US2538836A

US2538836A - Barrier grid storage tube

Info

Publication number: US2538836A
Application number: US54701A
Authority: US
Inventors: Arthur S Jensen
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1948-10-15
Filing date: 1948-10-15
Publication date: 1951-01-23
Anticipated expiration: 1968-01-23

Description

A. S. JENSEN BARRIER GRID STORAGE TUBE Filed Oct. 15, 1948 FOCUS Jan. 23, 1951 *ifi OUTPUT l f f f 1 f f l f 1 f f fr All' UNITED STATES PATENT OFFICE BARRIER GRID STORAGE TUBE Arthur S. Jensen, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application October 15, 1948, Serial No. 54,701

12 Claims.

This invention relates to cathode ray tubes, and in particular to storage tubes of the cathode ray; type.

A storage tube is one, in which a signal may be stored for a period of time and subsequently reproduced. In one type of storage tube, thereis Iutilized a target structure comprising a thin difelectric sheet having on one side thereof a metal Lsignal plate. The target sheet is mounted within the tube and is scanned by an electron beam focussed upon the exposed surface of the dielectric sheet. A pattern of electrostatic charges may be established on the insulating surface of the dielectric sheet when signals are applied to the metal signal plate of the target during the li'sCanning of the dielectric surface by the elec- ;tron beam. The established charge pattern on the dielectric surface is used to vary or modulate 1e secondary emission from the dielectric surfface due to the scanning of the-surface by the electron beam. An output signal of the tube is that obtained by the collection of the modulated 1 secondary emission from the insulating target surface by a collector electrode. A tube of this type is disclosed in the application of R. L. Snyder, Jr., filed July 24, 1945, Serial 606,812.

One type of storage tube as described above is that having a ne mesh barrier screen closely spaced from the exposed dielectric surface of the target. This ne mesh screen is maintained during tube operation at a common D. C. potential With the metal signal plate in contact With the opposite side of the dielectric sheet. When the insulating target surface is scanned by the electron beam, the dielectric surface will change in potential until an equilibrium potential is reached, at which the number of secondary electrons leaving the dielectric surface and passing through the barrier screen is exactly equal to the number of primary electrons striking the surface. When the dielectric surface has reached equilibrium potential, the secondary electrons in excess of the number of secondaries leaving the target will collect in the form Vof a space charge between the barrier screen and target surface and rain back onto the dielectric target surface in a manner to be redistributed on the target surface and change the potential of the unbombarded parts of the dielectric surface. These redistributed electrons tend to neutralize charges already on the target surface and make impossible any accurate comparison of signals from scan to scan.

To minimize this redistribution effect, the barrier screen is closely spaced from the target sur- (Cl. Z50-164.)

Z face. The spacing of the barrier screen from the dielectric surface is a compromise. If the spacing is too great, redistribution eiects Will shade the signals, introducing more interline crosstalk,

5 and reduce the resolution. If the spacing is too small, whenever negative signals are applied to the plate, very negative portions of the target surface surrounding the beam spot may, by a coplanar grid effect erect a potential barrier outside the screen, over which many of the secondaries cannot go. As a result they will be collected by the barrier grid screen, and their absence from the secondary beam each scan will cause a positive signal to appear on the collector.

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

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

It is also an object of this invention to provide a storage tube in which shading of the signal by redistribution of secondary electrons on the target surface is eliminated.

It is a further object of my invention to provide a storage tube in which there is eliminated the potential barrier against signal electrons provided by negatively charged areas on the target surface.

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

Figure 1 is a cross sectional view of a storage tube according to my invention.

Figure 2 is a partial, sectional View of a modification of the target Astructure ofthe tube of '40 Figure 1 according to my invention.

Figures 3 and 4 show cross sectional views of other modications of target structures according to my invention.

