US2498082A

US2498082A - Gun structure for cathode-ray tubes

Info

Publication number: US2498082A
Application number: US792797A
Authority: US
Inventors: Ralph E Johnson
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1947-12-19
Filing date: 1947-12-19
Publication date: 1950-02-21
Anticipated expiration: 1967-02-21
Also published as: DE838472C; FR975875A; NL83421C; GB662757A; BE486327A; NL143880B

Description

Fe 2L 395@ l R. E. JOHNSON 2,498,082

GUN STRUCTUE FOR CATHODE-RAY TUBES Filed Dec. 19, 1947 INVENTOR RALPH E I :nz-11455151 --multiplier stage over an appreciable area. y y prevent the production by the multiplierA section lPatented Feb. 21, 95@ '1n-1:35; -L-L GUN STRUCTURE FOR CATHODE-RAY l TUB I Ralph E. `lohnson, Lancaster, Pa., assigner to Radio Corporation of America,V a corporation of Delaware v Application December 19, '1947, Serial No. 792,797 11 claims. (c1. 25o-.175)

This invention relates to television pick-up tubes and in particular to a novel gun structure for use in such tubes. y

One type of television pick-up tube has an electron gun for forming an electron beam which can be scanned across a target transverse to the axis of the electron gun. The beam is oneof relatively low velocity in which the electrons are speeded up to 200-300 volt velocity and then deaccelerated to practically zero velocity. The target has a glass surface across which the beam is scanned magnetically. The beam striking the target surface reduces its potential to anequilibrium value determined by the p-otential of the cathode of the electron gun. At this equilibrium potential, the electron beam is repelled by the target surface to form a return beam which is accelerated back to'- Ward the gun cathode. A charge pattern of positive areas is established upon the target surface corresponding to a light image focused: on the tube. When the scanning electron beam reaches an elemental positive area of the target, it will lose electrons to discharge the positive area .to equilibrium potential at which point the remainder of the beam is repelled. In this manner, an electron beam of uniform density scans the target while a non-uniform or modulated beam is returned to the cathode end of the tube Where it is collected and amplified as the output signal.

The modulated return beam is picked up and amplified Aby a plural stage multiplier in which the end of an accelerating anode cylinder serves as the first multiplier stage. Secondary electrons emitted by the action of the return beam upon the surface of the rst multiplier stage pass into van,

electrostatic eld which pulls them over tothe second multiplier stage where they strike to produce a greater number of new secondary elecf trons. These additional electrons in turn pass on to succeeding stages giving rise at each stage to a greaternumber of secondaries which are finally collected as the video signal of the tube.

Due to the introduction of several factors, the return beam will scan the surfaceof the first of a non-uniform brightness of the -Vide'o signal of the tube, the second stage must collect secondyaries uniformly from all parts of the scanned area of the first stage as well as provide the same mul-l tiplication for electrons coming from the scanned area of the rst stage. In orthicon pick-up tubes of the type described above it has been difficult to consistently produce uniform vcollection by the second multiplier stage of secondary `electrons,

2 emitted from the first stage, which would resul in a video signal giving a uniformly bright picture.

It is, therefore, an object of my invention to provide -an improved television pick-up tube.

It is also an object of my invention to provide an improved structure for a multiplier section of a television pick-up tube providing an improved video signal.

It is a further object of my invention toprovide an improved structure for the multiplier section of a television pick-up tube which will result in uniform collection of the signal between the rst and second multiplier stages. v

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing, in which:

Figure l is a longitudinal.cross-sectional view of a television pickup tube incorporating the details of my invention; and

Figure 2 is an enlarged longitudinal view with parts broken away to show details of the multiplier section of the television pickup tube of Figure l.

