US2403239A - Target electrode for electron discharge tubes - Google Patents

Description

A. ROSE Filed Aug. le, i941 July 2, 1946.

INVENTOR AL BERT ROSE Y www ATTORNEY Patented July 2, 1946 TARGET ELECTRODE FOR ELECTRON DISCHARGE TUBES Albert Rose, East Orange, N. J., assigner to Radio Corporation of America, a corporation of Dela- Ware Application August 16, 1941, Serial No. 407,132

9 Claims.

My invention relates to electron discharge tubes and more particularly to novel electrode structure for use in tubes of the low velocity electron beam scanning and electron image types.

In television pickup tubes utilizing high velocity electron beam scanning with optical or electron imagev projection on a surface other than the surface scanned by the high velocity beam it has been proposed to utilize double-sided mosaic-type targets, as disclosed in British Patent 442,666. Such target electrodes are exceedingly diiiicult to construct, the task of forming a structure having over 100,000 electrically discrete highly insulated metal plugs extending through the target and insulated from a signal screen embedded in the target being a tedious and difficult manufacturing problem. In addition, such electrodes, even though prepared with exceptional care, produce a spurious signal not representative of the electron or optical image, due to electrical or mechanical nonuniformities over the target surface.

In my copending application, Serial No. 357,- 543, filed September 20, 1940, I described and claimed a tube and electrode structure having lhighsensitivi-ty and substantially uniform signal I will here describe an generating properties. improved electrode structure which is particularly adapted for use in such television transmitting tubes therein described, the method of manufacture of such electrodes being more fully described and claimed in my application, Serial No. 407,131, filed concurrently herewith.

It is an object of my invention to provide a television transmitting tube having higher sensitivty and lower distortion than tubes constructed heretofore. It is a further object of my inventionto provide -improved electrode structure for a television transmitting tube utilizing low Velocity electron beam scanning which is capable of developing exceptionally high sensitivity with substantial elimination of all spurious signal, and

. it is a still further object to provide a television 2 the following description and accompanying drawing in which:

Figure 1 is a longitudinal cross-sectional view of a television transmitting tube utilizing an electrode made in accordance with my invention, and

Figure 2 is a greatly enlarged sectional view of a portion of the target structure and associated signal screen shown in Figure 1. The tube shown in Figure 1, as well as the operation thereof, is fully disclosed in my above first-mentioned application and will be described below only in rough outline.

Referring specifically to my tube structure shown in the `drawing, the tube comprises an evacuated envelope I enclosing at one end an electron gun structure 2 and at the opposite end a semi-transparent photocathode I3 of the conventional type with a target electrode 4 intermediate the electron gun and photocathode positioned to be scanned on one side by an electron beam from the electron gun and to have an electrostatic image formed on the opposite side. This electrostatic image may be formed onthe rear surface of the target 4 by projecting light, such as represented by the object arrow 5, through the lens 6 upon the photocathode 3 to liberate electrons therefrom, or the light may be projected directly upon the rear surface of the target in accordance with my prior teaching.

The electron gun assembly is preferably of the low velocity electron beam generating type, as described in my joint patent with Harley A. Iams, No. 2,213,175, and comprises a cathode 1, a control electrode 8 connected to the usual biasing battery and an anode 9 provided with a beam limiting 'aperture through which electrons may be directed toward the target 4, the anode 9 being maintained at a positive potential with respect to the cathode by a battery or potential source I0.

Intermediate the electron gun 2 and the target 4 I provide a pair of deflection plates II, a pair of shield plates I2-I3, one on either side of the deflection plates II having slots for the passage of the electron beam, and a conductive wall coating I4. The plates II are connected to a source of deflection potential and to the shield plates I2-I3 and coating I4, through a center-tapped resistance of l to 10 megohms. A magnetic coil I5 of slightlyv larger diameter than the envelope I extends over and beyond the space between the `electron gun 2 and target -4 to develop a magnetic focusing field and may extend beyond the photocathode 3 to focus electrons from the photocathode upon the rear surface of the target 4. The electrons from the photocathode 3 are accelerated by an electrostatic field such as generated by the wall coating I6 operated at a positive potential with respect to the cathode 3 by a battery or other potential source I1.

Deflection of the electron beam in a direction normal to that produced by the plates II is accomplishedby a pair of deflection coils I8, as well known in the art. The deection coils I8 may, of course, be replaced by a second pair of deflection plates and, in fact, any other type of.

electron gun and associated deflection structure capable of resolving an electron beam having a velocity approaching zero adjacent thetarget may be used in the place ofthose described.

In accordance with my invention the target comprises a very thin imperforate target sheet I9 of homogeneous electrically .semi-conducting material, the properties of which will be `discussed in considerable detail later, supportedas Vshown in Figure 1 in a plane normal to the longitudinal axis -ofthe envelope I. ,and consequently normal to the projected electron beam. .Contiguouswith thetarget sheet I!!v of .semi-conducting material and preferably radjoining .exceptionally small areas of the Yrear surface of thesheet I. provide a Signal screen 20. coextensive ,with .and very closely spaced over the nonsupported areas with respect to the rear surface of the semi-conducting sheet. The enlarged fragmentary section of Figure 2 shows the general form of the. target sheet I9 and closely associated signal .screen 20 as constructed in accordance with my invention.

