US2074829A - Electron beam tube - Google Patents

Description

March 23, 1937. Y J. M. CAGE ELECTRON BEAM TUBE Filed June 21, 1955 .a 9 M m m fi WM Cm Jwm UH, b

UNITED STATES PATENT OFFICE ELECTRON BEAM TUBE John M. Cage, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application June 21, 1933, Serial No. 676,875

13 Claims.

The present invention relates to electron discharge devices, more particularly to those devices which utilize a beam of electrons deflected electrostatically within the tube.

Tubes of this character usually employ a source of electrons together with a focusing arrangement, an anode for receiving the electrons which is charged positively with respect to the electron source, and an intermediately positioned elecw trode for moving the electron beam over the face of the anode or other electron-receiving member. The focusing device may take the form of a hollow cylinder surrounding the source of electrons and is provided with a cutaway portion or slit through which the electrons move on their way to the anode.

In the design of these prior art tubes, it was considered necessary to perform all of the focusing action at a position as close to the cathode as possible rather than at any other position on the theory that, if the electrons are once focused or constrained to a beam of proper shape, they will remain in this constrained condition throughout their travel toward the anode. However, it was subsequently found that notwithstanding their initial focussed condition, the electrons tend to spread or disperse when the beam is electrostatically deflected across the tube and a considerable distortion is introduced into the translating properties of the tube. According to the prior art teachings, the cylindrical member, which surrounds the cathode, was vested solely with the focusing function and, for this purpose, was charged negatively with respect to the cathode. In these tubes, the remaining electrodes, including the deflecting members, were either so biased with respect to the cathode or else were entirely disconnected therefrom so as to preclude any concentrating efiect on the moving electrons.

Contrary to the accepted principles of design of the prior art tubes, I have discovered that, when the deflecting members are connect-ed to the cathode through a biasing battery of suitable polarity and potential, they perform not only their usual function of sweeping the electron beam across the tube but, in addition, effectively serve to ,focus the electrons and thus assist in maintaining the original concentrated shape of the beam during its transverse excursions. Thus, an additional focusing effect is introduced into the tube at a position intermediate the cathode and anode, which is exercised simultaneously with the deflecting action, serving to eliminate the normal tendency of the electrons to disperse or spread before they reachthe anode. In accordance with my invention, it becomes possible entirely to eliminate the usual focusing member surrounding the cathode and to rely entirely upon the focusing effect offered by the deflecting members, thus permitting the elimination of a member heretofore considered essential, although it is preferred to employ the usual form of focusing electrode in addition to the combined focusing and deflecting members.

An object of the present invention is to improve the operation of/beam tubes and, in particular, to improve the focus of the beam so that the latter will maintain a highly concentrated form as it sweeps across the electron receiving surface. The manner in which this object is carried out and the invention, itself, will be apparent from the following description when perused in connection with the accompanying drawing in which Fig. 1 is an elevational view of anelectron beam tube improved in accordance with the present invention; Fig. 2 is a diagrammatic view of the tube shown in Fig. 1 and connected in an electrical system for amplification purposes; while Fig. 3 shows in diagram, a modified form of electron beam tube utilizing a fluorescent screen and connected in suitable circuits.

Referring more particularly to Fig. 1, numeral I designates an envelope which may be either highly evacuated or contain a small amount of gas, the envelope terminating at the bottom, as shown, in a reentrant stem 2 and a four-pillar press 3. The envelope is of the tipless type, hence, an evacuating tubulation 4, which communicates with the interior of the envelope, is provided within the stem 2. As will be seen from the drawing, the entire electrode structure is supported in a symmetrical manner entirely from the press 3. This structure includes an indirectly heated cathode 5, seen morev clearly in Figs. 2 and 3, containing a filamentary heater 6, preferably of tungsten. The cathode conveniently takes a form of an elongated cylinder and is preferabiy coated with electron-emitting material, such as alkaline earth oxide. concentrically surrounding the cathode, there is a cylindrical member I, also of elongated form which is provided with a narrow slit 8 for the egress of electrons on their way to electron-receiving members described hereinafter. There is an electrode 9 positioned in proximity to the slit 8 for the purpose of accelerating the electrons. This electronaccelerating member is preferably constituted of two angular pieces of elongated'form which are joined together only at their ends by metal straps It so that the longer legs of the angular pieces are in the same planes, and leaving a rectangular slit lliinlinewiththeslitfl.

