US2839679A - Half-tone memory tube - Google Patents

Description

June 17, 1958 v F. H. HARRIS HALF-TONE MEMORY TUBE 6 Sheets-Sheet 1 Filed May 16, 1952 FRANKLIN H. HARRIS BY Q6 6&7@-

ATTORNEY F. H. HARRIS HALF-TONE MEMORY TUBE June 17, 1958 6 Sheets-Sheet 2 Filed May 16, 1952 INVENTOR FRANKLlN H. HARRIS ATTORNEY 6 Sheets-Sheet 3 PR I MARY BEAM CURRENT R E Z INVENTOR FRANKLIN H. HARRIS BY 95M ATTORNEY June v17, 1958 Filed May 16. 1952 D E D. R 5 A S B O \II NMMM IO EOT I EBL T WDW T b EN( BAC E m NOC E E RRL ENE 2 F I .S FELD I `DEF Tcl. A T WV I I 0 NEN OIn V NGE OV ERM 2 a IUE I OOL PSE 4 T h h3 O O O h 2 I n .nv w 2 2 .T .T E .n Z hl Nz .aS-q ghomjm 0.". PZmDo .mz -I POTENTIAL DIFFERENCE BETWEEN CONDUCTING MEMBER AND HOLDING CATHODEJVOLTSI June 17, 1958 F. H. HARRIS HALF-TONE MEMORY TUBE l l 6 sheets-sheet 4 Filed May 16. 1952 @M 295mm d N. 295mm INVENTOR FRANKLIN H. HARRIS ATTORNEY June 17, 1958 F. H. HARRIS HALF-TONE: MEMORY' TUBE 6 Sheets-Sheet 5 Filed May 16, 1952 INVENTOR FRANKLIN H. HARRIS ATTORNEY F. H. HARRIS 2,839,679

HALF-TONE MEMORY TUBE 6 Sheets-Sheet 6 BY Q WL# ATTORNEY June 17, 1958 Filed May 16. 1952 sooo/LL AMP. IN.2

POTENTIAL DIFFERENCE BETWEEN CONDUCTING MEMBER AND GATHODE United States Patent Gflce 2,839,679 latented Julien, ross -This invention relates generally to cathode Vray storage systems, and more specifically to a cathode ray storage v system capable of recording, storing,'and reproducing, for an indefinite numberof repetitions, amplitude gradation information signals. This invention also specifically relates to an arrangement whereby a relatively high output current for reading purposes `may be 'obtained from a cathode ray storage system.`

Cathode ray tubes are well known in the prior art which will perform the sequential functions of an information device, namely recording, storage, reproduction and erasure of an information signal. These'electron discharge tubes operate upon the vprinciple of translating received information into charges and coincident potentials upon the surface -of VIan insulating material and subsequently recovering this information by exploring the surface with a focussed 'electron beam. As a 'result of the extremely high impedance offered by an insulator to the flow of current to and from a ,charged area, in the absence of a reproduction operation the charges and coincident potentials initially impressed upon the dielectric will persist without substantial deterioration over a period of time during which the information which they represent may be made available. When elicitation of the information stored Vis accomplished, however, by scansion of the insulating surface With an electronreading beam, the information Vbeing recovered either as a secondary emission current from the Vdielectric orV as capacitively induced current due toV a shift in potential of the initially impressed' potentials, in the prior art theV information bearing charges and potentials are progressively destroyed with vsuccessive repetitions of the reading operation. Thus in the prior art information reproduction can only be effected by destructive reading.

`In the copending application of Andrew V; Haefl', Serial No. 768,790 led August l5, 1947, there is taught as a departure from the prior art, the employment of a holding Abeam which counteracts the tendency of thev charges and coincident potentials to deteriorate because of destructive reading. Briefly, in the tube disclosed by Haeif a pattern of charges and potentials is impressed upon a continuous dielectric surface, each charge and its coincident potential representing a single resolvable information item. Thereafter the surface of the dielectric is exposed either continuously or intermittently to a holding beam of electrons. This holding beam in falling' on the dielectric target has a first ephemeral action ofY driving individual areas, coextensive with the initial charges representing information items, upwards or downwards in potential to one of twopotential limits, the course taken corresponding to the sense ofthe initial potential of the area as measured relative to la critical potential. The holding beam also has a second enduring action of stabilizing the information pattern as so modified by continuously regenerating the charges and potentials at the respective limits to which they have been driven. Binary information onceV established upon the Surface of the dielectric may thus be reproduced an un` 'teristics of the' Vtarget under bombardment by Va holyi 2 Y limited number of times without destruction of the pattern, the tendency towards deterioration occasioned `by the action of the reading beam vbeing counter-acted by the self restoring actionv of the holding beam.

Since each charge represents a single vresolvable item of information, andv since each area loccupied by an initial charge is driven by the holding beam to one 'of two potential limits, the Haef device is not yadaptable"to the faithful reproduction o f received information signals representing intermediate tones between black and w te without special expedients seriously limitingtheamiint of information handled; Y

Further, 'in the prior art devices the reading b.

m Produces a shift Vin potential of the information a as scanned, during the course of which'shift a utilizable reading current of secondary emission electrons isprof duced. ln these devices, however, the magnitude 'of reading current obtained Ais onlyv a relatively smalll per? centage of the primary current of the reading beam.

It is `therefore an object of this invention to provide an arrangement whereby an amplitude gradation informa-V tion signal may be stored in a cathode ray storage device and may be reproduced therefrom an unlimited number of times. It is a yfurther object of this invention to provide arrangement whereby in a cathode ray storage ydevicea reading output current may be obtained vof .relatively high magnitude compared to prior art devices.r :Y

It is a further object of this invention to providestolfage targets for a cathode ray storage device whereby'received information signals having amplitude gradatioiis may be stored and be repetitively reprdLICed, Without deterioration, by va relatively` high magnitude output Vcur! rent compared yto prior art devices. i It is a further object of this invention to provide a' method whereby amplitude gradation information sigl' may' be recorded, stored and" repetitivelY Ieprod'uA d without deterioration at a relatively high magnitudeo'ut-` put current compared to prior art devices. Other and further lobjectsand features of the present invention will become apparent upon a careful consideration' of the following description when taken together with the accompanying drawings which illustrate tyl? l features of the invention and the manner in which invention may' Abe considered to operate.

In the drawings. Figure l is a diagrammatic representation of an` illustrative embodiment of the present invention and operating circuits therefor; I v

Figure 2 is an enlarged representation of a portionof the surface of an illustrative type of target, utilizing'a back plate, as viewed from the cathode end of thefstor'age tube;

Figure 3 is an enlarged representation of a portion of the surface of a Vvariant type of target, utilizing 'a rn V h l and Figure 3a is a cross'sectional view of Figure; Figure 4 is 4a graphical representation of Vthe ,current characteristic of an isolated dielectric particle .under bombardment of a` beam Vof electrons; Figure 5 represents graphically the current characng beam of ,electrons at various selected voltages;

Figure 6 is a representation, as shown by a greatly enlarged cross sectional view of a portion of ya mesh ,type target, of electrical conditions in the vicinity `of Vthel target `surfaceifor various holding voltages; *i l Figure 7 is a greatly enlarged elevation o f a dielectric cell embedded between the wires of a mesh type target, and is illustrative of the different capacitance zonesv within a single dielectric cell;V

Figure 8 is a cross secti rralyiew of Figure 17 tak n -Figures 9-10' are"graphical representations of modes ofoperation of the tube to produce negative writing;

Figure 11 is a graphical representation of a mode of operation of the tube to` produce positive writing; and Y Figure 1'2 is a representation, as shown by a greatly enlarged cross sectional view of a portion ofthe mesh type' target, ofelectrical conditions in the vicinity of a target surface produced by a reading beam.

Briefly Vthe objects of invention set forth above are achieved by employing in la holding beam type of storage discharge device a storage target having, by'way of contrast tothe homogeneous exposed target faces utilized in theprior art, a heterogeneous face exposed to the electron`beams,"this face consisting of a member of electrically 'conducting material and a myriad of separated dielectric masses `dispersed throughout the surface of the conducting memberV in sufficient concentration so that a considerable number of `these dielectric masses appear within. an area approximating in dimension the spot of a focussed electron beam. As 'will be explained hereafter, by virtue of this arrangement potentials, representing information items, when once impressed upon the mentioned areas by the writing beam will not be'altered in their values by the action of the holding beam and yet will be retained thereby. Amplitude gradation informationitems may thus be stored for an unlimited period and be reproduced an unlimited number of times. Further, by virtue of this arrangement, reading signal currents may be obtained without disturbance of the information pattern and having magnitudes approximating the value lof reading'beam primary current.

lReferring now to Figure 1 there is shown an electron discharge storage device or tube generally designated by 20,V comprising an evacuated envelope 21 of glass brother material, with an after section 22, midsection 23 land forwardsection 24. Positioned in the rearward region of after section22 and within envelope 21 are located three cathodes, namely writing cathode 25, holding` jcathode 26 and reading cathode 27, the aforementioned cathodes 'being capable of heating to an electronemitting temperature by means not shown. Cathod'es 25,"26 and 27 are separately connected to 'the negative terminals of variablevoltage supplies 28, 29 and 36, respectively, lthe positive terminals of which are conriected to ground. Cathodes 25, 26 and 27 are thus always maintained, with respect to ground, at a negative adjustable direct voltage. Holding cathode 26 in addition may have an alternating voltage impressed thereon by closure of a switch 31 which completes a connection to alternating signal source 32.