The drawing discloses a storage delay tube o the cathode ray type comprisingan envelope l0 of glass or any other suitable material. Within the envelope is positioned an electron gun for the purpose of forming and focussing an electron beam upon a target electrode 30. The gun structure consists of a thermionic cathode l2, surrounded by an apertured control grid I4 and an anode electrode I6, for accelerating the electron emission from the cathode I2 and focussing the electrons of the beam onto the surface of a target 30. 'IWo pairs of deecting coils represented by 3 a yoke structure 20, are positioned along the beam path between the anode electrode I6 and target 30. Asis well known in the art, these pairs of deflecting coils each produce electromagnetic fields at right angles to each other and to the path of the electron beam. It is understood that these coils respectively will have varying currents applied, say by a saw-tooth generator, to produce line and frame scansion; or appropriate currents may also be applied to the deflection coils for producing spiral scansion of the target, or line scansion only, as may be desired. The means for producing different types of scansion of the target surface 33 arealso well known in the art and need not be further described.

The operation of the tube of Figure 1 depends upon a signal being generated by the emissionV of secondary electrons from the target 3B, when bombarded by the electron beam. A collector electrode in the type of tube shown, comprises an apertured plate 22 mounted in the enevelope l0 coaxial with the electron beam path. A conductive coating 24, applied to the inner surface of the envelope IB, extends from a point adjacent the collector electrode 22 to beyond the target electrode 36. The conductive coating 24 is connected to a sourec of D. C. potential (not shown) and during tube operation maintains a uniform electrostatic eld between the collector 22 and target 30. Outside of yoke 20 there is placed a solenoid 26, which, during tube operation, produces a strong magnetic focussing eld parallel to the axis of the tube l0.

The target electrode 30 (Figure 1) comprises essentially a metal support plate 32 mounted transverse to the path of the electron beam and which may function as the signal plate of the target electrode. On the surface of the signal plate 32, facing the electron gun, is xed a dielectric layer 34, such as mica. Target 30 may also be formed, using an aluminum signal plate, having a dielectric surface of aluminum oxide, facing the electron beam, formed, for example, by anodizing the aluminum. Howev-er, the construction of the target 3i] need not be confined to either of these described forms, but may also comprise any other appropriate insulating materials such as titanium dioxide, or silicon dioxide, for example, deposited in any manner such as evaporation or thermal decomposition upon a conductive signal plate. As shown in Figure 1, closely spaced from the exposed surface of the mica layer 34, there is a fine mesh screen 36, or a barrier grid, which may be mounted a few mils from the surface of the dielectric layer 34 by a supporting ring 38, mounted on the target 3U. The screen 36 is maintained during tube operation at the same D. C. potential as signal plate 32. As shown in Figure 1, screen 36 and plate 32 are connected to the same ground terminal with, however, an impedance 31 separating plate 32 from screen 36.

As the surface of dielectric 34 is scanned by the electron beam, secondaries are emitted and drawn away toward collector electrode 22, which is maintained during tube operation, at a high positive potential relative to the potential of surface 34.

The principle of electrostatic storage on an insulating surface has long been known and used in television pickup tubes, such as the iconoscope. If an insulating surface is bombarded by an electron beam, the secondary emission ratio will vary with the energy of the bombarding electrons. If the energy is such that the secondary emission ratio is greater than unity, then the potential of the insulator target surface will become more and more positive and will change with respect to the electrode which collects the secondaries until the number of secondaries leaving the target surface is exactly equal to the number of primary electrons striking the target plus the secondary electrons falling back on the target surface. 'The target surface potential at which this action takes place, is known as the equilibrium potential. The secondary electrons which return to the target surface, first collect in the form of a space charge and then rain back on the insulating target surface, charging the unbombarded parts of the surface to a negative potential. Thus, a charge pattern is built up on the surface between the bombarded and unbombarded target portions and in the absence of any applied signal. rlhe returning secondary electrons, redistribute themselves and partially neutralize-any positive charges already formed on the surface, thus making any comparison of signals from scan to scan impossible.

