US3002124A - Display storage tube - Google Patents

Description

P 1 R J: SCHNEEBERGER 3,002,124

DISPLAY STORAGE TUBE Filed April 9, 1956 5 Sheets-Sheet 1 WITNESSES. INVENTOR Robert J. Schneeberger.

ATTORNF Sept. 26, 1961 Filed April 9, 1956 Total Electrons Leaving Total Electrons lncldenr 3 Sheets-Sheet .2

oi'ar'e Primory Energy Vpr- Kev Fig. 6.

o z 4 B IO l'z l'4 [6 Primary Energy Vpr-Kev Fig.7.

able to be able to modify the charge image. or :image element by element.

tracted from the storage grid by use of only beam.

" shown in FIG. 1; V

i FIG. 3 is a detailed structure showing another possible I FIG. 1; and

3,002,124 r DISPLAY STORAGE TUBE Robert J. Schneeberger, Pittsburgh, Pa., assignor to Westrnghouse Electric Corporation,-East Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 9, 1956, Ser. No. 576,847 5 Claims. (Cl. 315-12) This invention relates to electron discharge devices and more particularly to devices in which an input signal may be stored in the form of a charge and later converted into an output signal.

This invention is more particularly directed to viewing storage tubes Which are cathode ray tubes in which the visual display'of an image may be maintained, for an extended period of time after writing has ceased.

Storage type cathode ray display tubes have been proposed and utilized for display systems such as in television and radar in order to obtain brighter images and to display different types of information simultaneously on the same viewing screen.

In general, the storage tube consists of a foraminated storage grid member which controls theelectron flow between a flooding electron gun and a luminescent screen. A space distributed charge image is created on the surface of the storage grid by an intensity modulated scanning or writing beam and 'the'charge image recorded on the grid controls the flow of "electrons to the screen from theflooding beam and thus a light-image of the charge ,'pattern is produced on the screen. I

The charge pattern on the storage grid may be created by secondary emission from the surface of the grid. In most devices the. scanning or writing beam is only able to change the charge in only one direction. It is', therefore, necessary to erase or removethis information by pulsing the storage grid or the cathode ofvthe flooding gun to erase the whole frame. another gun for erasing in a-line by line method but this adds problems in the structure of the tube.

I There are many applications where it would be desirerase. the

' It is, accordingly, an object ofmy invention to provide an improved storage tube. I It is another object to provide an improvedtype storage tube in which information may be .added or subone writing It is another object to provide an improved storage tube in which a charge image may be stored and then the charge image modified to add'or subtract'infonnation.

It is possible to use r d S at s )Pa e fQ" second electron gunstructure 30. The second electron These and other objects are-efiected by my invention as will be apparent from the following description taken in accordance with the accompanying drawings throughand in which:

FIGURE 1 is a composite structural and-schematic;

'showing of a storage tube and associated-circuitryembodying my invention;

' FIG. 2 is a detailed structural view of the storagegrid structure of a storage grid that may be embodied in FIG. 1;

FIG. 4 is a detailed structure showing another possible FIG. 5 is a detailed structure showing another possible structure of a storage grid that may be embodied in tion of my invention.

Patented Sept. 26 1961 Referring to FIG. 1 there is shown a storage tube comprising an evacuated envelope 10 having a cylindrical body portion 12 having a first tubular extension or neck portion 14 mounted substantially coaxial with cylindrical portion 12. A second tubular extension or neck portion 16 is mounted with its axis at an angle with respect to cylindrical portion 12. The extensions 14 and 16 are positioned near one end of cylindrical portion 12. 'Positioned within the extension 14 is an electron gun 20 of suitable design comprising at least a cathode 22 and an accelerating grid 24 for generating a flooding electron beam. Lead in conductors are provided for each of the elements of the gun 20 to the exterior of the envelope 10.

