US3838309A

US3838309A - Direct view storage tube having a lateral field neutralizing electrode adjacent the storage grid

Info

Publication number: US3838309A
Application number: US00263561A
Authority: US
Inventors: J Ogland
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1972-06-16
Filing date: 1972-06-16
Publication date: 1974-09-24
Anticipated expiration: 1991-09-24

Abstract

A transmission type direct storage tube in which a lateral field neutralizer electrode is positioned adjacent the transmission storage electrode to confine the lateral effects of charge on the storage electrode to individual mesh openings in the storage electrode and prevent interaction with neighboring mesh openings.

Description

nite States atet 1 Ogland 1 Sept. 24, 1974 [54] DIRECT VIEW STORAGE TUBE HAVING A 3,302,054 1/1967 Courtan i 315/12 LATERAL FIELD NEUTRALIZING 3,459,990 8/1969 Brooke et a1, 315/12 X 3,480,482 11/1969 Picker 315/12 X ELECTRODE ADJACENT THE STORAGE GRID Inventor: Jon W. Ogland, Glen Burnie, Md.

Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed: June 16, 1972 Appl. No; 263,561

Assignee:

References Cited UNITED STATES PATENTS 8/1965 Davis 315/12 X PULSE 45 \49 PULSE SOURCE J SOURCE T Primary Examiner-Carl D. Quarforth Assistant ExaminerP. A. Nelson Attorney, Agent, or Firm-W, G, Sutcliff [57] ABSTRACT A transmission type direct storage tube in which :1 lateral field neutralizer electrode is positioned adjacent the transmission storage electrode to confine the lateral effects of charge on the storage electrode to individual mesh openings in the storage electrode and prevent interaction with neighboring mesh openings.

9 Claims, 8 Drawing Figures SIGNAL SOURCE Emma 39241914 COLLECTOR ELECTRON PA'BH A cHAEEED CHARGED TO- 12v PHOSPHOR SCREEN DIRECT VIEW STORAGE TUBE HAVING A LATERAL FIELD NEUTRALIZING ELECTRODE ADJACENT THE STORAGE GRID BACKGROUND OF THE INVENTION The transmission type direct view storage display tube provides significant improvement over a conventional cathode ray tube in that it provides means for viewing an image for a substantial length of time without degradation of the image. It has been found in these storage tubes that there is a loss of resolution in the storage mesh. It is also found that the brightness of small spots on the display such as a moving target, does not increase linearly but logarithmically with deposited charge and saturates at a low level. It is found that a small target attains a much lower brightness than larger target on a radar screen. The number of shades of gray is also greatly reduced. This deficiency is particularly detrimental in that small spots or targets require higher brightness than larger spots for visual detection.

SUMMARY OF THE INVENTION A transmission type of direct view storage tube is provided in which a lateral field neutralizing electrode is utilized for confining the charge field associated with a single aperture in the transmission storage electrode to that aperture without affecting a neighboring aperture control.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of this invention, reference may be had to the preferred embodiment, exemplary of the invention, shown in the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a direct view transmission type storage tube in accordance with the teachings of this invention;

FIG. 2 is an enlarged view of the storage assembly of FIG. 1;

FIG. 3 illustrates the brightness built up of an enlarged spot or area in a storage tube in comparison with the built up of a small spot in a prior art type structure;

FIG. 4 illustrates roughly the field pattern within the storage grid assembly of a prior art structure;

FIG. 5 illustrates the field configuration in a storage tube in accordance with the teachings of this invention;

FIG. 6 is a perspective view of a modified storage target assembly that may be incorporated into FIG. 1;

FIG. 7 is a perspective view of another modified-stor age target assembly that may be incorporated into FIG. 1; and

FIG. 8 is a sectional view of another modified storage target assembly that may be incorporated in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a direct view transmission storage tube is shown. The tube comprises an evacuated envelope 10. The envelope 10 is comprised of a tubular body portion '14 connected by a tapered portion 16 to a tubular neck portion 18 of smaller diameter than the tubular body portion '14. The body portion 14 is closed at its other end by a face plate portion 20 of a suitable radiation transmissive material such as glass.

The other end of the neck portion 18 is closed by a suitable base portion 19 containing lead-in members (not shown) for applying voltages to the electrodes provided within the envelope 10.

