US2748312A - Cathode-ray storage tube system - Google Patents

Description

May 29, 1956 c. D. BEINTEMA 2,748,3l2

cATHoDE-RAY STORAGE TUBE SYSTEM Filed Nov. 19, `1954 \\\\\\\\\\\\\\\\\\m w 5R ivan/2% nited States Patent 2,748,312 CATHODE-RAY STGRAGE TUBE SYSTEM Chester Donald Beintema, Santa Monica, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application November 19, 1954, Serial No. 470,025 3 Claims. (Cl. S15- 12) This invention relates to cathode-ray type storage tubes and more particularly to an electron gun for flooding a storage tube target electrode with electrons having substantially the same normal velocities near the target.

An electron gun called a ood gun is generally provided in a cathode-ray type storage tube to reproduce information stored on a target electrode. The flood electrons are thus directed over the entire surface of the target electrode. In order to cause electrons to impinge upon the target electrode with substantially uniform normal velocities it is necessary to make the usual at cathode source of the ood electrons as small as possible, to position the source as far from the'storage target as possible and to place the source as near the symmetrical axis of the tube as possible. Itis undesirable and impracticable to put the ood gun exactly along the axis of the tube because a small and relatively high Voltage recording or writing beam is directed along that axis whereby electrostatic focusing may be employed to direct the writing beam to any elemental area upon the target electrode.

In the design of a tlood gun for a cathode-ray type storage tube all three of the above recited designed procedures are necessary but all are undesirable. One disadvantage of a small electron source is that the available current is small. When the iiood gunis spaced a considerable distance from the storage tube target electrode, positive ion generation prevents a good reproduction of the charge pattern. For the same reason magnetic deection and 2,748,31l2 .Patented May 29, 1956 In accordance with the invention an annular cathode is positioned about the symmetrical axis of a storage tube to provide a symmetrical nood electron beam having a uniform normal velocity distribution. An annular current control electrode is positioned about the cathode to control the current therefrom and to focus the ilood electrons.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which an embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawing is for the purpose of illustration and description only, and is not intended as a definition of the limits of the invention.

Fig. 1 is a sectional view of a direct-viewing storage tube incorporating an embodiment of the electron gun of the present invention; and

Fig. 2 is an enlarged broken-away sectional view of the ood gun shown in Fig. 1.

Referring to Fig. 1, there is shown a storage tube 19 having an evacuated envelope 12 and provided with a deection voltage generator 14 which is synchronized with a recording voltage generator 16 through a lead 18.

Envelope 12, which may be made of glass, has a neck portion 20 enclosing a writing beam electron gun 22 and an enlarged portion 24 having a at viewing end 26. The viewing end 26 may be made of any suitable clear glass and coated internally of the envelope 12 with a focusing of the writing beam cannot be used without interfering with the efficient operation of the ood gun. Furthermore, actual symmetry of the llood gun can never be achieved when it is not placed on the symmetrical axis of the tube. Voltages representing the normal velocities of the flood electrons impinging upon the target electrode then may vary as much as five volts out of one hundred. This does not appear to be a large variation; however, recent developments in direct-viewing storage tubes have made it imperative that this variation of the electron voltages be minimized.

Secondary emission current from the target electrode is utilized in direct-viewing storage tubes to illuminate a phosphor viewing screen. The secondary emission current in a tube then rises from zero to thatrequired for full brilliance when voltage of the target electrode storage surface varies as much as three volts. This voltage control range has recently been expanded to as large as fifteen or twenty volts; however, the production of ood electrons having uniform normal velocities when approaching the target is still. extremely desirable. This is particularly true, when reasonable accuracy is required in producing intermediate illumination for half-tone operation. For a storage tube utilizing intermediate illumination, reference may be made to a copending application Serial No. 459,403, iled :September 3 0, 1954,- and assigned to 'the assignee of the present application, by E. E. Herman and G. F. Smith,jen ti t led, Direct-Viewing Half- Tone Storage Device. V

It is therefore an object of the invention to provide an yimproved ood gun for a cathode-ray type storage tube.

