US3319103A - Storage tube target structure having attached shield ring - Google Patents

Description

Mw HW G. D. CARTWRIGHT ETAL. wm

STORAGE TUBE TARGET STRUCTURE HAVING ATTACHED SHIELD RING Filed Nov. l2, 1964 United States Patent "i 3,319,1.03 STRAGE TUBE TARGET STRUCTURE HAVTNG ATTACHED SHHELD RTN@ Gerald D. Cartwright, Lititz, and )Edgar M. Smith, Miliersviille, Pa., assignors to Radio Corporation of America, a corporation of Delaware Filed Nov. l2, 1964, Ser. No. 416,433 2 Claims. (Cl. 313-68) This invention relates to display storage tubes and particularly to an improved target for such tubes.

One type of display storage tube comprises an envelope having a glass fa-ceplate coated on its inner surface with a phosphor screen that emits light when subjected to electron impingement. In the operation of this type of storage tube, useful light emission fr-om the screen occurs `only `during a Viewing cycle.

To provide such useful light emission from the screen, the tube includes a perforated target closely spaced from the screen. An electrostatic charge pattern produced on the target modulates a relatively low velocity electron beam which iioods the screen during the viewing cycle to provide -a viewable display on the screen corresponding to the electrostatic charge pattern.

The charge pattern on the target is produced during a writing -cycle of operation and is effected by scanning a relatively high velocity electron beam across the target. The high velocity electron beam is modulated by signals derived from a lig-ht pattern to be viewed, to vary the velocity of the beam in response to the modulating signals. Such variations in the velocity of the beam, charge an insulating coating on the target by the phenomenon of secondary electron emission to produce the charge pattern thereon.

The charge pattern so produced on the target is not destroyed by the relatively low velocity viewing beam during the viewing cycle. Consequently, the target, when once charged, is effective to produce a display throughout a relatively long period. However, the charge pattern on the target can be destroyed fully during a later erase cycle.

Erasure of the electrostatic charge pattern on the target is accomplished by the viewing beam in combination with the appli-cation of a positive erase pulse to a conducting mesh substrate on which the insulating coating of the target is deposited. When so pulsed, the viewing beam becomes an erase beam. Such a positive pulse causes electrons of the beam to be attracted to the target and to land thereon. A negative charge is thereby produced on the target which effectively cuts off the viewing beam, and thereby erases the display image.

Several serious problems are involved in a display storage tube of the type described. One problem is related to the modes of operation of the tube and other problems arise as a consequence of unavoidable errors in applying an insulating charge-holding coating to the target of the tube.

The problem related to the modes of operation of tube arises as a consequence of the fact that the erase beam does not discharge all portions of the target in an erasing operation. The portions not erased are located at the periphery of the target and produce a peripheral or annular ring of intense illumination upon the phosphor screen during a subsequent viewing operation. Presence of the bright ring is objectionable for several reasons. Light from the ring may be reflected internally from glassair surfaces, and thus make information at low light levels difficult to see. The ring also makes it `difficult for the observer to become dark adapted. Furthermore, under conditions of low ambient illumination, the bright ring adds unnecessary light, thus making the observation of other close-by instruments diicult.

3 ,3 l 9, l @E Patented May 9, 1967 ICC The bright ring is caused by contraction vof the viewing beam diameter by the erase pulse so that the beam diameter is appreciably smaller than the diameter of the target. During an erasing operation, therefore, the erase beam covers a smaller area of the target than is exposed to the viewing beam in a viewing operation and the writing beam in a writing operation. This leaves a peripheral region of the target in which charges produced during the writing operation are incapable of erasure by the erasing beam. After a number of writing, viewing and erasing cycles, the charge built up in the peripheral region of the target becomes large enough to produce the bright ring in a subsequent viewing operation.

