US2846604A - Storage tube - Google Patents

Description

Aus. 5, 1958 l.. E. FLQRY v2,846,604

y STORAGE TUBE Filed April 50, 1953 TTORNIY gun.

r.L8-445,604 Patented" Aug. 5, 1958 STORAGE TUBE Leslie E. Flory, Princeton, N. l., assignor to Radio Corporation of America, a corporation of Deiavvarc Application April 30, 1953, Serial No. 352,221 4 Claims. (C1. 31asn This invention relates to electron discharge devices and more particularly to storage tubesA for the conversion of one type of electric signal to another type of electric signal. Y

The invention relates specifically to a storage tube having an insulating target upon which a signal charge ypotential is established. The signal charge potential is used to provide output signals from the tube, corresponding'to the charge pattern, which are utilized to provide a Visual picture of the pattern. One storage tube of lthis type is that described and claimed in a copending U. S. Patent application Serial No. 29,746 lof L. Pensak, filed May 28, 1948, now abandoned, `and assigned to the same assignee as the present invention.v

' yOne modification of this type of storage tube utilizes a target electrode including a thin film of insulating material. An electron beam is directed against one side of the target film to provide a charge pattern on the insulating film. A second electron beam is scanned over the charge surface of the target film to discharge the film and provide an output signal from the tube, which may be amplified and applied to a viewing tube to provide a visual picture of the charge pattern on the storage tube target.

Such a storage tube essentially comprises a first electron gun, called a writing gun, directed at one side of the target electrode and a `second electron gun, known as the reading gun, directed at the opposite side of the target electrode. yBoth the reading and writing Aguns are on substantially the same axis with the target electrode positioned intermediate the guns and substantially normal thereto. In tubes of lthis type, the target electrode normally inn cludes a supportingfmesh screen coated with an electron pervious aluminum film on one surface of the screen and ya thin lfilm of insulating material on the exposed sin Iface Iof the aluminum film. The insulating film being exposed to the electron beam produced by the reading The writing gun provides a high velocity electron beam that penetrates Ithrough the aluminum rand insulating films lto discharge the surface of the insulating film to the potential of the aluminum film. The reading gun normally sees the insulating film surface and provides a relatively low velocity electron beam that is scanned over the insulating target film to drive the insulating film to an equilibrium potential that is normally positive with respect `tothe potential of 'the aluminum film. Secondary electrons emitted from the insulating film surface, when the reading electron beam impinges thereon, are collected by a collector electrode. tron. beam yof the writing gun is modulated by a signal applied thereto, there is established on the target surface a charge pattern that corresponds tothe incoming signals that are applied to the writing gun. When the reading gun scans the charged target surface, the reading beam charges, the surface from point topoint, to the equilibrium potential `by an amount depending upon the charge laid down by the writing gun at that point. This charging i .of the target surface provides a signal in the output cir-v When the elec-y cuit connected to the collector electrode that can be amplified and/or applied to a viewing tube so that the charge pattern on the storage tube is visually presented.

Since the surface of the target electrode, on the reading gun side, operates at an equilibrium potential with respect to ythe collector electrode, an effect known as redistribution of secondary electrons takes place. This effect, which is that of secondary electrons from a vbombarded area being unable to reach the collector electrode, and consequently these secondary electrons return toan unbombarded area of the tar-get electrode, results in a shaded signal over the surface of the target electrode. The redistribution of the secondary electrons alsov causes a non-uniform discharge of the charged area.