'Y In alignment `with and forward of writing cathode 25 in the order named, in mutually spaced relationship are positioned a grid 33, and accelerating and focussing assembly34 and a deliecting assembly 35, `the aforementioned elements comprising a conventional electron gun. Portions of assembly 34 are connected to `the maximum potential terminal 35 4of a high voltage supply 36, 'the negative terminal 37 of which is connected to ground. Assembly 34 is vthus maintained at a positive potential with respect to ground. Electrons emitted from writing cathode 25 are resolved by accelerating and focussing electrode assembly 34 into la writing beam of small cross sectional diameter and high intensity, the intensity of which may be modulated by information signals, particularly of the amplitude gradation type, originating with an information signal source 38 and impressed upon grid 33 in the form of varying voltages. During passage through deflecting assembly 35 the writing beam may be subjected to further control, this time as to assumed direction, by voltages originating with horizontal sweep s-ignal source 39 and vertical sweep signal source 40 and impressed upon horizontal deflecting plates 41 and verti-` cal deecting plates 42 respectively. By proper selection andtsynchronization of the horizontal andvertical sweep slgnals, the writing beam may be calnised to trace `out desired scansion patterns upon a cross sectional area of 'forward"section 24; One scansion"`pattern found suitincremental vertical step at the left hand pattern border,

preparatory to sweeping out the next odd horizontal line.

Similar to the arrangement for the writing beam, in alignment with and forward of reading cathode 27 in the order named, in mutually spaced relationship are positioned a grid 43, an accelerating and focussing electrode assembly 44, and a deecting assembly 45, the elements mentioned forming another conventional electron gun. Accelerating and focussing electrode assembly 44 is Amaintained at positive potential with respectA to ground `by a connection to maximum potential terminal 3S. Electrons emitted vfrom reading cathode 27 are resolved 'by assembly 44 into a reading -beam Iof high intensity and approximately the same cross sectional diameter as the writing beam. `l`he reading beam thus produced may be deected in directionby signals originating with horizontal sweep signal sour-ce'46 and vertical sweep signal source 47 and ,impressed as voltages upon horizontal deflecting plates 4S and vertical deflecting plates 49 respectively. Thus, by selection of proper horizontal and vertical sweep signals the reading beam in a manner similar to the writing beam may be caused to sweep out desired scansion patterns on a cross sectional area of forward section 24. Further, the intensity of the reading beam may be controlled by voltages limpressed on grid 43. `In the present embodiment grid 43 may be connected Vby two position switch 50 to cathode 27 for ordinaryl reading or alternatively to radio frequency signal source 51 for a purpose to be later more fully described( Turning to a consideration of the holding beam electronV gun, in 'alignment with and forward of holding cathodeV 26 in the order-named, in mutually spaced relaltionship are positioned a Vgrid 52, an accelerating anode 53, al focussing electrode 54Y and another accelerating anode 55;' Anode 53, electrode 54 and anode 55 together resolve electrons emitted from holding cathode 26 into a [holdin-g beam. Focussing electrode '54 is maintained at a negative potential with respect to ground byr a` connection Vto cathode 26. Accelerating anodes 53 and 55 are maintained at `a high positive potential with respect Vto ground by connect-ions to maximum potential terminal 35. Electric elds are thus established between focussing electrode `54 and'n accelerating anodes 53 and 55, which fields in effect form a powerful electron lens. The converging action of this lens produces a cross-over and subsequent wide divergence of electron rays originally mildly diverg-ing from the holdingubeam axis. As a result, the holding beam electrons in forward section 24 are uniformly diffused at a Ilow area density across a major portion of the tube bore.

The holding beam intensity may 'be controlled by the magnitude of voltage applied to grid 52. In the embodiment shown, by means of two position switch 56, grid 52 may be either directly connected to cathode 26, or may be connected to `the negative terminal of bias supply 57, the positive terminal of which is connected 'to cathode 25. The lformer-and latter connections produce on and off conditions-for the holding beam respectively.

For convenience and space economy the writing, holdi ing and reading electron guns may be supported in a tri- Collector `electrode '58,rn; 1ybeA conveniently composed` of .a layer ofconduotingmaterial,l typicallyaquadagdeposited on the inside surfaceof envelope 21. For proper shielding, collector electrodeis. maintainedat a' positive potential with respect to ground by a connection to terminal 35 of high voltage supply 36.

Coaxially spaced forward of collector electrode 58 and insulated ltherefrom is located a similar ring or collector electrode 59 of smaller axial extent. Ring 59, which also may be conveniently composed of a layer of aquadagl deposited on envelope 2l, is maintained ata lower potential to ground than collector electrodef by a connection to a median terminal 6i) of high voltage supply 36.` Ring 59 performs the function of parallelization of the rays of the various electron beams passing therethrough, with the result that each ray :of a given electron beam impinges at the same angle, upon a subsequent cross sectional area ofthe tube.

Within section 24 and forward ofring 59, a plurality of members 6de cooperate to support a target 61. concentric with the axis of tube 20.` Target 61 is charac# terized by a substantially 'planar face` exposed to the electron beams, which face is normal to the tubevaxis, and occupies the larger part of thetube bore atthat location. The entire `area ofthe target face is subject to bombardment by the electrons ofthe diffused holding beam.

Considered from the point of view ofits electricalproperties, target 61 is constituted of two distinct portions, namely `a flat, continuously-connected, conducting member 62 coextensive with the target 6l` itself, and an' ag; gregate of dielectric material appearing as a multitude of minute, mutually separated dielectric massesdistributed throughout the exposed target face. While Figure l shows dielectric masses 63 Vdeposited upon conducting member o2, alternatively the masses may partially oc'- cupy interstices in conducting member 62. Regardless of the particular structure adopted, the eiective facing of the target as viewed by the electron beams presentsa heterogeneous appearance, being a composite'jof a myriad mutually spaced `dielectric material exposures ntermingled with exposures of the `conducting member.

Conducting member 62 of target 61 is connected by lead 64 to one end of an impedance 65, typically a resistance, the other end of which is` connected to ground. Under bombardment by the various electron beams conducting member 62 suffers a net loss or enjoys a net gain of electrons.

ln the case of a net gain of electrons a negative current (equivalent to positiveVV electron ow) will be produced, flowing from ground through impedance 65 and lead 64 to conducting member 62, alongthe particular impinging beam (in a direction opposite to electron 'movement) to the cathode from which the beam emanates, through the associated variable voltage supply, and back to ground. In the case of a net loss ofy electrons from secondary emmission, some ofthe secondary emitted electrons will travel from the surface of conducting member 62 to the more positive collector electrodes 58 or S9. A positive current thereby will be produced owingV from conducting member 62 through lead 64', impedance 65, high voltage supply 36, collector' electrodes 5S `or 59, and back to conducting member 62' alongV a path opposite in direction to that of the secondary emitted electrons. With either type phenomenon, coeval with the ilow of current .a voltage will `be created between the upper terminal of impedance 65 and ground. When the target face is subjected to the scanning action ofl the writing or the reading beam, the voltage generated thereby' across impcdance 65 is of a lluctuating nature which may be coupled to utilization device 66 by a condenser 67 connected between the input of device 66and the ungrounded side of impedance 65. When Vdesired, for a purpose to be later more fully described, by closure 4of switch 68, a filter 69 tuned to the frequency of radio frequency signal source 51 may be connected betweenthe input of utilization device 66 and ground.

It will be understood of course that electromagnetic:`

Ground to terminale35 r +1200 volts.`

Ground to terminal 60 |200` volts:

Ground to cathode 25l -600 Volts.-

Ground to cathode 26 -190 to 210 volts:

Ground to cathode 27 19040-210 volts.

Writing beam current at target-- l().microamperes.l

Reading beam current at target l-S microamperes:

Holding beam currentiat target-.. microamperes-f.

Size of focussed reading and n i writing beam spot ortargeL- 40 milsV diameter (.04

inch).