In the operation'of the tube of Figure 1, theV electron beam strikes the dielectric surface V34 with sucient velocity to produce a secondary emission ratio greater than unity and drives the surface of the dielectric 34 to an equilibrium;4 potential as described above. To obtain this condition, the screen 36 and signal plate 32 in the specific tube described of Figure l, are maintained at a potential about one thousand volts positive relative to the cathode l 2 of the electron gun. f

Since surface 34 of the target is an insulator,I the only source of current to it is the primary beam, and the only drain of current from it is the secondary electron emission. At the equilibrium potential of the target surface, these two must be-equal. The barrier grid or screen 35 `'functions as a virtual collector, so that the equilibrium' potential for the target surface 34 is established with respect to screen 36 and not toA the actual collector electrode 22.v Wherever the beam strikes the dielectric 34 with a velocity'sufcient to initiate a secondary emission ratio greater' than one, the potential of the elemental area under bombardment of the surface of dielectricI 34 is brought to thisv equilibrium potential, whichexists at a few volts positive with respect to the screen 36, because the initial velocity of most of the seco-ndary electrons is suiiicient to lift themv over a field of several volts. The exact potential is not very definite, because it is affected by space charge conditions and the geometry of screen 36 and nearby electrodes. However, at this equilibrium potential, a number of secondary electrons just equal to the number of arriving primaries are suiiiciently energetic to penetrate the negative screen 36. not return to the target, as appropriate elds outside screen 36 urge them away and toward the collector 22 as the secondary beam. Meanwhile,

the excess secondary electrons are ,not sufcientlyf energetic to penetrate the negative barrier screen 36 and fall back onto the target surface 34.v

These excess secondary electrons are restricted;

in their motion by the close proximity of the screen 36 to the dielectric surface, so that their redistribution to portions of the target not direct-v These secondaries do,

of the A. C. signal to be recorded. The exposed to make the comparison at some arbitrary phase insulator surface of the dielectric 34 is capacitiverelation. The tube may be used in various other ly coupled to the signal plate 32 and also to ways where storage of information is desired. the screen 36. When a signal voltage is irn- Signals may be impressed on the signal plate pressed on a signal plate 32 it also appears, 5 while the beam scans a predetermined pattern somewhat diminished in amplitude, on the reover the target. The signal Vand beam may then cording surface 34 of the target. be shut off, leaving the information stored on If an A. C. signal is applied to plate 32, so that the target. At any desired later time, the bean it is driven negatively relative to the equilibrium may be turned on and, with no new signals impotential, any elemental area under bombardpressed on the signal plate, scanned over the patment of the surface of dielectric 34 at the time tern so that the signal impressed during the first by the electron beam, will also be driven negaoperation will be reproduced. The signals need tively relative to the screen. Under these connot be recorded in one scansion and utilized in ditions, a positive field between screen 36 and the next scansion. Signals can be stored at one the negative target plate 30 will be presentedto 15 time and reproduced at any later time merely the surface of dielectric 34 and, therefore, the by shutting off and turning on the beam at thesecondary electrons, released by the impact of desired time. In some tubes of the type dethe beam electron, are drawn away from the surscribed, Signals have been Stored on the target face of dielectricv 34. since the number of sec- Surface up to one hundred hours withlittle loss ondary electrons from the surface of dielectric 2o 0f definition. As will be evident from the de- 34 is greater than the number of primary ele'oscribed method of operation,'recorded signals may trons from thefelectron gun, there is a net loss be combined with new signale, enabling one to of negative charge and the elemental target suruse the tube for Carrying out Complex operaface area under the striking beam will become tions by suitable combination of Signals. more -positive until it attains equilibrium poten- In tubes 0f the type described in Figure 1, tial. If, however, the elemental area of surface there are essentially three related problems in- 34 is driven positive with respect to the screen Volved in the deSign of the tube, particularly With 36 at the time'of bombardment, due to an inregard tothe target Structure of the tube. coming signal driving plate 32 in a positive di- One problem, for eXample, is that 0f screen reotion, a, more'negative field bei-,Ween screen 35 30 disturbance caused by the successive intersection and target is' "presented to the surface 34 and 0f the beam by the Wires of Screen 36 which gen#l the secondaryemission is suppressed. Since no erateS a disturbing Signal HoWeVer, it Can be secondary electrons leave the target surface ele- Shown, and itis true in practice, that if the ratio ment, there isa net gain of negative charge and 0f the Screen Wire diameter to the beam spot the potential of the target surface under the striksize is small, that the Screen disturbance is not ing primary beam will drop in potential until troublesome. In tubes of the type shown in Figthe elemental area attains equilibrium potential. ure 1, Where the beam diameter is between l0 and Thus, as the beam is deeoted across the sur- 15 mils, it has been found thata wire diameter face 34, while `a signal is impressed on the signal of one Inil iS sufficiently line t0 give a SatiSfaetory plate 32, it will cause each element of surface area 40 screen disturbance ratio and yet provide Suit strikes to come to equilibrium potential, recient nieohanieal Strength for a Self-Si'illliortingv gardless of the potential the surface would Sereen. otherwise have due to the influence of the signal Another problem, involved in tubes of this typey plate. This action, then establishes a potential iS that of the S0 Called Co-planar grid effect. difference between the signal plate 32 and the 4i Whenever negative signals are applied to the sigdielectric surface element under the beam, which nal plate 32 of the tube of the type Shown inv will cause thev element to have a potential dif- Figure 1, very negatively charged areas Will be ferent from 'that of the equilibrium potential, built up on the dieleotrio target surface by the when the beam moves off of the surface element Scanning beam These negative portions ofthe and the signall plate 32 returns to the potential 50 target Willl Create a negative field in front of the of Screen 36, 1f the beam scans a, long path over target surface, Vwhich will act as a potential bar.` the target surface 34, while a fluctuating Voltage rier suppressing the Secondary emission from the is impressed on the signal plate 32, a bandof target surface and yforcing back to the screen 3B, charges, as wide as the beam, will remain on the the Secondary Signal eleetrons passing through path when the beam is eut oir. 1f the signal: c5 screen. 36- As a result of this coplanar grid effect. plate 32 returnsto the potential of screen 3s', the portlons of the Secondary emission forming the potential along/the path will vary in proportion signal current is collected by the screen 36, and to the signal vonage impressed during the beam their absence from the secondary beam on suctransit. The secondary emission from the dicessive scans will cause positive Signals t0 appear electric surface, however, fluctuates according to in the collector circuit.