The shading of the signal is due to the fact that the secondary electrons from a bombarded area return to an unbornbarded area to gradually drive the unbombarded area more negative resulting in a gray signal and destroying the contrast ratio between signal and non-signal areas. The non-uniform disch-arge of the charged areas results from the fact that :an attractive field of a nonbombarded area is sometimes much stronged with respect to a bombarded area than the field of the collector electrode so that some portion of the signal is lost to the unbombarded areas. n

Furthermore, in storage tubes of this type, it has been the practice prior to this time to separate the reading and writing signals by resorting to frequency discrimination. in other words, in order to separate the reading signal from the writing signal it has been necessary to modulate the reading gun with high frequency signal and .placethe carrier signal thus generated thro-ugh a band pass amplifier and lat-er rectify the signal from the amplifier to recover the desired video signal. t g

The frequency separation of the reading and .writing beam has been necessary due to the fact that, prior to this time, the output' signal has been obtained fromthe supporting mesh of` the target electrode. Since this has been true the writing beam, having the greater electron density, produces anoutput pulse ofV a much greater amjplitude than the remx the output cir-cuit temporarily inactive if the .frequency discrimination were not present. The obvious solution for this problem is to take output signal from a col-, lector eieetrode other t* target electrode. However, this has proved to be impractical prior to this time due to the fact that a .j ...anni areas of the target electrode are positive with respect to the bombarded areas, and are much more closely spaced from the bombarded areas, so that secondary electrons from-a bornbarded larea tend to go to an unbombarded area of the target electrodev instead of the collector electrode, thus resulting in a low degree of mcdulationof the collecto current.`

It is therefore ank object of this invention to provide a newand improved storage tube. Y Y

A further object Iof this invention is to provide a new and improved storage Vtube wherein the effect known as redistribution of the secondary electrons is substantially eliminated.

A lstill further object of this invention isto provide a new and improved storta-geV tube wherein there isrno necessityrfor providing Afrequency discrimination between the reading and Writing signals. g

These and other. objects have been accomplishedrin' accordance with vthis invention by providing a storage tube having a high velocity electron beam` froma first electron gun known Ias a writing gun, `and a relatively low velocity electron beam from a second electron gun known a beamen-d could possibly ren-:ler

thin insulating film. Intermediate the insulating lm and the reading electron gun is provided a barrier grid. Adjacent the barrier grid is a focusing electrode and a eollector electrode. When desired, the mesh screen may be eliminated and the barrier grid may be utilized to support the thin metallic film and the thin insulating film.

The novel features which are believed to be characteristie of this invention are set forth with particularity in the appended claims. The invention itself will best be understood by reference to the following description taken in connection with the accompanying drawings in which:

Figure 1 is a transverse sectional view of a storage tube constructed in accordance with this invention;

Figure 2 is an enlarged sectional View of the barrier grid and target electrode structure of the device shown in Figure 1; and

Figure 3 is an enlarged sectional view of a modification of the barrier grid and target electrode structure constructed in accordance with this invention.

The tube of Figure 1 comprises an elongated tubular envelope having tubular neck portions 12 and 14- on a common axis. Coaxially mounted within the tubular extension 12 of the elongated envelope 10 is a writing gun 20, of conventional design, comprising a cathode 16, a control grid 18, and a tubular focusing and accelerating electrode 22. Mounted within the other tubular extension 14 of elongated envelope 10 is a second electron gun structure 30 including a cathode cylinder 32, a control electrode 34, and an accelerating and a focusing electrode 36 arranged substantially coaxially along the tube axis. A target electrode 40 is mounted within the enlarged portion of envelope 10 and is substantially normal to the axis of the tube. The target electrode 40, which is supported by a ring 39, is shown more clearly in Figure 2 and will be explained in detail in the description connected thereto.

Adjacent the target electrode 40 and on the reading gun side thereof, is a barrier grid 41 that is closely spaced from the target electrode 40. Closely spaced adjacent the target electrode 40 is a focusing electrode cylinder 43 that is shown as a conductive coating on the envelope wall. Extending into the focusing electrode 43, but spaced therefrom, is a collector electrode 45 that is supported in the envelope 10 by means of support rings 46 as shown. Other conventional support means may be utilized for the various electrodes when desired. The envelope 10 is coated with a grounded conductive coating 24 and 24 to prevent a voltage gradient from being developed along the walls of the envelope, and also to provide focusing of the electron beams as is well known.

A yoke 26 surrounds the tubular extension 12 adjacent the envelope portion 10 and a second yoke 38 surrounds extension 14. The yokes 36 and 38 each comprise two or more magnetic coils for providing desired types of beam deflection for the electron beams from the electron guns and 30.

