US2268195A

US2268195A - Electron discharge device

Info

Publication number: US2268195A
Application number: US307255A
Authority: US
Inventors: Robert C Winans
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1939-12-02
Filing date: 1939-12-02
Publication date: 1941-12-30
Anticipated expiration: 1958-12-30
Also published as: US2268194A; US2268196A; DE862640C; BE444002A; US2245581A; US2268197A; DE883938C; NL72884C; GB545835A; US2268165A; NL68166C; BE444006A; GB545689A; FR52414E; FR881705A

Description

Dec, 30, 1941. R. c. wlNANs I ELECTRON DISCHARGE lDEVICE 2' sheets-sheet 1 Filed Dec. 2, 1959 COL L INA TING E L E C 7' RODE MODULAT/NG EL ECTRODE A T TORNE V Dec. 30, 194,1. R. c. wlNANs 2,268,195

E LECTRON DISCHARGE DEVICE Filed Dec. 2, 1939 2 Sheets-Sheet 2 ATTORNEY Patented Dec. 30, 1941 ELECTRON DISCHARGE DEVICE Robert C. Winans, Chatham, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation o! New York Application December 2, 1939, Serial No. 307,255

(Cl. Z50-27.5) i

l2 Claims.

This invention relates to electron discharge devices and more particularly to electrode systems, such as disclosed in the application Serial No.` 307,232, `filed December 2, 1939, of Myron S. Glass for producing a concentrated stream of electrons in electron beam discharge devices.

Electron beam discharge devices comprise, in general, an electron source, a target, such as a fluorescent screen, spaced from the source, an electrode system for` forming the electrons emanating` from said source into a beam, an electrode system for focussing the electron beam upon the target, and means for controlling the beam whereby the` current to the target may be modulated. The efliciency Yand operating characteristics of such devices are dependent primarily upon the fields extant between the various electrodes. The determination of such fields is a rather involved problem with the result that such devices heretofore have been designed mainly upon a qualitative basis in the lightof experience. Under such circumstances, not only are the operating characteristics unpredictable with accuracy but the efficiency in general is low principally because but a relatively small percentage of the electrons emanating from thesource are concentrated into the electron beam.

One general object of this invention is to improve the efficiency and operating characteristics of electron beam discharge devices. Y

More specifically, objects of this invention are: To produce a concentrated stream of electrons having a sharply dened line focus;

To increase the efciency of the electron beam forming system in discharge devices whereby a large proportion of the electrons emanating from the electron source is concentrated into a thin beam of rectangular cross-section;

To enable the focussing of a thin electron beam ,of rectangular cross-section upon a line focus;

To obtain efficient and uniform modulation of a rectangular electron beam without disturbing the focussing thereof; and

To enable the construction of electron beam discharge devices having accurately predictable operating characteristics.

In one illustrative embodiment of this `invention, an electron beam discharge device comprises an elongated cathode, an electrode system for concentrating the electrons emanating from the cathode into a thin rectangular beam, a fluorescent screen, and an electrode system for focussing the electron beam to produce a sharp, clearly defined line upon the fluorescent screen.

In accordance with one feature of this invention, the beam forming system comprises a pair of electrodes having elongated apertures or slots in alignment with one another and the cathode and having opposed surfaces conforming to equipotential boundaries of elds of predetermined magnitude and configuration whereby a large percentage of the electrons emanating from the cathode is concentrated into a beam passing through the elongated slots.

In accordance with another feature of this invention, the focussing system comprises a plurality of electrodes having surfaces conforming to equipotential boundaries of fields of predetermined magnitude and configuration whereby the electron beam is brought to a clearly defined line focus upon the fluorescent screen.

In accordance with a further feature of this invention, one of the electrodes of the beam forming system is so constructed and arranged that variations in the potential of this electrode, with respect to the cathode, will produce corresponding variations `in the length of the line focus upon the fluorescent screen without substantially altering theintensity of the electron beam.