' The cathode 22 of the electron gun 20 is connected through a resistor 44 to ground. The cathode 22 is also provided with a suitable voltage from a source 46 for generating negative pulses. The amplitude of these pulses may be of the order of 8 volts and of any given time interval. The voltage source 46 acts as a switch and thus determines whether the flood gun 20 is in erase or read operation. The accelerator electrode 24 of the gun 20 is'connected to the positive terminal of a battery 52 with the negative terminal of battery 52 connected to ground. g

Mounted within the other tubular extension .16 is a gun 363 may 'be referredto as the writing gun and in;

eludes at least a cathode 32, control grid 34 and acceleratingelectrOde 36 for generating a pencil beam. Lead-in conductors. are provided from each of the electrodes oj f the gun 30 to the exterior of the" envelope 10. The cathode 32 of'the electron gun 30 is connected through a a resistor 51 to the negative terminal of a voltage source represented by a battery 50 which may be of the order of.5,00 0 volts. The positive terminal of the battery 50 is connected to ground. The cathode 32 is also corinected to the positive terminal of a battery 54 which maybe of the orderv of 30 volts. The negative terminal of the battery 54 is connected to the' control grid The cathode 32 isalso-c'onnected to the negative terminal of a voltage source represented by a battery 58which may be of the order of 500 volts. The positive terminalof the battery 58 is connected to the accelerating electrode tion 12 and thereby provides a second anode electrode common to the electron gun structures 20 and 30. The

coating 40 is connected on the exterior of the envelope 10 to the positive terminal of a battery 42. The negative terminal of the battery 42 is connected to ground. The battery 42 may be of the order of 20 volts. g

' Positioned on the opposite end of the cylindrical body ,portion 12 of the envelope 10 with respect to the electron guns 20 and 30 is an end plate member 70 of a transparent material such as glass which closes off the end of the body 12. Deposited on the inner surface ofthe transparent end plate 70 is a coating 72 of a luminescent material. The coating 72 may be a phosphormaterial such as zinc sulfide. An electron permeable coating74 of a conductive material such as aluminum is deposited on the exposed surface of the phosphor layer 72.- A lead in connection is provided from the exterior of the envelope '10 to the aluminum coating 74 so 'as to provide means of applying a suitable potential to the phosphor screen 72. The coating 74 is'connected to the positive 3 terminal of a suitable voltage source represented by a battery '76. The battery 76 may be of the order of 15,000 volts with the negative terminal connected to ground.

Positioned adjacent and parallel to the phosphor screen 74 is a foraminated storage grid 86 which consists of a conductive back plate 82 which may be in the form of a fine wire mesh screen which may have 200 to 400 openings per linear inch. The storage grid 80 shown in FIG. 2 is comprised of the conductive wire mesh back plate 82 having a coating 84 several microns thick of a material which exhibits the property of electron bombard ment induced conductivity such as barium fluoride, lead oxide, magnesium fluoride, arsenic trisulfide and antimony trisulfide. The coating 34 should also have sec ondary emissive properties such that the secondary emission ratio is less than one and should be as close as to aero as possible within a selected range of electron bombardment energies. The foraminated back plate 82 may be made by any process. The coating 84 which may be .of a glassy type as illustrated in FIG. 2 should be such as to have a high lateral resistance to preserve definition and also sutficient transverse resistance to avoid leakage during a predetermined length of time. Other properties desired in the storage grid 80 in order to practice my invention may be best described in connection with the operation of the device. The storage grid 80' is connected to ground potential by means of a suitable lead-in to the back plate 82.

Positioned adjacent to and parallel to the storage grid 80 is a collector grid 90 which is in the form of a line wire mesh having a similar number of openings as the storage grid 80. A lead-in is provided to the collector grid member 90. The collector grid 90 which is of similar area to the storage grid 80 is provided with an annular metallic member 92 on the side thereof facing the electron guns 20 and 30 and is at a similar positive potential of about 215 volts above ground as the collector grid 90 for collimating the electron beams so that the flooding electrons approach the storage grid 80 normal to the surface thereof. The potential may be suplplied by means of a battery 86.

To explain the operation of the described tube and the operation of the storage grid therein it is necessary to refer to the graphs illustrated in the drawings.

The storage electrode 80 consists of the conductive back plate 82 to which a suitable potential is applied.