One prior attempt to solve this problem has involved the application of an appropriate pulse to an electrode of the tube during the erasing operation for the purpose of widening the erasing beam. However, this solution to the problem is not entirely satisfactory because of the expense involved in providing the components required for the beam widening pulse, and because the components, as well as the electrical circuit required for their use, may be subject to malfunctions.

With respect to the several problems presented by unavoidable errors in applying the charge-holding insulating material to the target, one problem concerns a buildup of electrical charges on an annular portion of the target that overlaps an imperforate target support flange. This annular portion is so close to the area of the target to be coated that frequently during a target coating operation, some of the coating material is also unavoidably deposited on the annular portion. This annular portion is exposed to the relatively wide area scan of the writing beam and consequently acquires an electrostatic charge from the writing beam. The annular portion referred to is outside of the target area ooded by the viewing and erasing beams, and, therefore, the electrical charge thereon is built up during each writing operation. This charge nally becomes sufficiently large to produce a voltage break-down between the target and phosphor screen. By voltage break-down is meant a voltage difference between the target and the screen of such magnitude as to cause electron ow from the target to the adjacent screen. Since the target is apertured, such electron flow is free to occur from the charged imperforate annular portion, to the screen through apertures in the target. Such voltage break-down or ashover produces a bright annular light response on the phosphor screen. When this voltage break-down occurs during a viewing operation, it seriously affects the fidelity of the display presented on the screen.

Another problem occurring because of unavoidable error in coating the target is due to failure to coat the target completely. Such failure to coat the target cornpletely may arise from excessive care in attempting to avoid coating the annular region of overlap of the target and its support ring. In this situation, a portion of the target mesh adjacent to the support ring may remain free of target coating. Such uncoated portion is objectionable for proper tube operation.

Accordingly, it is an object of the invention to provide sa structure that simply and economically avoids the problems discussed in the foregoing.

lt is a further object to provide adjacent to the target of the display storage tube, conductive means that avoids the problems of (l) bright ring on the phosphor screen, (2) voltage break-down between the target and screen, (3) undesired excitation of the screen through uncoated portions of the target.

The invention accomplishes the foregoing objects by mounting on the target, a ring made of electrically conducting material for covering certain regions of the target structure. The regions covered are those that give rise to the problems mentioned, and which are on the side of the target facing the direction from which the writing, viewing and erasing beams originate. In one example, the conducting material is in the form of a metal ring. The ring is so shaped and positioned on the target that it intercepts electron beams at peripheral portions of the target and effectively dissipates the resultant electrical charges at such portions through a suitable lead. In this way, operation of the tube is free from the several effects of a bright ring, voltage break-down between the target and screen and undesired excitations of the screen.

The invention will be described in greater detail by reference to the folowin'g description taken in connection with the accompanying drawing in which:

FIG. 1 is a schematic View of a display storage tube in which the present invention may be used; and

FIG. 2 is a fragmentary sectional view of the tube depicted in FIG. 1 and shows the novel target including the conducting ring of the invention.

The display storage tube shown in FIG. 1 includes an evacuated glass envelope 10, having a glass faceplate 11. On the inner surface of the faceplate 11 is a phosphor viewing screen 12. The phosphor viewing screen 12 is bonded to a transparent conducting film 14 made of metal or metallic compound such as tin oxide, for example, deposited on the inner surface of the glass faceplate 11. The conducting film 14 is normally maintained at from about 5,000 to 10,000 volts positive with respect to ground. Located next to the phosphor screen 12 is a storage target assembly \15 consisting of a storage grid 16 and a conducting mesh 18. The storage grid .16 cornprises a relatively thin deposit on the mesh 18 of an insulating material such as magnesium tiuoiide having a thickness of the order of several microns. The insulating material is sufficiently thin so that the size of the openings in the conducting mesh 18 remain substantially unchanged. The storage grid or screen 16 is in effect a multiplicity of `storage elements capacitively coupled to the conducting mesh 18.

