US2503949A - Storage tube - Google Patents

Description

April 11, .1950 A. s. JENSEN ElAL 2,503,949

STORAGE TUBE Filed April 23, 1948 H mm 5/6/1071 l NV EN TO R S ARTHUR S. TENsEN 8/ LESLIE. E. Fm: Y

4 TO R N EY Patented Apr. 11, 1950 STORAGE TUBE Arthur S. Jensen and Leslie E. Flory, Princeton,

N. J., assignors to Radio Corporation of America, a corporation of Delaware Application April 23, 1948, Serial No. 22,848

8 Claims.

cation of R. L. Snyder, Jr., filed July 24, 1945,

Serial No. 606,812.

vTubes of this type utilize a magnetic field, established parallel to the tube axis and perpendicular to the target surface, for focusing the scanning electron beam on the target and for directing the secondary emission back along the beam path toward the collector electrode. Tubes using the same field for focusing both the primary electron beam and the secondary emission are subjected to a non-uniform collection of secondary emission over the target surface. Also due to the design of such tubes, changes made to improve secondary collection from the target tend to cause defocusing of the scanning electron beam and loss of resolution of signal.

Furthermore storage tubes of this type utilize appropriate electromagnetic coils for establishing the required scanning fields. Elimination of the scanning coils would greatly reduce the weight as well as the power requirements in large tubes of this type. Also, a long magnetic scanning field is impractical for tubes having targets two inches and more in diameter. Consequently, in large storage tubes, of the type described, magnetic scanning'has proved impractical.

Accordingly, it is an object of this invention to provide an improved storage tube of the oathode ray type.

' It is also an object of this invention to provide a storage tube having uniform collection of signal from the target surface.

It is a further object of this invention to provide a storage tube in which the collection of signal current from the target 'is not dependent upon the electron beam focusing fields.

' It is also an object of this invention to provide" a storage tube in which the primary electron beam is little effected by the collecting fields for the signal electrons.

The novel features whichwe believe to be characteristic of our 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 55 path of'the electron beam.

ranged so that the beam passes through the cen- I with the accompanying drawing, in which the single figure is a cross-sectional View of a storage tube according to the invention.

The drawing discloses a storage delay tube of the cathode ray type comprising an envelope ill of glass or any other suitable material. Within the envelope is positioned an electron gun for the purpose of forming and focusing an electron beam upon a target electrode 30. The gun structure consists of a thermionic cathode l2 surrounded by an apertured control grid l4. Mounted in alignment with the opening through the control grid is an anode electrode It for accelcrating the electron emission from the cathode l2 and for focusing the electrons of the beam onto the surface of a target 38 mounted at the other end of the envelope l0. Two pairs of deflecting plates, respectively l8 and 20, are positioned along the beam path between the anode electrode l6 and the target 30. As is Well known in the art, these pairs of deflecting plates each produce electrostatic fields at right angles to each other and to the path of the electron beam. It is understood that these plates respectively will have varying voltages applied, say by a sawtooth generator, to produce line and frame scansion, or appropriate voltages may also be applied to deflection plates l8 and 2B for producing spiral scansion of the target, or line scansion only, as may be desired. The means for producing different types of scansion of the target surface 3i] are well known in the art and need not'be further described.

The operation of the tube depends upon a signal being generated by the emission of secondary electrons from the target 30, when bombarded by the electron beam. A collector electrode 28, in one type Of tube as shown in the figure, comprises an open-ended cylinder 28 mounted in the envelope l5! coaxial with the electron beam path and so that the beam will pass through the cylinder 28 after deflection by plates l8 and 20. Electrodes 22, 24 and 26 are shield electrodes formed from apertured metal discs and spaced along the Electrode-24 is artral aperture thereof and it forms an electrostatic shield between the plates [8 and the plates 2t. Similarly, electrode 26 is an apertured disc coaxially spaced along the electron path and positioned between the deflection plates 20 and the collector electrode 28. Shield electrode 2% not only shields collector 23 from the signal of deflection plates 20, but also maintains axial symmetry of the electron lens fields in the target section to eliminate one type of astigmatism in the beam. Shield electrodes 24% and 26 may be maintained at the potential of the focusing anode electrode I6.