The invention and the foregoing and other features thereof willbe understood more clearly and fully from the following detailed description with referen-ce to the accompanying drawings in which:

Fig. 1 is a perspective view of an electron beam discharge device illustrative of one embodimentA of this invention, a portion of the enclosing vessel being broken away to show` the electrode assembly more clearly;

Fig. 2 is a View in section along plane 2--2 ofl Fig. 1;

Fig. 3 is a circuit diagram illustrating one manner in which the device shown in Fig. 1 may be operated;

Fig. 4 is a diagram illustrating the configuration of the cathode and the electrodes of the beam forming system in the device shown in Fig. 1;

Figs. 5 and 6 are diagrams showing the configuration of illustrative equipotential lines to which surfaces of electrodes in the device shown in Fig. 1 may conform in accordance with this invention; and

Fig. "I is a diagram illustrating the electron paths and the determination thereof to produce a focus upon the fluorescent screen.

Referring now to the drawings, the electron beam discharge device shown in Fig. 1 lcomprises an evacuated enclosing vessel I0 having at one end thereof an inwardly extending stem II from which a unitary electrode assembly is supported, the stem terminating in a press I2 in which leading-in conductors for the various electrodes are sealed.

The electrode assembly comprises a cathode I3, a modulating electrode I4, a colliinating and focussing electrode I5 and a pair of focussing and accelerating electrodes IBa and IBb, all of these electrodes being maintained in spaced relation by a pair of parallel insulating members I1, such as mica discs, supported from the stem by a pair of rigid uprights I8. Y

The cathode I3, as shown more clearly in Fig. 2, may be of the equipotential indirectly heated type and comprises an elongated metallic sleeve having a dished surface I9 coated with 'thermionic material, and a heater filament 2l] within the sleeve and insulated therefrom. The cathode sleeve may be fitted in aperturesin the insulating discs I1 and secured in position by brace wires 2| affixed to the sleeve and to the discs. Electrical connection to the cathode sleeve andthe heater filament 20 may be established throughl leading-in conductors 22.

V'Ihernodulating .electrode I4 may be formed of sheet metaLfor example nickel, and comprises plane divergent rarms 23 extending at equal angles with respect toa plane :i1-:c passing centrally through the cathode, and U- shaped end portions24, similar, for example triangular, por tions of :themodulating electrode being cut away as shown in Fig. 1 The electrode I4 may be supported by a plurality of rigid wires 25 fittedin apertures in the insulating discs I'I, one of the wires 25 being connected to one of the uprights I8, as by a conductor 26, having a leading-in conductor 21 connected thereto. The colliinating electrode I comprises two identical halves, for example metallic blocks, positioned by rods 28 fitted in apertures in the insulating discs I1, the two halves being equally spaced on opposite sides of the plane .os-x to define an elongated aperture 29 in parallel alignment with the dished surface I9 of the cathode. As shown clearly-in Fig. 2, the two halves of the electrode I5 have identical

curved surfaces

30 and 3|, respectively, the configuration of which will be described fully hereinafter. A suitable potential may be applied to the collimating electrode vI5 through leading-in conductors 32.

The focussing electrodes I6a and IBb, which may be metallic blocks, are alike, `are equally spaced on opposite sides'of the plane :r-x'to define an aperture 33 in alignment with the aperture 29, and have similarly curved surfaces 34. The electrodes I6a and IIb are positioned rby rods v(itl tted in apertures in the insulating discs I1, one ofthe rods having a leading-in conductor 35a connected thereto and another of the rods being connected to one of the uprights I8 by a tie wire 36,'this upright having a leading-in conductor 35D connected thereto, whereby a potential may be impressed between the electrodes Iaand I6b.

A portion of the inner side ,wall of the vessel `II) is coatecLas indicated lat 31,'to form a fluorescent screenin alignment with the

apertures

29 and 33.

During operation of `the device, as illustrated 1in Fig. 3, thepheater filament 20 may be energized, as by a battery 3,8, and the collimating .felectrode I5 may'be maintained at a positive potential withrespect to the cathode I3 through a :potentiometer including a battery 38 and a resistanced. The modulating electrode I4 may be biased negatively with respect tothe cathode by the potentiometer, and the modulating potential may be impressed between the cathode I3 and modulating electrode I4 as shown. The yfocus- 4sing electrodes Ia and IEbV are maintained at a .positivepotential with respect to the cathode I3 .and are electrically balanced with respect to the .cathode by resistances 4I.