the charging demand. If no change of charge A third problem involved in tubes of the type is required by an elemental surface of the didescribed in Figure l, is that secondary emission. electric, the secondary electrons Vreleased therewhich falls back onto the target surface, will tend from are equal in number to the impinging beam to go to other more positive areas of the target electrons. If a negative charge is supplied to surface. This redistribution of electrons will tend the dielectric surface 34 to bring the elemental to discharge areas of the target other than that target area to equilibrium potential, secondary which the beam is striking. This redistribution emission is suppressed until the demand has been effect produces a shading of the signals and resatised. If a positive charge is needed to bring duces the resolution of the output signal of the: the target area to equilibrium the secondary tube. i emission is a maximum until full charge is To effectively solve the problems of a co-planar achieved. grid eiect and a redistribution effect in tubes oi"v One adaptation of the tube disclosed may be this type, several types of target structures may in signal comparison, where both signals are not be utilized. The screen 36 may be modified in;

available simultaneously, or where it is desirable several different ways to eliminate these effects.V

For example, in Figure 2, there is disclosed :a type of target .structure in which the metal backing plate 32 has one surface covered by the dielectric v layer 34. Plate 32 `and dielectric layer 34 correspond with the identical structures of Figure 1. However, in place of the negative grid screen 36 in Figure l, there is substituted a thick screen 62, which may be made up of a cellular structure formed by vintersecting walls or strips of conductive material.