The target electrode 30 comprises essentially a metal support plate 32 mounted transverse to the path of the electron beam and which functions as the signal plateof the target electrode, on the surface of the signal plate 32, facing the electron gun, is fixed a dielectric layer 34, such as mica.

Target 30 may also be formed, using an aluminum 1 signal plate, having a dielectric surface of aluminum oxide, facing the electron beam, formed, for example, by anodizing the aluminum; However, the construction of the target 39 need 'notbeconfined to either of these described forms, but may be of any other appropriate insulatingfmaterial such as titanium dioxide, or silicon dioxide, for example, deposited in any manner as evaporation" or thermal decomposition upon a conductive signal plate. In the figure, closely spacedfrom the exposed surface of the mica layer 34, is a fine mesh screen 36 or barrier grid Whichmay be mounted'by a' suppo-rtingring 38 at'a few'mils from the surface: of'the dielectric' la-yer 34.

As the dielectric surface 34 s scanned by the electron beam, secondaries-are emitted and drawn away toward collector electrode 28 which is maintained during tube operation, at a high positive potential relative to 'the potential of surface 34.

Since the secondary-electrons from the dielectric surface 34 are emitted in all directions at rela tively low'velocities, electrodes 40 and 42; maintained during tube operation at different poten tials; are used'to form afocusing electrostatic field for directing the secondary emission into the collector cylinderZBZ. A shield electrode 44" comprising an ap'ertured' plate is'mounted', as is shown, andshields collector electrode 28 from voltage fluctuations produced on the' target 30* by incoming signals:

The principle of electrostatic storage on an in'- sulating surfa'cehaslongbeen known an'd' used in television pickuptubes, such as the iconoscope. If an insulating-surface is bombarded by an electron beam, the secondary emission ratio will vary with the energy of the bombarding;elec-' If the energy is such that'the secondary emission ratio is greater than unity, then thepotential of the target surface will change with I trons.

respect to the electrode which collects the secondaries, until the net number of secondaries leaving the target surface is exactly equalto the number of primaries arriving there. The sur-- face potential, at which this action takes place; is known as the equilibrium potential. Theremaining secondary electrons-collect in the' form of a space charge and win back on the insulating surface, charging the unbombarded parts or-the surface to a negative potential.

of any applied signal.

Several :ways'have been attempted in the-past toeliminate electron redistribution" effect.

Thus, a: charge pattern is'blIilt up on thesurface in the'absen'ce:

The returning" electrons,- of' course, partially neutralize any charges alreadyon the surface" and," thus would. make any come Screen 36; mounted close to 'th'e insulating 'surface 34, eliminates this redistribution effect, as described below.

In operation, the electronbeamistrikes'the di electric surface 34 with'sufficient'velocity to produce a secondary emission ratio greater than unity. To obtain this condition; the screen 33 and signal plate 32 inthespecific tube described" are maintained at a potential about eleven hundred volts positive relative to the cathode l2 of the electron gun, which may be considered to be at ground potential. Wherever the beam strikes the dielectric 34, the potential of the elemental area of the surface under bombardment becomes the same or nearly the same as that of the screen 33, that 11s,; equilibrium.1conditions exist only at this potential.

Since target surface 34 is an insulator, the only source of current to it is the primary beam, and theonly drain of current from it is the secondary electron emission. At equilibrium, these two must be equal; and any deposition or re- .moval of charge on the surface 34 will appear as primary beam. must return 1 to the target $511K+ face to maintain the target surface 3 -at. equllie brium potential.

The barrier-grid" or screen36 functions: as 'ai virtual collector, so that the equilibrium potential of the target surface 34 1sestablishedwith respect to: screen 36andnot to the actual co1-= lectoi' electrode 28. At this potential, a'number of secondaries, just equal to the number of .ar-"

riving primaries,.are sufficiently energetic to pene- These cannot return to the i trate screen 36. target, as appropriate'fields:outside the screen urge them. away and toward: collectorl28 "as the: secondarybeam; Meanwhile, the excess electrons are not sufficiently energetictosreach screen 36 from surface34, and are restricted: in their -motion by the close proximity of thescreen to the dielectric surface 3 so thattheir redistribution:

to portions of the-target not directly under-the beam is considerably reduced:

In normal operation, the screen 36 over the target surface 34-ismaintainedat a direct cur- 1 rent potential and the-conductor'plate 32 is con- 1 nected to asourceof the signal to be recorded.