An electron sensitive coating 22 is provided on the inner surface of the face plate 20. The electron sensitive coating 22 is a display screen and may be of a suitable phosphor material which emits visible light in response to electron bombardment. A suitable phosphor material is zinc sulfide. The phosphor coating 22 is also provided with an electrically conductive coating 26 of a suitable material such as aluminum. A lead-in 28 is provided to the exterior of the envelope 10 from the electrically conductive coating 26 and is connected to a suitable potential source 15. The potential applied to the conductive coating 26 may be about 10,000 volts positive with respect to ground.

Disposed adjacent to the phosphor screen 22 is a suppressor electrode 24. The suppressor electrode 24 may be positioned at a distance of about one-eighth inch from the screen 22. The suppressor electrode 24 may be of a suitable electrical conductive material such as nickel which is formed into an electroform mesh with about 500 lines per inch. The suppressor electrode 26 may be connected by a lead-in member 23 to a suitable potential source 35. The potential source 35 may be of a potential of about volts with respect to ground. A voltage source 55 may also be connected to the lead-in member 23 to provide operation of the electrode 24 in the manner set forth in US. Pat. No. 3,088,048 by Ogland et al.

A storage grid 30 is disposed adjacent-to the suppressor grid 24 and on the opposite side thereof with respect to the screen 22. The storage electrode 30 is comprised of a mesh 29 of a suitable electricallyconductive material such as nickel having about 500 lines per inch. The mesh 29 is provided with a coating 31 thereon suitable insulating material such as magnesium fluoride. Other suitable materials are silicon monoxide, calcium floride, and aluminum oxide. This coating 31 is disposed on the side of the mesh 29 remote with respect to the screen 22. It may have a thickness of about 2 micrometers. The storage grid 30 is disposed at a distance of about 0.01 to 0.03 inch from the suppressor electrode 24. The storage grid 30 is also connected to a suitable vpotential source 41 by means of a lead-in 13. The potential source 41 may provide a potential of about 15 volts with respectto ground. Again, a voltage source 53 maybe connected to the lead-in 13 to permit erasure in a manner set forth in US. Pat. No. 3,088,048.

A'field neutralizing mesh 40 is disposed on'the opposite side of the storage grid 30 with respect to the screen 22. The field neutralizing mesh 40 is positioned at a distance of .about 0.002 inch from the storage grid 30. Here again, the field neutralizing electrode 40 may be of electrical conductive material such as nickel having about 500 lines per .inch. The field neutralizing electrode 40 is connected by a lead-in member 42to a suitable potential source 45. The vpotential source 45 may provide a potential of about 5 volts with respect to ground.

A collector mesh 47 is disposed on the opposite side of the field neutralizing mesh 40 with respect to the screen 22. The collector-mesh 47 is positioned at a distance of about'0.2 inch fromthe field neutralizing mesh 40. The collector mesh 47 is of an electrical conductive material such as nickel having about 500 lines per inch. The collector mesh 47 is connected by a lead-in member 43 to a suitable potential source 49. The potential source 49 may provide a potential of about 250 volts with respect to ground.

A writing electron gun 50 is positioned within the neck portion 18 for generating and directing an electron beam onto the storage grid 30. The electron gun 50 generates a pencil-like electron beam of a small spot size. The electron gun 50 may be of any suitable construction to provide such a beam and comprises at least a cathode 52 and a control grid 54. The cathode 52 may be connected to the negative terminal of a suitable potential source 51 of about 2,000 volts with the positive terminal connected to ground. The control grid 54 is connected to an input signal source 57 with a suitable bias as illustrated. Horizontal and vertical deflection plates 56 and 58 are provided for deflecting the electron beam from the electron gun 50 over the storage grid 30. Suitable deflection voltages are applied to the deflection plates 56 and 58 to scan the writing electron beam from the electron gun 50, over the storage grid 30. Alternatively, a magnetic yoke may be applied to provide electromagnetic deflection of the electron beam.

