US2644906A

US2644906A - Electron beam discharge device

Info

Publication number: US2644906A
Application number: US241433A
Authority: US
Inventors: Ralph J Bondley
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1951-08-11
Filing date: 1951-08-11
Publication date: 1953-07-07
Anticipated expiration: 1970-07-07
Also published as: BE513449A; FR1061233A

Description

July 7, 1953 RfJ. BoNDLEY 2,644,906

ELECTRON -BEAM DISCHARGE DEVIE FiledAug. 11, 1951A 2 sheets-Sheet 1 Inventor: Ralph J.v Bondleg,

bym

lHis Attorneg.

July 7, 1953- R. J. BoNnLEY 2,344,906

LEc'rRoN BEAM DISCHARGE DEVICE Filed Aug. 11, 1951 2 sheetsshu't `2 ANODE JRFACF Figa Ralph J. Bcvndleg,

His Attorn eg.

Patented July 7, 1953 f E 2.644.906 ELEcrRoN BEAM DISCHARGE DEvIoE Ralph J. Bondley,-Scotia, N. Y., .assignor to General Electric Company, a corporation. of New York ' Application August y11, 1951, Serial No. 241,433

My invention relates in general to electron beam discharge devices such as, for zexample, cathode ray television picture reproducing tubes, and in particular to electron guns for such devices. .y Electron beam discharge devices'of the cathode ray tube type, as heretofore constructed, have not been capable of the most desirable performance, particularly with regard to lthe control characteristics of the beam Aintensity modulating control electrode therein. The control electrode, in general, has-in the past consisted of a flat disk defining a small aperture closely7 spaced to the electron emissive cathode. When the aperture in such a control electrodedisk is made very small, the beam control by cont-rol electrode potentials is good, i. e., beam current is easily cut off and is linearly. modulated by, changes in control electrode potential, but .the maximum beam current passed by the control electrode is insufficient, for example, 4to vprodueea desired degree of brightness on.v a fluorescentpicture screen. When the aperture is made. larger, the maximum beam current is sufficient but the cutoff point is remote and linearity of control is poor. That such large aperture control electrodes are only fairly effective is evident from the fact that most cathodey raytubes exhibit beam intensity control characteristics similar to those of variable mu amplifying devices, having remote beam cutoff points requiring a negative potential as great as f4() to -80vvolts to stop beam current flow. The mutual conductance is valso low and variable; this requires, then, a high electron beam intensityv from cutoffl to full intensity, and a linear relationship betweenithe control electrode potential and beam intensity is not obtained over the full range of variation.

The elimination of the foregoing shortcomings of conventional control electrodes and electron gun arrangements in electron beam discharge devices hasbeen the object of attempted im'- provement in the beam intensitycontrolcharacteristics of electron guns, such attempted improvements in a few instances being directed to the employment of a Wire lmesh or grid over an apertured element as: a control electrode through which the electrons pass-and byxthe potential of which the inten'sityofv the electron beam is controlled. Such a grid affords a much lower negative cut-off potential considerably increases the mutualconductanceof the device. However, if the grid-covered aperture vis made small, a satisfactorily large. area of Acathode vemissive sur.-

. gaclaims (c1. 31a- 82) face is not utilized and a full intensity `of the transmitted electron beamis precluded at negative potentials. At positive potentials, the grid current formed by electrons collected by the control f electrode becomes so great that the transmitted beam consists ofk only a small portion of'the electrons emitted by the cathode, so that the emissive capabilities ofthe cathode are severely taxed and its useful life shortened. Enlargingv the grid covered kaperture kgives .increased beam current at negative potentials Vbut spoils the beam focus, enlarging the beam area or spot size and, further, results in a grid pattern'or image which appears on the screen of a television picture tube, for example. vFor these reasons, the few attempts at the use of a grid type control electrode have not provided a satisfactory solution to the problems outlined above, are not commercially Aacceptable and have not found use in the very important eld of home television receivers. A f

It is therefore, ran objecty of lmy invention to provide a new and improved electron beam discharge device. f

rIt. is another object of kmy invention to provide an electron beam discharge device having a control grid operable in the negative bias region with substantially'full utilization of electrons emitted from the cathode and affording adequate g maximum beam currentv for television picture purposes. y f

It is another object of my invention to provide an electron beam discharge device having high y mutualr conductance and a low negative potential cutoff point whereby relatively low amplitude signals may cause electron beam modulation over the range from beam cutoff to yfull intensity.