A thickscreen of a type similar to t2 shown in Figure 2 has a somewhat critical relationship between the thickness and the width of an aperture opening of the screen. The shielding effect of sucha screen -92 must not only neutralize the coplanar negative fields extending `from the target surface but also such a screen 32 must not prevent thecollection of the secondary electron emission from the surface of dielectric 3i. Ascreen having a ,ratio of Screen thickness to aperture width equal to 1:1 would almost completely shield the surface of dielectric 3d from the positive fields of electrodes 2li and 22 so as to prevent the collection of secondary emission from the surface of dielectric 311. Thus, little or none of the secondary emission from the dielectric surface 34 would leave the target. A tube, similar to that described above and shown i-n Figure l, and in which the maximum signals impressed upon the signal plate 32 may range up to `-100 volts peak voltage, could be operated with a thick screen having a ratio of screen thickness to aperture width equal to less than lzl 4and .more than 1:2. If the screen used, had a ratio of Y thickness to aperture width less than 1:2, the tube would not operate ,successfully within the aboverange of maximum input signals, but would have to be confined in operation to a lower input signal voltage range. That is, a screen having .a ratio of screen thickness to aperture width equal to less than 1:2 would not effectively eliminate the coplanar grid effects or redistribution eiects of a tube operated with the above maximum input signals.

VThe edges of the strips forming 'the :cellular screen 62 are of approximately one mil thickness so Vas to provide a satisfactory screen disturbance ratio as described above. For a tube, similar to that described for Figure l, thesignals impressed upon the signal 'plate 32 are less than lo() volts peak voltage. In such cases, the dielectric layer 34 nof the target of Figure 2 is preferably one mil in thickness while .the thickness of screen '32 may vary from 2 mils'tolO mils, with 'the ratio cf screen thickness t0 aperture width Tbeing lbetween 1:'1 and 1:2.

The screen 62, of 'the target structure vshown in Figure 2, will effectively eliminate the coplanargrid effect described above as well as prevent redistribution of secondary electrons on lthe target surface. The size of the openings of screen 62 must be as small as, or preferably smaller than, the size of a picture element, or spot lsize of a vprimary beam. A picture element can `be defined as the smallest element that can be resolved by the tube. The picture element may also ybe dened as the spot size of the primary :electron beam when it strikes the target surface. In a tube, similar to that shown in Figures 1 and 2,

the spot size is between 10-15 mils in diameter.l

fectingfbcth the redistribution of secondaryeiectrons .on the target surface I,and the emission iofA seoondaries from the elemental areas. If the width of the openings of screen 62 are approxi,- mately 4 mils, then the spot `size of the primarybeam of the tube will simultaneouslycoverfsevelalof the mesh openings. v

A highly successful type of screen corresponding to screen 36 in Figurel which hasbeen found to give excellent results in tubes of the type `of Figure 1, is a woven stainless steel screen having approximately 230 openings Iper inch and woven from wire of one mil diameter. Such a screen-,is shown in Figure 3 in which the screen 39 is Woven from warp wires :42 running vertically in the ng@ ure, and Woof wires il runninginto the plane of the paper of Figure 2. The wires are one mil .in diameter, so that the effective thickness of the screen .is approximately 2 mils. I 4have foundthat when this woven screen 39 is placed in conv--v tact with the dielectric surface 34 as shown in the ligure, that portions of the screen are suffi-V ciently spaced from the dielectric surface to eliminate the co-planar effects andredistribution effects described above, Since there are approximately 230 wires per inch, the distance between successive lwires or width ofr aperture is around 3 mils. The spot size of the primary electron beam Of the tube will cover between ten and twenty-,ve openings of this screen at one time. This number of openings in the mesh 39 would; represent a picture element. Since portions of the mesh screen 39 are ,spaced at least 2 kkmils from the dielectric surface 34, this woven mesh-` screen provides an effective screen fol-*eliminating* the co-planar grid effects land redistribution effects, described above, for a tube in which the amplitude of the signals applied to the signal plate 32 ,are less than 100 volts.