"In the specific illustration the back plate 82 is connected 'to ground. This initial ground potential is also the poten- "tial of the surface of the insulator coating 84. 'The potential on the insulator surface may be referred to as V If the flooding electron gun 20 is pulsed by the erase voltage source 46, the cathode 22 is driven to a potential of 8 volts negative with respect to the storage grid 80. At this low voltage, the primary electrons will strike the grid 80 at energies below the first crossover of the layer 84. Fewer secondary electrons are emitted than there are primaries incident and the surface of the layer 84 of the storage grid 80 will assume a negative potential approximately equal to the '8 volt potential of the cathode.

In FIG. 6, the curves 102 and 104 illustrate the secondary emission characteristics of two representative materials. The curves 102 and 104 are a plot of the secondary emission ratio as a function of the primary "or bombarding electron energy. The primary energy is .expressed in electron volts. The secondary emission ratio is simply the ratio of the secondary electron emission current to the primary emission current. The curve 102 is representative of most suitable target materials evaporated in a suitable manner to give a smooth surface.

--It is to be noted that the secondary emission ratio is .the second crossover is found at higher values.

The curve 104 is representative of a material such as carbon in the form of soot. In this type of material there are no crossover potentials and the maximum ratio is always less than unity. t is found that many other materials when deposited as a sooty deposit have a secondary emission ratio of less than unity over a large range of primary voltages. In my application it is desirable that the secondary emission ratio of the layer 84'be less than unity and as close to zero as possible.

It is seen from the curve that an insulator layer 84 of a suitable material having characteristics similar to curve 102 or 104 will have a ratio of secondary emission below unity at a potential of 8 volts. Therefore, the grid will on bombardment with electrons of 8 volts energy collect electrons on the surface and the surface will be charged negative with respect to the back plate 82. It should also be noted that at this potential, the electrons will not penetrate the layer 84 and there will be no electron induced bombardment conductivity effect within the material. This bombardment may come from the flooding gun 20 supplied with an erase pulse of negative 8 volts. After the erase pulse, the potential of the cathode 22 of the gun 20uis reduced to zero, and the electrons from the flooding gun cannot penetrate the storage grid .80.

The layer of material 84 must be of a material which exhibits the property of bombardment induced conductivity. These materials (such as barium fluoride, lead oxide, magnesium fluoride, arsenic trisulfide and antimony trisulfide) are normally insulators of resistivity of about 10 ohm cm. When the material is bombarded by electrons or other particles, the material becomes conducting if an electric field exists between opposite surfaces of the material. These bombarding electrons must be of sufficient energy to penetrate the layer and cause generation of charged carriers therein. These charged carriers are drawn to the electrodes due to the field and constitute a conduction current which may be .in excess of the current of the bombarding electrons.

In FIG. .7 there is "a typical plot of bombardment induced conductivity ratio in an insulating layer as a func tion of bombarding voltages for various potentials between the front and back surface of the layer 82.

In electron bombardment induced conductivity of materials, it is found that conduction starts in most insulating materials only when almost completely penetrated by the electron beam. The voltage at which bombardment induced conductivity begins in a given material can be determined primarily by .the thickness of the layer. In my specific embodiment it is desirable that the portion of the family of curves illustrated in FIG. 7 be used for primary voltage where the bombardment induced conductivity ratio is less than unity.

When the writing gun 30 is turned on, an electron beam produced by the .gun 30 will scan a raster on the surface of the storage grid 80. The electron beam from the gun 30 will be modulated with an input signal so that a charge pattern will be recorded on the storage grid 80 corresponding to the video signal received. The video signal may, for example, correspond to an image or scene scanned at another location by a device such as a pickup tube or a radar set.

The cathode 32 of the electron gun 30 is maintained at a negative potential of about 5100 volts by a battery 50 and the video modulation is superimposed thereon. The video modulation may have a voltage swing of 20 volts. The potential diiference of 5100 volts between the cathode 32 of the Writing gun 30 and the storage grid 80 causes the electron beam to write in the following manner. The potential of 5100 volts is of such a value that the potential is beyond second crossover of the material in the layer 84. If the layer 84 is of the type that does not have a second crossover, then the secondary emission ratio is still less than one. The secondary emission ratio beyond second crossover is less thanunity.