On the opposite side of the electron-accelerating electrode 9 from the cathode 5, there are a pair of parallelly disposed metal plates 12 of a fiat rectangular configuration, arranged symmetrically with respect to the slits 8 and H, the purpose of which is to deflect the electron beam in a controllable manner, also to focus the electrons during deflection, as will be described hereinafter. For receiving the electrons, there may be provided a pair of physically separate auxiliary anodes l3 which are also conveniently made of angular pieces spaced apart to leave a slit It in line with the slits of the other electrodes. Directly in back of the slit l4, there may be .provided the main electrode or anode l 5 for receiving the electrons under certain conditions. The electrode l5 conveniently takes the form of an elongated hollow cylinder provided with a slit 16 which faces the slit M in the auxiliary anodes l3.

The manner in which these electrodes are supported from the four-pillar stem is clearly shown in Fig. 1, and a detailed description of the electrode supports is believed to be unnecessary since the arrangements of the supporting members are multifarious and will readily occur to those skilled in the tube manufacturing art. Furthermore, the manner in which these electrodes are supported within the tube does not constitute a part of the present invention. It is sufficient to state that the electrodes and their supports are mounted in a symmetrical and rigid manner-within the tube, utilizing rods of glass H, where necessary, to insulatingly support one electrode from another. Each electrode is preferably supported at two places so as to lend rigidity to the structure as a whole and suitable leading-in conductors are taken through the stem to the exterior. A getter cup l8 containing magnesium or similar material is conveniently secured to one of the support rods in a position as to be heated by a high frequency coil placed around the envelope during evacuation so that the magnesium may be flashed and the envelope relieved of deleterious remnant gases.

The electrodes are given the usual gas-denuding and eat treatments prior to placing within the envelope and the tube evacuated to a high degree, for example, less than a few microns of mercury, by attaching a vacuum pump to the tube 4. When the proper degree of vacuum has been obtained, the tube 4 may be sealed OE Within the stem 2, as is well known in the art. It will be understood that, if desired, instead of being operated as a high vacuum tube, the envelope may contain one or more inert gases, such as argon or neon, at reduced pressure. A base, not shown, of the usual construction is usually provided about the lower end of the tube. A number of contact plugs may be molded in the base and connected to the various leading-in conductors; these plugs register with terminals in a socket for conveniently making electrical connection with the electrodes within the tube.

Fig. 2 shows one form of circuit which may be employed with the improved tube. As shown therein, there is a source of electromotiveforce'20 of direct current voltage which may comprise a battery, and various taps taken from the source to the electrodes. The negative terminal of the battery is connected by a conductor 2| to the focusing member I and an intermediate terminal is taken to the cathode 5 by a conductor 22. The battery 20 may also provide energy for the heater 3 through a conductor 23, which is connected cathode cylinder 5 so that a return for the heater energy is provided through the cathode wire 22. The extreme positive terminal of the battery 20 is connected to the electron-accelerating electrade 9.

The combined electron-deflecting and electronfocusing members l2 arebridged by a high resistance 24 and the mid-tap on this resistance is connected by means of a conductor 25 to an intermediate terminal on the battery, as shown. Separate conductors 26'are taken from both ends of the resistance 24 so that these conductors are connected directly to the respective electrodes I2. The physically separate auxiliary anodes l8 are likewise bridged by a high resistance 21 and a conductor 28 taken from the mid-tap on the resistance back to a terminal on the battery which is less positive than the terminal connected to the electrode 9. A conductor 29 is also provided between the battery 20 and the main anode l5, theterminal on the battery towhich the conductor 29 is connected being preferably more positive than the terminal to which the conductor 28 is connected.