A suitable potential .by the fields extant between the modulating and collimating electrodes and are formed into a `beamjpassing through the aperture 29. The electrons in the beam then come under the influence of the fieldsextantibetween the colliinating and focussing electrodes and finally impinge upon the screen 31 whereby a fluorescent image is pro- Vduced upon the screen. The intensity or size of the image ,may be controlled by varying the potential of the modulating electrode.

Stated objectively, operation of the device requires that the electrons emanating from lthe cathode be formed into a beam, that the beam be directed upon the screen and that the beam be capable of modulation. Efficient and satisfactory` operation requires that a large proportion of the electrons emanating from the cathode be concentrated into the beam, that the beam be brought to a sharp focus upon the screen, and that modulation of the beam may be effected by small changes in the potential of the modulating electrode.

The concentration of the electrons into the beam and the focussing of the beam and modulation of the beam are dependent primarily upon the fields extant between the several electrodes. The attainment of fields of the configuration and intensity necessary for efficient and satisfactory operation of the device in accordance with a feature of this invention will be apparent from the following considerations.

The fields to be considered, it will be apparent,

v are two dimensional, that is, fields which satisfy the Laplacian equation for two dimensions, i. e.

2 ZQS- 'antw-0 These fields also are symmetrical with respect to the plane :zz-r, that is, if the direction perpenwhere V y) is the potential at any point, having coordinates a: and y and wir) is the potential along the axis as a function of the axial distance In order that electrons will be subjected to a field directing them toward the .r plane and a beam will be formed, it is necessary that there be moderately small potential gradients inthe vicinity of the cathode and that there be a gradient toward the :c plane, which increases progressivelywith distance from the cathode. From Equation 1 it will be seen that these requisites are satisfied if p has a positive second derivative and if this derivative is a function of the distance Hence, p (0:) should be at least a second degree equation in and preferably Aa higher than second degree equation. A variety of functions, of course, will satisfy these conditions. For simplicity of calculation, however,

`relation `Equation 1 then may be used to determine the the simple functions, i. e. powers of a: are preferable. Two illustrative general functions are p(m)=K and ()==eKa: (2)

where K is an arbitrary constant, e is the Nape- `rian base, and n is` 2 or greater.

The desired potential distribution may be obtained by making two electrodes of such configuration that the opposed surfaces thereof con-` form to equipotential surfaces resulting from the desired distribution. As pointed out heretofore, for two-dimensional fields the equation for the iieldmust satisfy the Laplacian equation az-d yz (3) where `V, is the potential at any point (5c, y) in the field. This equation willbe satised by the expressed by Equation l above.

equation for the requisite equipotential surfaces. As` a specific illustration, the case Where :Kw4 may be used. Then, from Equation 1,

The last term, Ky, is small and may be neglected `the coordinates a, o,

V(a, o)=Ka4=K:L4-6Kx2y2 and solving for y vt y: 6 :c2

This is `the equation for a family of curves B, illustrated in Fig. 5, asymptotic to straight lines A, y=`.407`,`making an angle of 22.2 degrees with the :r axis. Each curve B corresponds to a particular value of a in Equation 6, the values of a being indicated on the a: axis. An accurate solution of Equation 4, that is without neglecting the g4 term, gives the asymptotes A as straight lines making an angle of\22.5 degrees with the m axis. The error involved in neglecting this term, it will be seen, is very small and, therefore, the approximate solution may be utilized for practical purposes.

For the axial potential distribution Mr) :fmt it can be shown, following the method used for giur) =K:1:4, that the equation for the equipotential `lines is This, it will be apparent, is the equation for a family of curves C, illustrated in Fig. 6, asymptotic to two lines A` parallel to the a: axis and spaced Hence, equipotential surfaces will be generated f by moving the equipotential lines of Figs. 5 and 6 parallel to themselves, that is7 the equipotential surfaces will be such that any right section conforms to the same equipotential line.