,ConsideringV now in more detail the target 61 ofelec tron discharge storage device 20, Figure 2` representsin' greatly enlarged cross section an elevation of a` plate type' of target :as viewed from the` cathode end of tube 2li.I That portion of Figure 2 designated as 62a representsav member comprised ofV electrically conducting material', the member having a substantially Yplanar viewed surface. Ir'ronev variety of plate target, member 622i consists of a sheetV of mirror finished-v aluminum, thi'sf'particul'ar rna-te;'Y

rial being selected because ofY its high ,andA stablefsec'iond-ary emission characteristics.Y tion of the information pattern. impressed uponY the target Y is desired, alternatively member 62a may -consistofa thin, transparent, conducting film, producedupon a transparent: glass backing, for example,A asdiselosed in Patenti 2,064,369 granted to O. H. Biggs on December 15, 193.6.

Regardless of the composition of member 62a, those portions of Figure 2 designated. as63a represent. massesof dielectric material taking the'form ofrandonr size-.para

ticles evenly distributed, in adhering relationship,- overIk the surface of member 62a, in sufficient. scarcity of num-- ber so that most of the particles are mutually separated. Within the aggregate, the individual particles of dielectric vary in size below an upper limit mean diameten;` typically 10 microns. Because of their smallY size. and close proximity many hundred dielectric particles are-lcon centrated in a given focussed beam spotarea. Particles 63a m-ay be composed of any suitable dielectric material;

In practice a phosphor having the chemical formulaV (ZnSzAg) and designated by the Radiov Manufacturers. Association as P-ll has been found to be satisfactory.

The plate type target described above may be'manuf factured in the following manner. The conducting. member 62a is first chilled below the dew point so thatV a thin film of water condenses on itsv surfage. Member' 62a is then placed in an enclosed space into which is. blown a quantity of dielectric particles 63a` which scatter to form a nely dispersed cloud. The airborne particles are allowedv to settle upon the surface of the conducting,- member 62er untilV a proper concentration has been reached, at which. time member 62a. is removed and the. water lilm hitherto clinging to its face is driven oli". During the evaporation of the film a capillary elect is produced which capillary action forces the dielectric particles into intimate contact with member 62a, thus causing the dielectric particlesto adhere strongly'to member 62a.

While the'plate type of targetthas'proved satisfactory in the 'practice of the present invention, as a result ofthe random sizeA and unpredictable distribution of the Vdielectric particles within a unit'area, which term is used y to designate a subdivision of the target spaceaequalrinl WhereY visual obser-va-v 7 ties of `a unit area 'of target face vary'slightly from one unit area to another. Thus'from the electrical point of view, the surface of the target is to a minor'degree irregular, which roughness may inject a certain amount of noise into the performance of the storage device.

Referring now to Figure 3, the figure represents an enlarged elevation, viewed from the cathode end of tube 20, of a portion of a mesh type target, which mesh type target by Way of comparison to the plate type target hitherto described,-exhibits, electrically speaking, a face comprised of highly uniform unit areas. Conducting member 62b in this case comprises a finely wovenfmesh, typically 230 meshes to the inch, of a material having good secondary emission characteristics, typically lstainless steel. In contrast to the usual mesh structure wherein the strands, viewed edgewise, follow sinusoidal paths, as is more clearly shown in Figure 3a, the wires of my mesh, viewed edgewise, extend in a straight line except in crossing point vicinities where they are exed under an intersecting wire. All of the wires are thus for the major portion of their length tangential to a plane surface lying above the mesh.

, The interstices of the woven mesh are partially or fully closedby a coating of particles of dielectric material, the sum of which particles within an interstice will hereafter be denominated as a dielectric cell. Cells 63h occupy spaces within the interstices preferably below the level of the aforementioned tangential surface. VThose parts of Figure 3 designated as 64b represent cavities in the dielectric cells, the cavities being produced incidental tothe manufacture of the target. The presence or ab sence-of cavities 64b is a matter of indiierence, since dielectric material if located in the central portion of the cell would have a negligible effect upon the operation of the device.

. While dielectriccells 63b are considerably larger in size than dielectric particles 63a shown in Figure 2, the cells 63b having a typical dimension along a side of 2 mils, the cells are still suciently small that a considerable number, typically 67, will be centered within an information unit. As a result of the orderly array of cells within the interstices of the woven mesh the electrical properties of the target face, considered in terms of information units, will be uniform.

In practice a satisfactory mesh target has been made from a 230 mesh stainless steel screen with its interstices lled by (Zn2SiO4zMn) phosphor, designated by the Radio Manufacturers Association as P-l.

' The mesh type target may be manufactured in the following `manner. An ordinary nely woven mesh is passed between two polished rollers which atten its surface. A suspension of dielectric particles in acetone, having dissolved therein a plastic acetate binder, is then sprayedragainst the back side of the mesh at an acute angle to its plane. By depositing the dielectric material on the screen from an acute angle, the dielectric particles build up within the interistices rather than accumulating upon the back surface of the wires where their presence would be ineffective. During Vthe spraying action the mesh is rotated around an axis normal to its surface to insure an even dispersion of dielectric material. The dielectric particles accumulated in the interstices will be bonded together and to the wires by the acetate binder upon evaporation of the acetone. The front side of the mesh is utilized as the storage surface of the target.

Turning now to principles generally applicable to electron discharge storage devices, it' is obvious that the smallest subdivision of a target face capable of resolution by a focussed electron beam, and also the subdivision at which resolution inevitably takes place, is an area of the target face equal to the spot size of a focussed electron beam; which area has hitherto been described as a unit area. To the primary electron beam each unit potential whichmay be termed respectively as a mean charge and a mean potential, and which 'have the same effects Vas uniform charges and potentials distributed evenly throughout theaun'it' area. While the charges and potentials throughout a given unit area willappear to the electron beam as a homogeneous entity, diiferences between mean potentials successively scanned are easily detectable. It is thus seen that each mean potential displayed by a unit area'represents a Ysingle and separate information item.

It willalso be obvious that if within each unit area the storage surface is electrically irregular a number of subareas may exist within each unit area relatively heterogeneous in charges and potentials, although this variation is not seen by the electron beam. In such a case the resolvable mean charge'and coincident mean potential of a unit area are the resultants, of the plurality of subarea charges and potentials within the unit area.

For purposes'of Vclear distinction in terminology, as to this tine -level of phenomena which actually exists though it is rresolvable to the electron beam, the sub areas will be termed elemental dielectric portions, the charges and potentials assignable to each portion, the simple charges and potentials, and the entirety of the various simple charges and potentials throughout the storage surface as the charge and potential patchwork` respectively. By way of comparison, analogous features at the level of phenomena resolvable to the electron beam are the unit area, means charge, mean potential andthe charge and potential patterns. Transposing these latter mentioned features into terms of information which they represent, the storage surface acts as an information accumulator consisting of a bank of information units, each of which exhibits an information item, the conguration of these information items being an information pattern. i

Considering more specifically the modes by which th present tube may be considered to operate it is necessary to consider the phenomenon by which information initially established on the target may be indefinitely preserved. Figure 4 represents graphically the variance of current flowing to the surface of an isolated elemental dielectric portion in dependence upon Vthe potential between said portion and a bombarding electron source. Since' the elemental dielectric portion is specified. as isolated, the values of the current curve of Figure 4 do not include the current effects resulting from the interactions between adjacent dielectric portions of thc target face, which interactions practically occur. Primary electrons emitted from the source and absorbed by the dielectric surface are considered to contribute a negative current to the dielectric portion. Secondary electrons, produced by bombardment of the surface by the primary beam and escaping from the'surface to collector electrodes 58 and 59 or to conducting member 62, are considered to furnish a positive current to the dielectric portion. The ordinate of Figure 4 thus represents the sum of two simultaneous currents or the net current to the surface of the dielectric portion. The abscissa of Figure 4 represents the potential of the bombarded dielectric portion with respect to the electron source which is considered to be at zero volts. Point c on theabscissa of Figure 4 represents the state of the conducting member of the target, which member is maintained atVc volts.

Assume now that an elemental portion of dielectric can be given at will any potential along the abscissa of Figure 4, and that the portion is bombarded for a momentary interval of time with electrons vfrom the source, which electrons necessarily have an energy equal to that of the impressed potential. As the potential of the portion is raised positively from zero volts, initially primary electrons will be absorbed by the dielectric surface, resulting in a negative net current as shown by the current curve between points'h and a. Bombardment of the dielectric surface by the` primary beam electrons, however.

produces secondary emission electrons, the numbers of which increase at a faster rate with the increasing potential than the number of primary electrons absorbed. At point a on the curve conditions are such that for an incremental increase of potential, the additional secondary electrons escaping from the surface as a result of the increase, exceed in number the number of additional primary electrons absorbed.