Thetarget sheet I9 as indicated above is of semi-conductingmaterial .and I .have found that the specific resistance of the material used should lie withinv the rangeof 109 to 1'012 ohm-centimeters. Preferably an exceptionally thin .homogeneous sheet. of vitreous material such as glass having the. desired specific resistance and being smooth and. of uniform thickness may be used. I have. found that a thin sheet/ of yglass having aspecific resistance of approximately 101 .ohm-centimetersv is suitable, and the' glass described below which I have usedmay be formed in. very thin films.

In .constructing thisportlon of `my structure the glass may be blown to form a very thin cylindrical Vglass iilm which is laid upon an insulated wire mesh screen having crimp promontories which extend above the plane of the screen and to which the glass film is sealed by heating the glass film supported by the insulated screen until the. glass film is in va semi-molten state, whereupon by surface tension the sheet. of glass contracts, to form a. substantially plane surface.

I have made such assemblies wherein the thickness'of the sheet of glass is less than 0.0002",

vwhich is less than one-fourth the thickness of mica usually used for high resistance types of mosaic electrodes. Following the assembly of the semi-insulating sheet I9 and signal screen 20, these parts are supported in the tube as shown in Figure 1 and an electrical connection made from the Ysignal rscreen to a translating device such as 4whereas larger areas between the very small support areas are uniformly spaced with respect to then signal screen 20. To provide small areas of contact I utilize a woven wire mesh screen, which is ,not rolled or flattened to an appreciable degree, so that the'wires of the woof and the wires of the warp are: substantially nonplanar. I have found that it is desirable to utilize Woven wire mesh having alternate points of the crossed mesh wiresextending above the normal plane of the the thermionic tube 23 and tothe positive tery,

minal of the potential source I1 through an output impedance 24 andthenceto the battery I9 vat a point substantially equal to or slightly positive with respect to the'potential applied tothe cathode 'I. The wall coatingv II` is shown connected through an auxiliary potential source to maintain this coating slightly negative with respectto the signal screen: 20, `although these electrodes maybe operated at'the same potential.

While one method of forming such an electrode Vassembly is fullydisclosed-in 4my concurrently ,in temperature than the glass sheet I9.

signal screen. Thus the crimp promontories of the crossed wires extend bothabove and below the main body of the screen. It is -also desirable to provide a, thin coating of vitreous enamel or other vitreous material .on the side of the signal screen adjoining the thin glass sheet I9 for the purpose of bonding the screen to the sheet over the raised promontory areas, and to insulate the screen from the thin glass sheet. 'While a thin glass sheet may be fused to an enameled rolled line mesh screen, the .thin glass tends to be drawn into the Vholes by surface .tension of the glass working along the enamel. This is .especially true if `the .temperature of the ring exceeds a predetermined maximum. In addition, the electrical properties of such a structure are unsatisfactory, especially when the wire screen is. greater than mesh per inch. While such an assembly is satisfactory when utilizing a relatively coarse mesh screen, such as `less than 150 mesh per inch, the structure is far from satisfactory when utilizing a iine mesh screen .such as from 150 to 250 mesh per inch. I have found that, when utilizing my method of shrinking the glass by surface tension, the thin vsheet of glass is substantially tangent to the surface of the woven wire mesh at the crimp promontories. Thus it is important that the screen following weaving should not be rolled to the .extent of iiattening the normal crimp that appearsY in the wires, in fact, the existence of the crimp permits the glass to `touch only at the crimp promontories, leaving the exposed surface of the glass sheet'planarV and spaced, on the average, far .enough from the signal lscreen'to provide the desired target capacitance as described in my rst above-mentioned application.

Briefly, the woven Wire screen 20' is etched to provide a relatively high light vtransmission such as 65% and coated on at least the side which is to face the glass sheet 20 with a thin film of enamel having high electrical insulation properties. Such an enamel should have a specific resistance higher than that of the glass sheet 20, andk may be 5` 1012 ohm-centimeters or higher. This resistance is considerably above that of the thin `glass sheet I9 which may range from` 109 to 1012 ohm-centimeters. The enamel likewise preferably has a softeninfg'point which is lower The enamel may be applied by spraying to a thickness of .0001 to .0002" followed by firing to vitrify the enamel, whereupon the thin glass sheet |81 is supported on the' screen 20 and fired for a Yperiod of time and at such a temperature that 5 rture softening point than that of the thin glass, it tends to adhere to the thin glass sheet at the points of contact, such as at the crimp promontories.V The promontories tend to prevent the glass from drawing into the interstices of the mesh and the firing is controlled over such a short period of time or at such a low temperature as t` prevent such action. Firing for too long a period of time or at too high a temperature permits the thin glass to sink into the holes of the screen. The glass for the sheet I9 and the enamel for the wire mesh may have the following composition, these percentages by weight resulting in materials which have the above desired properties.

