US2811667A

US2811667A - Electron gun

Info

Publication number: US2811667A
Application number: US474078A
Authority: US
Inventors: George R Brewer
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1954-12-09
Filing date: 1954-12-09
Publication date: 1957-10-29
Anticipated expiration: 1974-10-29

Description

Oct. 29, 1957 G. R. BREWER ELECTRON GUN 2 Sheets-Sheet 1 Filed Dec. 9, 1954 E El T/ Oct. 29, 1957 G. R. BREWER 7 ELECTRON GUN Filed Dec. 9. 1954 2 Sheets-Sheet 2 Wmwz.

lira/Vii United States Patent ELECTRON GUN George R. Brewer, Palos Verdes Estates, Califl, assignor to Hughes Aircraft Company, Culver City, Calif a corporation of Delaware Application December 9, 1954, Serial No. 474,07 8

4 Claims. (Cl. 315-) This invention relates to electron tubes anl more particularly to an electron gun for producing an electron stream having a substantially uniform diameter over its length.

Electron guns which are employed to produce relatively high current electron streams generally comprise a cathode having an emission surface which is concave with respect to the electron stream, a frusto-conical focusing electrode disposed concentrically about the cathode, and a dish-shaped anode having an apertured central surface concave with respect to and spaced from the cathode emission surface. All of the gun electrodes are properly shaped and maintained at appropriate potentials to cause substantially all of the electrons which emanate from the cathode to be focussed toward the same point along a predetermined electron stream path. The gun electrodes are thus employed to develop a solid cylindrical electron stream having a uniform current density over its cross section. When the gun actually accomplishes this result, an electron stream thus produced is said to be well collimated.

It may be particularly desirable to produce collimation of electrons in an electron stream which is magnetically focused in a manner well known in the art as Brillouin flow. This is true because the Brillouin type of flow allows a given stream to be focussed or confined with a magnetic field of a minimum strength. However, unless certain precautions are taken in the design of the gun and magnetic field system, electron stream diameter variations invariably occur in Brillouin flow. These variations physically manifest themselves as a plurality of ripples which are disposed at periodic stationary intervals along the stream causing the stream to expand and contract. When such a stream is employed in a microwave tube, such as a traveling-wave tube, the eificiency of the tube is generally reduced by the stream diameter variations. These ripples can also cause a number of other deleterious effects on traveling-wave tube performance. in such a case it is therefore definitely desirable to minimize these variations by producing a collimated electron flow.

At present it is frequently the practice to design the several electrodes of a converging beam gun as though the anode aperture through which the beam passes were not in existence. A focusing error is thus produced which impairs a collimated electron flow. This focusing error results in a large part from the distortion of the electric field lines, imparting undesirable transverse motion to the electrons.

An object of the invention is, therefore, to provide an improved electron gun for a traveling-wave tube.

Another object of the invention is to provide an electron gun for developing a substantially collimated flow of electrons.

in accordance with the invention as second anode having a cylindrical internal surface is disposed adjacent to and partly within the dish-shaped anode of an otherwise more or less conventional converging beam gun. The

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electric field contiguous to the gun cathode may thus be modified in such a way as to be more uniform and thus to improve collimation when the second anode is maintained at a potential somewhat greater than that of the dish-shaped anode.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description taken in connection with the accompanying drawings, in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Figs. 1 and 2 are sectional views of different embodiments of an electron gun constructed in accordance with the invention;

Figs. 3 and 4 are schematic views of an electric field distribution which may exist contiguous to the cathode of the gun shown in Figs. 1 and 2; and

Figs. 5 and 6 are diagrammatic views of an electrolytic tank which may be employed in the designing of the gun of the present invention.

Referring to Fig. 1, an electron gun 10, which may, for example, have a perveance of 2x10 or higher, is shown comprising a cathode 12 which is provided with a filament 14, a focusing electrode 16, a dish-shaped anode 13, and a second hollow cylindrical anode 20.

Cathode 12 is itself a hollow cylinder having an enclosed end portion 22 concave with respect to its electron stream and which serves as an electron emissive surface. The emissive surface may be, for example, spherical. Filament 14, which has a helical shape,is surrounded by a concave dish-shaped block of a refractory material 24. Block 24 and filament 14 are thus situated concentrically within the cylinder of the cathode 12 adjacent the end portion 22.

Focusing electrode 16 is disposed concentrically about the cylinder of the cathode 12 having a frusto-conical internal focusing surface 26. Focusing electrode 16 also has an aperture 28 in a disc-shaped end portion 29 through which a concave sheet 30 forming the central portion of dish-shaped anode 18 extends. Dishshaped anode 18 is likewise provided with an aperture 32 through which electron fiow is produced.

Cylindrical anode 20 is disposed partially within the concave sheet 30 in order that an improved collimation of electrons along a predetermined path, e. g., a path 34, may be produced. Cylindrical anode 20 is provided with an encompassing supporting disc 31 with which cylindrical anode 20 may be mounted in an electron tube.