The current curve thus becomes positive going, crossing the zero line at o, `at which point the total number of secondary electrons escaping fromv the surface of the dielectric equals the total number of primary electrons absorbed by it The potential V at which cross-over point o occurs is known as the critical voltage, and it is determined only by the type of dielectric material.

When the potential is increased somewhatbeyond the value of V0, the current curve enters the positive region in which the number of secondary escape electrons exceeds the number of primary electrons absorbed. From point o up to a point b the current curve is still positivegoing. At point b, however, the net current starts to decrease with increasing potential. The explanation for this change of trend is as follows: For potentials below point b, secondary electrons once emitted from the dielectric surface are drawn away from it to the higher potential rings S or 59 or the higher potential conducting member of the target. At point b, however, the potential of the dielectric portion is approaching the potential of the conducting member. Conducting member 62 begins to exert a suppressive effect upon the emitted secondary electrons. As a result considerable number of secondary electrons instead of escaping the rings S3 or 59 ultimately fall back on the dielectric. As to these electrons their current contribution to the dielectric surface szero. Another factor in limiting the positive equilibrium voltage of the dielectric may be the capture, at low incident velocity, of secondary electrons from member 62.

Beyond point b, the net current to the dielectric surface rapidly decreases as more and more secondary emission electrons return to their point of origin. With a rather small potential increase beyond point b the current curve again crosses the zero line at a point p where the number of completely escaping secondary electrons just balance the number of primary electrons absorbed. If the potential is again increased slightly beyond point p the current curve goes downwardly into the negative region until a point d is reached where substantially all the secondary electrons emitted by the dielectric surface are recaptured by the surface. Beyond point d, the current curve levels olf in the negative region, the value of the current curve being equal to the value of primary electron beam current impinging on the dielectric surface.

So far it has been assumed that the primary electron beam plays upon the surface of the elemental dielectric portion for just a momentary interval; consequently the current conditions heretofore portrayed have been characteristic only of this momentary interval. It will now be necessary to consider the effect of the electron beam upon the dielectric portion for a longer period of time.

Dielectric materials are characteristically insulating materials through which conduction currents will not flow. When, therefore, by the bombardment of a beam, electrons are gained by or lost from the surface of a dielectric portion, the resulting electrical u'nbalance cannot be restored by an offsetting current flow to or from the conducting member. The effect of an electron 'gain or loss upon the dielectric surface is thus quasi-permanent, manifesting itself respectively as a negative or positive charge and an accompanying negative and positive potential upon the dielectric. Further if the exposure of the dielectric surface to an electron beam is 'a continuous one, the acquisition of charge and potential upon the dielectric surface due to gain or loss of electrons, becomes a cumulative process, each incremental charrge deposit of a given sense causing an incremental change in 'potential' of the surface in the same sense which, by its effect on the beam, in turn increases in the same sense the next incremental cha'rge deposit on the dielectric portion. Obviously because of this accrual feature characteristic of the interaction between the dielectric surface and the beam, ultimately the surface will be driven to a state of equilibrium with the beam such that the net electron current is zero.

Referring again specifically to Figure 4, assume that an isolated elemental dielectric portion is given an initial potential between h and o. The net current to the dielectric surface in this region is negative in character representing an absorption of electrons by the surface. Exposure to the beam for a first interval of time thus producesV a negative charge and accompanying negative potential on the surface. The negative potential once established reduces the velocity of bombarding primary electrons, cutting down the number of secondary electrons emitted. As a result, duringv a second interval of time, under exposure to the electron beam, the dielectric surface accrues an even larger negative charge and accompanying negative potential. The sequence just described occurs continuously throughout a span of time with the result that the dielectric surface is driven downwardly in potential as shown by the arrow S1 until point h is reached. At point h the dielectric surface is substantially at the same potential as the source of bombarding electrons. No electrons will reach the surface from the electron source, with the result that at the surface there will be no electron loss or gain. Point h therefore represents a state of equilibrium of the elemental dielectric portionl with the electron beam. Very minute currents from positive ion bombardment have not previously been considered, but they restrain the dielectric from going more negative than point h.

In a similar manner where the elemental dielectric portion is given an initial potential between o and p, under continued exposure of the electron beam, positive charge and accompanying positive potential will build up on the dielectric surface. The dielectric surface will thus be driven upwardly in potential as shown by arrows S2 and S3 until point p is reached. At point p the voltage Vp between the elemental dielectric portion and the source of electrons is high,A typicallyv about two volts above the potential of the conducting member Vc. Primary electrons from the source thus will strike the surface at a high velocity, being absorbed thereby, and producing' by their collision with the surface a large number of secondary electrons. 0f the secondary electrons emitted, however, only that number equal to the number of irnpinging primary electrons will escape to the higher potential collector electrodes 58 or 59. The excess of emitted secondary electrons will be recaptured by the dielectric surface because of the suppressive effect exerted by lower potential conducting member 62 upon electron escape. Capture at low incident velocity of secondaries from member 62 may also depress the potential of the dielectric. Above point p, the surface would be urged back towards point p as shown by arrowv S4, by recapture of an excessive number of 'secondarily emitted electrons. Point 'p therefore represents another state of equilibrium of theA elemental dielectric portion with the electron beam.

Byrway of summary it can be said that in an electrical discharge'devic'e of the type described, the surface of an elemental dielectric portion under continued exposure' to an electron beam will be driven to one of two stable potentials, the alternative state assumed having the same sense as the initial voltage of Vthe dielectric portion,l

measured from a midway critical potential. y

Turning now to the practical application of the phenomenon 'outlined above,V Figures 5 and 6 will be considered. FigureS graphically portrays the current characteristics of a dielectric portionV under the action. of an adjustable potential holding beam. Figure 6 pictorially @essere represents electrical conditions at a target fa'ce subjected to this holding beam.

Referring to Figure in more detail, the abscissa of the graph represents potentials, measured (in contrast to Figure 4) with reference to the conducting member of the target. The ordinate of Figure 5 represents net current to the dielectric surface in a similar manner to the ordinate of Figure 4. Figures 9, and 1l will use the same conventions.

Assume now that a voltage V113 for the holding cathode 26 has been selected which voltage V113 is known to produce unlimited persistence. Assume further that the target face has been erased in such a fashion that all the elemental dielectric portions of the face have dropped to a potential at the point h3. A positive-writing beam, intensity modulated by amplitude gradation information signals is next caused to scan the surface of the target. As a result individual dielectric portions are raised to various potentials. The potentials of individual dielectric portions are thus scattered in a range fromV point 113 to point p. Subsequent to the traverse of the writing beam, the entire target face is ooded, as may be seen in Figure l, with holding beam electrodes. Under the exposure of the holding beam at voltage V113, portions of dielectric having a potential to the left of point o3 will be driven down in potential to point h3. Conversely portions of dielectric to the right of o3 rise upward in potential to point p. The separate dielectric portions having reached these states, the pattern so established is continuously regenerated by the holding beam, since any drift from the two states induces an immediate selfrestoring action. By the medium of a holding beam therefore, a two potential patchwork may be unlimitedly preserved even in the presence of a reading beam whose action tends to deteriorate the patchwork.

Conditions at the target face for unlimited persistence of the two potential patchwork are pictorially represented in section 6b of Figure 6. For Figure 6 as a whole, numbers 62b designate individual Wires of a mesh type target and numbers 63h designate masses of dielectric in cross section view, which masses within the mesh interstices form dielectric cells. By means to be later more fully described, the surfaces of portions 70 of the cells 63h are negatively charged and the surfaces of other portions 71 are positively charged. Secondary emission from 6211 is not shown in Figure 6.

In the section 6b of Figure 6, electrons emitted from the holding cathode `and following path 72, strike wire 6211l of the conducting member with a velocity of V113 electron volts. Since the surface of portion 70 is at a potential corresponding to the point 113 in Figure 5, electrons following paths 73 on entering the region of the target face will be repelled away from portion 70. Portion 70 in Figure 6b thus remains at point h3, neither gaining nor losing electrons. Primary electrons following paths 74 will be attracted ,to portion 71 having a potential at point p, the electrons striking the surface of portion 71 at high velocity and being absorbed therein. As a result of the collision between the primary elecirons and the surface of portion 71 a considerable number of electrons will be secondarily emitted from portion 7l, the secondary electrons following paths 75 as shown. Of these secondary electrons following paths 7S a certain fraction will escape to collector electrodes 58 and 59, shown in Figure 1, this fraction of escaping electrons being just suicient in amount to balance the number of primary electrons absorbed by the surface of portion 71. The remainder of the emitted secondary electrons will be recaptured by portion 71 as shown by dotted path 76. Portion 71 therefore remains at point p, neither gaining nor losing electrons. The information represented by the charges on 70 and 71 is permanently retained.

l. Il

4 electrons.