Referring now to Figures 1 and 2 the target electrode 40 comprises a wire mesh 56 having several hundreds mesh per linear inch. Supported on the mesh 56 is a thin film of a conductive coating 54 that may be of a material such as aluminum. Supported on the conductive coating 54 is a thin film of insulating material 52 that may be of a material such as magnesium fluoride, or silica. The target electrode 40 is supported substantially normal to the axis of the reading and writing guns 20 and 30. The target 40 is supported in the envelope 10 by means of ring electrode 39 that is sealed through the envelope 10 as shown.

The barrier grid 41 should be closely spaced adjacent the target electrode 40 in order to provide a planar electric field as close to the surface of the insulating surface 52 as possible. In fact, as shown in Figure 3, the barrier grid 41 may be a part of the target electrode 40'.

The optical transparency of the barrier grid 41 should be as large as possible and still provide a substantially planar electric eld adjacent the target 40. The extent of the electric field is determined by the spacings between the target electrode 40 and the barrier grid 41 as well as the optical transparency of the barrier grid 41, both of which may vary. Nevertheless, mesh sizes of 200 to 400 apertures per linear inch with spacing of 0 to .5 mil have proved to be satisfactory. These sizes are not intended to be limiting but are merely representative of successful operation. i

The operation of the tube is substantially that in which input signals are applied to the control grid 1S of gun 20 to modulate the electron beam of the gun while it is scanned over the surface of target 40. The aluminum target film is maintained in the order of a volts negative with respect to ground. The electron beam of gun 30 is normally unmodulated and may be scanned over the surface of the magnesium fluoride film 52 in a substantially rectangular raster formed by normal frame and line television scansions. The beam of electron gun 30 will strike the dielectric target film S2 at energies in the order of 1,000 volts. These energies are between the first and second crossover potentials for the magnesium fluoride screen so that, at every point on target film 52 Where the beam of gun 30 strikes, there is initiated a secondary electron emission from the film which is greater than unity. The secondary electrons are drawn away and collected by the positive, with respect to the target electrode 40, collecting electrode 45. The loss of secondary electrons from film 52 raises the potential of the scanned target surface positively to a point close to the potential of barrier grid 41 which is held at 100 volts negative with respect to ground potential. This potential is an equilibrium potential and is that state at which the number of secondary electrons leaving the target surface is equal to the number of primary electrons of the beam of gun 30 striking the surface.

The modulated electron beam of gun 20 strikes the target with energies in the order of 9,000 volts which is sucient to causethe electrons of the beam of gun 20 to penetrate through the target lms 54 and 52 as described in the above cited copending application of L. Pensak. The magnesium fluoride film 52 is a material which is normally insulating, but which becomes condufv tive when struck by high velocity electrons at those pointsy where the electrons penetrate substantially throughtlvn magnesium fluoride film.

Thus, during tube operation, the modulated high velo@L ity beam of gun 20 provides conductivity between the4 aluminum film 54, at a -150 volts, and the exposed` scanned surface of the magnesium uoride lrn 512, at equilibrium potential which is close to the potential of the barrier grid 41. Thus, the beam of gun 20 will dit charge the positive surface of film 52 toward the negative potential of the aluminum film 54. Thus, the scanning of. the high velocity modulated beam of gun 20 over the surface of target 40 establishes on the exposed surface of film 52 a negative charge distribution or pattern corre-- sponding to the sequential input signals applied to thel control grid 18 of gun 20.

The beam of gun 30, on scanning over the charged surface of the film 52, will drive the areas discharged by the beam of gun 20 back toward equilibrium potential, i. e. the potential of the barrier grid 41. When the beam of gun 30' strikes a negative area of film 52, the secondary emission from this point is instantaneously greater than that from a positive area. This instantaneous charging of any one point of lm 52 results in the flow of secondary electrons through the focusing electrode 43 to the collector electrode 45 thus forming a corresponding pulse in the output circuit 49. This voltage may bedetected and amplified in any well-known manner.