Filament 14 is heated by a filament battery 36 which is connected thereacross. Cathode 12 and the negative terminal of battery 36 are maintained at ground potential by a suitable connection thereto.

Focusing electrode is may be maintained a few volts negative with respect to ground by a focusing electrode source of potential 38. Depending upon the position and exact shape of focusing surface 26, focusing electrode 16 may be maintained negative or at zero potential with respect to the potential of cathode 12.

Dish-shaped anode'ls is maintained at a relatively large positive potential with respect to that of focusing electrode 16 by means of a first anode source of potential 40. Cylindrical anode 20 is maintained at a still larger positive potential by a second anode source of potential 42.

Through empirical studies of electric field patterns in an electrolytic tank, which method of examination is well known in the art, it has been found that best collimated electron flow is produced in the range of depending on the design the electrode 20 and its disposition with respect to anode 32, where V2 is the voltage of cylindrical anode 20 and V1 is the voltage of dishshaped anode 18. Some improvement in electron collimation can be obtained when V2 is simply larger than Vi; however, if V2 is more than twice as large, the improvement may be negligible, and the performance may even be impaired.

The electrodes of the gun 10, excluding the second anode 20, may be designed according to the procedure related in U. S. Patent No. 2,268,165, granted December 30, 1941, to C. V. Parker et al. The cathode 12, the focusing electrode 16, and the first anode 18 may also be maintained at appropriate relative potentials as set out in the Parker patent.

The principal reason for using the cylindrical anode 20 arises out of the fact that for a relatively high perveance gun the size of the first anode aperture diameter is of the same order of magnitude as the distance from the first anode to the cathode. This generally decreases the focusing capabilities of the dish-shaped anode 18 by causing a distortion of the electric equipotential lines from their desired spherical shape. However, this effect is essentially negligible in relatively low perveance guns.

The gun is shown again in Fig. 2 with both a differently shaped focusing electrode 120 and first anode 122. Focusing electrode 120 performs the same function as focusing electrode 16; however, focusing electrode 120 consists simply of a frusto-conical metallic sheet. The structure of first anode 122 differs from that of dishshaped anode 18 in that the interior of the dish-shaped outer wall of the first anode 122 is partially filled with metal in order to improve the performance of the gun 10 and shaped to accommodate a portion of cylindrical anode 20.

The schematic diagrams of Figs. 3 and 4 illustrate in particular how the cylindrical anode 20 may be emloyed to improve the collimation of stream electrons when maintained with a potential range set out in the inequalities of the expression (1). Cathode 12, focusing electrode 120 and first anode 122 are shown in both Figs. 3 and 4 whereas cylindrical anode 20 is only shown in Fig. 4. A plurality of dashed lines 33 in Fig. 3 and 44 in Fig. 4 represent equipotential lines which would be produced by the electrodes shown in the respective Figs. 3 and 4.

In order to produce a collimation of stream electrons, it is necessary to direct the electrons emitted at the end portion 22 of cathode 12 toward a common focusing point, viz. points 46 in Figs. 3 and 4. However, in Fig. 3 where cylindrical anode 20 is not employed, the transverse forces resulting from the distortion of the equipotential lines 33 will cause electrons at the outer edge of the stream to be directed toward the focal point 46 nearer the cathode 12 than a focal point 47 to which the electrons at the center of the stream will be directed. This is true because the equipotential surfaces represented by the dashed lines are not concentric; giving rise to transverse defocusing fields which act upon the stream electrons. The density of electron emission in Fig. 3 likewise varies with the radius of the electron stream, whereby collimation may be exceedingly poor. In extreme cases a large number of the electrons may be caused to strike the anode. However in Fig. 4 where cylindrical anode 20 is employed, all of the equipotential lines 44 are substantially parallel and good collimation is produced. The magnitude of ripples in Brillouin flow may thereby be reduced and the efficiency of microwave tubes, such as, for examp tra elin -w v tubes, ma be thu increase n o e to bta a p ope ly ped d spa ed se ond anode and to obtain the optimum potential at which it should be maintained, an electrolytic tank design procedure may be most effectively employed. Such a tank is illustrated in Figs. 5 and 6 and is filled with water 151 up to a water level 153. A first conductive sheet 152 in the tank represents one-half of the cathode surface 22. A second conductive sheet 154 illustrated only in Fig. 6 represents focusing electrode 120. A third conductive sheet 156 represents first anode 122, and a fourth conductive sheet 158 is employed to represent the structure of cylindrical anode 20. Suitable potentials are applied to

sheets

152, 154, and 156, and the potential applied to sheet 158 is varied as the electric field pattern is detected by a probe disposed in the water 151.