Figures 1 and 2 disclose a television pickup tube for transmitting high frequency signals'corresponding to an external scene optically viewed by the tube. The tube shown in Figures 1 and 2 is one form of a type of television pickup tube which utilizes an electron image of the scene Viewed by the tube and a low velocity scanning beam of electrons for providing the video signal. In greater detail, the tube consists of a glass envelope I0 of tubular shape having an enlarged section II at one end. The enlarged section II is closed by a flat glass face plate I2 having deposited on the inner surface thereof a transparent photoelectric film I4 which is sensitized in a Well known manner to provide an emission of photoele-ctronswhen an exterior scene is focused optically upon the surface of the face plate I2. Mounted within the enlarged envelope portion I I is a glass target 22. In Aone embodiment of my force substantially parallel to the axis of the tube to direct thephotoelectrons in parallel paths between Athe photocathode I4 and target 22. An

electrode I8 of tubular form surrounds the paths of the photoemission from electrode I4 between the photocathode I4 and target 22. 'Ihe tubular electrode I8 is maintained, for example, at around 300 volts positive relative to the photocathode I4 and acts as an accelerating electrode for the photoelectrons passing between the photocathode I4 land the target22. A ne mesh collector screen is closely spaced from target 22 and is maintained, for example, at cathode potential during normal tube operation. The `screen `mesh 20, which during tube operation will be several volts positive relative to target 22, provides a collector for secondary electronsfwhich are emitted from the glass target 22 by the incident primary photoelectrons from the photocathode I4. It is understood that the specific voltages mentioned are only examples of those utilized in one successful forrl of my invention .but that others could'be use At the opposite end of the -tubular envelope IB there is positioned an electron gun structure comprising a thermionic cathode electrode 26 (Figure 2), a control grid 28, an accelerating electrode 3B, and a limiting aperture electrode `32 all fixed to a tubular support member `2 4. `The gun structure is described in greater details `in Vmy copending application Serial Number 9,420

led February 19, 1948. This electron gun structure provides a beam of electrons which is formed into electron ray of small cross-section.

-I'Iflfle electron'ray or `beam is focused on the glass 'target 22 by an electrode 36, which may com- `iting aperture 34 of electrode 32, the uniform axial magneticeld of coil I6 directs the beam substantially parallel to the axis of the tube. The beam 35 is caused to scan the surface of target 2 2 by two pairs of magnetic deflection coils represented by 4U. YEach pair of coils is arranged on an axis normal to the other. The

fields of coils 4G are perpendicular to each other `and to the axis of tube IU. Deflection coils 40 will have periodically varying voltages applied thereto, for example, by saw tooth generators "(not shown) of suitable frequency, to produce Aline and frame scansion. Due to .possible mis- 'alignment of the gun structure 24, relative to the tube axis, an alignment coil 39 is provided to maintain a small magnetic field perpendicular to the axis of the electron beam. Rotation of coil 39 around the stem of the tube envelope l0 will tend to correct any misalignment of the electron beam relative to the axis of the tube as it Aleaves the electron gun 24.

The wall coating electrode 36 during tube operation is maintained positive relative to the cathode potential, for example, at around 200 volts. Adjacent to the target 22 and between it and the focusing electrode 36 is a tubular electrode 38, which is maintained at close to cathode potential. Both the 4potential of the target 22 and of the cathode 26 are identical during tube operation. The beam in leaving the electron gun structure 24 is slowed down somewhat when passing into the lower potential iield of electrode 36 and upon approaching target 22 is slowed down completely to essentially zero velocity by the decelerating iields of electrode 38 and of target 22. The low velocity lbeam 35 upon scanning the insulator surface of target 22 will lower the potential of the target to an equilibrium value of several volts negative relative to cathode potential, at which point the incident electron beam 35 will 4be turned back or reflected as a return beam 31. Thesecondaiy emission Vfrom the photocathode side of target 22 leaves on the surface of the glass target a charge pattern corresponding to the image of the 'scene focused on the -photocathode I4.