A source of controlling voltage is applied across the conductors 26 which may conveniently be considered the input circuit, and a current indicating or measuring device connected across the conductors 30 which may be considered the output circuit of the device.

The operation of the tube is clear from the circuit relations shown in Fig. 2. The filament 6 serves to heat the cathode 5 which, on account of being electronically active, emits a profuse stream of electrons which move through the slits 8, H, and either strike the auxiliary anodes I3, depending upon the amount of deflection given theelectron beam en route, or else pass through the slit it between the auxiliary anodes and through the slit it into the anode l5. Due to the elongated configuration of the various slits through which the electrons pass, the electrons take the shape of a beam in relatively thin sheet form.

It is apparent that the electrode I, being connected to the negative end of the battery 20, introducesa fieldwhich acts symmetrically along the length of the cathode and on all of the electrons because the cathode is of an equipotential character. This electrode therefore serves to focus the electrons, i. e. to constrain them to a desired shape, depending upon the shape of the slit 8. It is also evident that the electrode 9, being connected to the positive end of the battery 20, produces a positive field in the region of the cathode which serves to give to the electron beam, as it emerges from the slit 8, an extremely high velocity. Thus, it may properly be said that the cathode 5, the negatively-charged focusing member 'l, and the positively-charged accelerating electrode 9, together constitute an electron gun" which causes a fast-moving and highly concentrated beam of electrons to emerge from the slit ll.

This electron beam, after leaving the slit II, is electrostatically deflected by the electrodes l2, i. e. given' a transverse or sweeping movement across the tube, depending upon the magnitude of the control voltage applied to the conductors 26. As the beam moves across the tube in response to the electrostatic field exercised by the electrodes l2, variable portions of the beam strike the electrodes I 3 and I5. Thus, in the arrangement shown in Fig, 2, when the beam is deflected toward the left-hand electrode l2, practically all of the beam strikes the left-hand auxiliary anode I3 and little, if any, of the beam will pass through the slit l4 and reach the anode I5, and practically none of the beam will strike the right-hand anode l3. When the beam is deflected the other way, i. e toward the right-hand electrode l2, it traverses the slit H, at which time all of the beam temporarily passes into the anode l and, upon further deflection, the beam strikes the right-hand auxiliary anode I3. It is obvious that, as the beam impinges on either the lefthand anode I3, the main anode [5 or on righthand anode I3, depending upon the amount and direction of deflection given the beam by the electrodes l2; the respective electrodes I 3 become variably charged, both in magnitude and polarity. These variations of electrical charge can be detected or measured by suitable devices and instruments connected to the conductors 30 or to the conductor 29.

It is apparent that, due to the manner in which the electrodes are disposed with respect to one another and, consequently, the manner in which the tube operates, the variations of the output current not only represent a satisfactory reproduction of the voltage variations applied to the deflecting electrodes l2 but also amplify these voltage variations to an extremely large degree. Thus, the device has application as a radio frequency and audio frequency amplifier in radio and other intelligence communication circuits and, in addition, can be designed to detect radio signals efficiently.

Electron beam tubes employing intermediately positioned deflecting electrodes are, of course, well known to the prior art. My specific improvement in a tube of this sort consists in employing the deflecting electrodes not only for the usual deflecting function, but also for assisting the focusing member 1 in maintaining the original shape of the electron beam as it passes through the tube on its way to the anodes l3, l5.

It will be understood that, in order for the tube to provide a satisfactory reproduction of the voltage variations applied to the conductors 26 in terms of current variations at the conductors 30, it is necessary that the electrons maintain the same position within the electron beam as it moves through the tube until it finally reaches the electron-receiving surfaces.