Returning .to a; *consideration of Figure 5 it would appear from the above discussion `that the voltage of the holding cathode 26 shown in Figure l could be altered by variable voltage supply 28 within fairly wide, albeit reasonable limits, with an indefinitely prolonged holding action still resulting. lf for example the voltage of the holding cathode is changed from point 113 to point h1 the previously outlined sequence of events seems to apply. as properly in the latter case as in the former. The above discussion however ignores the interactions between portions 70 and 71, which interactions are significant as limiting factors on the practical range of variance of the holding cathode voltage productive of unlimited persistence.

If the voltage of the holding cathode is adjusted down- Wardly to point h1'(i. e. there is an increase in potential between the holding cathode and the conducting member) an occurrence known as positive spread results.

kConditions amicable to positive spread are shown in section 6c of Figure 6. Portion 71 is still at the potential of point p of Figure S'but portion 70 is now at the low potential of h1. As a result a rather strong electric lield exists, running between portions70 and 71. This cross field has a iirstieffect of deflecting electrons following path 77 away from the outlying surface of portion 70, upon the margin of which the electrons normally would replenish the negative charge, and towards an adjacent part of portion 71 where the electrons build up the positive charge. If the negative charge upon the margin of portion 70 is not replenished, the voltage of the margin drifts upwards due to the occasional arrival of positive ions which deposit positive charge. Thus since positive charge is gained on one side and negative charge lost on the other the boundary between portions 70 and 71 creeps towards the latter.

As a second effect, since a higher holding voltage exists, namely V111, the voltage gradient of the electric lield across the boundary between portions 70 and 71 is increased as compared to Figure 6b. If the gradient becomes strong enough, in the vicinity of the boundary, by a mechanism known to the artv as eld emission, electrons, representing a negative charge loss, will be pulled out of the surface of portion 70, as shown by path 78, and will be transferred over to portion 71. A shift of the boundary line at the expense of portion 70 ensues. i Conversely if in Figure 5, the voltage of the holding cathode is adjusted upwards to point 115, an opposite condition to positive spread, namely negative spread, will be produced. The nut-:chanismV of negative spread may best be understood by reference to section 6a of Figure 6. Primary electrons in the holding beam as they bombard the target are moving with the rather slow velocity of V electron volts, thus producing, as compared with the situation in 6b a low number of secondarily emitted Where electrons following path 79 fall upon the margin of portion 71 near portion '70, the negative charges on portion 70 suppress secondary emission to the extent that insufficient numbers of secondary electrons escape to maintain positive equilibrium. These primary electrons following path 79, therefore, produce a negative rather than a positive charge upon the margin of dielectric portion 71 adjacent to portion '70. The area of negatively charged surface is thus gradually enlarged, portions 70 encroaching upon portions 71.

Because of the occurrence of positive and negative spread, for unlimited persistence the satisfactory range ot variance of holding cathode voltage is rather narrowly prescribed. A proper holding voltage for unlimited persistence is represented, in Figure 5 by point 113, the current curve for which contains approximately equal areas under the curve for the negative and positive regions of net current tothe surface of the dielectric. For a variance of approximately plus or minus 5% from point h3, as represented by points 114 and h2, unlimited persistence @essere may still be maintained. BeyondY the limits demarcated by points h4 and h2, the information pattern on the target face deteriorates over a period 'of time. I'he rate of deterioration, however, may be'. regulated in accordance with the adjustment of the holding' cathode voltage In certain applications icontrollable persistence of this' sort may be desirable as for example where successive radar echoes are retained upon a screen to indicate the velocity of an observed target.

Referring again to Figure l, commonly for operation of the presently described information storage device, the holding beam is maintained in a' continuous' on condition by a direct connection of holding gridv 52 to holding cathode 26 through switch 5'6. For certain modes of operation, however it may be desirable to turn off the holding beam. This may be accomplished by the connection of grid 52, by means of switch 56, to holding cathode 26 through bias supply 57.

The above decsribed' holding phenomenon is characteristic both of systems capable of storing binary information only and of a system which additionally may indenitely store amplitude gradation information. It is now necessary to consider the mechanism of the present invention by which amplitude gradation information may be utilized.

Turning specifically to the features which permit retention and reproduction of amplitude gradation information signals, Figure 7 is a greatly enlarged elevation, viewed from the cathode end of the tube of a single dielectric cell 63b Within aninterstice of the mesh type target. For the purposes of illustration the body of the dielectric cell 63b may be thought to be divided into a set of elemental dielectric portions 80, 81`and 82, each portion being a Volume ha'ving' one surface contiguous to wires 6217 and vone surface a subdivision of the entire exposed surface of the dielectric cell. The mode of division selected may be more clearly seen by reference to Figure 8. Portions 80, 81 and 82 are so selected that their suriicial areas are equal.

It will be obvious to those familiar with the art that each of portions 80, 81 and 82 may be considered as anv elemental capacitor, wires 62b and the exposed surface of the portion being analogous to the plates of a condenser separated by a layer of dielectric. Since the exposed surfaces of portions 80, 81v `and 82 have equal areas, the values of capacitance for portions 80, 81 and 82 are dependent only on and are inversely proportional to the mean spacings mb, mil and" mi2, respectively, between the opposed surfaces of the portion. Portion 82 therefore is equivalent to a smaller elemental capacitor than portino 81 which in turn is equivalent to a smaller elemental capacitor than portion 80. For the purposes of the invention it is Adesirable that the surface capacity per unit area vary relatively continuously over a wide range. In Figure 8, the surface capacity of portions 80, 81 and 82 varies over a range of about 6 to l. Further subdivision would show a much greater range in high capacity adjacent 62b.

Assume that in Figure 7, dielectric cell 63b is initially in an erasedcondition. If a. writing beam is traversing the target face passes over cell 63B, the writing beam will dwell upon the cell for a short interval of time t determined by the equation amount of simple charge Q will be deposited on the surfaces of portions 80, 81 and 82 in accordance with the relation Q'=K1]t where I is the instantaneous current density of the writing beamand'Klkis Va constant. lecollectingl the fundamental equation for a capacitor where V standsrfor potential and C for capacitance, it is seen that since portions 8i), 81 and' 82 have diierent capacita-nce values, although the quantity of simple charge deposited on the surface of each portion is the same, the simple potential exhibited by the various portions will differ. Since t for usual operation is a constant, the simple potential, immediately .after Writing, of any dielectric portion may be expressed in the form V=KT From the above expression it is evident that Within each of a plurality of cells 63b, the mean potential of the group of elementalv capacitors 80, 81 and 82 will be proportional to I, butthat the deviation of any individual elemental capacitor from the mean potential depends on C, the value of its capacitance. High capacity portion 8b will undergo the least shift from erased potential, 81 and 82 ,progressively greater shifts.

It Will be apparent to one familiar with the art that the division of cell 63h into merely three elemental capacitor portions, is a crude approximation to reality, and that to properly accord` with fact each dielectric cell should be considered subdivided into many elemental capacitors, each having a diiferent capacitance value.

In the region of proportional operation of the invention, the charging current, which may be a current modus lated electron beam, charges some but not all dielectric portions from the erase-d potential to a value beyond the critical' potential. The unmodulated writing beam would charge a substantial fraction of the target dielectric area, in some cases half, beyond' critical. Should the positive or negative 'modulation peaks reach values which charge all or none of the dielectric area beyond the critical value, such a condition would represent peak clipping andresult in signal distortion.

Under the mean or unmodulated value of charging current, about half the target area will, under the holding beam, be converted to the other equilibrium potential. The resulting mean potential will correspond to that produced by the writing beam, and will be equal thereto where the critical potential is midway between the two equilibrium potentials.

Under positive peak modulation, an initially negative target area will be almost all charged above critical potential, and its mean potential resulting from holding beam operation will be raised considerably above that produced by thev writing operation. Correspondingly, negative peak modulation charging of a negative target area will charge only a small fraction of the target area (K2 is a constant) above critical potential, and the resulting mean potential of the area under the holding beam will be lower than that produced by the writing operation.

While the mean values of target area potential arenot identical, after holding beam operation, with those pro'L duced by the Writing operation, their relations are sub-k stantially linear and the final pattern represents the imposed signal as .amplified in modulation amplitude. rhis.

inherent amplification is a ve'r'y desirable fact-or.V

It will be appreciated by those familiar with the art that the analysis centering'a'bout -Figure 7 is highly simplied and is only utilized to convey in a general Way an understanding of the operation of the device. The' applicants invention'of course is independent of any theoretical principles which may be applied to explain its.

operation.

The plate type target shown in Figures l and 2 manifestly presents areas whose surface capacity varies over a substantial range due to dielectric thickness and to the Varying. effectV of dielectric aggregate area on fringe ca-V l pacity. The latter, normally effective at the edge of large condensers, is operative throughout the areas of the small dielectric aggregates.