The insulating surfacel34 is thereforecapacitively coupled to the signal plate'32 andalso has capacity to the screen 36; When asignal voltage-is impressed on signal plate 32: it also'appears', somewhat diminished inamplitude, .on the re-- cording-surface 3 10f the'target.

If a signal is applied to plate 32 so that it is driven negative relative to the equilibriumor screen potential then any elemental area of sur- .face 34 under bombardment, at the time by the electron beam, will also be-negative 'relative'tothe screen. Under these conditions, a positive field between screen 36 and target 3il'willbe presented to the surface 34 and, therefore;'all of the secondary electronsreleased bythe impact of the beamzelectronyare drawnaway- Sincethe number of secondary electrons is greater: than "the; number: of: primaryv electrons; .there its a net' loss of. negative charge andvthe elemental-surface area:- v becomes more :positive:- If, however, the element of surface 34 is positivewith respecttothe screen at' the' time of tbombardment :due-to an incoming signal driving plate' 32 in-a positivedirection, a

negative field exists betweenthe'surfa'ce 34 and the screen 36 so that fewer secondary electrons are. sufficiently energeticto penetrate the "screen and more-'aremeturned' to the. element of surface from whence theycame.v Since there are'less secondary" electrons "leaving than: primary elec.-- trons arriving at the element, thereQis a net gain:

of negative charge and the potential of the surface becomes more negative.

When target plate 32 is at the potential of screen 36 or a little positive thereto, there are just as many secondary electrons escaping through the screen as there are primary electrons arriving. This condition of equilibrium exists at a potential a few volts positive with respect to the screen 33 because the initial velocity of most of the secondary electrons is sufficient to lift them over a potential barrier of several volts. The exact potential is not very definite, because it is affected by space charge conditions, the geometry of screen 36 and nearby electrodes, and the energy distribution of the secondary electrons. Fortunately, the value of the equilibrium potential has no great influence on the operation of the tube as long as it remains practically constant.

As the beam is deflected across the surface 34, while a signal is impressed on the signal plate 32, it will cause each element of surface area it strikes to come to screen potential, regardless of the potential the surface would otherwise have due to the influence of the signal plate. This action, then establishes a charge between the signal plate 32 and the surface element which will cause the element to have a potential different from that of the screen 36 when the beam moves ofi of the surface element and the signal plate 32 returns to equilibrium potential. If the beam scans a long path over the target surface 34. while a fluctuating voltage is impressed on the signal plate 32, a band of charges, as wide as the beam, will remain on the path when the beam is cut oif. If the signal plate 32 returns to equilibrium potential, the potential along the path will vary in proportion to the signal voltage impressed during the beam transit.

When the electron beam scans the target 30, some of the secondary electrons directed toward collector electrode 28, are from the solid parts of the screen, which intercept the beam current. The rest of the secondary electrons come from the surface of the dielectric. leased from the screen are essentially constant in number, because the beam current is fixed and the secondary emission ratio over the screen is quite uniform. Also, the screen 36 is fine enough to average out geometrical effects. The secondary emission from the dielectric surface, however, fluctuates according to the charging demand. If no change of charge is required by an element. the secondary electrons released therefrom are equal in number to the impinging beam electrons. If a negative charge is to be supplied to the dielectric surface 34 to bring the elemental target area to equilibrium potential, secondary emission is suppressed until the demand has been satisfied. If a positive charge is needed to bring the target area to equilibrium the secondary emission is a maximum until full charge is achieved.