The glass of target 22 has an electrical resistivityjust low enough to permit a charge on one surface to unit-,by conduction in a frame time of about'l/soof a second with an opposite charge on the other surface. Also, the thickness of target :22 is small enough to prevent sideways spreading by conduction of these charges during a frame time. The close spacing of the target `22 from the ne mesh screen 20 increases the scene'brightness lof `the charge .image `produced on sthe surface of target 22 by lthe photoelectrons from the photocathode I4. If no light vfalls upon the -photocathode I4 glass target 22 will be charged to an equilibrium potential, negative relative to the cathode 26 lby the incident scanning beam as mentioned above. This equilibrium screen potential is a stable operating point, and is the vpotential the target 22 assumesin Vthe'absence of light and in the presence of the beam.

Photoelectrons from an illuminated spot on the photocathode I4 are guided in :axial parallel paths by the 'magnetic `field of coil 'IE tothe target j22. When the photoeleetrons strike target y22 at an electron velocity of approximately 300 volts, vthey will eject a ygreater number of secondaryelectrons from a target area and leave the target Varea positively charged. The collecting potential of the fine mesh screen 20 is small `and ,in the order `of several volts positive relative to the equilibrium potential of target 22. Yet the collecting -eld of screen 22 is high because of its close spacing from target 22, Aso that the secondary electronsfrom the target 22 are valmost completely collected by screen 23. The glass of target 22 is sufliciently thin, that each positive area formed on the -photocathode side will attract incident `electrons to a corresponding varea on the beam -side of the target. The low velocity scanning beam 35 will deposit suflicient electrons to neutralize the positive charges on the target 22, and to reduce the potential of each positive area of the target to the negative equilibrium. target potential. During the following Aframe time, the positive and negative charges on the opposite sides of the glass target 22 unite by conduction. 'The neutralization of the charges on the two sides and the charging up by the photoelectrons are simultaneous as the scanning 'beam35 passes over the positive areas of target 22, electronsare subtracted from the beam to neutralize or discharge these positive areas, as described above. The remaining portion of the beam is reected from the discharged areas of the screen 22. Thus the return beam 31 is lmodulated by subtraction of electrons according to the positive charge pattern maintained on the glass target 22. The return beam is a maximum for regions of no light and a minimum for regions of high light .area5- Electrode 32 is the first stage of a multiplier `section describedin greater detail in Patent No. '2,433,941 of Paul K. Weimer, issued January 6,

electron emitting or dynode surface.

y1948. Electrode 32 is formedv preferably fromia silver magnesium sheet to provide a secondary The multiplier section also includes a second multiplier stage 44, a third multiplier stage 46, a fourth multiplier stage 48, and a fifth multiplier stage 52, each maintained during tube operation at progressively higher positive potentials. Secondary electrons from the dynode or first multiplier stage 32 will be collected in turn and amplified by secondary emission from

successive stages

44, 46, 48, and 52. Electrode 50 is a collector` maintained at a higher positive potential than the fth multiplier stage 52 for collecting the resulting amplified secondary emission from the several multiplier stages.

The returning modulated beam strikes the first multiplier stage ordynode 32 to produce a secondary emission greater 'than that of the incident beam 3l. Secondary electrons will be attracted to the second stage 44 maintained during tube operation positive relative to cathode potential at, for example, 600 volts. The secondary electrons strike the second stage surface 44 at velocities high enough to knock out more secondaries. These in turn pass on to the succeeding

stages

46, 43, and 52, maintained at successively higher positive potentials and from which the electrons are collected by collector electrode 50 to form an amplified video signal of the tube. Multiplier stages 44, 46, and 48 each consist in this particular tube, of a thirty-two blade pinwheel, stamped from silver magnesium alloy. By forming the blades 45, 41 and 43 of the respectively consecutive pinwheels in an opposite sense, y

an opaque surface is seen by the approaching electrons while the secondary electrons are drawn through slots shown between the blades to the next succeeding stage. The three

stages

44, 4B, and 48 each respectively have a 90% transmission screen 5l, 53, and 55. These screens 5|, 53, and 55 each serve to prevent the relatively negative potential of the preceding stage from suppressing the collection of secondaries through the slots by the succeeding stage. The third stage 4S is maintained at approximately 880 volts, the fourth multiplier stage 48 is maintained at y1160 volts While the secondary emitting surface of the fifth stage 52 is maintained at 1450 volts. The collector electrode 50 comprises essentially an annular` ring having a metal screen 51 stretched across its center. The collector electrode 50 is maintained during tube operation at approximately 1500 volts. All of these voltages are merely illustrative of those used in one tube made according to my invention.