In both type of target surface shown, it might be desirable for some purposes that the dielectric-surface area present relatively uniform distribution of area over the range of.surface -capacity employed in the proportional writing operation. It is suflicient for most applications that the surface present a substantially continuous distribution of area with capacity. It will be understood, of course, that should a target lack in any particular unit area an appreciable fractional area having a surface capacity per unit area lying intermediate of its intended capacity variation range, writing signal amplitudes which would just charge such areas beyond the critical holding potential will be distorted.

Having described the mechanism by which half tone information items when once written may be indefinitely retained upon the target face, the various modes of operation of the tube may be discussed by which the information is initially impressed upon the sensitive storage surface. Depending on the operative parameters the writing action may be either of the negative or positive variety.

Employment of negative writing requires Vthe preliminary step of producing a uniform background by driving all the dielectric portions of a selected area to Vthe upper stable limit of potential. This step may be accomplished by positive erasure in a way later described. The area is then scanned by the writing beam, the constituent electrons of the beam having a slow enough velocity to result in deposition of negative charges on the dielectric portions. Since the dielectric portions initially Yare at the upper stable limit of potential, suiciently slow electrons can only be obtained by adjusting writing cathode 33, shown in Figure l, upward in potential until the voltage between the cathode and the background is less than V0, the critical voltage. When writing is accomplished, information items will be manifested as unit areas having negative potentials relative to the background.

Conversely when positive writing is employed, the elemental dielectric portions of a selected area of the target face by negative erasure are uniformly driven to produce a background at the lower stable limit of potential. The writing cathode 33 hence must be adjusted downward in potential until the voltage between cathode 33 and the background exceeds Vo. The constituent electrons of lthe writing beam thus will have a high enough velocity to positively charge the dielectric portions as the beam scans the selected area. In the case of positive writing therefore, items of information will be manifested as unit areas at a positive potential relative to the background.

Whether negative or positive writing is utilized, translation, of received information signals into charges on the target face is accomplished by impressing the signals in the form of voltages on grid 33 which controls the current density of the writing beam.

Usually it is desirable to maintain the holding beam in an on condition during the writing process, since by doing so the background to the writing will not drift in potential. Continuous operation of the holding beam, however introduces a new problem. Obviously when either positive or negative writing is used, at least some of the newly charged dielectric portions must be driven from their original potential to a point beyond the cross over voltage for the holding beam. Otherwise upon cessation of the writing current, all the portions within a unit area would lapse back to their original potential and the information carried by the unit would be utterly eifaced. The holding beam itself however may prevent a movement of the dielectric portions beyond its own cross over voltage, and for the following reason. Under bombardment by both the holding and the writing beams a given dielectric portion'has a current curve which is the sum of the two curves of the same portion produced separately by the two beams. .The summation curve deviates by dips and rises from the writing curve, and if a dip or rise should chance to cross the zero line, a spurious stable state will appear at this point. With such a spurious point dielectric portions influenced by the writing beam will charge in contrast to the background until they reach the point where theyY will stick, unable to change potential further to pass beyond the cross over voltage of the holding beam. As a result, upon cessation of the writing beam and under holding beam exposure these portions will slidel back to the potential of the background whence they came.

In order therefore to produce effective writing, the writing beam must have suicient current density and voltage to both impress a contrast charge against the background and to override the influence of the holding beam, which causes the summation current curve to cross the zero line at an additional point. Modes of writing now will be described which satisfy these requisites.

Figure 9 shows operating parameters for one form of negative writing. Dielectric portions of the target initially exist at the potential of point p. The dot-dash curve lz4-a4-04-b4-p represents the current curve for a dielectric portion under the action of the holding beam utilized. The voltage between the conducting member and the holding cathode is adjusted -to a value 'Vh4 which, as may be seen from Figure 5, represents a limit for unlimited persistence holding, voltages of lesser magnitude than Vh4 causing negative spread. With theholding voltage Vh.; utilized the distance between points h4 and o4 measured along the horizontal -ordinate is greater than the distance between o4 and p.

The full line curve passing through points he, as, r and o6 represents the current curve of dielectric portion under bombardment by the writing beam. The writing curve is shown as having considerably greater displacement along the vertical ordinate than the holding curve since the writing beam has a much greater current density than the holding beam, typically 6000 microamperes per square inch as compared to 10. The summation curve is represented by the line a4fgr.

Since the critical voltage is dependent only on the nature of the dielectric material, the distances (measured along the horizontal ordinate) of h4 to o4 and h6 to o8 are the same. As mentioned heretofore the distance o4 to p is less than h4 to o4; therefore it is less than h6 to o6. Point h6, however is located just below point o4 in potential. As a result dielectric portions existing at the stable potential p with respect to the holding beam, will appear to the writing beam as if existing at a point r which is in the negative region of the writing curve. Hence under bombardment by the writing beam the dielectric portions by the absorption of electrons will be negatively charged. As a result of the accrual feature hitherto described, all the portions in a unit area will differentially drop in potential along the dotted line summation curve. Upon cessation of the writing beam, those portions which have dropped below o4 will be driven to point h4 by the holding beam, and those which have not dropped so far will rise again to point p.

lt will be noted that if point h6 is shifted an extent to the left of its presently shown position, the background dielectric portions existing at the potential of the point p will not appear to the writing-beam as being in the negative region of the writing curve, and no negative writing can occur. Conversely if point h6 is shifted from its presently shown position to the right of point o4, the summation curve will cross the zero line to the right of point o4. This point of zero current for the summation represents a spurious state of stability which prevents eiective writing for the reasons hitherto described.

Referring to Figure lO a mode of negative writing is disclosed in which the direct voltage of4 theholding cath- 'ode need not be adjusted away trom the value Vh3, which Voltage as shown by Figure is in the middle of the holding region for unlimited persistence. The current curve for a dielectric portion subjected to a holding beam alone of direct voltage Vhs is shown by the full line curve h303b3p. The current curve for a dielectric portion subjected to the action of the writing beam alone is shown by the curve hsaro. The summation curve resulting from the action of both the holding and the writing beams is represented by the dotted line curve oafgr. Y

lt will be noted that the summation current crosses .the zero line at point f, and that point f therefore represents a spurious stable state. If the holding voltage is kept at the steady value Vha, any dielectric portion originally existing at the background potential p, no matter how quickly it is negatively charged by the electrons of Ithe writing beam, will drop no further in potential, 'than the point E, at which point it remains until subjected to the holding beam alone at which time it returns to p. Dielectric portions arrived in the region of point f, however, can be induced to further drop to point h3 if the holding voltage is transiently rocked upward in potential so that the holding curve cross over point o3 moves past point f as shown by 0.3. A transient rocking characteristic of this sort may be imparted to the holding voltage by superposing an alternating component Vs upon the steady negative voltage Vh3. As shown in Figure l, in practice this may be accomplished by capacitively coupling holding Acathode 26 to alternating signal source 32 by closure of switch 31. Of course any other fluctuating signal having an upward and downward movement may be substituted for VB.

Returning to Figure upon application of alternating voltage Vs the limits of shift of the holding cathode upward and downward in potential are represented by the points h3 and h3 respectively. The current curve corresponding to the upward limit of the holding cathode is portrayed by the dot dash line h3 a3 03 b3 p.

As the holding voltage shifts upwardly, the cross over point o of the current curve will move rightwards of point f. Upon the happening of this event dielectric portions at the potential of point f will appear to the electrons of the holding beam as in the negative region of the holding current curve. As a result these dielectric portions will begin to drop down in potential. Subsequent to rising to its upper limit, h3 a3 o3' b3 'p, the holding curve as an entirety shifts downward in potential. The dielectric portions to the left of point 0'3 will then ride the curve, to use an analogy, like a Surfboard onpa wave, dropping in potential all the while until the portions will fetch up at point h3. Upon removal of the alternating voltage Vs, these dielectric portions, as a result of positive ion currents, will move upwards in potential to point h3. Y

By the employment of a shifting holding voltage it is seen that separate dielectric portions within a unit area, initially impressed with varying negative potentials relative to the background, may beurged to one of two potential limits dependent yon the quantity of charge acquired. Negative writing is thus possible without disturb ing the adjustment of the direct holding beam as fixed to operate in the middle of the unlimited persistence holding region.

vWith either negative or positive modes of writing, the writing speed' of the device is limited to the rate in scan at which the writing 'beam can place sufcient charges on the dielectric portions so that after exposure to the holding beam the written trace perceptiblycontrasts against the background. In its turn, the'rapidity with which a single dielectric portion can be charged depends upon the effective charging current of the writing beam. For negative writing the effective charging current equals primary beam current minus secondaryV emission current; for positive writing it is the reverse, secondary emission current minus primary beam current. Primary beam current is Va' fixed quantity; secondary Vemission current however may vary from close to zero to several times the primary beam current, depending on the magnitude of writing voltage. Obviously, while the effective chargingcurrent for negative writing cannot exceed the primary beam curf rent, for positive writing, the yeffective charging current may be several times that value. Positive writing therefore permits a faster writing speed than negative writing, and it is preferable on that account.