In the known coherent pulse radar system, the signal pulses are transmitted at the beginning of each line scansion at the peak of the sawtooth wave controlling this scansion. Echo sig nals from both moving and stationary objects are received and mixed with a continuous wave signal of a local oscillator to produce a heterodyned signal which is impressed on the target 30 in certain phases of the line scansion, of the storage tube, depending upon the distance of the objects from the transmitter receiver. The successive echo signals from fixed objects will be beat signals of fixed amplitudes, while the echo sig- Those electrons renals from moving objects will be beat signals whose amplitudes vary with the Doppler frequency. A charge put down on an elemental area of the dielectric surface 34 by the electron beam, when the potential of the surface was changed from the equilibrium value by a signal echo from a stationary object, will maintain the elemental area at equilibrium potential during succeeding line scansions of the target. However, echo signals from moving objects arriving with a changing amplitude will cause the elemental target areas struck by the beam to change in potential. Thus, the secondary electron emission will be constant from the target areas charged during the reception of signals from fixed objects, while the secondary emission will vary from target areas charged during reception of signals from moving objects. This varying secondary emission will be the alternating current component of the secondary emission current in the output circuit of the tube. Thus, only signals from moving objects will be detected by the storage tube described.

The dielectric layer 34 of the target 30 must have a sufficiently high product of resistivity and dielectric constant so that an appreciable amount of charge cannot leak through the dielectric layer 34 between scansions. Also there must be so little surface leakage across the dielectrio, and the successive lines of the scan must be sufficiently spaced relative to the spot size of the beam, that the beam cannot remove the charge that was deposited when it previously scanned a neighboring line.

The spacing of screen 36 from the mica surface M is not too critical. However, if the spacing is too great there will be a redistribution of the secondary emission which does not penetrate the screen but falls back onto the target surface and which will tend to discharge positive areas of the target other than that which the beam is striking. This effect will tend to produce a shading of the signals and also reduce the resolution of the signal collected. Also if the spacing is too small, whenever negative signals are applied to the signal plate 32, the very negative portion of the target surrounding the beam spot may create a negative field which will erect a potential barrier producing a negative grid effect suppressing the secondary emission and forcing the secondaries back to the screen 36. As a result these electrons will be collected by the screen 36 and their absence from the secondary beam on each scan will cause a positive signal to appear in the collector circuit.

The screen electrode 36 in one form of the tube shown in the drawing, comprises a mesh screen having 230 openings per inch. A suitable material for screen 33 has been found to be stainless steel, which provides sufiicient strength to support the screen at its desired spacing from the dielectric surface 34. As the electron beam scans across the wires of the screen 36, a noticeable modulation of the secondary electron signal collected by the electrode 28 is produced due to the difference in secondary electron emission of the screen 36 and the dielectric siu'face 34. Since it is desirable that the secondary emission ratio of the screen 36 be approximately unity, during tube operation, the stainless steel screen 33 is coated by a layer of sputtered gold or other suitable materials having a secondary emission ratio close to unity such as carbon, for example.

The transmission of screen 38 is approximately ano e-49 '2": 50percentiof the impinging;electron-beam. However; if a coarser. meshscreen were used, there would-Joe a greaterredistribution of the secondary emission electrons suppressed. by the screen. Thewscreen 3% functions as an electrostatic shield to :prevent the redistribution of' secondary electrons, to other positive portions of the dielectric surface.

As the-dielectric surfacet l of the target is bombarded with electronsof'the primary beam, the-secondary electronsfrom the dielectric surfacefiitr also fromathe wire mesh screen 36 will be emitted in all directions. These secondary electrons which leave the target area must be properly focused and directed toward the collector-electrode iii in an efficientm'anner. To acthis, electrodes 40 and-d2 are provided. Electrode tilcomprises a conductive coating on the-innersurface'of the envelope wall it: and conforms'to the. cylindrical shape of thetube envelope. Coating dil' extends so-that one end overlaps'andencloses the barrier-grid til-B and the dielectric target surface 3%. Electrodes dll and 42 may also be cylindrical metal electrodes mounted Within the tubeenvelope. Wall electrode 40 is preferably maintained at the potential of the barrier grid 35; Electrode Z2 is spaced from electrode-"ill and is maintained in the specific tube described at a potential of between one hundred and two hundred volts positive to the potential of electrode 46. The arrangement of electrodes at and 52 provides a converging focusing electrostatic field therebetween, when appropriate voltages are applied during tube operation. This converging field, between electrodes 46 and d2, will tend to direct the secondary emission from the target 3! toward the axis of the tube. envelope I9.