The woven mesh 39, of Figure 3, forrnedfrom the one `mil stainless 'steel wire, providesjsuili.- cient stiffness and rigidity to prevent any dis-y turbancedue to vibration of the screen 39 causedby large `charges on the elemental areas oftheY target surface 34 which tend to attract or ,repel The size of the mesh of the screen .3,9 maybe varied as long as this ratio is approximately maintained," to provide the optimum screening the dielectric surfaces. However, the maximuml size of the mesh is limited bythe picture element as described above. i

Figure 3 also shows a means V:for Lassembling.

the componentparts of target 39 together, "The invention vis not limited to this specific structure;

of 'Figure 3 but the mountingstructure is shown only by Way of example. The rtarget'parts 3lf are mounted within a supporting ring 44 having,

at one end a flanged lip 118. The woven wire. mesh 9 is first welded to the flanged ring portion 48,. The thin mica vor dielectric-disc '341 isf then dropped into the supporting ring illuso as:

to rest in contact against the wire mesh 39. The metal signal plate 32 is of an annular shape and is also inserted into the :support ring 4'4 and heldf tightly in contact against the dielectric disc v34.'- Since incoming signals are applied .to `the signal plate 32 the signal plate is insulatedly spacedatl 47 from the .support ring 1114. To maintain A.the

Screen 39 of Figure 3,Y has signal plate 32 in this spaced relationship relative to support ring 44, a second ceramic or insulating disc is inserted into the open end of support ring 44.. The insulator 54 tightly ylits the inner circumference of the support ring Y44 and is apertured atV 52 to permit the passage therethrough of a stud 54 integral with the signal plate y32. In this manner the stud 54 of the signal plate maintains the signal plate insulatingly spaced from the support ring 44. To tightly maintain the several parts of the target structure in close contact, spring ngers 5E are welded to the edge of the support ring 44 and extend into spring-pressed contact with the exposed surface of theinsulator disc 5U. The support ring 44 with the component parts Vof the target 3% held therein as shown in Figure 3.may be mounted within thetube envelope l0 in any desired manner'and 'in the position shown in Figure l. As shown in Figure 3', lead 33 may be connected to the signal-plate 32 by welding or other means to the projecting stud 54. Also the barrier grid 39 is connected into its circuit by fastening conductor 35vto the support ring 44 as shown.

"Another type off target structure which will tend to eliminate the co-planar grid eiect and the redistribution effect in tubes of the type shown in Figure 1, is that indicated in Figure 4, in which there is utilized a pair of screens 64 and 65 superimposed, one upon the other, upon the surface of the dielectric layer 34. The apertures of screen E4 need not match the apertures of'screen 66. These two screens 54 and 6B are an approximation to the thick cellular screen shown in Figure 2. The use of two screens doubles the barrier thickness and thus permits the use of a wider mesh than can be effectively used with the single ywoven screen shown in Figure 3. Thus, if both woven screens 64 and 66 had 100 mesh per inch and were made from one mil diameter wire, the transmission ratio of each screen would be 81 percent and of the two would be approximately 65 percent, which is a higher transmission ratio than that provided by the 230 per inch mesh woven screen of the target of Figure 3. However, the effective screen thickness is now 4 wire diameters or about 4 mils in thickness.

The eiective aperture width of the double screen grid of Figure 4 would vary from 9 mils in the case where the apertures of one screen are matched with the apertures of the other screen to approximately 41/2 mils depending in what position one screen was placed upon the other. Thus the ratio of screen thickness to aperture Width for the double mesh screen, shown in Figure 4, would be within the range of 1:1 to 1:2, given above as that necessary for successful tube operation. The two screens 64 and 66 are mounted in contact with each other and may be held in a supporting structure against the dielectric layer 34 in a manner similar to that shown in Figure 3.

While certain specic embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein Without departing from the spirit and scope of the invention.

What I claim is:

1, A target electrode comprising a metal support plate, an insulating layer covering one face of said support plate, a mesh screen mounted in contact with said insulating layer, the ratio of the-thickness of said screen to the Width of an opening of said screen being between 1:2 and 1:1.