Figure ll discloses graphically operating parameters v suitable for positive writing. Full line haasosbgp represents the current curve for a dielectric portion subjected to electrons from a holding beam adjusted in'potential holding region for unlimited persistence. Full line hflaq'oqbqp similarly represents the current of a dielectric portions subjected to a writing beam of voltage Vhq. Under bombardment of both beams the summation curve of the portion is shown by dotted line fgjp.

writing is fairly obvious. Initially all dielectric portions of a selected area by negative erasure are driven to a stable background potential at the point h3. To the'positive Writing beam, however, the dielectric portions scanned will appear in the positive region of the summation curve. As a result during the interval of time the writing beam dwells on the dielectric portions in alunit area, the portions will be driven upward in potential from ha, the varying capacitancesof the portions causing in which the holding beam is maintained in an on writing voltage used, while not critically curtailed, fmustz Y be within a prescribed range. In upwards adjustment the writing voltage must be kept below thevalue where the summation curve would cross the zero current linevat` an extra point thereby introducing a spurious stable state. In downward adjustment the writing voltage must be kept above the point where for agiven rate of scan the electrons of the writing beam would positively saturate all dielectric portions they bombard'during the period of dwell.

Figures 9, 10 and 11 have dealt with modes of writing condition during the action of the writing beam. If.V desired, however, writing is feasible with the holding beam turned o as shownV in Figure 1, by connection of grid 52 to holding cathode 26 through switch 56 and bias supply 57. kkIn such case if the dielectric surface isat a posi-k tive background potential, the writing cathode is adjusted upwards in voltage until the writing beam electrons produce negative charges upon impact with the' dielectricY portion. j Conversely, if the dielectric surface is at negative background potential the writing cathode is adjusted downwardly in voltage until the writing Ibeam electrons have sui'lcient velocity to cause positive charges on the dielectric portions. Y

As has been mentioned heretofore the means by which a reading current is induced is a distinctive feature of the storage tube presently disclosed. It will be recalled that a unit area includes within its bounds a multitude of dielectric masses separated by conducting interspaces, the interspaces being exposed zones of the conducting member. It will further be recalled that by the action of the writing beam an information unit will display a mean potential caused by the aggregative effect of numerous dielectric portions having simple potentials distributed in value around the mean potential. This mean potential,

standing for an information item, is linearly amplied'byf an exposure to the holding beam when the separate di- In Figure 10the sequence of events producing positiveV electric portions shift to one or the other of two stable limits.

When an information unit is traversed Vby the reading beam, primary electrons impinge upon the exposed interspaces of the conducting substance and are absorbed therein. The striking velocity of these primary electrons is sucient to cause secondary emission of electrons from the conducting substance. Of the entirety of secondary electrons initially emitted from a given interspace, however, the number of electrons which inally escape is regulated by the simple potentials on adjacent dielectric portions. The dielectric portions thus control the secondary emission current from a contiguous interspace in a manner analogous to the way a grid controls the cathode cur rent in an ordinary triode.

Conditions at the target face during bombardment by the reading beam are pictorially shown in Figure 12. As in Figure 6, Figure 12 represents a greatly enlarged cross section of small part of mesh type target with numbers 62]) designating individual wires of the mesh and numbers 6317 designating dielectric masses, which masses within the mesh interstices form dielectric cells. Primary electrons of the reading beam follow paths -83 as shown. Paths of reading beam electrons which impinge upon the dielectric masses are omitted. Paths also are not shown for electrons which though secondarily emitted by the exposed wire surfaces do not escape therefrom.

In section 12a of Figure l2, the entire surface of the dielectric mass is negatively charged, as represented by the minus signs in portions 70 and 71. The influence of portions 70 and 71 does not extend to primary electrons of the reading beam following paths 83. The primary electrons therefore impinge upon the wire 62b with a velocity given by the voltage between vthe conducting member and the'reading cathode. As a result of this collision the primary electrons are absorbed and secondary electrons are emitted. The fields associated with portions 70 and 71, however, prevent any of these emitted lsecondary electrons from escaping from the region of the wire; instead all of these initially emitted electrons vultimately fall back on the wire surface, furnishing a zero current component.

In section 12b of Figure l2, dielectric mass 63b is half negatively charged as shown by portion 70 and half negatively charged as shown by portion 71. Portions 70 and 71 exert an opposite inuence upon electrons secondarily emitted from wire 62h, portion 70 suppressing escape and portion 71 encouraging it. As a resuit,` of the entirety of electrons secondarily emitted, approximately half escape as shown by paths 84.

In the section 12C of Figure l2 the entire surface of dielectric mass 63b is positively charged as shown by the plus signs on portions 70 and 71. Both portions 70 and 71 therefore encourage the escape of electrons secondarily emitted from wire 62h. As a result, substantially all the electrons secondarily emitted will finally escape as shown by the greater num-ber of paths 84.

With the reading beam voltage used, each primary electron upon collision with the wire surface produces on the average more than one secondary emitted electron. It follows that if all the emitted secondaryelectrons are permitted to escape, as shown by section 12e of Figure l2, secondary emission current will exceed primary beam current. In practice it is found that asV an'V incident to this multiplicative effect, the useful range of modulation of the reading current exceeds the magnitude of primary beam current by an appreciable factor, typically 20%.

If the number of secondarily produced electrons escaping from a given interspace within a Vunit area is dependent on the simple potentials of adjacent dielectric portions, it is obvious that the number of electrons escaping from a unit area as a whole will be proportional to` the mean potential of the information unit, the vmean potential being measured relative to the bakgIQurid pos tential. Since the mean potential of a unit area typies 'an amplitude gradation information item, the quantity -of escaping, secondarily produced electrons also is a measure ofthe information item impressed upon the unit area. Now, as it was earlier discussed, primary electrons --absorbed by the conducting member will produce a negative current component flowing to the conducting .member from impedance 65 and secondary electrons es- '.caping from the conducting member cause a positive cur- .rent component owing from the conducting member to the impedance 65. During a scan in which successive information units are traversed by the reading beam, the negative current component remains the same, equalling in magnitude the number of reading beam electrons absorbed by the conducting interspaces of the unit. The negative current component thus furnishes a background level signifying zero information. Concurrently, the positive component'of the current fluctuates Vin accordance with the amplitude gradations of the meanpotentials on the unit areas, instantaneously equalling in magnitude the number of escaping secondary electrons from the unit area which at that timerthe reading beam bombards. Thus when an information item is elicited by the reading beam the item will be manifested as the positive component of the net current owing from the conducting member to impedance 65.

In the embodiment of the invention disclosed in Figure 1, the net current flowing from conducting member 62 passes through impedance 65, generating a voltage across the terminals of the same. From the net signal thus produced, by the utilization of a voltage level transposer, an output potential may be obtained linearly related to the positive current component. ln the present case the voltage level transposer tak-es the form of a capacitor 67 coupling the Vungrounded terminal of impedance 65 to utilizationV device 56.

It will be noted that the reading principle of operation requires only that the reading beam bombard the interspaces of a unit area. No bombardment of the interspersed dielectric masses is necessary, since the dielectric portions can perform their regulating function solely by virtue of the enduring charges and potentials regenerated on them by the holding beam. Incidental impingement of electrons on the dielectric masses is unavoidable, however, as the reading beam dwells on the unit area. Reading beam voltages outside the holding voltage range would thus tend to destroy the information pattern regenerated by the holding beam. Destruction of this sort in turn curtails the number of effective reading repetitions. It is desirable therefore to adjust the voltage of the reading cathode to a value similar to the Vvoltage of the holding cathode. By such adjustment the reading beam in acting on the dielectric masses will supplement the action of the holding beam.

Although having the same voltages, both reading and holding beams are preferably employed simultaneously. Under these conditions, when a given unit area is scrutinized by the reading beam, although both holding beam and reading beam electrons move toward the unit area with the same velocity, the number of electrons impinging on the scrutinized unit area is much greater than for surrounding unit areas subject only to bombardment by the holding beam. This greater density of the electron concentration striking a pin-pointed unit area results in the production of a differential current indicative of the information item carried by the unit area.