Also positioned within the neck portion 18 of the envelope is a second electron gun assembly 60 which may be referred to as a viewing or reading gun and it performs the function of reading and erasing information on the storage grid 30. The reading electron gun 60 provides a large area beam so as to substantially flood the entire area of the storage mesh 30. The flood gun 60 includes at least a cathode 62 and a control grid 64. The cathode 62 may be connected to ground potential. A wall coating 66 extends from just in front of the electron gun 60 to near the collector grid 47. The wall coating 66 is connected to a suitable potential source 68 and is operated at a potential of about 70 volts positive with respect to ground. The wall coating 66 is an electrically conducting coating of a suitable material such as aquadag and is normally referred to as a collimating electrode for directing the flooding electron beams onto the storage grid so that the electrons approach substantially normal to the surface of the storage grid 30.

In the normal operation of a device prior to the writing operation, the storage grid 30 and the back plate or backing electrode 29 may be pulsed to a potential of about 20 volts positive with respect to ground by means of the potential source 53. The field neutralizing electrode 40 may be at a potential of about 5 volts. The flooding electron gun 60 will direct electrons onto the dielectric storage surface 31 causing the surface to charge to a potential of about ground. The potential difference across the dielectric at this time is about 20 volts. It is also customary in the operation of this device to pulse the suppressor electrode 24 to a negative potential of about 80 volts by the source 55 in order to prevent any electrons passing through to the phosphor screen 22 during the erase cycle. On removal of erase pulse, the electrode 29 will return to a positive potential of about l5 volts. The electrode 40 will be returned to a potential of about 5 volts. The charge stored on the dielectric surface 30 changes from ground potential to a negative potential approximately equal to about 5 volts negative with respect to ground due to capacitive coupling. This voltage is normally adequate to cut off the tube.

During the writing operation, the electron gun is modulated by the signal from the signal source 57 and generates a small pencil type electron beam which is deflected over the storage grid 30 by means of the deflection plates 56 and 58. The cathode 52 of the writing gun 50 is generally operated at a potential of about L500 to 2,500 volts negative with respect to ground. The signal source 57 modulates the control grid 54 of the writing gun 50 in accordance with the information to be written onto the storage grid 30. The collector grid 47 is operated at a positive potential of about 250 volts. in those areas, where the electrons from the modulated electron beam land on the storage grid 30, the electrons have sufficient velocity to produce a greater number of secondary electrons than incident primary electrons. Thus, more electrons leave the storage grid 30 than arrive on these elements of the storage grid struck by the writing electron beam and the surface 31 of the storage grid 30 assumes a less negative charge. The secondary electrons emitted from the storage grid are attracted to and collected by the collector electrode 47. Thus, the storage mesh elements may be charged to any potential intermediate between the storage grid cut off voltage and zero volts. In this manner, a storage charge pattern is written onto the storage grid 30 by the writing gun 50 in accordance with the modulation applied to the control grid 54 of the writing gun 50 from the signal source 57.

In the viewing operation, the viewing gun 60 provides a low velocity electron beam which floods the storage grid 30. A display with high brightness and long duration is possible because of the high viewing gun current which floods the entire screen during the viewing cycle as opposed to the conventional cathode ray tube where the electron beam excites any one screen element for less than a microsecond as it scans the area of the screen. The coating 66 collimates the flooding electron beam so that the electrons approach the storage grid 30 substantially normal to the surface. The collector grid 47 and associated coating 66 serve to accelerate the electrons in the viewing beam and to repel any positive ions which may be generated within the volume between the electron guns 50 and 60 and the storage grid 30. The potential charge on the storage layer 31 of the storage grid 30 determines the number of viewing electrons passing through the apertures in the storage grid When the charge about an aperture in the storage grid 30 is such as to allow passage of electrons, these electrons are accelerated by means of a positive potential of about 75 volts applied to the suppressor electrode 24 and the l0,000 volts applied to the coating 26. These electrons passing through the storage grid 30 then bombard the screen 22 causing the emission of light therefrom.

In order to appreciate the function and operation of the field neutralizing electrode 40 in this reading operation, reference is made to FIG. 3. Curve 70 shows the brightness built up of an enlarged spot or area of about three-sixteenths inch diameter versus charge on the storage grid 30 of a prior art device. Curve 70 indicates that the brightness built up charge is substantially linear. Curve 72, on the other hand, shows the brightness obtained when the beam is focused so that the area of charge is about 0.020 inch of a prior art device. Curve 72 shows that the brightness obtained does not rise linearly with charge built up and in fact reaches an early saturation in brightness. The utilization of a field neutralizing electrode 40 as taught by this invention results in the curve 72 for a small area spot being modified so that its brightness will more closely follow that of curve 70.