It is a further object of invention Ato proy vide an electron beam discharge device wherein electron beam intensity is linearly related to control electrode potential.

And it is a still further object of my invention to provide an'electron beam discharge de- Y vice wherein the electron beam area or'spot size is substantially'independent of beam intensity.

In fulfilling the aforesaid objectives, my invention in a preferredform thereofv is embodied in Y anode in turn being spaced from an electron receiving element, such as a fluorescent screen, within the envelope. The cathode includes a circular disk portion carrying a thermionically emissive surface and means, such as an electrical heater, for heating the emissive surface. The control electrode includes a body member having a flaring passageway therethrough, the y smaller entrance to the passageway being substantially the same in size as the cathode emissive surface and located in registry with and adjacent to the surface.

effect present in previous grid controlled electron beam discharge devices was caused mainly by a flattening out of the electric field4 adjacent the grid which precluded good focusing of the beam.

I provide a sharply convergent electron lens orv electric field configuration adjacent the grid in order to cause convergence of electrons pervading the relatively large area grid into a small area electron beam. By the s-pecial flaring geometry given the control electrode body portion and a special configuration given the anode member, the operating potential supplied to the control electrode and the anode form the required electron-converging electric field near the grid which not only causes convergence of electrons after passing through the grid but provides a high mutual conductance control characteristic. The anode, then, in one form includes'a hollow cylindrical member closed at the end thereof adjacent the control electrode by a generally semispherical cup-like member extending toward the control electrode and having a limiting aperture on the common axis. `The electric field present between the control electrode and the anode during operation causes electrons passing through the grid to be focused on the axis in the limiting aperture so that a sharp electron beam, of sufficient beam current for television .purposes and fully modulatable by small signals applied to the control electrode in the negative bias region, passes through the anode to strike the electron receiving element or screen.

The features of my invention believed to be novel are pointed out with particularity in the appended claims. However, for a-better understanding of the invention, together with further objects and advantages thereof, reference should be had to the following description takenin conjunction with the accompanying drawings, where- Fig. l is an elevational view, partially in section, of an electron beam discharge device of the television picture tube type embodying my invention; Fig. 2 is an enlarged elevational view in section of the electron gun portion of the electron beam discharge device of Fig. 1; Fig. 3 is a perspective view of a portion of theelectron gun of Fig. 2 showing the construction of the control electrode grid; Fig. 4 is a graph illustrating the equi-potential contours of the electric field between the control electrode and the anode of the electron gun of Fig. 2 during operation; and Fig. 5 includes two curves illustrating the relationship between control electrode potential and electron beam current for a prior art type beam discharge device and for a beam discharge device embodying the present invention.

Referring now to the drawings,-I have shown in Fig. 1 an electron beam `discharge device of the television picture tube type comprising an evacuated. envelope I having-.a neck'portion 2 madel of The smaller entrancev to the flared. passageway is also covered with a grid of fine uni` formly spaced parallel wires. The detrimental.