In arriving at a correct design for the various electrodes of the gun, one must vary two parameters, the position and shape of the focusing electrode so as to achieve the desired potential distribution along the beam boundary and the shape, position, and potential of the second anode 158 relative to' the anode in such a Way that the electric gradient adjacent to the surface of the cathode is uniform over the cathode surface. These desired shapes can be obtained as follows: In the electrolytic tank 150 the required boundary conditions at the edge of the beam are: (a) the electric gradient normal to the beam boundary must be zero, and (b) the potential must be continuous across this boundary. These conditions are fulfilled by placing a dielectric strip 157 along the desired beam boundary with a seriesof thin spaced metallic strips 161 which can be held at the desired potentials by external circuitry indicated by 159. The presence of the dielectric strip 157 insures that no current flows normal to the beam boundary in the tank, thus forming the analog of the zero normal field condition. In addition, the metallic strips 161 can be adjusted in potential to provide a potential distribution along the inner surface which is identical to that produced on the outside of the dielectric strip 157 by the focusing electrode 120, thus satisfying the second boundary condition. The potential along the edge of the beam should follow that found by Langmiur and Blodgett, Phys. Rev. 24, page 49 (1924), which is where o: is represented by the series of Langmiur and -a is the value of a at the anode, or any other desired and suitable distribution law. The position and shape of the conductive sheet 154 may then be determined in the manner indicated in Fig. 6 where 163 indicates a metering device for ascertaining the potential distribution along the metallic strips 161.

The shape and potential of electrode 15% is adjusted while measuring the electric gradient normal to the cathode at several points therealong until this gradient is substantially uniform. Under this condition it can reasonably be assumed that the emission density will be uniform, the equipotential lines being essentially concentric with the cathode emission surface and the focusing properties of the gun being that desired. The dependence of emission current density on the normal potential gradient contiguous to the cathode emission surface may be observed by reviewing the emission equation. For example, using planar equations a i the fir t anod l ag a L s e fir seed erasin is the. gradient immediately adjacent the cathode in the absence of space charge then E0 2 Jo K(, (4) Thus if E0 is uniform over the cathode surface,.then Jo will also be uniform over the cathode surface.

It is then evident that a converging beam electron gun may be constructed which will give a collimated electron stream with a minimum disturbance of rippling in an axial magnetic field. This is made possible by the uniform emission current density provided and by the symmetrical focusing which may be obtained in practicing the present invention. e

What is claimed is:

1. An electron gun for producing a substantially collimated flow'of electrons, said electron gun comprising a thermionic cathode having a concave emission surface with respect to emitted electrons, a focusing electrode disposed about said cathode, an anode having a central dish-shaped surface disposed partially within said focusing electrode, said central portion of said anode being concave toward said cathode, and a second anode having 1 one end disposed partially within said dish-shaped anode,

said second anode having a cylindrical internal surface extending throughout its length to permit an electron flow therethrough, whereby equipotential surfaces with curvatures substantially concentric to the emission surface of said cathode may be produced by the application of appropriate voltages to said anodes, said cathode, electrode, and anodes being concentrically aligned with each other.

2. An electron gun for developing a substantially collimated flow of electrons along a predetermined path, said electron gun comprising a thermionic cathode having a substantially concave spherical electron emissive surface with respect to emitted electrons, a focusing electrode disposed about said cathode having a substantially frustoconical internal focusing surface converging inwardly toward said cathode, a dish-shaped anode having an apertured central surface disposed concave toward said cathode and disposed partially within the frusto-conical surface of said focusing electrode, a hollow cylindrical anode disposed about said path having one end disposed partially within the central portion of said dish-shaped anode, means for maintaining said dish-shaped anode at a first predetermined potential with respect to said cathode, and means for maintaining said cylindrical anode at a second predetermined potential with respect to said cathode larger than said first predetermined potential said cathode, electrode, and anodes being concentrically aligned along said predetermined path.

3; An electron gun for producing a substantially collimated flow of electrons along a predetermined path, said electron gun comprising a thermionic cathode having an emission surface concave with respect to emitted electrons, a focusing electrode disposed about said cathode, a dish-shaped anode having a central dish-shaped surface disposed partially within said focusing electrode, said central portion of said dish-shaped anode being concave toward said cathode, a second anode having one end disposed partially within said dish-shaped anode, said second anode having a cylindrical internal surface extending throughout its length to permit electron flow therethrough, and means for maintaining said second anode at a potential. greater than that of said dish-shaped anode with respect to said cathode and less than twice the potential of said dish-shaped anode with respect to said cathode whereby equipotential surfaces substantially parallel to the emission surface of said cathode may be produced said cathode, electrode, and anodes being concentrically aligned along said predetermined path.

4. An electron gun for developing a substantially collimated flow of electrons along a predetermined path, said electron gun comprising a thermionic cathode having a substantially spherical electron emissivc surface concave with respect to emitted electrons, a focusing electrode disposed about said cathode having a substantially frustoconical internal focusing surface converging inwardly toward said cathode, a dish-shaped anode having an apertured central surface concave toward said cathode disposed partially within the frusto-conical surface of said focusing electrode, a hollow cylindrical anode disposed about said path having one end disposed partially within the central portion of said dish-shaped anode, and means for maintaining said cylindrical anode at a predetermined potential from 1.1 to 1.5 times the potential of said dish-shaped anode with respect to said cathode.

References Cited in the file of this patent UNITED STATES PATENTS Szegho Oct. 30,