In FIG. 4, there is illustrated the field configuration in a prior art type storage tube wherein the charge is provided on the storage mesh of about negative 6 volts on one portion and negative 12 volts on another portion. The effect of this field configuration on the electron in the flooding beam is also illustrated. The lateral charge field effect between the charge areas extends over several apertures of the storage mesh and substantially reduces possible electron penetration and actually accelerates the electrons laterally. The result is that a reduced quantity of electrons will be able to penetrate the storage mesh and there is electron dispersion and loss of resolution. The bending of the field at the border line between the areas charged to different voltages is unavoidable in the prior art devices since the equipotential lines must converge along this border line in order to comply with the voltage change.

FIG. 5 illustrates the field configuration in a transmis sion storage tube which utilizes a field neutralizing electrode 40. The equipotential plane formed by the field neutralizing electrode 40 is placed close to the storage surface of the storage grid 30 and the field pattern shown in FIG. 4 is modified to that shown in FIG. 5. The field deformation caused by the non-uniform charge on the storage surface is confined to the particular mesh or wire of the border line. The field in front of all the other meshes is undisturbed. The brightness of a small spot will be the same as that of a large spot. The lateral movement of the electrons will not exceed one mesh opening. The mesh openings in the storage grid are about 2 mils and are therefore smaller than the beam thickness of the writing beam. The spot size, therefore, will be the same as the beam thickness. The field neutralizing mesh 40 should be positioned as close as possible to the storage surface of the storage grid 30 in order to limit the field deformation to one mesh opening. The spacing between the storage grid 30 and the field neutralizing mesh 40 should preferably be of the same order of magnitude as the mesh openings. This close spacing requires a high degree of precision in stretching the two screens. Actual contact, however,

between the two surfaces poses no problem, since the storage surface 31 is a good insulator.

A further improvement or complete elimination of disturbing lateral field and proximity effects can be obtained by physically merging of the equipotential plane and the storage surface. Such a structure is illustrated in FIG. 6. A storage and neutralizer mesh assembly 73 is shown in which two mesh openings 75 are illustrated.

The storage electrode portion is comprised of an elec trical conductive back plate mesh 76 with the storage surface 77 provided thereon. The field neutralizer electrode portion is provided by depositing a conductive mesh 78 on the storage surface 77 as illustrated. The field neutralizer mesh 78 thus surrounds each mesh opening 75 and provides a unipotential surface immediately at the storage surface and prevents the fields from the charges of one mesh opening extending into the neighboring mesh openings. In this manner, complete independence of the individual mesh openings is secured.

FIG. 7 illustrates another modified storage and separator neutralizer electrode assembly 79. The assembly 79 comprises a conductive backing electrode mesh 80. A storage surface 82 is provided and a groove 84 is provided in the insulating surface 82 and exposes a surface 86 on the conductive storage backing electrode 80. The exposed surface 86 of the backing electrode provides the equipotential surface. This arrangement is possible since the DC voltage level of the backing electrode may be chosen quite freely and therefore equal to that of the neutralizer mesh.

FIG. 8 illustrates another modification in which a storage, neutralizer and suppressor electrode assembly 93 is provided which comprises a backing electrode mesh 90 of electrical conductive material and a storage surface coating 92 covering the entire mesh 90. The side walls of the apertures are tapered to expose the side walls to the reading and writing beam. A field neutralizing electrode 94 is provided on the surface of the coated mesh facing the electron gun structure and a suppressor electrode 96 provided on the opposite surface of the storage mesh and both

electrodes

94 and 96 are insulated from the mesh 90 by means of the storage coating 92. In this manner, the three

separate grid electrodes

90, 94 and 96 may be provided in one unitary structure.

Although the present invention has been described with a certain degree of particularity, it should be un derstood that the present disclosure has been made only by way of example and numerous changes may be resorted to without departing from the spirit and the scope of the invention.