A collector electrode 20 is located next to the target 15 and spaced about one-eighth inch therefrom. rI'he collector electrode 20 includes a metallic cylinder 22 supporting a grid structure 24, for-med of a fine metal mesh. The ne mesh collector grid 24, the storage target assembly 15 `and viewing screen 12 are closely spaced and in parallel planes.

The viewing section of the storage tube includes a cathode shown schematically at 26, a pair of grids 2S and 30, a conducting coating 32 on the inner part of the envelope 10, and a hollow metallic cylinder 34. The hollow metallic cylinder or electrode 34 serves to collimate the electrons emitted by the cathode 26. When impressed with a positive pulse, this section becomes the erasing section.

The writing sction of the tube includes a cathode 36, four grids 38, 40, 42, 44 and electrostatic deflection plates 46 and 48.

The functions of the Writing, viewing and erasing sections are as follows. The writing section provides a highly focused beam of high velocity electrons which are scanned across a relatively large area including the storage Agrid 16 and an annularimperforate region 49 (FTG. 2) where the storage grid overlaps a target supporting flange 50, for producing an electrostatic charge pattern thereon. The viewing section, on the other hand, produces a flood beam of electrons of relatively low velocity directed to an area smaller than that affected by the writing beam. During a viewing operation, the electrons of the flood beam either pass through the storage grid and land on the phosphor screen 12, or are repelled by the storage grid to the collector 20.

The flood beam of electrons of the viewing section is also utilized in erasing the display produced by the writing section. The erasing operation takes place after a viewing operation has been completed. During erasure,

the viewing beam is changed to an erasing beam by :applying a larger positive potential to the mesh support 18 of the storage grid 16, to cause electrons of the beam to be attracted to, and land on, an area of the storage grid. This area is smaller than that affected by the viewing beam because of the reduced size of the erase beam occasioned by the increased positive potential applied to the mesh 18. The landing electrons do not have sufficient energy to produce a secondary emission in excess of unity. A negative charge consequently is produced on the storage grid 16, which effectively cuts off the viewing beam and erases the displayed image.

In one example of operation of a display storage tube of the type described, the following exemplary potentials may be applied to the several electrodes thereof:

Volts Conducting film 14 +5,00() to +10,000 Conducting mesh 18 0 to +5 Collector 20 +150 to +210 Collimater 34 +32 to +150 Electrode 32 +10 to +50 Cathode 26 0 Cathode 36 1.500 to 2,000

Before the erase operation is started (i.e. during a normal viewing operation), the conducting mesh 18 of the target may be +2 volts, for example, and the initial charge on the storage grid 16 may be 0 volts. In carrying out the erasing operation, the potential of the erasing mesh 18 is raised to +10 volts by means of an erase pulse, and the storage grid 16 is raised to +8 volts. Under these conditions, the viewing beam electrons charge the storage grid 16 until the voltage drops to 0. Removal of the positive pulse from the conducting mesh d8 drops the storage grid 16 to +8 volts. This negative voltage on the storage grid effectively cuts off the viewing beam and thereby erases the charge pattern on the storage grid 16.

The erased pulse that is applied to the conducting mesh 18 effects a reduction in the diameter of the viewing beam. Thus, the viewing beam has a larger diameter in a viewing operation than in an erasing operation. The area scanned by the writing beam has a still larger diameter. As a consequence of this difference in the diameter of areas bombarded by the beams, two concentric annular portions of the storage grid 16 are not erased during an erasing operation, but continue to be built up in potential by the writing beam so that during a viewing cycle, one annular portion produces a bright ring on the viewing screen 12 and the other annular portion gives rise to Voltage breakdown. This is objectionable in that the bright ring and the variable display produced by the breakdown may be considered to be a part of the displayed image, and in that their high degree of brightness may make it diicult to view the desired image.

Prior attempts to solve this problem have not been entirely successful. One of such attempts has concerned the bright ring and has involved the application of a positive pulse to the target mesh 18. While this has widened the erase pulse, it is characterized by several objections including the need for expensive external equipment that may fail in operation.