The first multiplier stage 32 is located sufficiently outside of the focusing coil i6 that the magnetic field of this coil at this point is Weak enough to allow secondaries to be drawn away from the first stage 32 and toward the second stage 44. To aid in this procedure, a persuader electrode 42 preferably maintained at slightly negative voltage relative to the voltage of the rst stage 32 is arranged to enclose the dynode surface 32 of the first stage at one end thereof. The persuader electrode 42 not only prevents the secondary electrons emitted from surface 32 from impinging upon the glass wall Ii) of the tube but also acts to provide a practically eld free space above the surface of the-dynodeI 32. Thus, the secondaries emitted from the surface 32 will tend to pass from the surface into the -area' of low signal. ing is used to refer to this effect regardless of o. 7 k'ment of multiplier electrodes 32, 42, and 44,

electrons of varyingr velocities.

fleld'free space until they are seen by the field trode. The scanning of the first dynode surface isdue to displacement of both the incident and the return beam caused by the crossed electrostatic and magnetic fields in the region of the 'decelerating electrode 38, as Well as a helical mo- .tionretained by the return beam after its passage through the deilecting fields of coils 40.

' In order for the second stage 44 to not introduce additional spuriousv signal, it must provide .the same multiplication for electrons coming from each part of the first stage scanned by the return beam. If this requirement is not satisfied,

the portion of the scanned area of the first stage whose secondaries are used less efficiently will appear in thetransmitted picture as a lighter The term multiplier shadwhether it is caused by variation in secondary emission ratio of the first stage or by inefcient collection and multiplication of the secondaries from the first stage.

In trying to get the maximum possible total signal, it becomes increasingly difficult to get uniform collection, so that imperfections in co1- 4lection that mightv not be so noticeable under lesser conditions are not more exaggerated. Such imperfections in collection might be due to a nonuniform field between the dynode surface 32 and the first stage multiplier electrode 44. The spurioussignal, therefore, does not necessarily come from the laddition of unwanted signal but rather is due at this stage to a subtractive effect resulting from an inability to collect with the same uniformity over the surface of the dynode 32. The returning electron beam 37 striking the dynode surface 32 produces a splatter of secondary maximum collection, the lower velocity electrons are more susceptible to imperfections in the collecting field, which may either hinder or aid their collection selectivity over the dynode surface 32,

`and which introduces more severe shading problems.

The arrangement of a relatively flat dynode surface 32 enclosed within the field of the persuader electrode 42 is such that the secondaries produced at the surface of dynode 32 will first Areproduction of the signal, that will rise up into the collecting field of the second stage 44. Electrons of -lower velocities and representing in a greater proportion the spurious signal will not be collected and will fall back and be suppressed by .the field-of the persuader electrode 42. 'manner of operation, the shading or spurious signal introduced by the multiplier is reduced In this until it is not objectionable. The voltage of the persuader electrode 42 is adjusted Within several volts of that of the first stage 32 to give the most uniform shading.

@ne disadvantage of the symmetrical arrangeofthe second stage 44 asvare those secondaries At conditions of v .armagnac coming from points closer to the edge of 'dynode surface 32. This is due, not onlyito the .fact that vsecondaries from :the center must Irise farther .to enter the collecting '.field of electrode 44, lbut rthat the axial magnetic field of coil lf encourages this situation. This problem is partially solved by locating dynode surface `32 within the relatively weak fringe field of solenoid I6.