I claim:

1. A transmission storage display tube comprising an evacuated envelope and having therein a phosphor display screen, an apertured storage grid comprised of electrical conductive mesh having an insulating coating on at least one surface thereof opposite with respect to said phosphor screen, a writing electron beam means disposed on the opposite side of said storage grid with respect to said phosphor display screen for generating an electron beam and depositing a charge image on said insulating coating, a flooding electron beam means also disposed on the opposite side of said storage grid with respect to said phosphor display screen for generating and directing electrons through the apertures in said storage grid and modulated by the charge image thereon into incidence with said screen, and a field neutralizing electrode positioned adjacent to said storage grid between said electron beam sources and such storage grid, said field neutralizing electrode having a plurality of apertures and positioned with respect to said storage mesh so as to restrict the lateral field due to a charge on the storage mesh to substantially each of the individual apertures within said storage grid.

2. The tubes set forth in claim 1 in which the apertures in said field neutralizing electrode are aligned with the apertures within the storage grid.

3. The device set forth in claim 2 in which said field neutralizing electrode is positioned at a distance of less than 0.002 inch from said insulating coating on said storage grid.

4. The device set forth in claim I in which said field neutralizing electrode is positioned on the insulating coating on said storage grid and the apertures in said field neutralizing electrode are aligned with the apertures in said storage grid.

5. The device set forth in claim 1 in which said insu lating coating covers the entire surface of said mesh, said field neutralizing electrode positioned on the surface of said storage grid facing said electron beam sources and the apertures in said storage grid having tapered sidewalls to provide an aperture at the surface of said storage grid facing said electron beam sources of a first dimension and an aperture of a second dimension greater than said first dimension at the opposite surface and a suppressor grid provided on the opposite surface of said storage mesh with respect to said neutralizing electrode, said suppressor grid having a plurality of apertures with the apertures aligned with the apertures in said storage grid and insulated from the conductive mesh of said storage grid by means of said insulating coating.

6. A transmission storage display tube comprising an evacuated envelope and having therein a phosphor display screen, a storage grid comprised of an electrical conducting mesh with an insulating coating provided on at least a portion of said electrical conductive mesh, a writing electron beam means for depositing a charge image on said insulating coating, a flooding electron beam means for generating and directing electrons through the apertures in said storage grid onto said screen, said flooding beam modulated by the charge image on said storage mesh, said insulating coating covering a portion of the interstices of said mesh facing said electron beam sources and extending into the apertures in said mesh such that the insulating coating associated with each aperture is isolated with respect to the insulating coating of other apertures and a portion of the electrical conductive mesh between the insulating coating exposed to said electron beams to provide an equipotcntial grid in substantially the same plane as the insulating coating on said storage mesh.

7. A transmission storage display tube comprising an evacuated envelope and having therein a phosphor display screen, a combination apertured grid electrode provided adjacent said phosphor display screen, a writing electron beam means for depositing a charge image on said combination electrode, a flooding electron beam means for generating and directing electrons through the apertures in said combination electrode onto said screen, said flooding beam modulated by the charge image on said combination electrode, said combination electrode comprised of an electrical conductive mesh, an insulating coating provided over the entire conductive mesh to form an insulated coated apertured grid member whose interstices taper to a smaller dimension at the surface facing said electron sources, a first electrical conductive coating provided over the surface of said insulated grid facing said electron beam source and a second electrical conductive coating provided on the opposite surface of said insulated grid facing said display screen.

8. A transmission storage display tube comprising an evacuated envelope and having therein a phosphor display screen, an apertured storage grid comprised of electrical conductive mesh having an insulating coating on at least one surface thereof opposite with respect to said phosphor screen, a writing electron beam means disposed on the opposite side of said storage grid with respect to said phosphor display screen for generating an electron beam and depositing a charge image on said insulating coating, a flooding electron beam means also disposed on the opposite side of said storage grid with respect to said phosphor display screen for gener: ating and directing electrons through the apertures in said storage grid and modulated by the charge image thereon into incidence with said screen, and a field neutralizing electrode positioned on the insulating coating on the side of said storage grid facing said electron beam sources, said field neutralizing electrode having a plurality of apertures thereby aligned with the apertures of said storage grid.

