US2316214A - Control of electron flow - Google Patents

Description

April 13, 1943.

z. J. ATLEE ETAL CONTROL OF ELECTRON FLOW Filed Sept. 10, 1940 INVENTORS. I I Zed Jfltlee fi'azzfilffibbol'l Patented Apr. 13, 1943 CONTROL F ELECTRON FLOW Zed J. Atlee, Ehnhurst, and Frank R. Abbott, Maywood, Ill., assignors to General Electric X-Ray Corporation, Chicago, 111., a corporation of New York Application September 10, 1940, Serial No. 356,158

, 8 Claims.

Thisinvention relates in general to electron flow and has more particular reference to the control of electrons and the focusing thereof upon an electron target, the invention more especially relating to the focusing of electrons upon the anode target of an X-ray tube.

An important object of the present invention is to provide means whereby electrons may be caused to flow in predetermined paths from an electron source, whereby to focus the same upon a target, such as the anode target in an X-ray tube.

Another important object is to utilize means establishing an electrostatic potential field having equipotential force lines, in accordance with a desired pattern, extending between the electron source and a target upon which it is'desired to cause electrons to impinge, whereby to direct electrons emitted by said source along predetermined paths and thus to, focus the electrons on the target.

Another import-ant object'is to provide a cathode comprising an electron emission source havsion source and formed and arranged to produce, in the vicinity of said source, a field of force comprising equipotential lines having portions extending substantially normal to the direction in which it is desired to cause electrons emitted by said source to travel; a further object being to form the cathode as a cup having the electron emitting source, preferably a filament, disposed in said cup adjacent the bottom thereof so that the walls of the cup may aid in establishing equipotential force lines adjacent the filament for the purpose of causing electrons emitted by the filament to travel therefrom in a predetermined direction.

The foregoing and numerous other important objects, advantages and inherent functions of the invention will become apparent as the same is more fully understood from the following description, which, taken in connection with the accompanying drawing, .discloses preferred embodiments of the invention.

Referring to the drawing:

Figure 1 is a diagrammatic view illustratin the present invention;

Figure 2 is a section taken substantially along the line 22 in Figure 1; and t Figure 3 is an enlarged sectional viewof electron flow control apparatus embodying the invention. l a

To illustrate the invention, the drawingshows a co-operating cathode ll comprising an electron ing .cupped means in association with said emisemission source and. an anode l3 comprising a target l5 on which it may bedesired to cause electrons emitted at the cathode to impinge. It will be understood that the invention, while not necessarily restricted to electron flow control in X-ray tubes, is particularly welladapted for controlling the flow of electrons to and impingement thereof on the anode target of an X-ray tube. Consequently, the drawing illustrates a cathode l l and an anode I3 of types adapted for assembly in co-operating facing relationship in' the envelopeof an X-ray tube, the cathode H and anode l3' shown in Figure 1 being illustrated in the relative positions occupied thereby when assembled in a sealed envelope to form an X-ray tube.

As shown, the cathode ll comprises a cupshaped element I! having a flat bottom l9 and cylindrical walls 2| defining a pocket 23 opening toward the anode l3. The electron emitting source comprises an elongated filament 25 mounted on suitable conducting supports 21 which extend through openings in the bottom [9 and support the filament adjacent the bottom of the cup, the conductors 21 serving also to connect the filament with a suitable source of filament exciting energy. a

It will be noted also that the anode [3 comprises a preferably cylindrical socketed element having cylindrical walls 29 defining a socket 3| having an open end facing the cathode, the target l5 being mounted in inclined position at the bottom of the socket in position to project X-rays 33 therefrom laterally through a window 35 formed in the cylindrical anodewalls 29 when the target is energized and constituted as an X-ray source by impingement thereon of electrons emitted by the filament 25.

t is desirable, if not essential, to control accurately the flow of electrons emitted at the cathode of an X-ray tube and to cause the same to travel predetermined paths toward the target IS in order to insure that the electrons impinge upon a predetermined area of the target, thus to influence of a potential difference maintained between the anode and. cathode. Maintenance of such potential difference between anode and cathode results in the establishment of an electrostatic field of force between the anode and cathode, the intensity of electrostatic forces effective in such field being dependent upon the potential difference maintained betweenthe anode and cathode; and the configuration of the field, insofar as its constituent forces are concerned, is determined, at least to some extent, by the shape of the anode and cathode structures.

Investigation has shown that electrons emitted from a source such as the filament 25 tend to travel in paths extending at right angles to the equipotential planes of force comprising the electrostatic field in which the electrons are present.

