US2888605A - Electron gun - Google Patents

Description

y 1959 v G. R. BREWER 7 2,888,605

ELECTRON GUN Filed Feb; 23, 1955 2 Sheets-Sheet 1 May 26, 1959 Filed Feb. 25; 1955 G. R. BREWER ELECTRON GUN 2 Sheets-Sheet 2 WWW ,drmeux 2,888,605 Patented May 26, 1959 ELECTRON GUN George R. Brewer, Palos Verdes Estates, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application February 23, 1955, Serial No. 489,906

1 Claim. (Cl. 315-15) This invention relates to electron tubes and more particularly to an electron gun for producing an electron stream having a substantially uniform cross section over its length. This application is a continuation in part of application Serial No. 474,078, filed December 9, 1954, now Patent No. 2,811,667.

Electron guns which are employed to produce relatively high current electron streams generally comprise a cathode, a partially 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 focused along similar or parallel paths. The gun electrodes are thus employed to develop an electron stream having a uniform current density over its cross section. When the gun actually accomplishes these results, an electron stream thus produced is said to be well collimated.

It may be particularly desirable to produce collimation of electrons in a solid cylindrical 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 focused 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, undue 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 efficiency 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 an electron gun as though the anode aperture through which the beam passes were not in existence. A focusing error is thus produced which impairs 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 a second tubular anode is disposed adjacent to and partly within the aperture of the dish-shaped or first anode of an otherwise substantially conventional electron gun. The electric field contiguous to the gun cathode may thus be modified in such a way as to be more uniform thereby 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 two alternative embodiments of the electron gun of the present 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 Fig. 2; and

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

Referring to Fig. 1, an electron gun 10, which may, for example, have a perveance of 2 10- or higher, is shown comprising a cathode 12 which is provided with a filament 14, a focusing electrode 16, a dish-shaped or first anode 18, and a second hollow cylindrical anode 20.

Cathode 12 is itself a hollow cylinder having an enclosed end portion 22 which serves as an electron emissive surface. Filament 14, which has a helical shape, is surrounded by a concave dish-shaped block 24 of a refractory material. 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. Dish-shaped first anode 18, which may, as shown, be axially separated from and adjacent to the focusing electrode 16, is provided with an aperture 32 through which electron flow is produced.

Cylindrical second anode 20 is disposed partially within the concave sheet 30 forming anode 18 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 by means of 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 16 may be maintained at ground potential. Depending upon the position and exact shape of focusing surface 26, focusing electrode 16 may either be maintained negative or at zero potential with respect to the potential of cathode 12.

Dish-shaped first anode 18' is maintained at a relatively large positive potential, V with respect to that of focusing electrode 16 by means of a first anode source of potential 40. Cylindrical second anode 20 is maintained at a still larger positive potential, V 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 electron collimation can be obtained when V is simply larger than V however, if V 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 anodeZt), may be designed according to the procedure related in U.S; Patent No. 2,268,165, granted December 30, 1941, to C. V; Parker etal. 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 a size comparable to the distance from the first anode to the cathode. This generally decreases the focusing capabilities of the dish-shaped anode 18 by causing a distortionof the electric equipotential lines from their desired shape which is a group of lines parallel to the cathode surface. However, this effect is essentially negligible in relatively low perveance guns.

An alternative embodiment of the gun is shown in Fig. 2 with a differently shaped cathode 12 and first anode 122. Cathode 121 is spherically shaped, that is, its end portion 22 is concave to provide a converging electron stream. Focusing electrode 120 is disposed adjacent end portion 22 of cathode 121 and performs the same function as focusing electrode 16. The structure of first anode 122, which is disposed axially adjacent to and may, as shown more clearly in Fig. 1, be axially separated from the focusing electrode, differs from that of dish-shaped anode 18 of Pig. 1 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 to accommodate a portion of cylindrical second anode 20.

The schematic diagrams of Figs. 3 and 4 illustrate in particular how the cylindrical anode 20 may be employed to improve the collimation of stream electrons when maintained within a potential range set out in the inequalities of the Expression 1. Cathode 12, focusing electrode 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 121 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 while the electrons at the center of the stream will be directed toward a focal point 47. This is true because the equipotential surfaces represented by the dashed lines are not concentric, or, more generally, are not geometrically similar to the emissive cathode surface; giving rise to transverse defocusing fields which act upon the stream electrons. The density of electron emission in Fig. 3 likewise varies over the electron emissive surface of the cathode, 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 example, traveling-wave tubes, may be thus increased. It is to be understood that guns in accordance with the invention can also be used in other magnetic field systems than Brillouin flow.

In order to obtain a properly shaped and spaced second 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 indicated zit-151.

In arriving at acorrect design for the various electrodes of the gun, one must vary two parameters. The position and shape of the focusing electrode must be varied to achieve a certain desired potential distribution along the beam boundary which is generally that known in the art as the Langmuir distribution, and described in US. Patents Nos. 2,268,165 and 2,268,197. The shape, position, and potential of the second anode 158 relative to the anode must be varied 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 15% 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 series of 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, this 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 129, thus satisfying the second boundary condition. The potential along the edge of the beam should follow that found by Langmuir and Blodgett, Phys. Rev., 24, page 49 (1924), which is where -a is represented by the series of Langmuir and ot,, is the value of on 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 158 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 where:

J is the current density;

K is a constant;

V is the first anode voltage; and L is the first anode spacing.

. If the potential gradient is the gradient immediately adjacent the cathode in the absence of space charge than electron emissive surface, a focusing electrode disposed about said cathode having substantially a frusto-conical internal focusing surface converging inwardly toward said cathode, adish-shaped anode having an apertured central surface convex with respect to said cathode disposed adjacent and entirely externalito -said focusing electrode,

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 sym metrical focusing which may be obtained in practicing the present invention.

By considering the section of the gun 10 in Fig. 2 as a section of translation instead of a section of revolution, a gun for producing a wedge-shaped or rectangularly shaped stream of electrons may be realized. A planar gun likewise may be constructed using a flat cathode. Hollow or solid stream guns may also be constructed by treating the sections of Figs. 1 and 2 as sections of revolution about an axis outside of the cathodes 12 and 121.

What is claimed is:

An electron gun for developing a substantially collimated flow of electrons along a predetermined path, said electron gun comprising in the following order along said path a thermionic cathode having a substantially flat a hollow cylindrical anode "disposed contiguously about saidpath for a portion of 'the ljen'gth thereof and having the end thereof closest to said cathode 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,

whereby the equipotential surfaces between said cathode "emissive surface and said anodes are substantially geo- 'metrically similar in shape to said cathode emissive surface.

References Cited'in, the file of this patent UNITED STATES PATENTS 2,233,299 Schlesinger Feb. 25, 1941 2,268,165 Parker et a1. Dec. 30, 1941 2,268,197 Pierce Dec. 30, 1941 2,303,166 Loica Nov. 24, 1942 2,509,763 De Gier May 30, 1950 2,581,243 Dodds Ian. 1, 1952 2,782,333 Benway et al. Feb. 19, 1957 2,797,353 Molnar et a1. June 25, 1957 2,811,667 Brewer Oct. 29, 1957 FOREIGN PATENTS 151,377 Australia I.; May 12, 1953

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