Another disadvantage ofthe multiplier section was that the flat surface which was conventionally `usedfor dynode 32 often was tilted at an angle to the multiplier axis during during .tube assembly. Only a very slight tilt `was sufficient to Vproduce lnon-uniform collection of secondaries .from .surface 32 by the second stage electrode `M. The Atilt, which could not always be vcontrolled in .tube assembly, .would be sufficient to .result in a video signal producing a picture with a :larger amo-unt of shading on one .side than the other.

This was caused by `the `fact that the edge of -the flat dynode surface 32 'tilted toward the collector M, exposed to a greater extent the areas -of the dynode surface adjacent'to the tilted edge l :to the strong field of collector 44. Also, areas of the dynode surface 32 on the opposite side' were shielded to a greater extent from the field .of collector 44 by the edge of dynode 32 tilted away from the collector M. Thus lower velocity secondary electrons were collected from the areas `of dynode surface `32 closer to the edge tilted toward collector mi to'produce'uneven shading on the received picture.

To produce more uniform collection of secondary electrons from 4the center of dynode Vsurface 32 and to eliminate the disadvantages of a tilted nrst stage dynodesurface I have devised the 'rst stage dynode electrode 32 shown in Figure -2. This electrode 32 is constructed withla spherical surface as is graphically'indicated. This use of a spherical surface for the first stage eliminates the non-uniform collection of secondaries by the second stage Ml. Any slighttilt of the spherical -dynode surface 32 relative to the commonraxisfof the multiplier would coincide v.with the .spherical curvature of surface 32 so that there would be .no tendency to raise one portion of surface 32 above another portion. Furthermore, the provision of a spherical surface keeps the center .portion of the dynode surface 32 high so that -there is eliminated any'non-uniform collection of .secondaries by the second stage 44 from I.this `center area as described above. .structure the collecting eld of the second stage Thatis, with this 44 is more uniformly effective overall portions lof the dynode surface f32.

The curvature of the dynode surface 32 is somewhat critical since it is 'possible to increase the `spherical curvature of the surface to a point vwhere all of the dynode surface 32 will be eie'ctively seen by the collecting eld Vof the second stage lill. As pointed out above, this is an undesirable condition since shading 1and spurious 'signal will result due to the collection 'by `the second stage of an excessive amount of low velocity-secondary electrons .from the first stage. Thus, .the

surface .of Vthe dimensions described above, suc- :.cessfully use a .secondstage dynode wheel of approximately 11/2 diameter.

While certain specific embodiments have been illustrated and described, vit will be understood 'that various changes andv modifications may be made therein without departing from the spirit and scope ofthe invention.

What I `claim-as'new is: 1. An electron discharge device including a Vmultiplier unit comprising a dynode plate having one face of a material which will emit secondary electrons when struck by primary electrons, and a collector plate `electrode mounted opposite the other face of said dynode plate, said collector electrode extending beyond the periphery of said dynode plate, said dynode plate V-having a curvature rawayfrom lsaid .collector plate.

f2. For use in a .cathode ray discharge device, a multiplier unit comprising a dynode plate having 4a convex surface formed of a material providing 'secondary emission when struck by primary electrous .of an electron discharge, and a collector .plate electrodemounted .coaxially to said dynode plate and mounted opposite the other side of said :dynode plate, said collector plate extending radially beyond .the periphery of said dynode plate.

.3. For use in acathode ray discharge device, arnultiplier unit comprising a dynode plate having va convex surface, `formed of a material providing secondary emission when struck by an -zelectron discharge, an annular collector plate .electrode coaxiallyfspaced from the side of said dynode plate opposite to said convex surface,

.and a tubular vmember supporting said dynode iplate and .coaxially extending through said annular collector electrode, said annular plate extending radially beyond the periphery of said dynode plate.

4. Foruse in a cathode ray tube, a multiplier -unit comprising a spherically shaped dynode p1ate, the convex surface of said dynode plate .ibeing of a material providing secondary emission when struck by primary electrons, and an annular collector plate coaxially mounted oppo- .site the concave side `of said spherical dynode .plate and extending beyond the peripheral edge ,of said'spherical dynode tocollect the secondary emission from said convex surface.