In greater detail, When the electron beam of-gun 30 scans the target 40 there will be two types of areas which it strikes. vThese areas are (1) the areas thatliave been bombarded by the writing gun Z0 beam and therefore are at equilibrium potential (close to the potential of the barrier grid 41 say -98 volts), and (2) areas bombarded by the gun 30 which will therefore be at the same potential as the backingl plate 54, i. e. .150 volts. Since the secondary electrons that are dislodged from the bombarded areas (-150 volts) see the barrier grid 41 they are immediately attracted away from target electrode 40 Since the barrier grid field is a planar field and is extremely close to the target 40,v redistribution of dislodged secondary electrons is substantially eliminated because of the close proximity of the field that attracts the electrons toward the collector 45.

The vast majority of the dislodged secondary electrons that reach barrier grid 41 pass through the barrier grid and are focused by focusing electrodeA 4.3 andare attracted to the grounded collector electrode 4S. Thus, as the beam of gun 30 scans the charge pattern established by the beam of gun 20 on film 52, there is provided a succession of signal pulses which vary in amplitude as determinedvby the potential of the areas charged.

By controlling the beam current of gun 30, it is possible to control the storage time ofthe charge pattern on film S2. Thus, if the beam current of gun 30 is sufficiently small, the charge pattern on target film S2 may remain for a determinable length of time before it is entirely erased. In this manner then, the scanning rate of the beam of gun 20 may, when desired, be different from that of the beam of gun 30. Furthermore, the pattern or scanning raster established on film 52 may be written down by a different type of scansion than the reading raster of gun 30. For example, the writing gun 20 may be operated to provide a radar raster of the charge pattern, which is laid down on the target 52 in several seconds, while simultaneously the pattern may be continually read off by the reading gun 30' with a television scan with a frame time of lo of a second. The potential of signal plate 54 is approximately 150 volts negative with respect to the collector electrode 45 by being connected toa source of potential (not shown). Whenever the high velocity electrons from writing gun 20 penetrate the metal film 54 and the thickness of the dielectric film 52, the writing gun can discharge the charged areas. In other words, the exposed surface of the dielectric film 52 facing the reading gun 30 is constantly scanned by a relatively low velocity electron beam from the reading gun 30 which scans the target between the first and second crossover potentials thereof.

Due to the presence of the barrier grid 41, 'a substantially planar electric field is closely adjacent the target electrode 40. Due to this planar field, the secondary electrons from target 40 are either (l) knocked loose With sufiicient energy to go to the collector electrode 45 or (2) the secondary electrons have insufficient energy to penetrate the planar field of the barrier electrode 41 so thatr these secondary electrons return to the target electrode 40 at substantially the same spot as that from which they were dislodged. Because of the arrangement of the electrodes on the reading gun side, all of the secondary electrons that go through the barrier grid 41 are attracted to the grounded collector electrode 45.

Since the current to the collector 45 can be influenced by the writing beam only, through the charge stored in the insulator 54, the separation of the reading and writing beams is accomplished without any requirement of frequency discrimination between the reading and writing signals. This is true because of the fact that, due to the planar field of the barrier grid 41, the output may be obtained from the collector electrode 45 that is separate and distinct from the target 40.

The barrier grid 41 functions as a virtual collector electrode so that the equilibrium potential of the target surface 52 isk established with respect to the grid 41 and not to the actual collector electrode 45. At the`equilibrium potential, a number of secondary electrons, equal to the number of primary electrons arriving at the target 52, are sufiiciently energized to penetrate the barrier grid 41. The secondary electrons that penetrate'the barrier grid 41 cannot return to the target electrode 41 as appropriate fields outside the barrier grid 41 urge V.them away and towards collector electrode 45 as the secondary electron beam. Meanwhile, the remaining secondary` electrons, i. e. those not suiciently energized to reach barrier grid 41 from the target surface 52, are restricted in their motion by the close proximity of the grid 41 to the dielectric surface 52, so that their redistribution to other portions of the targetis eliminated, i. e. the remaining secondary electrons return to substantially the same spot from which1they were originally'dislodged.