In this manner; the secondary emission originating from the insulating surface 3 5 and the screen. will be directed toward the axis of the tube from all positions of the target surface struck by the'electron beam. This arrangement providesa uniform-collection of secondaries from all of thertarget surface so that there is little or no resultant shading of the signal.

The-collector"electrode 28- is operated at the highest potential :in the tube so that as the secondaries are directed'back toward the axis of the tube they will pass into a region close to electrode 23 at which they come into the field of the lens formed by collector 25 and the aperture in electrode Those secondary electrons, emitted principally from the central portion of the t will tend to pass down the axis of the tube toward the cathodeof the electron gun. To prevent these secondary electrons from passing completely through the collecting cylinder 28 and hence being lost and contributing by their abloncea positive disturbance signal, shield electrode 23 is positioned so that'when it is maintained at a potential somewhat negative to the target surface QM, electrostatic mirror. Thus electrode .25 will pro-. a negative barrier field to the secondary el. ctrons and prevent their passage through the collector cylinder 23 and secondaries attempting to pass through the collector :cylinder 28 will be reflected back and swept up by the positive field of electrode-7J8.

Electrode 54i is provided with an aperture large enough for the passage therethrough of the secondary signal electrons. However, it is arranged as is shown in the figure to electrostatically shield the collector electrode 28- from the fluctuelectrode 25 wi1l act as an ati-ng potentials applied: onathe; signal plate .32: of the target electrode-by incoming signals; Fur= thermore, the mirror:or.barrier electrode 26 serves to shield the collectorelectrode 28 from the.fluc.- tuating scanning potentials applied to the pair of deflection plates 20.. This. shielding of the collector electrode ZBQthHS prevents 'thev picking up of'spurlous signalsproducedby capacity efiects with other electrodes in the tube havingrfiuc-.-- tuating potentials applied during. tube operation...

The various voltage and potentialvalues indl-. cated in the figure and describedabove are given only as illustrativeof-those used'in a successfully; operated tube of thistype; Theinvention is not, confined to these values and other potentials may? also be used successfully.

We have described'the operation of our storage: tubes-in connection" with a moving target. indi cating radar system, but this is by Way. ofjexe amplenon'ly; The improved storage :tube may be used-in various other-"ways-wherei storage of ;in-. formationis: desired. .Signals may beimpressedf on the. signal platewhile' the :beam scans a presdetermined pattern over. the target. The "signal: and beam may-then beshut off, leaving the information stored in the target. Atany desired later time the beam may-be turned on and, with no new signals impressed on the signal plate,- scannedover the pattern so-that the signal impressed during the-first operation will be re-- produced? The .signalseneed'not be recorded one scansion andutilizedfin the next scansion. Signals canbe stored atone time and reproduced-.- at. any later time; merely: by shutting. 01p and some tubes-of the-type described,- signals have been stored on the targetsu-rface up to one hun dred hours withlittle loss ofdefinition; As will be evident from the described method'of opera-- tion, recorded signals may becombined with new signals, enabling: one to =use thetube-for carrying out complex operations-by suitable :combination ofs gnals;

While certain specific-embodiments have been" illustrated and described, it will be understood that various changes andmodificationsmay be made therein without departing from-the spirit. and scope of the invent on.

What we claim as new is:

l. A -cathode raytube comprising an evacuatedl envelope, meanswithin said envelope for forming.- a'beam of electrons along a path, a target electrode mounted within said'envelope' in the path of said electron beam; said target electrode hav- -.ing a surface providing secondaryelectron cmission when struck by said electron beam, 9. collec tor electrode mounted-within: said envelope be tween said beam forming means and said target,

and a plurality of electrodes within said envelope: between said collector electrode and said target-- for focusing the secondar emission adjacent said. collector electrode.

