a focusing action on the electrons emitted thereby comprising a cathode base member of relatively passive metal and an electron emissive material of relatively low work function on said base member, said cathode including a metal surface of a metal selected from the group consisting of titanium, Zirconium, and thorium and extending flush with or higher than said electron emissive coating, said metal surface having an aperture therein through Which an area of said emissive coating is exposed, said active metal remaining clean at the operating temperature of the cathode so that the high work function of said active metal is maintained and the contact potential between the active metal and the emissive material is effective to focus the electrons emitted from the exposed area of said emissive coating.

References Cited in the file of this patent UNITED STATES PATENTS