US2840754A

US2840754A - Electron beam tube

Info

Publication number: US2840754A
Application number: US453537A
Authority: US
Inventors: Ernest G Linder
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1954-09-01
Filing date: 1954-09-01
Publication date: 1958-06-24
Anticipated expiration: 1975-06-24

Description

June 24, 1958 E. G. LINDER 2,840,754

ELECTRON BEAM TUBE Filed Sept. 1, 1954 v 2 Sheets-Sheet 1 l I II II- I l I I INVLINTOR. I I I I j I I I I I Emesiilznder Tram/z) June 24, 1958 E. G. L'INDER 2,840,754

' ELECTRON BEAM TUBE Filed Sept. 1,- 1954 2 Sheets-Sheet 2 I INVEI! TOR. Ernest 6'. Llnder ram Er United States Patent ELECTRON BEAM TUBE Ernest G. Linder, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 1, 1954, Serial No. 453,537

12 Claims. (Cl. 315-534) path. For example, in a velocity modulated electron tube v wherein, as is known, an electron beam is projected through one or more apertured cavities each defining at least one interaction gap, and where interaction between the beam and the gap or gaps is effected, the natural tendency of an electron beam toward transverse spreading during its travel necessitates the use of a beam of a relatively limited electron density. The limitation to such a density is required in order to prevent the transverse area of the electron beam from becoming larger in size than the apertures through which the beam must pass. This problem can not be satisfactorily solved by the use of apertures of increased size as this results in a less eflicient interaction between the beam and the interaction gaps as well as in a number of other undesirable effects; Thus it is desirable to have the electron beam in focus at each interaction gap. The size of the gap aperture may then be made very small; this makes for a more eflicient interaction. Thus the use of a beam which is in focus at each gap allows the use of a beam of a higher electron density than could heretofore be tolerated without a relatively great spreading of the beam.

The bringing of an electron beam to more than one focus has heretofore been accomplished by a plurality of magnetic or electrostatic focusing devices, one near each of the desired foci. tures are often designed in which the use of separate focusing devices near each of the desired foci are impossible or impracticable. For example, in velocity modulated electron tubes, known as klystrons, it is a common practice to posit ion'two resonant cavities adjacent each other and with 'part of the adjacent sides comprising a common wall. An electron beam in such a tube passes the interaction gap in the first cavity, continues into a short'free space, and then traverses the gap inthe adjacent second cavity without passing a free space inits path of travel between the gaps where it is practicable 'to refocus the beam. Similarly, in reflex klystrons utilizingan electrostatic reflector electrode it is desirable to have the electron beam present a minimum transverse area to the interaction gap during both the incident and the return passes of the beam. And in reflex klystrons of the magnetic reflector type it is desirable for the beam to bein focus at the entrance thereof into the magnetic gap and also in focus, .after reflection, within the interaction gap. The use of reflex klystrons of the magnetic refiector type is known in the art and is described for example, in greater detail in U. S. Patent No. 2,651,000 granted to E. G. Linder.

Accordingly, it has been desirable for some time. to

However, electron beam tube struc Patented June 24, 1958 ice . 2 devise improvements in electron beam tubes whereby an electron beam can be brought to more than one focus by a single focusing means located at substantially a single position within the electron tube.

It is an object of the present invention to provide an electron beam tube with improved means for utilizing a high density beam of electrons.

It is a further object to provide an electron beam tube in whichfocusing means is provided for opposing the transverse spreading of the beam so that it may be of verygreatly increased density and at the same time have a small transverse area at each of a plurality of predetermined positions along its path of travel.

It is a further object to provide a reflex klystron'of the magnetic reflector type with improved means. forutilizing a small spacing between the magnetic polesof the reflector means.

In order to accomplish these andother objects this inve'ntionwmakes use of electron optic principles to provide an 'astigmaticallyshaped electron beam with two foci. Each of the two foci are axially spaced and oriented 90 from the other. The use of astigmatically-shaped electron beams provides two focal points, or more properly, focal lines instead of the conventional single focus point. While this electron beam shaping is especially useful'in the aforementioned velocity modulated electron tubes, it may beused in any electron tube where it proves useful to have two positions of focus along the path of an electron beam. I

The astigmatically-focused electron beam is created by a shaped'electron gun. The electron gun may have a shaped cathode and a conventional focusing grid, a conventional cathode with a shaped-focusing grid, or both electrodes may be shaped. For example, instead of the conventional spheroidal cathode surface, an ellipsoidal cathodesurface may be provided. Concentric and, parallelto this ellipsoidal cathode surface is an ellipsoidal focusing grid. The grid may comprise either a mesh type or a single apertured electrode. When the electron beam from this 'gun converges at a large angle, so as to minimize transverse beam spread, and comes to a focus notat a single point as in the more usualconical focus but,

first focus. This second focus or focal line lies in a second plane passing through the minorvaxis of the ellipsoidal electrode, and intersecting the center of the first focal line at 90. This second focal line lies on the second aforementioned plane; thus the second focal line is perpendicular'to thefirst focal line but axially spaced from it.