insulating material, such as glass, and a flared portion 3 which in the arrangement shown is made of metal. The right end of envelope I is closed by a transparent wall 4 which carries or is coated on the inside with an electron receiving surface or fluorescent screen 5, capable of becoming luminescent when electrons impinge thereon. In order'thatit may be placed at a positive operating potential, screen 5 is connected through metal portion 3 and a conductive coating 6 on the inner surface of the right end of neck portion 2 to an. external connecting terminal 1. Within the neck portion 2 of envelope I there is provided anelectron gun which includes three electrodesa cathode 8 (visible in Fig. 2), a control electrode 9, and an anode I0. In general and as will be more fully explained, electrons constituting electric current are emitted by the cathode and are accelerated by the anode, passing therethrough to strike screen 5, being modulated in intensity by the control electrode 9 and focused into a narrow beam by the control electrode 9 and the anode i9. To allow for electrical circuit connections, the sealed left end of neck portion 2 is capped by an insulating and strengthening base I I through which a plurality of lead-in connectors I2 extend. The base II includes a centering projection I3 and a positioning lug I4 thereon in order that connectors I2 may engage a conventional connecting socket. An axial magnetic field may be provided in thel region to right of anode I0, as by a current-carrying coil or a permanent magnet, not shown, concentric with envelope I, to compensate for the effect of space charge repulsion and maintain the beam form of electrons passing through anode I0'. Conventional electromagnetic means, not shown, to deflect the electron beam after passage through anode I0 so as to trace a pattern or raster on screen 5 may be provided external to envelope I although it will be understood by those skilled in the art that electrostatic means may also be employed within the envelope in this respect.

Turning. nowv to Fig. 2 for a more complete descriptionof the electron gun structure, it is seen that thethree electrodes, 6, 9, and IU are rotationally symmetrical about and spaced in alignmentV along a common axis I5. Control electrode 9' includes a cylindrical sleeve I6 provided to facilitate the mounting and support of both the cathode 8 and control electrode 9. Sleeve I6 is mounted by means of a plurality of legs I1 extrorse from its surface which are embedded in glass bars I8 bonded to the inner surface of neck portion 2. Fitting within the right end of sleeve I6 and secured by bonding or the like, there is a body member I9 which contains a passageway 20 flaring or increasing in diameter and cross-sectional area toward anode Il! or in the direction of electron travel, the illustrated passageway 20 being of conical shape. The exact geometry of the flaring surface of passageway 20 need not be conical, however, and is correlated to the shape an-d spacing of anode I0 in a manner to be explained hereinafter so as to permit establishment of a focusing electric field. Received within sleeve I6 is an annular disk 2I abutting body member 20 and having an aperture 22 providing the smaller entrance to passageway 20. As shown more clearly by Fig. 3, aperture 22 is covered by a plurality of fine, uniformly spaced wires 23 forming a grid thereacross. In one constructed embodiment of the invention found to operate with the advantages described, wires 23 are made of tungsten .0006". in. diameter and spaced in parallel relation .006" from centerfto center.V kWhile' other size wires and spacings maybe successfullyem-;

ployed, it is preferable'that the-grid wires 23 kbe very small in diameter so that a'relatively great number Vof them'` may bej'employed 'across thev aperture 221without blocking an undue proportion ofthe aperture area. Control electrode 9 is electrically insulated by being supported from the glass wall 'of neck portion 2 and may be placed atv operating potential by a lead-in connector I2 connected by a conductor 24 to sleeveI 6. .i

Cathode 8 includes a diskl 25fforming the end of a heat conserving tube 26, the disk 25 being made substantially the same sizeasaperture 22 and positioned in axial alignment therewith'. `4vThe disk 25 carries'a thermionically electron emissive surface 21, coated with a material such as barium oxide or others known to the art, the area of the surface 21, disk 26, and aperture 2.2 being made great enough so that the emissi-veproperties of the surface 21 are not overtaxed lwhen the required maximum electron current flows therefrom. It is possible to provide a useful cathode area of such large Asize and still obtain aI small area, sharp electron beam containing substantially all emitted electronsbecause of the special focusing action, to be explained, ofthe electrode structure. In some cases it may be desirable to augment this focusing action by making surface 21 concave so that emitted electrons tend to have a component of Velocity toward axis I5. Means to heat substance 21 may beprovided in the form of a coiled velectric heater 28 positioned within tube 26. The operating potential, usuallyconsidered a ground or reference potential, of cathode 8 may be applied through another lead-in connector I2 which is connectedl to tube` 26 through a conductor 29, whilefheating current may be supplied to heater 28 through the series circuit of a lead-in connector I2 connected `directly thereto, tube 26, conductor 29, and the leadin connector I2 connected thereto.