Thus, inasmuch as. the surfaceof an electrode,

same form irrespective of the` electrode I 5.

such as the collimating or modulating electrode in the device shown in Fig. 1, is an equipotential surface, the desired field between two electrodes may be obtained by making the opposed surfaces of the electrodes of such configuration as to conform to equipotential surfaces of the desired eld, as determined hereinabove, and applying proper potentials to the electrodes. Thus, in a specific case, (:r)=K4, illustrated in Fig. 4, the arms 23 of the modulating electrode I4 conform to the zero potential boundary A and the surfaces` 30 of the collimating electrode I5 conform to the equipotential 12S-volt boundaryB (5 .units from the origin O, a typical unit being TU `and in` operation `of the device the collimating electrode I5 is maintained 125-volts positive with respect to the modulating electrode I4. The various hyperbolae in Fig. 4 represent equipotential lines in the region between the modulating and collimating electrodes, the potential of each line being as indicated inthe gure and corresponding to a value of K=% in Equation The cathode I3 is positioned within the modulating electrode I4 and the emissive surface I9 is shaped to conform to the equipotential boundary corresponding to the cathode position in the field. It` will be apparent that there will be a definite, moderately small, positive gradient at the cathode surface I9 which, as noted` heretofore, is one of the requirements for the efficient production of an electron beam. The electrons emanating from the cathode surface I9 enter a eld which has a gradient toward the plane .frx, are accelerated toward this plane, and are concentrated into a thin beam of rectangular section passing through the aperture 29 in the An approximate electron trajectory is indicated by the dotted line M in Fig. 4.

As indicated hereinabove, the cathode surface I9 conforms to an equipotential boundary in the field between the modulating and collimating electrodes. If a potential corresponding to the position of the cathode in the field is applied to the cathode, there will be a known positive gradientlat, the surface I9 of the cathode. If the cathode potential is increased, the gradient at the cathode surface becomes zero which corresponds to cut-01T of the cathode current. If the cathode potential is decreased, the off-cathode gradient increases.

off-cathode gradient obtains for this potential.

In practice, it is preferable to have the cathode ata xed potential and to vary the potential of another electrode, e. g. of the modulating electrode I4 as illustrated in Fig. 3. For some value of potential, negative with respect to the cathode, of the modulating electrode, the cathode potential will correspond to that of the equipotential p trode do not affect the action of the field with respect to concentrating the electrons` into a beam to an extent appreciable for practical purposes. The exact position of the cathode in any particular case will be determined, of course, by

Hence, if the limit condition occurs when the cathode and modulating electrode are at the same potential, maximum l Reducing this potential to that where the cut-off potential desired. Thus, if the cutoif potential desired is one twentieth of the potential of the electrode I5, the cathode surface line focussing may be obtained in accordance l with this invention by -following a procedure similar to that described hereinabove in connection with the beam forming system.

In general, elds which are suitable for the beamforming system, that is, such fields as correspond to potential distribution of ,1 (JL)=K:r7L and (:r) :eK are suitablefor the focussing system. Thus, in the specific embodiment illustrated in Figs. 1 and 2, the surfaces 3| -and 34 may conform to equipotential boundaries for the relation wat) =Kx. The surface 3| for simplicity, may conform to the same equipotential boundary as the surface 3B.

The parameters of the focussing system may be determined from the following considerations with particular reference to Fig. '7. For practical purposes, space charge is neglected, electrons are assumed to move in straight lines when in field free space and the electrons 'are 'assumed to remain the same average distance from the axis as they pass through the focussing eld. The electron path, then, may be represented by the line abc in Fig. 7, the electrons originating at a point O' at a distance do from rc1, the point 'at which the 'electrons enter the focussing field. The image point I (on the screen 3l) is at a distance d1 from x2, the point at which the electrons leave the focussing field. In the device shown in Figs. l and 2 the points indicated in Fig. '7 would be located as follows: O at the end of the slit 29 toward the screen 3l, :v1 at the point on the axis xwhere the surface 3| would intersect this axis if the contour were complete, x2 'at the corresponding point on lthe axis for the surfaces 33, and I at the screen 31.