The use of focal lines instead of focal points gives an added advantage. Beam spreading due to space-charge mutual-repulsion is reduced. In a convergent beam,.as the beam tends to come to a smaller diameter the electronsget closer together and the space-charge density I increases within the beam. Hence the space-"charge mutua'l-re pulsion become greater. This means that the transversecomponents of velocity of the electrons toward the beamjcenter tend to decrease as the beam becomes more constricted. If the electrons are converged toward a line focus rather than a point focus, the electron density (and thus the mutual repulsion) at the line focus is substantially less than the electron density at the point focus.

The current will be spread over the length of the line focus in the first situation while the current centrated at a point in the second.

will be con- 'While the invention ispointed out withparticularity inthe appended claims it may be best understood from the following detailed description and drawing where like numerals refer to like parts; The embodiments described are presented solely for illustrative purposes and not by way of limitation.

In the drawings:

Figure 1' is an illustration depicting the electron optic principle used in the invention.

Figure 2 is a longitudinal sectional view of a two-cavity type ofklystron.

Figure 3 is a transverse sectional view taken on line 33 of Figure 2. V

Figure4 is a longitudinal sectional view of'a klystron oflthemagnetii: reflector type.

Figure 5 is a sectional view of Figure 4 tal-xenon line 55.

Referring now to the drawing in greater detail there is shown in Figure 1 an illustration depicting the electron optic principle which is made use of in the invention. A surface 10, which is a section of the surface of an ellipsoid, isshown with

rays

12, 14, 16 and 13 emanating from it. The rays come to a first focus at a focal line 20 parallel to the major axis 22 of the sectionof the ellipsoid 10. The rays continue in a straight line, crossing over at that first focal line 20, and come to a second focus at a focal line 24 parallel to the minor axis 26 of the section of the ellipsoid.

It will be appreciated that an electron beam emanating from an ellipsoidal cathode surface will vnotfollow exactly in straight lines-as depicted'in Figure 1 if an un-neutralized space charge is present. Rather the mutual repulsion of the electrons within the beam due to space charge will cause the lines defining the edge of the electron beam to be divergently curved. Such a beam will tend to come to the respective focal lines at positions further from the ellipsoidal surface than shown. However, if the beam path is passed within an electrostatically shielded space, and this space is entirely free of any electric field except for that of the electron space charge, mutual repulsion will not occur within the space. As is more fully in the above-mentioned patent, positive ions will be entrapped in the beam path and space charge neutralization, will take place. Consequently, the electron beam will not be divergently curved by the space charge. Through the use of space charge neutralization the beam may be brought to precise focusing.

Figure 2 shows an improved two-cavity type of klystron embodying the invention which may be used as an amplifier. It will be appreciated that the k-lystron may be operated as an oscillator by providing a coupling aperture between the two cavities or by providing an external feedback line therebetween. The two-cavity klystron 30 is provided with an envelope 32 which may contain two

cavities

34 and 36. The cavities are provided with

interaction gaps

38 and 40 and coupling lines 42 and 44 for transferring energy to and from the cavities. An astigmatic electron gun structure 46 is positioned within the envelope 32 and at one end of the klystron; a collector 48, which may be connected to a lead 51, is provided. at the other end. The gun 46 has a cathode 49 which is in the shape of a section of an ellipsoid. It is shown connected to its lead 50. The cathode is provided with a heater and heater leads (not shown). An accelerating electrode 52 concentric with the shaped cathode, and connected to a lead 53, cooperates with the cathode to bring the electron beam 54 to a first thin-line focus, similar to the thin-line focus 20 shown in Figure 1, at the interaction gap 38 of the first cavity 34. The electrons in the beam then come to a. second thin-line focus at the interaction gap 40' of' the second cavity 36. The electron beam then continues on until it is collected by a collector 48 having a lead'51. The

apertures

58, 59, 60 and 61 in the cavities through which the beam passes are of a size and shapesimilar to that of the cross sectionalarea. ofthe electron: beam atv 4 those apertures. Thus, for example, as seen in Figures 2 and 3, the

cavity apertures

58 and 59 in the first cavity may be in the shape of a narrow slot which is elongated in a vertical direction so as to pass through it the electron beam which at that position has no appreciable horizontal width thickness but which has a vertical width. Similarly, the apertures 60 and 61 in the second cavity may also be in the shape of a narrow slot but which is elongatedin a horizontal direction so as to pass through it the electron beam which at that position has no appreciable vertical thickness but which has a horizontal-width.