Referring now to Figure 3 there is shown a modification of the barrier grid and target electrode structure constructed in accordance with this invention. In this modification, the insulating film 60 is supported directly on the barrier grid 61. The insulating portion 6ft in turn supports the thin film of conducting material 62. y

The operation and function o-f the Various portions of the target electrode and barrier grid 61 are similar to that described in connection with the embodiment shown in Figure 2 so that further description is not deemed necessary at this time.

Although the invention has been disclosed in connection with specific details lof preferred embodiments thereof, it must be understood that such details are not intended to be limitative of the invention except as is set forth in the accompanying claims.

I claim:

l. An electron discharge device comprising, an envelope, a target electrode having a plane surface within said envelope and including an insulating film having induced conductivity characteristics when struck by electrons at a predetermined velocity, a conductive material extending over one side of said film, an electron gun Within said envelope on the other side of said target for providing electrons at a Velocity below said predetermined velocity, a barrier grid adjacent said insulating film portion of said target electrode and intermediate said target electrode and said electron gun, and a second electron gun within said envelope, said second electron gun being on said one side of said target electrode and supplying an electron beam of electrons having velocities greater than said predetermined velocity.

2. A cathode ray tube comprising an elongated evacuated envelope, a writing electron gun in one end of said envelope for providing electrons above a predetermined' velocity, a reading electron gun in the other end of said envelope and arranged substantially coaxial with saidv writing electron gun, a target electrode having a planar surface arranged intermediate said electron guns and substantially normal to the axis thereof, said target electrode including a thin film of insulating material having induced electron conductivity characteristics when struck by electrons above said predetermined velocity, a conductor extending over said thin film on the writing gun side of said insulating material, a barrier grid closely spaced adjacent said target electrode and intermediate said insulating portion of said target electrode and said reading electron gun, a focusing electrode arranged adjacent said barrier grid and intermediate said barrier` grid and said reading gun, said focusing electrode having an aperture therein rfor the passage of electrons therethrough, and an output electrode intermediate said electron gun and said focusing electrode for the collection of secondary electrons from said target electrode. l,

3. A cathode ray tube comprising an elongated envelope, a target electrode comprising a sheet of dielectric material having a conductor extending substantially over one surface of said dielectric sheet, said dielectric sheet having a thickness that is pervious to electrons having velocities above a predetermined value, an electron gun in one end of said envelope providing electrons with velocities greater than said predetermined value for rendering said dielectric material conductive during passage of said electrons through said dielectric material, said electrons impinging on said conductive surface of said target electrode, a second electron gun in the other end of said envelope providing electrons with velocities less than said predetermined value, said electrons from said second gun impinging on the surface of said dielectric material, and a mesh electrode adjacent said dielectric material and intermediate said target electrode and said second electron gun, said mesh electrode being substantially the same size as said target electrode and substantially parallel thereto.

4. A cathode ray tube comprising an elongated envelope, a target electrode within said envelope and comprising a sheet of dielectric material and a conductor extending substantially over one of the planar surfaces of said dielectric sheet, said dieleetriesheet having a thickness that is pervious to electrons having velocities above a predetermined value, a mesh grid electrode in contact with the other of said planar surfaces of said dielectric sheet, said mesh grid electrode being substantially the same size as said target electrode and substantially parallel thereto, an electron gun in one end of said envelope providing electrons having velocities greater than said predetermined value for rendering said dielectric material conductive during passage of said electrons through said dielectric material, said electrons impinging on said conductive surface of said target electrode, and a second electron gun in the other end of said envelope and providing electrons having velocities less than said predetermined value, said electrons from said second gun irnpinging on said dielectric material.

References Cited in the file of this patent UNITED STATES PATENTS 2,503,949 Jensen et al. Apr. 11, 1950 2,711,289 Zworykin June 2l, 1955 2,728,020 Pensak Dec. 20, 1955 2,728,021 Blanks Dec. 20, 1955

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