The cathode 8 is mounted in insulated spaced relation from control electrode 9 by means of a flanged annular ring member `3l] bearing at the right on disk 2| and at theleft on an annular insulator 3l', preferably a heat resistant ceramic insulator, which surrounds Ytube 26 and'is axially xedv thereon by annular brackets'26a.` Anannular retaining ring 32 bears against theleft face of insulator 3l, being free of contact with tube 26, and is held in place by a flanged member 33 bonded to the inner surface of sleeve I6. The

cathode 8 is thus mounted in insulated andy and a limiting entrance aperture therein. In oneV form anode I0 includes a cylindrical'y member 34 mounted in insulated and axially spaced relation' from electrode S! by means ofradiallyl extending legs 35 embedded at their extremities in glassv bars 36, which in turn are bonded to the inner wall of glass neck portion 2. 'Cylindrical 'mem-y ber 33 is substantially closed at the end thereof .c

facing control electrode 9 by a cup-like member 31 decreasing or tapered in cross-sectionalarea toward controlelectrode Sand having a limiting; aperture 38 therein. on axis I4. 4The surfacegeom etry of the left end ofthe anodeg-shelLi. e;,of,. member 31, is chosen to produce adesired'fo`cusf1v ing field, as willbe' explained present1yonesatisfactory vsurfaceconfiguration .beingxwg'eneral-ly. semi-spherical as shown in the drawingsemlhe end of cylindrical member 34 remote fromcontrolelectrode 9 is partially closed bya-n anode disk 39' having a central exit'aperture 40 therein. `In

order to electrically connect anode I0, spring slips f 4I; are threaded through a flange42 von disk y39 so thaty they beary against coating 6, and-*thusv v connect anode I0 with' terminal 1', at the 'same time giving added supportfto the anode assembly.`

In the operationA of the electron beamv discharge device, heater 28 is energizedA by current supplied thereto' while control electrodeB-is biased to a negative potential rwithrespectito cathode 8.A Anode-|0 and screen 5L are supplied` with the full operating potential which'is highly" positive with respect vto the potential of'cathode 8, and which may be, for example, in the order of ten kilovolts. Electrons emitted! by surfacev 21 and pervading the gridlformed byfwires 23 are accelerated-by the positive anode potential and condensed or focused from :the relatively broady area of the aperture 22 into a narrow beam'passing through limiting aperture 38 by theelectric field created in the control electrode-anode space asa result of the geometric configurations, spac- Thosek ing, and potentials of these electrodes. few electrons not focused into the narrow beam are collected: at the outersurfaceof cup-like member 31 and those few -electrons passing through aperture 38A with `an appreciablelradial` component of velocity are collected at :the-inner surface of the anode shell, whilethe majority of the electrons pass axially in beam lforr'n through cylindrical member 34fandexitfaperture.

40'to strike, in most instances-*after defle'ction by well known means not shown, screen :.5 andcreatea luminescent spot or trace thereon. The

intensity of the electronbeam,` current," andthus the brilliancy of the luminescent spot orftrace ony screen 5, may be varied andl controlled by varying the-potential of the controlelectrode', such control being characterized by the factsr that, in spitev n of operation of the control grid in thefnegative biasgre'gionto substantially eliminate grid cur-fi rent, the maximum electron beam current'is suiiicientfor the maximum brilliancy requirementsy of television picture reproduction,y the mutual conductance is greatly increased to a high lvalue? and is relatively constant, and the beam intensity may be -varied over the full range from cutoff to maximumy by low amplitude control electrode potentialchanges;