It is another object of the invention to provide a storage tube electron gun ooding the storage tube target elecdielectric material 28 such as luminescent phosphor.

Writing beam gun 22 comprises simply a cathode 30 which is provided with a iilament 32, a current biasing electrode 34 having a central aperture 36, and an accelerating anode 38. Filament 32 is heated by means of a filament voltage supply 40 and an adjustable voltage supply 42 is connected between the positive terminal of filament supply 40 and biasing electrode 34 to control the writing beam current. Cathode 30 is maintained at a few thousand volts negative with respect to ground by an accelerating source of potential 44. Accelerating anode 38 is then maintained at ground potential as is the positive terminal of potential sourcel 44.

Positioned adjacent anode 38 and painted onto the internal surfaceof the envelope 12 is a conductive coating 46 which may be made of a commercial preparation known as Aquadag. Conductive coating 46 is also maintained at ground potential.

An electron flood gun 48, constructed in accordance with the present invention to produce an electron stream 49, is shown in the enlarged portion 24 of the envelope 12. Electron gun 48, which is employed to produce flood electrons, comprises a single annular wire filament 50 having a cathode sleeve 52 (see also Fig. 2) disposed thereabout and a refractory material 54 such as aluminum oxide disposed between the annular filament 50 and cathode sleeve 52. Annular filament 50 is connected across a source of potential 56 and cathode sleeve 52 is maintained at ground potential.

A substantially cylindrical current control electrode 58 is disposed about and partially encloses cathode sleeve 52 with 'an appendage or rim 59 extending axially through the. cathode. Control electrode 58 is maintainedat a suitable potential by means of a voltage supply 60'having its positive terminal grounded.

Amal velocities when they reach the target electrode.

Conductive coating 46 is maintained at a predetermined fixed potential to focus the flood electrons emitted from cathode sleeve 52. For this `purposing conductive coating 46 is generally maintained at ground or at a potential negative Iwith respect to cathode sleeve 52 by means of a potential source.

Filament 50 could be used without the cathode sleeve 52 to provide a directly heated cathode; however, the voltage drop across the length of the filament will generally be larger than tive volts. This will then cause a non-uniform cross-sectional velocity distribution and may defeat the purpose of the ood gun 48. Hence, an indirectly heated cathode, viz. cathodel sleeve 52 is preferably employed as a unipotential source of electrons. To this end an electron emissive material 66 is deposited on the cathode sleeve 52 more clearly as shown in Fig. 2.

Control electrode 53 need not have the exact shape illustrated, but may bc constructed simply of a single annular wire or as a hollow frustum of a cone or may have any other convenient shape. The potential of control electrode 53 may be adjusted by means of source 69 to control the maximum outer diameter of thc stream i9 and to control die current of the stream.

To the right of the flood gun 43 within the envelope i2, two conductive coatings 6% aud 70 are positioned along thc internal surface of the envelope 12 to aid in focusing the stream 49. Wall coating dii is maintained a few hundred volts positive with respect to ground by a source of potential 69 and coating 7l) is maintained at a potential somewhat less than that of coating 68 by a source of potential 7l. Between coating 7l) and the dielectric material 2&5 on the viewing cud 26 of the envelope i2, a screen grid electrode 72 is disposed adjacent a target electrode 7d. Target electrode 74, which is employed as a storage device, may comprise an electroformed mesh 76 having patches of a dielectric material 78 scattered over its left surface facing the guns.

Conductive coating 46 is maintained at a suitable potential to cause the internal surface of dood electron beam 49 to meet in a point 80 at the target electrode 74 whereby double illumination due to overlap is eliminated.

Screen grid 72 is maintained at the same potential as conductive coating 70 by a connection to the positive terminal of potential source 71. The output lead cf recording voltage generator 16 is then connected to target electrode 74.