Whereas heretofore, the deflecting electrodes had a tendency to diverge or spread the beam away from its original shape due to the electrostatic fields'introduced between deflecting members, in accordance with my invention, I have found that it is entirely feasible to so modify the deflecting members and their electrical circuits as to prevent this divergence or spreading out, and actually to provide an added focusing action at the position of the deflecting electrodes. This desirable effect is brought about by the use of a potential which may be either positive or negative with respect to the cathode, depending upon the distance between the plates, and which is applied in the same polarity and magnitude to each of the deflecting electrodes. For this purpose, the deflecting electrodes are electrically connected by a high resistance 24 and a conductor taken from the mid-tap of the resistance back to a suitable positive or negative terminal on the battery 20. As in the case of the focusing electrode 1', the deflecting electrodes also introduce a fleld of constant magnitude which acts symmetrically along the length of the cathode and on all of the electrons in the beam because the cathode is of an equipotential character and has no potential drop therein which might interfere with the focusing fleld.

The manner in which this improved connection operates to introduce an added focusing effect will be at once clear when the equipotential lines, suggested by the dotted lines, are considered. The positively-charged electron-accelerating electrode 9 produces a field within the space confined by the electrodes I2 which may take the form of a series of loops 3|, symmetrically arranged about the slit l I andextending for a substantial distance within the said space. This distance is determined by the difference in potential between the electrodes l2 and 9. There are similar equipotential lines which take the form of inverted loops 32 and also extend downwardly from the auxiliary anodes l3 for a considerable distance into the space between the deflecting electrodes I2. As in the case of the loops 3|, the distance that the loops 32. extend into the space and their configuration depend to some extent upon the relative potentials existing between the auxiliary anodes i3 and the deflecting electrodes l2. The electrical charges on the deflectlng electrodes, due to the current flowing from the battery through the conductor 25, produce equipotential loops 33 which extend toward one another in the manner roughly suggested by the dotted lines.

Now supposing an electron beam is moving toward theanode and is temporarily in the space between the deflecting electrodes. The individual electrons will tend to cross the respective equipotential lines at right angles thereto and in a direction from the lower potential line to the higher potential line. By noting the position of the arrows 3 4, which are drawn at right angles to the dotted lines, it will be seen that there is a constant force acting on the electrons which continually urges them toward the center of the space and away from the deflecting electrodes themselves. This continual urge tends to keep the electrons in the beam packed together and to maintain the original shape of the beam as it moves through the deflection zone. Consequently, that portion of the beam which strikes either one of the electrodes l3 or the main anode l5 represents an accurate proportion of the entirebeam determinable by the magnitude and polarity of the voltages applied to the deflecting electrodes. Therefore, the tube as a whole tends to amplify without distortion, and the current available at the output circuit conductors 3!] represents a faithful reproduction of the voltage variations applied to the conductors 26. It will be understood that the above-stated theory, based upon the position of equipotential lines, is offered purely for explanatory purposes and .I do not desire to be limited thereto. It is a fact, confirmed by many tests and regardless of theory, that, when the deflecting members are maintained at a'flxed average potential with respect to the equipotential cathode and different from the potential of the anode in accordance with my invention, the output current variations follow with much more faithfulness the changes in potential applied to the deflecting members than when the latter are not so connected.

While I have shown the combined deflecting and focusing electrodes ll being used in connection with a focusing member 1, it is obvious that the latter may be dispensed with, if desired, as suflicient focusing effect for ordinary purposes may ordinarily be obtained from the deflecting electrodes. However, it is considered preferable to utilize the focusing member I in addition to the improved deflecting electrodes.