Thus an efficiently focussed beam is available at each cavity, whereas in the case of the usual single-point focus gun a compromise is necessary and the focus is positioned intermediate the two interaction gaps.

Similarly, a reflex type klystron using an electrostatic reflector plate may use the astigmatic electron gun of this invention to advantage. The gun in this case would be oriented with respect to the interaction gap such that the first thin-line focus of the electron beam occurs atthe interaction gap during the incident passage of the beam past that. gap. The second thin-line focus occurs at the interaction gap, after the reflection of the electron beam, during the return'passage of the beam. The first thin-line focus may, for example, take the position of a vertical line. Both apertures defining the interaction gap would then include a vertical elongation to pass the incident beam; The second thin-line focus, during the return passage of the electron beam to the gap, would then be oriented 90 from the first thin-line focus and would take the position of a horizontal line. The aperture on the reflector electrode side of the gap would then have, in addition to the vertical elongation to pass the incident beam, a horizontal elongation to pass the return beam. That aperture would thus be in the shape of a cross; the aperture on the gun side of the gap is in the shape of a vertical slot so that it passes substantially all of the vertical incident beam but intercepts most of the horizontal return beam. Such interception of the beam is desirable since it substantially eliminates multiple transits and thus prevents hysteresis effects during the modulation of the beam.

There is shown in Figure 4 a side view of a reflex klystron of the magnetic reflector type. The magnetic reflex klystron 62 is provided with an envelope 64. The envelope 64 may be made of conductive material so as to shield electrostatically all of the space which it surrounds. A magnetic reflector 66 is positioned within the envelope 64; the reflector 66 has two, closely spaced pole pieces 68 which define between them a space 70 in which is produced a relatively dense magnetic field. A cavity 72, which has an interaction gap 74, is provided within the envelope and near the magnetic reflector. Within the envelope, on the side of the cavity remote from the reflector, is positioned the electron gun 76 which provides a dense shaped beam of electrons 78. The electron gun 76 comprises a cathode 80, which has its active surface in the concave shape of a section of an ellipsoid, and a grid 82, which is also in the shape of a section of an ellipsoid. The grid 82, which serves as an accelerating electrode, is positioned concentric with the cathode and with the major aXis of the ellipsoidal grid in registry with the major axis of the ellipsoidal cathode. This position also places the minor axis of the grid in registry with the minor axis of the cathode. It will be noted that the cathode 80 is positioned closely adjacent to the entrance side of the interaction gap 74. The'purpose of this is to prevent excessive spreading of the electrons as they are the magnetic reflector 66 is grounded, it is connected'to the positive terminal of the battery. The tube may in clude a modulating electrode 93, which is insulatingly supported between the pole pieces 68 and provided with a lead 94 so that the electrode may be connected to an external signal source.