The flood gun 48 is easily mountable within the enlarged portionk 24 of envelope 12 as shown. The fact that the tlood gun 48 may be mounted there is advantageous for two reasons. Firstly, the unusual proximity of the flood gun 48 to target electrode '74 reduces ion generation, and secondly, the writing beam developed by gun 22 may be usefully magnetically focused and deflected. Magnetic focusingl for the writing beam is actually provided with the storage tube 14) by a focusing solenoid 82 which is disposed about the neclt portion 29 of the envelope 12 near the Vaccelerating anode 33 of the writing beam gun 22. Adjacent to and to the right of solenoid $2 there are disposed four deflection coils 84., three of which are illustrated a-nd which are employed to dellcct the writing beam through the center of the annular ood gun 48 to elemental areas on the target electrode 74.

Focusing solenoid 82 is supplied with direct current by means of an adjustable potential source 86 and de'ection coils 84 are energized according to the output current of deflection voltage generator 14 to which the deflection coils S4 are connected.

The flood gun 48 has practically no limitations with respect. to available current. The fact that the flood gun i8 may be mounted symmetrically in the envelope 12 reduces the non-uniform normal velocity distribution of the flood electrons at the target electrode 74 inherent in thc'use of point sources or dat cathodes. Furthermore, even with the most crudely constructed flood guns of the present invention, it is unusual for the normal velocity distribution in volts over the cross"sectional` area of the flood beam at the storage tube target electrode 74 to vary more than one volt out of one hundred volts.

in the reproducing operation of the storage tube 10 the ood beam 49 is directed to the storage target electrode 74 and secondary electron emission from the target electrode 74 illuminates the luminescent viewing screen 28 according to the charge pattern recorded thereon. The uniform velocity spread over the beam 49 then permits half-tone operation whereby the brilliance of an elemental area on the viewing screen 2S is made proportional to the amount of charge stored.

What is claimed is:

l. In a cathode-ray type storage tube having a target electrode for storing an electrostatic charge pattern providcd thereon by a writing beam, an electron gun for producing a parallel, univelocity electron stream to liood the target electrode with electrons, said electron gun comprising an annular cathode symmetrically disposed about the path of the writing beam and adapted not to interfere therewith. a filament disposed within said cathode for directly heating said cathode, a refractory material disposed between said filament and said cathode within said cathode, a cylindrical current control electrode having two inwardly extending rims partially enclosing said cathode, focusing means disposed adjacent said control electrode on the side of. said cathode opposite the target electrode for focusing said stream, and means for maintaining said focusing means at a potential to cause the inner surface of the stream produced by said electron gun to meet substantially in a point at the target electrodc.

2. A cathode-ray type storage tube comprising an evacuated envelope having an axis of symmetry, a writing beam electron gun disposed on said axis at one end of said envelope for producing a writing beam of electrons, a target electrode disposed at the opposite end of the envelope, a flood beam electron gun including an indirectly heated annular cathode disposed symmetrically about said axis between said writing beam gun and said target electrode for producing a parallel, univelocity stream of electrons to flood said target electrode, and means for directing said Writing beam beyond and through said annular cathode to said target electrode, said annular cathode being disposed symmetrically about said axis whereby said cathode does not interfere with said writing beam.

3. A cathode-ray type storage tube comprising an evacuated envelope having an axis of symmetry; a writing beam electron gun disposed on said axis at one end of said envelope for producing a writing beam of electrons; a target electrode disposed at the other end of said envelope for storing an electrostatic charge pattern; and a flood beam electron gun disposed between said writing beam gun and said target electrode for producing a stream of electrons to llood said target electrode, said stream of electrons having substantially equal velocities normal to said target electrode, said flood beam gun including an annular cathode symmetrically disposed about said axis between said writing gun and said target electrode and Vbeing indirectly heated, an annular current control electrode disposed about and adjacent to said cathode between said cathode and said target electrode, focusing means disposed adjacent to said cathode between said writing beam gun and said cathode for focusing said stream, and means for maintaining said focusing means at a potential to cause the inner surface of the stream of electrons produced by said flood beam gun to meet substantially in a point at said target electrode.

References Cited in the file of this patent UNITED STATES PATENTS 2,462,569 Sziklai Feb. 22, 1949 2,548,789 Hergenrother Apr. 10, 1951 2,622,226 Theile Dec. 16, 1952 2,706,246 Klemperer Apr. 12, 1955 FOREIGN PATENTS 687,940 Great Britain Feb. 25, 1953

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