Consequently, b establishin an electrostatic field of force having predetermined characteristics with respect to the shape of constituent equipotential force planes, it is possible to direct electrons emitted from the filament 25 along predetermined paths, cutting said planes at right angles, and thereby direct the electrons emitted by the filament 25 with precision upon desired areas of the target. The electrons, of course, in transit from the emitting source to the target, will at all times be guided by the electrostatic field of force which extends between the cathode and the anode as a result of the potential difference applied therebetween in order to operate the X-ray apparatus. The shape of equipotential force planes is naturally different along the electron flow path, but by establishing the field of force with equipotential planes of predetermined shape, the travel of the electrons between filament and target may be accurately controlled, although it is more important to determine the electron fiow path at and adjacent the electron emission source in order that electrons, during the initial stages of travel, may be started off in a desired direction. For operation on voltages from 100,000 to 400,000 volts, the field strength on the electron emitting portion of the filament is preferably within the range of 2,000 to 10,000

volts per cm. v

, In theillustrated embodiment, the filament 25 comprises a substantially line electron emission source, and in order to produce the desired square sectional characteristics in the X-ray beam 33,as shown at 31, the walls 2! of the cup I! are formed and extended in the proportions substantially as shown in Figure 1 of the drawing, in order to establish, equipotential planes 39 having sectional shape substantially as shown. These planes adjacent the cathode cup have portions extending within the cup and adjacent the filament 25, the central portions of said planes being substantially normal to the vertical path between the filament and the target, but bending opposite the sides of the filament and passing thence outwardly of the edges of the cup. An electron emitted from the central portions of the filament will be directed toward the central portions of the target. Electrons emitted at the ends of the filament, because of the slightly curved shape of the force planes through which said electrons initially travel, will also be diverted slightly toward the central portions of the target. By using the principles of electron optics, the exact shape of the cup element I1 may be determined in order to produce equipotential force planes 39 at the cathode to insure electron impingement upon a target area such that the resulting Xray beam 33 may have theldesired square shape, as shown at 31. The particular shape and proportion of the cup is substantially as shown in Figure 1, in which the depth of the pocket 23 to its diameter is approxi- 7 mately as two is to three, with the filament length about one-third of the diameter of the cup.

Electron focusing control, of course, is not necessarily restricted to a single filament arranged in the cup I T, but, as shown in Figure 3, a plurality of filaments 6| may be mounted at the bottom of the cup, and an additional control may be accomplished by mounting the electron source in a depression or depressions 13 formed in the bottom of the pocket so that the filament means may be disposed in the plane of the bottom of the pocket. Where but one filament is involved, it preferably is mounted with its longitudinal axis extending diametrally with respect to the pocket 23. Where more than one filament or electron source is assembled in the cathode, the filaments may be disposed in any preferred position, th filaments usually being in parallel spaced relationship adjacently at the bottom of the pocket 23, and may be mounted in a common depression, or each in its own depression, asshown in lfigure 3.

By determining the shape of the member ll in the manner herein described, it is possible to control electron flow in orderto obtain any desired sectional pattern of the electron stream and thereby produce a beam of Y-rays 33 having a desired sectional shape.

It is thought that theinvention and its nu merous attendant advantages will be fully understood from the foregoing description, and it ,is obvious that numerous changes may be made in the form, construction and arrangement of the several parts without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the forms herein disclosed being preferredembodiments for the purpose of illustrating the invention. 7

The invention is hereby claimed as follows:

1 Anelectronic; device comprising an anode and a cathode embodying an elongated electron emission element and an electron guiding cup forming a cavity having an open end, facing said anode, and a closed bottom, said cavity being cylindrical and having a diameter to depth ratio as three is to two, said emission element being disposed in the cup substantially at the bottom of the cavity, the diameter of said cavity at the open end thereof being not less than two times the effective electron emitting length of said element.

2. An electronic device comprising an anode and a cathode embodying an elongated electron emission element and an electron guiding cup forming a cavityhaving an open end, facing said anode, and a closed bottom, said emission element being disposed in the cup substantially at the bottom of the cavity at a vertical distance from the open end thereof substantially equal to one and one-half times the effective electron emitting length of the element. a 3. An electronic device comprising an anode and a cathode-embodying an elongated electron emission element and an electron guiding cup emission element and an electron guiding cup forming a cavity having an open end, facing said anode, and a closed bottom, said emission element being disposed in an openpocket at the bottom of the cavity at a vertical distance from the open end thereof equal at least to the efiective electron emitting length of the element.

5. An electronic device comprising an anode and a cathode embodying an elongated electron emission element and an electron guiding cup forming a cavity having an open end, facing said anode, and a closed bottom, said cavity being cylindrical and said emission element being dis posed in the cup substantially at the bottom of the cavity and having an effective electron emitting length not less than one-third the diameter of the cavity.

6. An electronic device comprising an anode and a cathode embodying an elongated electron emission element and an electron guiding cup forming a cavity having an open end, facing said anode, and a closed bottom, said cup having a minimum diametral dimension, at the open end of the cavity, substantially equal to two and onehalf times the efiective electron emitting length of the element.

7. An electronic device comprising an anode and a cathode embodying an elongated electron emission element and an electron guiding cup forming a cavity having an open end, facing said anode, and a closed bottom, said cup having a minimum diametral dimension, at the open end of the cavity, at least equal to two times the effective electron emitting length of the element.

8. An electronic device comprising an anode and a cathode embodying an elongated electron emission element and an electron guiding cup forming a cavity having an open end, facing said anode, and a closed bottom, said cup comprising a flat bottom wall and a side wall defining said cavity, said side wall, at the open end of the cavity, extending at right angles with respect to said bottom wall.

ZED J. ATLEE.

FRANK R. ABBOTT.

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