Before new writing may be impressed upon the target face, the old potential patchwork must be cleared away lwhich process is called erasure. Erasure is either positive or negative depending on whether negative or positive writing respectfully is to be subsequently utilized. In the erasing operation al-l of the elemental dielectric portions of the entire target surface or a selected area thereof are driven to the upper or lower stable potential limit, as the case may be, the uniform potential pattern assae've 21 'thus produced representing a background .clear of information items. the unit areas by the 4writing beam will contrast again-st this zero information background.

Both positive and negative erasure may be accomplished by the holding beam. Where positive erasure is desired the voltage of the holding cathode is adjusted downwardly until the most negatively charged dielectric portions of the old potential patchwork appear in the positive current region of the holding curve. The negatively charged dielectric vportions are thereby driven to the upper stable limit at potential p. Dielectric portions already at point p are unaffected by the higher voltage of the holding beam since any trend towards a potential rise, caused by increased secondary emission, is counteracted by the suppressor grid effect of conducting member 62. f l

Negative erasure is an inherently slower process since in order to cause negative current at the surfaces of the dielectric portions charged, in the old patchwork, to the positive potential p, it is necessary to adjust the voltage of the holding cathodeupward of the chosen lower stable limit of potential. If nothing else is done, the dielectric portions initially at p will shift down to the voltage of the holding cathode and will drop no more. A further drop in voltage to the lower stable limit of potential may be induced, however, in these portions if the holding cathode voltage is now adjusted downward to the lower stable limit. The potentials vof the more positively charged dielectric portions follow the holding cathode in its drop.

It will be readily appreciated by those familiar with the art that the information storage device presently disclosed is nowise limited to the modes of operation described above. For example the tube may be operated in a manner permitting simultaneous -writing andreading on the surface of the target. When the Writing beam bombards the dielectric portions of a unit area, electrons of the beam will incidentally strike the interspatial zones of the conducting member. A secondary. emission current thereby is produ-ced which normally would mask a simultaneously produced reading current. lf however, as shown in Figure l, two-way switch 50 is operated to disconnect grid 43 from reading cathode 27 and to couple grid 43 to radio frequency voltage source 51, the reading beam primary current will be varied at the radio frequency signal rate. As a Vresult information items elicited by the reading beam appear as amplitude modulations upon a radio frequency carrier. For simultaneous read and write, filter 69 is connected to the junction of capacitor 67 and device 66 by closure of yswitch 58. Since filter 69 is tuned to the frequency of radio frequency voltage source Sl, lter 69 offers a high impedance to the reading signal modulated carrier but a low impedance to any other current. Reading signals may thus be separated from the concurrent masking signals incidentally generated by the writing beam.

Is is also obvious that if a focussed beam (of either the reading or the writing gun) is lirst adjusted to a voltage in the normal holding region and then continuously scans the target face, the focussed beam will regenerate the information pattern as effectively as the diffused holding beam hitherto described. Although each unit arca is only subject to the action of the focussed beam for a small fraction of the scanning cycle,l this fact is compensated for by the much greater current density of the focussed beam as compared to the diffused beam. Employment of the focussed beam has the advantage of giving the operator the option of utilizing selected areas rather than the entire target face as an information storage surface.

It follows from the above that the diffused holding beam gun may be entirely eliminated from the storage tube assembly, the beams from the remaining writing and reading guns, being utilized to perform all the necessary functions relative to information handling. In such a Potentials subsequently generated on 22 t two gun tube, a preferable mode of p sign only the single usual function of writing to the writing beam. The reading beam however is assigned .a multiplicity of functions. The reading beam if operated in continuous scansion at a voltage inthe normal holding region will both preserve the information pattern and produce a reading current. According to the wishes of the operator the period of operation for the reading ,beam may either exclude or include the writing interval.V In the latter case, of course the; voltage of the writing beam must be of a magnitude to override the holding effect of the reading beam. Erasure is preferably accomplished by a' continuous scansion of the reading beam while adjust/edl to an erasure voltage as described above. Ifsimultaneous reading and writing is desired'the density ofthe readingbeam is radio frequency modulated as described'above.A

Further by the employment of a time sharing principle;l` the information storage devices may be satisfactorily operated with just one focussed beam gun, the other two:

guns being eliminated from the assembly. -In such a one gun tube, successive writing, holding plusreading, andv erasure voltages are successively impressed for intervals upon the cathode of the single gun. Simultaneous reading and writing or the maintenance of a holding effect while writing, iny this case, of coursefi's it feasible.4 Manyapplications of the Y presently disclosed device will suggest themselves to those familiar with ,theart.l For example, the information tube may be employed for; the storage and presentation of signals received by radarV and sonar systems, for the preservation of television images and as an information accumulation unit ingelec.- tronic computers.

Asiwill be understood the subject invention is'caprableV of many embodiments and applications. The specific embodiments disclosed in this application are exemplary only and are not submitted for thenpurpose of defining the limits ofthe invention.

The invention described herein may be manufactured face portions being relatively small with respect tosaid elemental reading area, said substantially discrete dielecv tric surface portions having surface capacitydistributed throughout a substantial continuous range, said dielectric portions being distributed over the target surface with at least random uniformity in a density presenting in any elemental reading area said substantial range of surface capacity. f

2. The structure of claim 1 further including target charging means operative on a target charge pattern area selectively in dependency on the surface capacity of the dielectric portions thereof to charge said portions to potentials distributed through a substantial continuous range, said range in every reading area over the pattern area including a .common intermediate potential, and cathode means operative to supply electrons to the pattern .area from a potential negative of said common intermediate potential by ank amount equal the critical electron voltage for unitary secondary emission.

3. A cathode ray information storage device comprising, in combination, an evacuated envelope, a collectork p electrode and rst, second and third electron beam K sources disposedV within said envelope, separate spot focussing and deecting means for said iirst and second beams, diffusing means Vfor said third beam, means for intensity modulating said trst beam, an information storperati'on Ito as-f age target Within said envelope comprising an electrically conductive member and an aggregate of dielectric material adherent thereto to form therewith a source confronting facing including a plurality of minute sized substantially dielectric material exposures intermingled with exposures of said member, said dielectric material exposures occurring in substantial concentration for each target area of focussed electron beam spot dimensions.

A 4. A cathode ray information storage device as in claim 3 in which the conducting member has a continuous surface confronting said sources and the aggregate of dielectric material comprises a plurality of mutually spaced dielectric particles of random sizes adherent to said surface. v

5. VA cathode ray information storage device as in claim 3 Vin which the conducting member is a mesh and the aggregate, of dielectric material is adherent to the mesh to format least partial closures of the mesh interstices.

6. A cathode ray information storage device compris` ing, in combination, an evacuated envelope, a collector electrode and first, second and third electron beam sources disposed within said envelope, separate spot focussing and deecting means for said first and second beams, diffusing means for said third beam, means for intensity modulating said rst beam, an information storage target within said envelope comprising anV electrically conductive member and an aggregate of dielectric material'adherent theret'oto form therewith aa source confronting facing including a plurality of minute sized substantially discrete dielectric material exposures in termingled with exposures of said member, said dielectric material exposures occurring in substantial concentration for each target area of:

focussed electron beam spot dimensions, and anV output impedance coupled between said member and said collector electrode.

7. A cathode ray information storage device as in claim 6 further 4characterized by means for intensity modulating the second electron beam with an alternating carrier `signal and control means responsive to and operable in ac- 24 cordance with said signal, said control means being coupled to the output impedance to control the output thereacross.

S. A cathode ray information storage device as in claim 6 further characterized by means for selectively modulating the intensity of the third electron beam with a uctuating signal.

9. In a` charge storage device, selectively exploring charge reading means operative to detect the charge on a predetermined elemental reading area, and a charge storage target having a face Vpositioned operatively with respect to the charge reading means comprising substantially discrete dielectric surface portions and conductive surface portions, said dielectric surface portions being distributed over the target with at least random uniformity in a density presenting in any elemental reading area a multiplicity of dielectric Vsurface portions and a multiplicity of conductive surface portions, said dielectric surface portions having a surface capacity distributed throughout a substantially continuous range and distributed to present said substantially continuous range of surface capacity in any elemental reading area.

References Cited in thel tile of this patent UNITED STATES PATENTS 2,149,977 Morton Mar. 7, 1939 2,193,101 Knoll Mar. 12, 1940 2,324,504 Y Iams et al July 20, 1943 2,373,396 Hefele Apr. 10, 1945 2,415,842VL Oliver Feb. 18, 1947 2,500,633 Edwards Mar. 14, 1950 2,535,817 Skellett Dec. 26, 1950 2,549,072 Epstein Apr. 17, 1951 2,594,740 DeForest et al Apr. 29, 1952 2,640,162 Espenchied May 26, 1953 2,687,492 Szegho et al. Aug. 24,1954 2,757,233 Webley July 31, 1956 2,777,060 Waters Ian. 8, 1957

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