2. A target electrode comprising a metal sun- 10 port plate, an insulating layer covering one face of said support plate, a mesh screen mounted in contact with said insulating layer, the ratio of the thickness of said screen to the Width of 4an opening cf said screen being approximately 2:3.

3. A target electrode comprising a metal support plate, a thin dielectric layer covering one face of said support plate, an apertured screen mounted in contact with said dielectric layer, said screen having a cellular structure in which the width of the apertures are approximately equal to the thickness of said screen.

4. A target electrode comprising a conductive plate, means forming a dielectric surface on one face of said conductive plate, a first ne mesh screen in contact with said dielectric surface and second fine mesh screen superimposed on and in contact with said rst screen.

5. A cathode ray tube comprising an envelope, means within said envelope for forming an elec'- tron beam along a path, a target electrode within` said envelope mounted transverse to said beam path, said target electrode including a sheet of dielectric material intercepting said electron beam path, a wire screen in contact with the surface of said dielectric sheet intercepting said electron beam path, a metal signal plate in Contact with the opposite surface of said dielectric sheet, the ratio of the thickness of said screen to width of mesh opening of said screen being approximately '7. A storage tube comprising an envelope, a target electrode mounted within said envelope and including a support plate having a dielectric surface on one side of said support plate,

electron gun means spaced within said envelope from said target electrode for forming a beam of electrons and focussing said electron beam on said dielectric surface, an apertured screen ixed to said dielectric surface, the apertures through said screen being less than the spot size formed on said dielectric surface by said focussed electron beam.

8. A storage tube comprising an envelope, a target electrode mounted Within said envelope and including a conductive signal plate and means forming a dielectric surface on one face of said signal plate, electron gun means Vspaced within said envelope from said target electrode for forming a beam of electrons, means for focussing said electron beam on said dielectric surface, a woven mesh screen mounted in contact with` said dielectric surface, the mesh openings through said screen being less than the spot size formed on said dielectric surface by said focussed electron beam.

9. A storage tube comprising an envelope, a target electrode mounted within said envelope and including a conductive signal plate and means forming a dielectric surface on one face of said/3 signal plate, electron gun means spaced" Withii'i'fsaid envelope from said target electrode for forming' a: beam: of electrons',` means for" focussing said electron beam on said Adielectnicsurface, a woven mesh screenmounted contact with said dielectric surface, the mesh` openings through said screen being less than the spot size formed on sai'df dielectric surface by said focussed electron beam, the ratio of the tlnifeknessl of said screen to the Width of a meshy opening through said screen being less than 1:1.

l0. A storage tube comprising an envelope., a target'v electrode mounted Within said envelope and including a conductive signal plate and means forming a dielectric surface on one face of said signal plate, electron gun means spaced.

Within said envelope from said target electrode for forming a beam of electrons, means for focussingA said electron beam on said'l dielectric surface, a Woven mesh screen mounted in oontactl with said dielectric surface, the mesh openingslthrough said. screen being' less than thev spot size formed on`v said dielectric surface by said focussed electron beam, the ratio of the thickness of said screen to the width of a mesh opening through said screen being approximately 2:3.

11. A storage tube comprising an envelope', a target electrode mounted' Within said envelope and including a conductive signal plate and means forming a dielectric surface on one face of said signal plate, electron gun means spaced within said envelope from said target' electrode for forming a beam of electrons, meansl for focussing said electron beam on said dielectric surface, an apertured screen. mounted in contact 12%r withv said dielectric.' surface, said screen having:` a cellular structure in which the Width of the apertures in the screen andthe thickness of: the

screen are respectively less than the spot size formed on said dielectric surface by said fo cussed electron beam.

1'2'. A storagetube comprising an envelope, a' target eiectrode mounted within. said envelope and including a conductive signal plate and means REFERENCES CITED The following references are of record in the file of this patent:v

UNITED STATES PATENTS.

Number Name Date 2,280,191 l-Iergenrotherv e Apr. 2l, 1942' 30 2,281,280 GaborV Apr. 28, 1942 2,403,239 Rose July'Z, 1946 2,454,410 Snyder NGV. 23, 1948