As previously mentioned, priorart beam dis-f charge devices employing control gridsvhave'no't metwith success because current suiiicient for television purposes could not be obtained-in sharp Vbeam form unless the emissive areav was made of `a small elemental size and the' control grid operated in the positive bias region, so that the con trol grid collected a considerable proportion of emitted electrons and the emissive capabilities of' In the'cathode surface were thus severely taxed.

order to better explain the reasons lfor'the improved beam' intensity controlr characteristics of the present invention,r which providesrvr not yonly the -required current in beam-'form with the control electrode operated.- in the negative 'bias region, but Valso a high, constant mutual conduct-- ance and a low negativebeamcutoff potential,it..

7" is-t'obefpointedlout that I have provided av relativelylargev area for the emissive surface 21 and the grid-covered aperture 22 which serves as the smaller entrance to passageway 2li-an area con-V Secondly, I have provided a control grid to increaseY thev mutual .conductance of the discharge device, at. the same time realizing that thecontroll. grid4 shouldbe operated in the negative bias region inr order that it not draw appreciable grid current and unduly. limit the proportion of emittedv electronsr transmitted therethrough. Thirdly, I- have provided anv electron gunstructure which produces a relatively great positive potential gradient at. the emissive. surface with a negative potential on the control grid so that. a large numberY of electrons may be emitted. And, finally,v I have provided means by which electrons pervading the control grid over the wide area thereof are-focused into narrow beam form. In accomplishlng these results the full amount of the positlveoperating. potential is applied tothe anode i9, no other electrodes or extra focusing means intervening between the anode l and the controlelectrodeI 9'beingrequired. As a result, a strong positiveV potential gradient is established at the emssive surface 2'1 when the control electrode 9 is still slightly negative in potential so that adequate maximum current may be drawn from the cathode 8. A sharply convergent electric field near'the-control grid, producing the desired convergence and focusing; of the grid-pervading electrons'from' the broad grid area to the narrow area ofy limiting aperture 38,- is established by the operatingY potentialsv of theanode l0 andthe control electrode 9 due tothe special configuration of the control electrode and theanode, i. e., the passageway 20. in body member I9 flaring in the direction of electron travel and the external surface ofv theend of anode I0 which faces control electrode 9 alsoffiaring in the direction of electron travel.

The reasons for this focusing-of electrons. into a beampassing; through limitingv aperture 38 may' be explained' withreference to Fig. 4; The specialv configuration and spacing required for the body member I9 and the left end of anode III-in accordanceV with my invention may be conveniently determined by a known method calledelectrolytlcfl'eld plotting. By this method the electric fieldbetween two or more-conductive surfaces at different potentials may be plotted -by immersingfscale-models of the-surfaces with scaled spacings in an electrolytic bath and applying `an, appropriate potential to each. The bath is explored with a probe, placed at various intermediate potentials, and a null detector, the various equipotential lines being thus located and plotted between the surfaces. equipotential` contours existing between the surfaces of control electrode 9 and anode l0 which are shapedl and` spacedin accordance with thel illustrated embodiment of my invention, the surfaces of these being represented as labeled and only half shown for convenience since they are rotationally symmetrical. Lines 43 illustrate the equipotential contours; being labeled in fractions of cathode-to-anode potential, as plotted in. an electrohrtic bath. Dashed lines 44 represent paths of. electrons pervading the control electrode grid whichare computedandplotted bythe prin- Fig. 4 is such a plot of4 ciples of electronkinetics, and which, as an approximatiomcross equipotential lines 43 at right angles. VIt is seen from Fig. 4 that the surface configuration of `the electrodes produces a sharply convergent electric field in the region near grid Wires 23 and that electrons pervading any4 part of the grid are, for the most part, focused-into aperture 38'. The closer the electrons get to anode I0, the-greater is their velocity and kinetic energy so that because of their inertia, they are not as readily deflected from their paths of, travel by the'electric field near anode I0. Eorzthis'reason, the rather sharply curved field in the'region of the .9 cathode-to-anode equipotentialline 43 has little effect on the electron paths asV shown in the drawing. While I have shown in theA drawing a control electrode passageway 20 which is conical' in shape and a left end of anode l0 which is, semi-spherical in shape, it will be understood by those skilled in the art that other configurations may be employed with success by a predeterminedcorrelation of the configurations and electrode spacing in themanner above described as long as the passageway 20 is made flaring toward anode l0 and the end of anode I ll-facing` control electrode 9 is madetapering toward' control electrode 9.