smegma rise secondary emission being less than unity causestnega; tive charging of the surface of the storage grid 80 and this may be plotted with respect to the bombardment voltage. This is indicated by the curve 106 'locatedbelo'w the abscissa of the graph shown in FIG. 8. The bombardment induced conductivity positive charging of the surface of the storage grid '80 is plotted with respect to the bombardment voltages and is indicated by the family of curves above the abscissa. The family of curvesare for diflerent values of voltage across the insulator storage grid 84. v

The curves of FIG. 8 provide an analysis of the current of the storage grid 80 as a function of the primary bombarding voltage. The curve 106 is plotted'to'indioate the charging current due to the secondary emission ratio of the surface of the storage grid 80 which tends to charge the insulator surface of the storage grid 80 in a relatively negative direction. At equilibrium potential the net current to the storage grid 80 must be zero. If one draws above the abscissa a mirror image of the curve 106 indicated by the broken curve 108 then the intersections of the broken line 108 with the family of curves 'represent the primary voltages'at which various equilibrium potentials of the storage grid 80' occur. By modulating the energy of the primary beam from the electron gun 30 it therefor possible to modulate-the potential on the surface of the storage grid 80. If theprimary beam from the gun 30 is both scanned and modulated, it is possible to write a charge pattern on the grid 80 and; to either add'or subtract information at any elemental location thereon. g

It is also possible to operate with the writing beam of a potential so as to be between crossovers. This allows a greater charging current and allows'faster' writing. This ,would tend to charge the surface in a positive direction while the electron bombardment induced conductivity component would tend to charge in a negative direction provided the backplate were'at a negative'potential with respect 'to thepoten'tial of the front surface'ofthe insulator. The operating voltages given are only for-the purpose of explanation and may vary by large amounts depending on the operating voltage applied to the storage grid 80.

In FIG. 9 there is shown a plot of the equilibrium target voltage derived from FIG. 8 as a function of the writing beam modulation. The modulation is applied to the cathode of the writing gun 30 and the tube therefor employs what is known as an equilibrium writing by means of cathode modulation.

It should be pointed out that the tube as described is not limited to just one writing gun but any number of writing guns may be employed. It may be desirable by time sharing of information or other features to subtract information from the storage grid with one gun that has been placed there by another gun. By the described storage grid operation it is possible to either add or subtract information by the same gun.

There are several structures that may be utilized to control the parameters of the storage grid 80 with a degree of independence from one another. It should be noted that the capacity per unit area of the storage grid 80 is determined by the effective dielectric constant and the thickness of the layer 84. The elfective dielectric constant is less for a sooty deposit or coating 110 of a material exhibiting property of electron bombardment induced conductivity illustrated in FIGS. 4 and than for a glassy one illustrated in FIG. 2 of similar material. In storage tubes a too-high-a-capacity requires excessive writing and erasing beam current while too-low-a-capacity results in premature positive ion discharging and therefore small retention time.

The secondary emission ratio of the insulator surface is determined by the bombarding voltage and the material. A coating 109 such as carbon soot may be used to vary secondary emission on the glassy coating 84.

This is illustrated in FIG 3. --A"sooty depbsit sometimes has a lower ratio of secondary emission than aglassy deposit of the same material illustrated in FIGS. 4 and 5. Where complete penetration of the layer84 ,orv 110 .by

the primary beam occurs, the eifective secondary emis- V and the potential gradientwithinthe material. It is also a functionof the presence or absence of scattering materials adjacent either of the surfaces as illustrated in FIG. 5. 4

-While I have shown my invention in only a few forms, it will be obvious to those skilled. in the art that it is not so limited, but susceptible-of various-other changes and modifications without departing from the spirit and scope thereof.

-I claim as my invention: I 1. An electron discharge tube apparatus comprising an apertured storage grid, said storage grid comprised of an apertured conductive back plate and a potential storage target coating thereon, means for applying apo tential to said conductive back plate, a first electron gun having at least a cathode for providing an electron beam for flooding the entire surface of the target coatingof said target, means for producing an electron beam potential on'said flooding gun with respect to said target surface to produce emission of secondary electrons of a less number than incident primaries so that said'target surface tends to charge negatively with respect to said potential applied to said back plate to assume apoten tial value more negative than said backplate: potential, a second electron gun having at least. a cathode forfgencrating an electron beam of elemental area, means for sweeping said second electron-beam in a -.point-to point manner over said target surface in a predetermined ner, means for producing an electron beampotential'on said second gun with respect to said target surface to produce emission of secondary electrons of a less number than incident primaries so that said target surface tends to charge more negatively with respect to said back plate potential, the electron beam potential of said second electron gun of a value so as to penetrate said target surface to cause bombardment induced conductivity within said potential storage target coating so that said target surface tends to charge positively toward the potential of said back plate, and means for modulating the electron beam potential of said second gun by electric signals to set up on said surface an electric potential record of said signals.