Fig. 3 shows the application -of the invention to a cathode beam tube which utilizes a fluorescent screen 36. In this figure, the anodes I 3 and i5 have, of course, been omitted since the electrons now impinge upon the screen to form a light image, for visual or photographic purposes, of the voltage'variations applied to the conductors 26. In this figure, elements, which correspond to the elements shown in Fig., 2, have been given the same reference'characters. Thus, the deflecting electrodes II are bridged by a resistance 24, and a conductor 25 taken preferably from the mid-tap of the resistance to the battery 20. As in the case of Fig. 2, there is present in the space between the electrodes l2 equipotential lines, indicated roughly by the loops 3| and 33, which tend to converge the individual electrons of the beam passing through the tube into a packed condition, similar to the condition produced by the focusing action of the member I. The arrows 34 represent the direction in which the electron-constraining forces act and it will be noted that this force is always exercised toward the center axis of the tube. Unless the deflecting electrodes II are provided with a fixed average potential negative or positive with respect to the equipotential cathode and different from the anode, which potential is applied to both electrodes, of the same polarity and magnitude, the electron beam tends to spread and to be attracted toward the electrodes while the beam is being deflected so that the light image, as shown on the fluorescent screen, does not represent a faithful reproduction of the voltage variations applied to the conductors 26.

However, in view of the focusing eifect provided by the deflecting electrodes 12 in accordance with my invention, the electron beam maintains its original focused condition as it moves through the tube and, hence, the light image on the fluorescent screen varies exactly in accordance with the variations of voltage applied to the conductors 26. As was explained in connection with Fig. 2, the focusing member I may be entirely dispensed with and the focusing efiect obtained 'solely from the deflecting members, but it isv considered preferable to utilize the electrode 7 in addition to the focusing effect offered by the electrodes i 2.

While I have described and illustrated my invention in connection with an electron beam tube in which the cathode rays are obtained initially from an equipotential source of electrons and the beam takes on a more or less elongateconfiguratidn, it is to be understood that the invention is not limited to this form of cathode or shape of beam. Obviously, the combined deflecting and focusing function exercised by the .electrodes I2 is equally applicable to beams of any and all shapes, depending upon the shape of the openings 8, ll, etc. Furthermore, it is also evident that instead of an indirectly heated cathode as shown and described, any suitable and wellknown form of directly heated cathode, for example, a flat spiral filament may be employed, in which case the connection 22 is brought back to a point on the filament circuit which represents the average potential of the filament so as to retain the same symmetrical electrical conditions as are provided in the case of the equipotential cathode.

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

1. An electron beam tube comprising an envelope containing an electron gun including an equipotential cathode for producing electrons in a fast-moving focused condition, and a common means for deflecting the beam across the tube and for simultaneously maintaining the beam in its focused condition, and means for receiving the electrons.

2. In combination, an electron beam tube comprising an envelope containing an equipotential source of electrons, an anode, a plurality of combined electron-deflecting and electron-focusing electrodes positioned on opposite sides of the envelope between said source and said anode, a source of electromotive force, the negative terminal of which is connected to said source of electrons, and connections from a positive terminal on said source of electromotive force to the electron-deflecting electrodes, and a connection from a different terminal on said source to said anode.

3. An electron beam tube comprising an envelope containing an equipotential source of electrons, an electron-receiving member, an electronaccelerating electrode positioned between said source and said member, and a combined electron-deflecting and electron-focusing electrode interposed between said electron-accelerating electrode and said electron-receiving member.

4. An electron beam tube comprising an envelope containing a source of electrons including an indirectly heated cathode, an electron-receiving member, an electron-accelerating electrode positioned between said source and said member, combined means for sweeping the electron beam across the face of said member and for focusing said electrons, said means including a pair of electrodes disposed on opposite sides of the envelope, a resistance connected between said electrodes, and a conductor between said resistance and said electron source, said combined sweeping and focusing electrodes being charged to a different potential from the potential of said electron-accelerating electrode and fixed with respect to the average potential of said source of electrons.