This attempt, however, did not solve the voltage breakdown problem and the resultant current transfer to the screen, or undesired screen excitations through uncoated regions of the target. With respect to the voltage breakdown, it is found that during the coating application for producing the insulating coating comprising the storage grid 16, the coating material unavoidably collects on the region 49 of the inner surface of tiange 50 (FIG. 2). This region is exposed to the writing beam but beyond the range of the erasing beam. An attempted solution of the problem of voltage breakdown and resultant current transfer has involved utilization of a masking technique during application of the target coating 16 to the target mesh 18. However, it is important that all portions of the mesh 18 having apertures through which electrons may pass, be coated by the target material. To assure that the entire portion of target -rnesh 18 that is exposed to the several beams during tube operation is coated, it is impossible from a practical standpoint, to avoid coating application to the area 49 of the flange 50. When attempts -are made to avoid coating the area 49, adjacent areas of the target mesh 18 sometimes are shielded by the mask and remain uncoated.

According to the invention, the objectionable bright ring on the screen 12 and voltage breakdown, as well as objectionable effects from uncoated portions of the target mesh, are eliminated in an economical and dependable manner. As shown in FIG. 2, the means for eliminating the objectionable conditions referred to, may comprise a metal ring 52 fixed to an inturned flange portion 50 of the target support ring 54. The support ring 54 may be insulatingly fixed (not shown) to the metallic cylinder 22 supporting the collector grid 24, in any suitable way, such as by supporting means (not herein shown) described in co-pending application Ser. No. 90,795, of R. G. Spangler, and assigned to the assignee of the present application. Cylinder 22 may be fixed to the cylindrical collimating electrode 34 in a similar manner.

The ring 52 is preferably made of a non-magnetic metal or alloy, such as stainless steel. However, any metal or alloy useful in electron tube structures may be used, provided it can be welded easily or otherwise fixed to the support ange 50, and does not de-gas readily. Copper and aluminum cannot be welded easily and, therefore, are not preferred. Gold and platinum, in the absence of cost considerations, can also be used as the composition of ring 52. The thickness of ring 52 is signicant in view of the relatively close spacing of about one-eighth inch between the collector mesh 24 and the storage grid 16, and the relatively high voltage difference (i.e., from 145 to 210 volts) between these elements. The aforementioned distance of one-eighth inch is reduced at the facing regions of the ange 50 and the mesh 24 by a value substantially equal to the thickness of flange 50. This thickness is about mils in the interest of a rugged support for the target structure 1S, The ring S2 should be as thin as is consistent with ease of handling and spot welding and to avoid a further reduction in the space between the target support and the collector mesh 24. We have found that ring 52 may have a thickness of about 7 mils for good results.

The ring 52 should have a sufficiently large radial dimension, i.e., the dimension between its inner and Outer edges, to assure coverage of all target material deposited on the flange 50, during a coating operation. One way in which the insulating material of the storage grid 16 is applied, is by evaporation. To assure that the entire area of the target mesh 18, defined by the inner edge of flange S0 is coated, the evaporant is caused to overlap a portion ofthe flange 50. Since the exact degree of overlap cannot be determined readily, it is preferred that the outer periphery of ring 52 be closely adjacent to the cylindrical portion `of the support ring 54 with which flange 50 is integral and that the inner periphery of ring 52 extend slightly over the electron bombarded portion of the target structure 15. In this way, there is assurance that all peripheral portions of the target that could contribute to the formation of a bright ring and voltage breakdown, are covered by the ring 52. While ring 52 is therefore in a position to receive electrons from one or more electron beams, formed during tube operation, such electrons can be dissipated readily through lead 56 which is connected to the rings 54 and 52 (FIG. 1).