2. -A storage display device comprising a fluorescent screen, a first electron source for providing a beam of elemental area, a second electron source for providing a flooding electron beam, an apertured storage grid positioned between said screen and said first and second electron sources, said storage grid having a plurality of apertures therein and comprising a-conductive member with an apertured coating of material deposited thereon facing said electron sources, said coating of a material which exhibits the property of an insulator in the unexcited state and exhibiting the property of increased con ductivity when subjected to an electric field andelectron bombardment, said coating also providing a secondary electron emissive surface, means for biasing said second electron source negatively with respect to a potential applied to said conductive member to cause secondary emission of electrons from the surface of said insulator coating of a less number than the incident electrons to establish an electric field across said insulator, means for biasing said first electron source to a potential value nega tive with respect to said surface ofsaid coating to cause sccondary emission to charge the surface of said coating more negatively due to secondary emission and also due to the energy of the incident electrons penetrating said coating and the field impressed across said coating to cause said surface of said coating to charge in a positive direction and means for modulating the bias on said first electron source so as to cause the surface to charge in a negative or positive direction.

3. A storage display device comprising an image screen, a storage grid positioned adjacent said image screen, a writing electron gun disposed on the side remote from said image screen for providing an electron beam of elemental area, means for moving said writing beam in a point-by-point scanning sequence across said storage grid, a flooding beam for providing an electron stream flooding the entire area of said storage grid, said storage grid having a plurality of apertures therein and being comprised of a conductive apertured member with an 'i'tisulativecoating on the surface thereof facing said electron guns, said .insulative coating of a material to provide a secondary electron emissive surface and also exhibiting the property of electron bombardment induced conductivity, means for biasing said conductive member of said grid to a potential, means for biasing said writing gun negatively with respect to the potential applied to said conductive member to a value such that the electrons substantially penetrate said insulative coating and also of such a value that the secondary electron emission "from said surface is less than the number of incident electrons and means for modulating the bias on said 'writing gun to write a charge image on said surface correspondingto the modulation applied to said first gun and thereby controlling the electrons from said flooding gun bombarding said image screen.

4 4. In an electronic storage tube, said tube comprised of a perforated storage target having an electrical conductive layer at fixed potential and an insulating layer thereon, and an electron gun disposed on the insulating layer side of said target for scanning a predetermined raster thereon, the method comprising bombarding the iii surface of said insulating layer with an electron beam from said gun of a velocity to cause secondary emission from said surface and tending to charge said surface in a direction of higher potential difierence with respect to the potential applied to said conductive layer due to secondary emission from said surface and simultaneously to cause electron induced conductivity within said -in sulating layer tending to charge said surface in a direction of lower potential difierence with respect to said conductive layer to thereby obtain an equilibrium potential on said surface of said insulator and modulating the velocity of said electron beam to modulate the equilibrium potential on the surface of said insulating layer.

5. The method of operating a storage type tube, said tube comprising a perforated storage target having an electrical conductive back plate layer at fixed potential, said conductive back place having an insulating layer on one surface thereof, an electron gun for generating an electron beam disposed on the insulating layer side of said target and means for scanning said beam over said target point by point, said method comprising bombarding the insulating layer surface of said target with electrons of a given velocity from said electron gun to cause said insulating layer to seek an equilibrium potential depending on the charging effect of secondary emission from the insulating surface and an opposite charging efiect due to electron bombardment induced conductivity within said layer and modulating the velocity of the electrons within said beam to determine the equilibrium potential of elemental areas on the insulating layer surface of said target.

Knoll: Viewing Storage Tube With Halftone Display, RCA Review, December 1953, vol. XIV, No. 4, pp. 492- 501.

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