For any particular axial potential distribution it can be shown, by equating the change in lateral velocity necessary to bring the 'electrons to a focus at I to the change in lateral Velocity received in the focussing field (between surfaces 3| and 34) that f y1 is the distance the electron remains from the axis in theV focussing field, i. e. between m1 and x2,

e is the electron charge,

m is the electron mass, and

V is the potential at any point yr in the field.

For the particular case of (fr) =Kx4, Equation 8 becomes which, it will be noted includes four variables. :r1 and :r2 may be obtained directlyfrom the equipotential' surfaces used. Thus, if in the specic embodiment shown in Figs. 1 and 2 surface 3| is of the same configuration as surface 3), for which, as noted heretofore a=5, 501:5. If the ratio of the potentials upon the focussing and collimating electrodes is desired to be about 4:1, the surfaces 34| should conform to the curve B in Fig. 5 for which a=7. The actual voltage ratio will then be 3.84. Hence mz?. du may be estimated fairly accurately from experience. For the particular case under consideration, .do=4. Solving Equation 9 for these values of x1, :r2 and do, d1=8.52 units. Hence, for the values of x1, :v2 and do employed, in order that a line focus will be produced upon the screen 3l, the screen must be 8.52 units beyond x2.

It may be noted that the slit 23 in the collimating and focussing electrode I5 preferably is Ymade vof appreciable depth (for example, three units in in the specic embodiment described) to act as a limiting slit. The surface 3| is made such as to be four units from the outlet end of the slit 29. The enlarged slit 5|) in the electrode l5 is made large enough to accommodate a beam of maximum divergence allowable with the depths of the slit 29. The slit 33 in the electrode I6 is made of appreciable depth, e. g. 2 units in the specific embodiment described, to obtain a quick transition to a ii'eld free space, thus reducing distortion of the field.

In devices constructed in accordance with this invention, three forms of control of the electron beam are obtainable. In the specific embodiment illustrated in Figs. 1 and 2, because portions of the arms 23 of the modulating electrode I4 are cut away, the modulating electrode may be operated to suppress electron emission from the end portions of the cathode surface |9 before affecting emission from other parts of this surface. Inasmuch as the modulator electrode vencompasses only the ends of the cathode and is well in front of other portions of vthe cathode, the off-cathode gradient at the ends of the cathode may be made Zero before the gradient at other portions of the cathode is so reduced.

Hence, modulation by Varying the length of the line image on the screen 3l is effected Without substantial variation of its intensity. Thus, devices constructed in accordance with this invention may be used where such form of modulation is required, for example, in variable area recording systems.

If the modulating electrode is not cut away as shown, the off-cathode gradient will be uniform over the entire length of the surface I9 and the intensity of the image may be modulated without varying the length thereof. Devices of such construction may be used, for example, in variable density recording systems.

Finally, the electrodes |641 and |613 may be utilized as deflector plates and a sweep input potential impressed between them. For example, if a saw-tooth sweep potential is applied, the beam traces a rectangle upon the screen. If a modulating potential is applied to the modulating electrode of the cut-away construction shown in Fig. 1, the length of thisrectangle will be varied and a modulated ligure is produced upon the screen.

Although a speciiicembodiment'of the invention has been shown and described, it vwill be understood that it is but illustrative. For example, although the

surfaces

33, 3| and 34 have been shown as conforming to equipotential Vboundaries for an axial distribution of Max) :K

they` may conform to equipotential boundaries corresponding to q ()=eK.or to other powers of :c such as (),=K2, ()=K:c3, ()=Kx5,

etc. ,f l I `Other modifications in the specific embodiment shown and described may be `made without departing from the scope and spirit of this invention fas defined in the appended claims.

What is claimed is:

1. An electron gun. for electron discharge devices comprising a cathode having an elongated electron emissive surface, and means for concentrating the electrons emanating'from said cathode into a thin beam of rectangular cross-section, saidmeans including an electrode having an elongated slit in alignment with said emissive surface and a second electrode having divergng arms extending from adjacent the sides of said surface and toward said rst electrode, opposed surfaces of said electrodes being dished in the same direction and conforming to equipotential boundaries of a field symmetrical with respect to a plane passing through said slit and said surface, increasing progressively away from said surface and decreasing away from said plane.