9. A transmission storage display tube comprising an evacuated envelope and having therein a phosphor display screen, an apertured storage grid comprised of electrical conductive mesh having an insulating coating covering the entire surface of said mesh, a writing electron beam means disposed on the opposite side of said storage grid with respect to said phosphor display. screen for generating an electron beam and depositing a charge image on said insulating coating, a flooding electron beam means also disposed on the opposite side of said storage grid with respect to said phosphor display screen for generating and directing electrons through the apertures in said storage grid and modulated by the charge image thereon into incidence with said screen, and a field neutralizing electrode positioned on the surface of said storage grid facing said electron beam sources, and the apertures in said storage grid having tapered sidewalls to provide an aperture at the surface of said storage grid facing said electron beam sources of a first dimension, and an aperture of a second dimension greater than said first dimension at the opposite surface, and a suppressor grid provided on the opposite surface of said storage mesh with respect to said neutralizing electrode, said suppressor grid having a plurality of apertures with the apertures aligned with the apertures in said storage grid and insulated from the conductive mesh of said storage grid by means of said insulating coating.

Claims

4. The device set forth in claim 1 in which said field neutralizing electrode is positioned on the insulating coating on said storage grid and the apertures in said field neutralizing electrode are aligned with the apertures in said storage grid.

5. The device set forth in claim 1 in which said insulating coating covers the entire surface of said mesh, said field neutralizing electrode positioned on the surface of said storage grid facing said electron beam sources and the apertures in said storage grid having tapered sidewalls to provide an aperture at the surface of said storage grid facing said electron beam sources of a first dimension and an aperture of a second dimension greater than said first dimension at the opposite surface and a suppressor grid provided on the opposite surface of said storage mesh with respect to said neutralizing electrode, said suppressor grid having a plurality of apertures with the apertures aligned with the apertures in said storage grid and insulated from the conductive mesh of said storage grid by means of said insulating coating.

6. A transmission storage display tube comprising an evacuated envelope and having therein a phosphor display screen, a storage grid comprised of an electrical conducting mesh with an insulating coating provided on at least a portion of said electrical conductive mesh, a writing electron beam means for depositing a charge image on said insulating coating, a flooding electron beam means for generating and directing electrons through the apertures in said storage grid onto said screen, said flooding beam modulated by the charge image on said storage mesh, said insulating coating covering a portion of the interstices of said mesh facing said electron beam sources and extending into the apertures in said mesh such that the insulating coating associated with each aperture is isolated with respect to the insulating coating of other apertures and a portion of the electrical conductive mesh between the insulating coating exposed to said electron beams to provide an equipotential grid in substantially the same plane as the insulating coating on said storage mesh.

8. A transmission storage display tube comprising an evacuated envelope and having therein a phosphor display screen, an apertured storage grid comprised of electrical conductive mesh having an insulating coating on at least one surface thereof opposite with respect to said phosphor screen, a writing electron beam means disposed on the opposite side of said storage grid with respect to said phosphor display screen for generating an electron beam and depositing a charge image on said insulating coating, a flooding electron beam means also disposed on the opposite side of said storage grid with respect to said phosphor display screen for generating and directing electrons through the apertures in said storage grid and modulated by the charge image thereon into incidence with said screen, and a field neutralizing electrode positioned on the insulating coating on the side of said storage grid facing said electron beam sources, said field neutralizing electrode having a plurality of apertures thereby aligned with the apertures of said storage grid.

9. A transmission storage display tube comprising an evacuated envelope and having therein a phosphor display screen, an apertured storage grid comprised of electrical conductive mesh having an insulating coating covering the entire surface of said mesh, a writing electron beam means disposed on the opposite side of said storage grid with respect to said phosphor display screen for generating an electron beam and depositing a charge image on said insulating coating, a flooding electron beam means also disposed on the opposite side of said storage grid with respect to said phosphor display screen for generating and directing electrons through the apertures in said storage grid and modulated by the charge image thereon into incidence with said screen, and a field neutralizing electrode positioned on the surface of said storage grid facing said electron beam sources, and the apertures in said storage grid having tapered sidewalls to provide an aperture at the surface of said storage grid facing said electron beam sources of a first dimension, and an aperture of a second dimension greater than said first dimension at the opposite surface, and a suppressor grid provided on the opposite surface of said storage mesh with respect to said neutralizing electrode, said suppressor grid having a plurality of apertures with the apertures aligned with the apertures in said storage grid and insulated from the conductive mesh of said storage grid by means of said insulating coating.