5. An electron discharge device including a multiplier unit comprising ra dynode plate having one surface which emits secondary electrons when struck by primary electrons, and a collector electrode mounted coaxially with said dynode 'plate and spaced from said secondary emitting surface symmetrically with respect thereto, said collector electrode positioned opposite the other surface of said dynode plate, said secondary emitting surface havin-g a curvature away from fsaid collector electrode.

6. A cathode ray discharge device comprising viding an electron beam along a path, a target electrode within said envelope and mounted transversely to said electron beam path for returning part of said electron beam to said gun structure, and a multiplier unit including a spherically shaped plate Xed to one end of said gun structure, a tubular persuader electrode enclosing said plate, and a collector electrode spaced from said plate on the concave side thereof, said plate arranged with the convex surface thereof transverse to said electron beam path toI intercept the return portion of said electron beam, said convex surface being of a material providing secondary emission when struck by the primary electrons of said return beam.

7. A television transmitting tube comprising an evacuated envelope, an electron gun including a tubular support member mounted within said envelope and a cathode electrode enclosed within said tubular support member for providing an electron beam along a path, a target electrode within said envelope transverse to the path of said electron beam for returning a part of said electron beam along said beam path, and a multiplier unit including a spherically shaped plate closing the end of said tubular member adjacent said target electrode, a tubular persuader electrode having one end enclosing said spherically shaped plate and a collector electrode spaced from said one end of said persuader electrode on the concave side of said plate, said plate having an aperture at the center thereof for the passage therethrough of electron emission from said cathode, said convex plate surface being of a material providing secondary emission when struck -by the primary electrons of said return beam.

8. A cathode ray discharge device comprising an evacuated envelope, an electron gun structure within said envelope for producing an electron beam along a path including a cathode electrode and an apertured spherical plate spaced therefrom transversely along said beam path and arranged with the concave side of said plate facing said cathode electrode, a target electrode within said envelope transverse to said electron beam path for returning a part of said electron beam along said path, said spherical plate arranged between said cathode electrode and said target, the convex surface of said spherical plate being of a material providing secondary emission when struck by the primary electrons of said return beam, a multiplier unit including a persuader electrode mounted within said envelope between said spherical plate and said target electrode for 10 suppressing low velocity secondary electrons from said plate surface, and a collector electrode spaced from said persuader electrode on the concave side of said spherical plate.

9. A cathode ray discharge device comprising an evacuated envelope, an electron gun structure within said envelope for providing an electron beam along a path including a cathode electrode and a spherical dynode plate spaced from said cathode electrode and intercepting said electron path, said dynode plate having an aperture'for masking said electron beam upon its passage therethrough and arranged with the concave side thereof facing said cathode electrode, a target electrode mounted within said envelope on the convex side of said dynode electrode for intercepting said electron beam and for returning a part of said electron beam back along said path, and a multiplier unit including a tubular persuader electrode mounted between said dynode and said target electrodes and enclosing the convex surface of said dynode electrode for suppressing low velocity secondary electrons from said dynode plate and a collector electrode spaced from said persuader electrode on the concave side of said dynode plate.

l0. An electrode discharge device including a multiplier unit comprising a dynode plate having a convex spherical surface which emits secondary electrons when struck by primary electrons, and a collector electrode mounted coaxially with said dynode, said collector electrode mounted opposite the concave side of said convex dynode surface and symmetrically with respect thereto.

1l. An electron discharge device including a multiplier unit comprising a dynode electrode having a convex spherical surface which emits secondary electrons when struck by primary electrons and a collector electrode for said secondary electrons mounted opposite the concave side of said convex surface and symmetrically spaced from the periphery thereof.

RALPH E. JOHNSON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,559,460 Ruben Oct. 27, 1925 2,204,503 Langenwalter June 11, 1940 2,424,850 Roman July 29, 1947