To provide further assurance that all areas of the target that may remain bare of coating are covered by the ring 52, the ring is preferably provided with a downwardly extending lip 58 as viewed in FIG. 2. This not only assures coverage and desired shielding from the Writing beam of any uncoated portion of the target mesh 18 adjacent to the inner periphery of the ange 50, but also provides a round corner that contributes to a prevention of flash-over between ring 52, which is at the potential of target mesh 18, and the collector mesh 24.

It should be noted that the purpose of ring 52 is not only to cover the coated regions of the storage grid 16 that are incapable of satisfactory erasure by prior art techniques, but also to cover any peripheral portions of the target mesh 18 adjacent to the inner edge of flange 50, that may not receive insulating coating materlal'of which the storage grid 1o is composed, during a coating operation. If such uncoated portions of the target mesh '18 are exposed to the electron beams, formed during tube operation, results harmful to tube operation are l1kely to occur. This is due to the fact that the target mesh 1S and the storage grid 16 are impressed with appreciably different voltages during tube operation. For example, the erasure pulse applied to the target mesh 18 raises its potential to about +10 volts, while the storage grid 16 (i.e., the charge holding coating) is raised only to about +8 volts. Removal of the erase pulse from the target mesh 18, reduces the potential of this mesh to from .0 to +5 volts, while the potential of the storage grld 16 1s reduced to -8 volts. Since it is the -8 volts on the storage grid 16 that accomplish cut off of the viewing beam, it is apparent that the viewing beam is not cut off at uncoated peripheral regions of the target mesh 18, which are at from 0 to +5 volts.

Failure to cut oft the erase beam at the uncoated peripheral region of the target mesh 1S during an erase operation, results in permeation of electrons from the erase beam through such regions. The electrons passing through such regions strike an annular portion of lthe phosphor screen 12 causing the appearance of a bright ring thereon during the erase operation.

Any uncoated peripheral region of the target mesh 18 is also objectionable during the writing and viewing operations. This is because any bare peripheral region is free of charge-holding insulation, and the writing beam cannot impress an electrostatic charge pattern thereon. As a matter of fact, the normal +2 volt potential on the target mesh 18, when coupled with the normal 0 volt potential on the storage grid 161, results in an attraction of electrons from the viewing beam to the uncoated portion of the target mesh 18, which attraction adversely effects the number of electrons passing through the coated region of the target and reaching the phosphor screen 12. During the viewing operation, electrons of the viewing beam may pass through an uncoated peripheral region of the target mesh 18 without modulation by the electrostatic charge on the storage grid 16. Such unmodulated electrons may be relatively numerous due to the fact that the potential on the storage ,mesh 18 is +2 volts while that on the storage grid is 0i. The resultant relatively large electron penetration of the uncoated regions of the storage mesh 1S, causes the appearance of bright displays on portions of the phosphor screen 12 disposed in opposed relation to the uncoated regions of the storage mesh.

The ring 52 should preferably have an inner diameter that is no larger than the diameter of the erasing beam. Preferably, the ring diameter should be smaller than the erasing beam diameter by an amount related to the tolerance observed in mounting the erase (viewing) gun in the tube. One acceptable tolerance in this connection is about 0.1 inch for the type of tube here involved. This tolerance value is indicative of the fact that the erase gun may be acceptably positioned so as to cause its bea-m to extend in any lateral direction a distance of about 0.1 inch from an ideal location concentric with respect to the tube envelope. The ring 52, therefore, should preferably have an inner diameter that is smaller than the diameter of the erase beam, by a value of about 0.2 inch. In a tube wherein the erase beam diameter is 4 inches, the inner diameter of the ring 50 may be 3.8 inches.

It is apparent from the foregoing that the invention provides a relatively simple and reliable structure for avoiding undesired excitations of the phosphor screen of a display storage tube.