2. An electron gun in accordance with claim 1 wherein said opposed surfaces conform to equipotential boundaries of a field corresponding to a potential which increases away from said surface substantially according to the relation where El is the potential, K is a constant, is the distance from said surface and nis 2 or greater.

3. An electron gun in accordance with claim 1 wherein said opposed surfaces conform to equipotential boundaries of .a eld corresponding to a potential which increases away from said surface substantially according to the relation ran where E is the potential, e is the Naperian base, K is a constant, and is the distance from said surface.

4. An electron gun for electron discharge devices comprising a cathode having an elongated electron emissive surface, and means for concenf trating the electrons emanating from said surface into a thin beam of rectangular cross-section, said means including an electrode spaced from said cathode, having an elongated aperture extending parallel to and in alignment with said surface and having the surface thereof toward said cathode corresponding to a surface developed by a generatrix moving parallel to itself and conforming to a curve of at least a second degree function of the distance from said surface, and a second electrode having diverging arms extending from adjacent said emissive surface and asymptotic to said surface of said first electrode.

5, An electron discharge device comprising a ondderivative of which potential as a function of distance from said surface is positive and a function of said distance.

6 An electron discharge device in accordance with claim 5 wherein said opposed surfaces conform to equipotential boundaries of a field corresponding to a potential which varies substantially according to the relation where E is the potential, K is a constant, :c is distance from the cathode and n is 2 or greater.

'7. An electronrdischarge device in accordance with claim 5 wherein said opposed surfaces conform to equipotential boundaries of a field corresponding to a potential which varies substantially according to the relation where E is the potential, e is the Naperian base, K is a constant and :i: is distance from the cathode.

8. An electron discharge device comprising a cathode, an electron receiving element spaced from said cathode, means for concentrating electrons emanating from said cathode into a thin beam of rectangular cross-section, and means for bringing said beam to a line focus upon said electron receiving element including a pair of spaced electrodes having elongated apertures in alignment and having opposed surfaces symmetrical with respect to a plane passing through said apertures and conforming to equipotential boundaries of a field corresponding to a potential which increases away from said cathodeV and the second derivative of which as a function of distance from said cathode is positive and a function of said distance.

9. An electron discharge device comprising a cathode having an elongated electron emissive surface, an electron receiving element spaced from said cathode, and means for concentrating electrons emanating from said cathode into a thin beam of rectangular section having a line focus on said electron receiving element, said means including a pair of opposed electrode surfaces between said emissive surface and said element and flaring away from said emissive surface and including alsoa second pair of opposed electrode surfaces between said first surfaces and said element, said opposed surfaces being symmetrical with respect to a common plane, and each pair of opposed surfaces conforming to equipotential boundaries of a field corresponding to a potential which increases away from said emissive surface and the second derivative of which as a function of distance from said cathode is positive.

10. An electron discharge device comprising an elongated cathode, an electron receiving member spaced from said cathode, means for concentrating electrons emanating from said cathode into a beam having a line focus on said electron receiving member, and means for modulating the length` of the line focus without altering the intensity of the electron beam.

11. An electron discharge device comprising a cathode having an elongated emissive surface, an

electron receiving element spaced from said cathode, means for concentrating electrons emanating` from said surface into a beam having a line focus on said element, and means for varying the off-cathode gradient along said surface nonuniformly along the length of said surface, the variation being greatest adjacent the ends of said surface and decreasing progressively toward 'the center thereof. V

12. An electrondischarge devicecomprising a cathode having an elongatedl electron#y emissive surface, an electron receiving element spaced 5 from said surface,v and means for concentrating electrons emanating from said surface into a rthin beam having aline focusron said element,

said means including aY modulating electrode of substantially V shape; the apex of which is on the side of said cathode remote from said emissive surface and the arms of which diverge toward said element, intermediate portions of said armsV adjacent said apex being cut away.

RQBERT c. WINANs.`