.It will' now become apparent that since the grid-pervading electrons are condensed or focused .from thev broad area of the grid into the limited area of aperture 38, the electron beam after passing aperture 3B is solid in cross section, i, e., the pattern of the grid wires which exists in the beam as the electrons pass through the grid is eliminated. Thus, the beam which strikes the screen 5 is a concentrated solid beam, not weakened by a pattern ofthe grid wires in its cross section; this'is an. important feature in the successful .reproduction of television pictures.

Turning next to Fig. 5, I have shown the relationship between cathode-to-control electrode voltage, EG, in Volts to electron beam current in microamperes-for typicalprior art electron beam discharge devices by a curve45 and for an electron beam discharge device embodying the present invention by a. curve 46. These curves are plottedfor a typical cathode-to-anode voltage of tenkilovolts. As clearly. illustrated by curve 45, prior .art discharge devices require an extremely great negative` control electrode potential (greater than. 50 volts) tov cause beam current cutoff, and at the same time have a relatively low and non-uniform slope for the beam intensity control characteristic, i. e., a low, variable mutual conductance. By contrast, the foregoing features of the present invention provide a control characteristic allowing beam current cutoff at -10 volts controlv electrodepotential, at the same timeA possessing a steep, uniform slope for the beam current-control: potential curve 46, i. e., a high, constant mutual conductanceover practically the fullrangefrombeam cutoff to maximumcurrent. For example, an electron beam discharge device embodyingmy invention and having a beam current-control potential relationship such. as illustratedby curve 48-may be operated in a television receivingsystemwith all or several of the here-l toforey indispensible4 video amplification stages eliminated sinceamodulatingsignal l0 volts in amplitude. is; suflicient to vary beam intensity from cutoff. to maximum, i. e., screen brilliancy fromblack to maximumwhite.

In .the foregoing. disclosure, it has been shown that myA invention provides an improved electron beam discharge .device having a control electrode operable in the negative bias region,jat the same time possessing a high, constant mutual conductance, i, e., a sensitive, .linear-` control ofbeam current by control electrode potential. Further, theVv device may produce. electronrbeam-fcurrerit more than sufliciently great and focused "into a sharp,` concentrated beam for television purposes. It shouldbe understoodthatfthe inVentionis-not limited to the precise construction herein described in connection with the illustrative drawings but that other arrangements within the scope of the appended claims are to be considered within the preview of the invention.

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

l. An electron beam discharge device comprising ya cathode, a control electrode, and an anode mutually insulated and spaced in the order named along a common axis; said cathode including an electron emissive surface; said control electrode including a body portion having a flaring passageway therethrough increasing in cross-sectional area toward said anode, the smaller entrance to said passageway being adjacent and in registry with said surface, and a plurality of wires forming a gridacross said entrance; said anode including a substantiallyl closed shell having a limitingentrance aperture in the end thereof opposite said control electrodel and the cross-sectional area of said end gradually decreasing toward said control electrode.