5. In combination, an electron beam tube comprising an envelope containing an indirectly heated cathode, an anode, a plurality of combned electron-deflecting and electron-focusing electrodes respectively positioned on opposite sides of said envelope between said cathode and anode, a resistance between said electron-defleeting electrodes, a source of electromotive force, said cathode, anode, and resistance being connected to different potential terminals on said source of electromotive force.

6. An electron beam tube comprising an envelope containing an electron gun including an equipotential cathode for producing electrons in a fast-moving focused condition and a common means for deflecting the beam across the tube and for simultaneously maintaining the beam in its focused condition, and means for receiving the electrons, said common means including a plurality of metal members maintained at a fixed average potential with respect to said electron gun but independently operable by a control deflecting voltage.

7. An electron beam tube comprising an electron gun including an indirectly heated cathode for producing electrons in a fast-moving f0- 76 cused condition, a main anode, a plurality of auxiliary anodes positioned between said main anode and the electron gun, means including electrodes charged to a positive potential of fixed value with respect to said electron gun for defleeting the electron beam across the face of the auxiliary anodes and for maintaining the focus of said electrons, said auxiliary anodes being maintained at a fixed average potential with respect to said electron gun and different from the potential of the main anode.

8. An electron beam tube comprising an electron gun including an indirectly heated cathode for producing electrons in a fast-movingfocused condition, an anode for receiving the electron beam, and intermediately positioned electrodes for deflecting the beam across the anode and for simultaneously maintaining the beam in its focused condition, said anode comprising a plurality of physically separate members which are charged in a variable manner depending upon the extent of the beam portion which strikes the respective members.

9. In combination, an electron beam tube comprising an envelope containing a source of electrons, a main anode, a plurality of auxiliary anodes, and a plurality of electron-deflecting electrodes positioned on opposite sides of the envelope between said source and said main anode, resistances respectively between said auxiliary anodes and between said electron-deflecting electrodes, a source of electromotive force, the negative terminal of which is connected to the source of electrons, and connections between difierent positive terminals on said source of electromotive force and each of said resistances.

10. An electron beam'tube comprising an envelope containing an equipotential source of electrons, an electron-focusing electrode surrounding said source, an electron-receiving member, an electron-accelerating electrode positioned between said source and said member, and a combined electron-deflecting and electron-focusing electrode interposed between said electron-accelerating electrode and said electron-receiving member.

11. An electron beam tube comprising an electron gun including an indirectly heated cathode for producing electrons in a fast-moving focused condition, an anode for receiving the electron beam and intermediately positioned electrodes for deflecting the beam across said anode and for simultaneously maintaining the beam in its focused condition, said anode comprising a plurality of physically separate members which are charged in a variable manner depending upon the extent of the beam portion which strikes the respective members, said intermediate electrodes being maintained at a fixed average potential with re spect to said electron gun.

12. An electron beam tube comprising an envelope containing a source of electrons including an indirectly heated cathode, an electron-focusing electrode surrounding said source, an electronreceiving member, an electron-accelerating electrode positioned between said source and said member, and a common means arranged on the opposite side of said electron-accelerating electrode from said electron-focusing electrode for simultaneously deflecting and maintaining the focus of the electrons, said means including an electrode for providing an electric field having a contour similar to the shape of the electron beam as it leaves said electron-focusing electrode.

13. An electron beam tube comprising an envelope containing a source'of electrons including an indirectly heated cathode, means for con straining said electrons to a beam of rectangular cr0ss-section, an electron-receiving member, an electron-accelerating electrode positioned between said source and said member, and a pair of metal plates arranged on the opposite side of said electron-accelerating electrode from said electron constraining means, said plates being positioned in planes corresponding to opposite sides of said beam, said plates having such positions with respect to the remaining elements of the tube and being adapted to be charged to such a fixed average potential with respect to said electron source that the electric field set up by said plates serves to maintain the initial focus given to the beam by said electron-constraining means and permit a free movement of the focused beam when a deflecting voltage is applied to said plates.

JOHN M. CAGE.

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