2-.. A cathode-ray tube comprisingan evacuated envelope, electron gun means within said envea. lopefor forming a beam of electrons along a path,

a targetelectrodemounted within said envelope to'intercept said beam of'electrons, said target. electrode having a surface providing. secondaryv electron emission when struckby said electron.

beam, an electrode within said envelope for collectingsaidsecondary electron emission, said collecting electrode having an aperturetherethrough for passage of. said electron beam, and electro-.v

static means forfocusing said secondary-electron.

emission. near-said collecting; electrode.

said collecting electrode having an aperture therethrough for passage of said electron beam, a plurality of electrodes enclosing the path of said electron beam for focusing said secondary elec tron emission near said collecting electrode.

4. A signal generating device comprising an evacuated envelope, electron gun means within said envelope for forming a beam of electrons along a path, a target electrode within said envelope intercepting said electron beam path, said target electrode having a surface providing secondary electron emission when struck by said electron beam, an electrode within said envelope for collecting said secondary electron emission, said collecting electrode having an aperture therethrough for passage of said electron beam, means for directing said secondary electron emission toward said collecting electrode, said directing means including a shield electrode between said target electrode and said collecting electrode and a plurality of electrodes between said shield electrode and said target for focusing said secondary electron emission into the aperture of said collecting electrode.

5. A cathode ray tube comprising an evacuated envelope, electron gun means within said envelope, electron gun means within said envelope for forming a beam of electrons along a path, a target electrode mounted within said envelope to intercept said beam of electrons, said target electrode having a surface providing secondary electron emission when struck by said electron beam, an electrode within said envelope for collecting said secondary electron emission, said collecting electrode having an aperture therethrough for passage of said electron beam, means for directing said secondary electron emission toward said collecting electrode, said directing means including an apertured shield electrode mounted within said envelope coaxially with said collecting electrode and between said collecting electrode and said target and a plurality of electrodes between said shield electrode and said target for focusing said secondary electron emission through said shield electrode into the aperture of said collecting electrode.

6. A cathode ray tube comprising an evacuated envelope, electron gun means within said envelope for forming a beam of electrons along a path, a target electrode mounted within said envelope to intercept said beam of electrons, said target electrode having a surface providing secondary electron emission when struck by said electron beam, an electrode within said envelope for collecting said secondary electron emission, said collecting electrode having an aperture therethrough for passage of said electron beam,

electrostatic deflection means for causing said electron beam to scan said target surface, means for electrostatically shielding said collecting electrode from said deflection means and said target, and a plurality of electrodes between said collecting electrode and said target for focusing said secondary electron emission into the aperture of said collecting electrode.

7. A cathode ray tube comprising an evacuated envelope, electron gun means within said envelope for forming a beam of electrons along a path, a target electrode mounted within said envelope to intercept said beam of electrons, said target electrode having an insulating surface providing secondary electron emission when struck by said electron beam, an electrode within said envelope for collecting said secondary electron emission, said collecting electrode having an aperture therethrough for passage of said electron beam, plate electrodes mounted within said envelope between said gun and said collecting electrod for causing the electron beam to scan said insulating target surface, a first shield electrode between said deflecting plate electrodes and said collecting electrode a second shield electrode between said collecting electrode and said target, and electrostatic means for focusing said secondary electron emission into the aperture of said collecting electrode.

8. A cathode ray tube comprising an evacuated envelope, electron gun means within said envelope for forming a beam of electrons along a path, a target electrode mounted within said envelope to intercept said beam of electrons, said target electrode having a surface providing secondary electron emission when struck by said electron beam, an electrode within said envelope for collecting said secondary electron emission, said collecting electrode having an aperture therethrough for passage of said electron beam, a plurality of plate electrodes mounted within said envelope along the path of said electron beam between said electron gun and said target, for causing said electron beam to scan th surface of said target, a first shield electrode mounted between said plate electrodes and said collecting electrode, a second shield electrode mounted between said collecting electrode and said target, each of said shield electrodes having an aperture therethrough for passage of said electron beam, and a plurality of electrodes between said second shield electrode and said target for focusing said secondary electron emission into the aperture of said collecting electrode.

ARTHUR S. JENSEN. LESLIE E. FLORY.

REFERENCES CITED The following references are of record in the

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