We claim:

1. A display storage tube comprising:

(a) a charge storage target structure, said structure including a target support ring having an nturned annular support ange with an inner edge, and an apertured storage member having one face xed t one face of said ange,

(b) means for producing a writing beam of electrons directed towards said one face of said storage member,

(c) means for scanning said writing beam across said one face for charging the same,

(d) a phosphor screen closely spaced from the opposite face of said storage member,

(e) means for directing a viewing beam towards said one face of said storage member, whereby electrons of said viewing beam selectively pass through the apertures in said storage member and impinge upon said screen for producing a visible display thereon, and

(f) a conducting ring fixed to the other face of said flange and having a lip extending over said edge, said ring extending over an annular portion of said apertured storage member adjacent to said edge, whereby said edge and said annular portion are shielded from the viewing beam and failure to coat said annular portion with insulating charge-holding material and failure to leave said edge uncoated are free from harmful effects upon the operation of said tube.

2. A display storage tube comprising,

(a) the charge storage target,

(1) said target comprising a metal mesh having acharge storage coating on a portion of one side thereof, another portion of said side failing to receive said coating during a coating operation, and a target support ring having an annular inturned metal flange supporting the periphery of said mesh and having an inner edge,

(b) a phosphor screen closely spaced from the opposite side of said target,

(c) means for scanning a writing electron beam across said one side `of said target including said another portion thereof that is free of coating material, said coating on the coated portion of said mesh acquiring potential from said writing beam that is different from the potential of said metal mesh free of coating,

(d) means for `directing to said one side of the target a viewing beam having a transverse area equal to the areas of said coated and uncoated portions of the target, and

(e) imperforate conductive means covering said uncoated portion of the target, said means comprising a conducting 4ring fixed to said flange, said ring having a lip extending parallel to said edge and covering said uncoated portion of said target,

(f) whereby said screen is preserved from electrons of said viewing beam that have not been modulated by the charge storage coating of said target.

References Cited by the Examiner UNITED STATES PATENTS JAMES W. LAWRENCE, Primary Examiner.

R. SEGAL, Examiner.

Claims (1)

1. A DISPLAY STORAGE TUBE COMPRISING: (A) A CHARGE STORAGE TARGET STRUCTURE, SAID STRUCTURE INCLUDING A TARGET SUPPORT RING HAVING AN INTURNED ANNULAR SUPPORT FLANGE WITH AN INNER EDGE, AND AN APERTURED STORAGE MEMBER HAVING ONE FACE FIXED TO ONE FACE OF SAID FLANGE, (B) MEANS FOR PRODUCING A WRITING BEAM OF ELECTRONS DIRECTED TOWARDS SAID ONE FACE OF SAID STORAGE MEMBER, (C) MEANS FOR SCANNING SAID WRITING BEAM ACROSS SAID ONE FACE FOR CHARGING THE SAME, (D) A PHOSPHOR SCREEN CLOSELY SPACED FROM THE OPPOSITE FACE OF SAID STORAGE MEMBER, (E) MEANS FOR DIRECTING A VIEWING BEAM TOWARDS SAID ONE FACE OF SAID STORAGE MEMBER, WHEREBY ELECTRONS OF SAID VIEWING BEAM SELECTIVELY PASS THROUGH THE APERTURES IN SAID STORAGE MEMBER AND IMPINGE UPON SAID SCREEN FOR PRODUCING A VISIBLE DISPLAY THEREON, AND (F) A CONDUCTING RING FIXED TO THE OTHER FACE OF SAID FLANGE AND HAVING A LIP EXTENDING OVER SAID EDGE, SAID RING EXTENDING OVER AN ANNULAR PORTION OF SAID APERTURED STORAGE MEMBER ADJACENT TO SAID EDGE, WHEREBY SAID EDGE AND SAID ANNULAR PORTION ARE SHIELDED FROM THE VIEWING BEAM AND FAILURE TO COAT SAID ANNULAR PORTION WITH INSULATING CHARGE-HOLDING MATERIAL AND FAILURE TO LEAVE SAID EDGE UNCOATED ARE FREE FROM HARMFUL EFFECTS UPON THE OPERATION OF SAID TUBE.

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