Referring now to Figure 5, one of the pole pieces 68 of the magnetic reflector 66 is shown in the background within the tube envelope 64. During the operation of this device, the grid 82 attracks electrons from the cathode and accelerates them in a shaped beam 78. The electron beam passes through the interaction gap 74 in the cavity 72 and comes to a first thin-line focus shown dotted at 95 at the magnetic reflector 66 and thus between the pole shown in the background of the figure and the other pole not shown. The reflector produces a substantially uniform magnetic field having flux lines extending perpendicularly through a hypothetical lamina in which the beam is intended to follow a turn-about path. In this figme these flux lines extend perpendicular to the plane of the drawing and across a gap, not appreciably thicker than this lamina, which is defined between the two opposed magnetic pole pieces. As is known, if an electron traveling along a straight path crosses a substantially planar boundary of a magnetic field, the electron, on entering the field, will be continuously deflected from the straight path in a perpendicular direction determined by the polarity of the magnetic field. If the magnetic field is substantially uniform, the path during deflection will be circular. The pole piece shown is presumed to be the south pole of the reflector; that is, the flux lines are presumed to point downward into the pole so that the direction of deflection will be toward the bottom of the figure. The field strength of the reflector and the velocity of the electron beam may be chosen so as to have the electron beam, as a whole, turn substantially 180 from its incident direction. This may be seen by looking at the paths of two portions of the beam each at opposite outer edges of the electron beam. We see a first portion 96 of the electron beam, at the upper edge of the beam, converging (downward in the figure) toward the same direction as the direction of deflection by the magnetic field. The electron beam 78 as a whole may be given such a velocity that the aforementioned first portion 96 will describe a circular path within the field of substantially less than 180. At the same time, with the same velocity, a second portion 97 of the electron beam, at the opposite edge of the electron beam, will describe, within the field, a circular path of substantially greater than 180. The velocity of the beam and the magnetic field may be chosen so that the path of the second portion within the field will be greater than 180 by the same amount as the path of the first portion is less than 180; Thus the beam as a whole will have traversed a course turning it around 180 and back toward a path which isparallel to and displaced somewhat to one side of its incident path. The progressive convergence of the beam toward a second thin-line focus 98 will be suspended when the electrons are in the turnabout gap. The reason for this being that the angle of convergence of electrons, with respect to each other, is the same at the place of entry of the beam into the field as is the angle at the place of exit. Thus the beam will then continue on toward its second thin-line focus which occurs at the interaction gap 74. As the beam has been laterally displaced during its passage through the magnetic field, the aperture 99 in the cavity wall 100 on the far side of the gun is further elongated on one side, the bottom part of the aperture in the figure, so as to pass the return beam into the interaction gap. But as the cavity wall 101 on the gun side of the beam is not so elongated, it intercepts the beam preventing its passage beyond the gap. By this means the beam is collected and multiple transits are substantially eliminated.

It will be apparent from the foregoing description of the invention that a novel means is disclosed for providing an electron beam having more than one focus. While examples have been given showing how this means may be utilized in some electron beam tubes, it will be appreciated that other uses of the invention within electron tubes may be had where it proves advantageous to utilize an electron beam focused to more than one position of focus.

What is claimed is:

1. An electron beam tube comprising an envelope containing an electron gun for producing a beam of electrons along a beam path, said electron gun including astigmatic focusing means for bringing said beam of electrons to a first thin-line focus and a second thin-line focus axially spaced from and perpendicular to the first thin-line focus, and means coupled to said beam path at each of said foci and adapted to coact with said electron beam at each of said feel.

2. An electron beam tube comprising a vacuum envelope,'an electron gun including a large-area, concave cathode and a cooperating grid for providing an electron beam having a first thin-line focus and a second thin-line focus axially spaced from and perpendicular to the first thin-line focus, said cathode being ellipsoidal in planes perpendicular to the axis of said beam and means at each of said foci adapted to coact with said beam.

'3. An electron beam tube comprising a vacuum envelope containing an astigmatic electron gun for producing a focused beam of electrons having a first thinline focus and a second thin-line focus axially spaced from and perpendicular to the first thin-line focus, and means adapted to coact with said beam comprising first aperture means at the first thin-line focus shaped to pass substantially only said beam at said first focus and second aperture means at the second thin-line focus shaped to pass substantially only said beam at said second focus.

' 4. A velocity modulated electron beam tube compris ing a vacuum envelope containing an electron gun for producing a substantially continuous astigmatically focused beam of electrons along a beam path, said beam having a first thin-line focus and a second thin-line focus axially spaced from said first focus, and resonator means comprising at least one pair of opposed conductive walls each formed with an aperture surrounding said beam path and spaced from the other wall to define an interaction gap.

5. In a velocity modulated electron beam tube, a vacuum envelope containing an electron gun for producing an asymmetrically shaped electron beam, said beam having a first thin-line focus and a second thinline focus axially spaced from and perpendicular to the first thin-line focus; first resonator means comprising a pair of opposed conductive walls each formed with an aperture surrounding the axis of said beam and spaced from the other to define a first interaction gap, said beam having its first thin-line focus at said first gap; and second resonator means comprising a pair of opposed conductive walls each formed with an aperture surrounding the axis of said beam and spaced from the other wall to define a second interaction gap, said beam having its second thin-line focus at said second gap.