2. An electron beam discharge device comprising a cathode, a control electrode, and an anode all rotationally symmetrical about a common axis and mutually insulated and spaced in alignment on said axis in the order named; said cathode including a circular electron emissive surface; said control electrode including a body member, a flaring passageway through said body member increasing in diameter toward said anode, and a plurality of fine wires positioned with uniform spacing forming a grid across the smaller entrance to said passageway, said smaller entrance being of substantially the same diameter as said emissive surface and spaced in adjacent registry therewith; said anode including a generally closed cylindrical shell having the end thereof facing said control electrode gradually decreasing in diameter toward said control electrode, and having a limiting entrance aperture on said axis in said end facing said control electrode and an exit aperture on said axis in the end remote from said control electrode; said control electrode passageway and `said anode decreasing-diameter end being correlated in shape and spacing'toprovide focusing of electrons pervading said grid through said limiting aperture when said control electrode and said anode are operated at potentialsrnegative and positive respectively with respect to said cathode.

3. An electron beam discharge device comprising a cathode, a control electrode, an anode, and an electron receiving surface all aligned and spaced in the order named along a common axis; said cathode, said control electrode, and said anode being mutually insulated from one another; said cathode including a thermionically emissive surface; said control electrode including a conductive surface flaring away from said axis in the direction toward said anode and defining a passageway therethrough in axial alignment with said emissive surface, and a plurality of fine equally spaced wires extending across the smaller entrance to said passageway; said anode including a tubular member, a rounded Cup-like electrode surface and said cup-like memberbeing correlated-.tofocus'electrons emitted by said surface and pervading said spaced wires on said axis at said limiting aperture with potentials vnegative and positive with respectto said cathodeon said control electrode and said anode'respectively, the

. majorityof said electrons passing throughy said limiting aperture substantially in rbeaml form 'to f said receiving surface.

4. An electron beam discharge device of the television picture type comprising 'an evacuated envelope having a neck portion, a flared portion,

a transparent wall closing said flared portion, and

a fluorescent screen on the inner surface of said Wall; an electron gun structure comprising an anode, a control electrode, and a cathode all rotationally symmetrical about and aligned in inn sulated and spaced relation in the order named v. from said screen on a common axis; saidcontrolv electrode including a sleeve mounted within said neck portion, a body member positioned in the end of said sleeve facingsaid anode, a conical passageway through said bodymember flaring toward said anode, and a plurality of fine uniformly spaced parallel wires rforming a grid across the smaller entrance to said passageway remote from said anode; said cathode including a heat conserving tube insulatably and concentrically mounted within said sleeve, an electrical heater element Within said tube, a disk closing the end of said tube adjacent said grid in spaced.

the outer face of said disk; said anode including a hollow cylindrical member,v a semi-spherical cup-like member closing the end'of said ycylindrical member facing said control electrode and extending toward said control electrode, a limiting entrance aperture being defined in said cuplike member on said axis, an anode disk. closing the other end of said cylindrical member, and an exit aperture being defined in said anode disk; said device being operable When said anode is at a high positive potential relative to said cathode, and said control electrode is at a negative potential with' respect to said cathode to focus electrons emitted by said emissive substance and pervading said grid on said axis at the limiting entrance aperture of said anode.

5. An electron gun for use in an electron beam discharge device having a screen for receiving an electron beam; said electron gun comprising a cathode providing a source of electrons, a control electrode through which said electrons first pass and by the` potential of which the intensity of said electrons is modulated, and an anode by the potential of which said electrons are accelerated and through which said electrons next pass to strike said screen; said control electrode comprising a body member, a passageway-through said body member flaring in the direction of electron travel, and a plurality of gri'dwires extending across the smaller entrance to said passageway; said anode comprising a substantially closed hollow shell member having the external surface of the end thereof facing said control ,c

zemgso majority ofsad electronszin: beamt form to` pausas through said anode'v apertures to. strike said.

Screen.

Ji. BONDLEY;

References-Cited inthe le of thisputent UNI'IEDSTATESPA'I'ENTS Number Name Date Gardneretal; May 4, 1937 Number 12 Name Date Mahl Nov. 26, 1940. Glass Dec. 30, 1941 Pierce Dec. 30, 1941 Schlesinger Dec. 29, 1942 Schantl et al Feb. 9, 1943 Moss July 12, 1949 Silverman Apr. 3, 1951