6. In a velocity modulated electron discharge device of the reflex type, a vacuum envelope containing an electron gun for producing an astigmatically focused electron beam having a first thin-line focus and a second thin-line focus axially spaced from and perpendicular to the first thin-line focus, resonator means comprising a pair of opposed conductive walls each formed with an aperture surrounding the beam axis and spaced from the other to define an interaction gap, and means adjacent the aperture on the farside of the gap from the gun fo rproducing a retarding field with a substantially' planar boundary on its side nearest said gun to reflect said electron beam back along a return path which extends back to said gap, said beam having its first focus on a line at said gap on the incident passage of said beam past said gap and having its second focus on a line at said gap on the return passage of said beam to said gap, said aperture on the far side of th gap being shaped to passvsubstantially only the incidentbeam focus line and. the return beam focus line, said aperture on the gun side. of the gap being shaped to pass substantially only the incident beam focus line.

7. A velocity modulated electron beam. tube of the magnetic reflex type comprising an electron gun for producing a beam of electrons with two foci along an axis, resonator means comprising a pair of opposed conductive walls each formed with an aperture surrounding said axis and spaced from the other wall to define an interaction gap, magnetic reflector means on the far side of said gunfrom= said gap for reflecting said electron beam, and astigmatic electronoptic means within said envelope for bringing said electron beam to one focus at said magnetic reflector means and the other focus at said gap.

8. A velocity modulated electron beam tube of thev reflex typecomprising a vacuum envelope containing an electron gun for producing a substantially continuous shaped beam of electrons, along an axis at a predetermined velocity, having a first thin-line focus and a second thin-line focus axially spaced from and substantially perpendicular to the'first thin-line focus, resonator means comprising a pair of opposed conductive walls each formed with an aperture surrounding said axis and spaced from the other to define an interaction gap, and means adjacent the aperture on the far side of the gap from the gun for producing a magnetic field transverse to said axis and having a substantially uniform field density, the entrance boundary of said field lying in a plane substantially perpendicular to said beam axis, said field having the polarity and flux density which for said velocity and the angle at which said axis transverses said boundary cause it to guide the electron beam along at substantially 180 of a circular path Within the field to redirect it onto a return path which extends: back to said gap and is substantially parallel to the path of the incident beam, said electron beam having its first'thinline focus at the circular path of said electron beam insaid magnetic field and its second thin-line focus on its return path at said gap.

9. A velocity modulated electron beam tube of' the magnetic reflex type, comprising a vacuum envelope containing an electron gun for producing a beam of electrons along. an axis at a predetermined velocity; said gun including astigmatic focusing means for bringing saidbeam to a first thin-line focus and a second thin-line focus axially spaced from and substantially perpendicular to the first thin-line focus; resonator means-including a pair of opposed conductive walls each formed with an aperture. surrounding said axis and spaced from the other to define a capacitive interaction gap; magnetic means.

having a pair of spaced magnetic poles adjacent. the aperture on the far side. of said gap fromv said gun for producing between the poles a magnetic field transverse.

to said axis, said poles having coextensive closely spaced edges on the end. sides toward the gun causing said. field to have a substantially planar boundary at the point where it is first traversed by said axis, said field having the polarity and flux density which for said velocity andv the angle at whichv said axis traverses said boundary cause .it to guide the electron beam along at substantially 180 of a circular path between the poles to redirect it onto a return path which extends back to said gap substantially parallel to the path of the incident beam; and a modulating electrode near the circular path of the electrons for electrically influencing said beam therein in response to an externally applied modulating signal to modulateradio frequency energy produced by the tube, said electron beam having its first thin-line focus at the circular path of said electron beam in said magnetic field and its second thin-line focus on its return path at said gap.

10. An astigmatic electron gun comprising a source of electrons and electrostatic accelerating electrode means 'for producing along a path a beam of. electrons having two line foci at right angles to each other and spaced from said source at successively greater distances along said path.

11. An electron gun as in claim 10 wherein said source is a cathode having an ellipsoidal surface.

12. An electron gun as in claim 11 wherein said means is a grid having an ellipsoidal surface uniformly spaced from said cathode.

References Cited in the file of this patent UNITED STATES PATENTS 2,075,717 Hehlgans Mar. 30, 1937 2,103,645 Schlesinger Dec. 28, 1937 2,124,270 Broadway July 19, 1938 2,227,033 Schlesinger Dec. 31,v 1940 2,567,674 Linder 1 Sept. 11, 1951 2,582,045 Lefferty Jan. 8, 1952 2,590,100 Heil Mar. 25, 1952 FOREIGN PATENTS 501,058 Great Britain Feb. 21, 1939' 572,540 Great Britain Oct. 12, 1945 580,860 Great Britain Sept. 23, 1946