Component Glass sheet Mesh enamel v Percent Percent Referring to Figure 2 which shows a structure made in accordance with this method, the wires 40 are covered with the thin lm of enamel 4I Y in such a manner that the crimp promontories 42 are not destroyed. It will be observed from the gure that the thin glass sheet I9 is in contact with the enamel screen only at the crimp promontories 42 and that the enamel 4I has become sealed to the glass sheet. While I have described an assembly wherein the Wire mesh screen may be coated over its entire exposed area with a' thin lm of enamel, the useof such an insulated screen requires the use oi an auxiliary electrode to collect secondary electron emission when utilized as shown in Figure 1. In the absence of a longitudinal magnetic field, such as developed by the coil I5, the electrode coating I6 may serve to collect secondary electrons ydeveloped by impinging primary electrons from the photocathode 3. If, however, the longitudinal magnetic eld is utilized, I have found it desirable to provide a relatively coarse wire mesh screen between the photocathode 3 and the target 4 to collect the secondary electrons. However, to avoid the use of such an auxiliary electrode the signal screen 20, as indicated in Figure 2, may be enameled on only one side, that is, the side adjoining the thin glass sheet I9.

While I have described the use of an intermediate thin coating of enamel upon the wire mesh of insuflicient thickness to destroy the crimp promontories, the thin glass sheet may be sealed directly to these Crimp promontories without the use of such an intermediate enamel nlm. For example, the above steps, with the exception of those relating to the application and iiring of the enamel, may be performed, whereupon the thin glass upon ring and shrinking by surface tension becomes attached directly to the crimp promontories. Such electrode structure, however, is more diiiicult to utilize, inasmuch as the glass of the sheet I9 is of relatively low electrical resistance and it is therefore necessary to provide substantially uniform electron beam velocity over the entire scanned areas.

The assembly comprising the sheet I9 and con.- tiguous signal screen 20 is supported in the tube I and connected as described above, and in operation high velocity electrons representative of an electron image of the picture to be transmitted are focused on the rear surface of the thin target sheet I 9 to form a positive electrostatic image on this rear surface by secondary electron emission. Other methods of forming a positive electrostatic image on the target sheet I9 may be utilized, such as by scanning the rear surface of the sheet with a modulated high velocity electron beam or by providing a ydiscontinuous coating of caesium or other light sensitive material on the rear surface of the sheet which, by emission of photo-electrons, forms an electrostatic image consisting of positive charges representative of the optical image to be transmitted. 'Ihus in the latter case the light from the object 5 may be focused directly on the rear surface of the target sheet I9, as also shown in Figure 1. The opposite or front Surface of the target sheet I9 is then scanned with an electron beam preferably of low velocity to develop a train of signals representative of the gradation in light and shade areas of the optical image.

The mode of operation may be considered in more detail taking into account the following assumptions: First, that the electron beam scans an area A of beam cross-sectional area on the front side of the sheet I9; second, that a corresponding area B on the opposite side of the sheet I9 and opposite area A receives a portion of the electrostatic image, and third, that the intensity of the electrostatic image over the area B is zero. The scanning beam such as the beam of low velocity electrons charges the area A in a negative direction to cathode potential so that no further electrons from the beam can be collected over the area A and no signal is transmitted to the signal screen 20. Now let picture electrons such as from the cathode 3 be projected on the surface of the sheet I9 opposite A and over the area B at such a velocity as to charge it positively by secondary emission. The charge on B may be stored continuously by making the signal screen 2D suliciently positive with respect to the photocathode 3 to collect the secondary emission on the signal screen 20, provided the side opposite the sheet I9 is not insulated. Since the area B is positive, the space around B and including an area of the signal screen 20 will also be positive by electrostatic influence. Points very near B will have almost the same potential as B and, in particular', the area A directly opposite B and on the scanned side of the target sheet I9 will have almost the same potential as area B. A portion of the electrons of the scanning beam in passing over the area A will therefore be able to land and deposit enough charge to reduce the residual charge on the area A to the potential of the cathode 1. During the subsequent frame time and prior to the next scanning of the area A by the electron beam, the positive charge at B and the negative charge at A unite by conduction through the sheet of semi-conducting material to return both areas A and B totheir original uncharged states. Assuming the proper selection of specic resistance for the material of sheet I 9, very little transverse redistribution of the charges from area to area of the target Will result. If, however, the specic resistance of the material is too high, such as 1013 ohm-centimeters or more, the neutralization of the charges on opposite sides of the sheet I9 is not completed by conduction during a single frame time, and the electrostatic picture side or rear surface of the sheet I3 will tend to charge up to the signal screen potential. However, if the semi-conducting material is chosen with the proper specic resis/tance, a positive charge on area B will be neutralized by the negative charge on area A during a single frame period.

Further, I have found that four conditions should be satisfied if. the semi-conductor is to operate eniciently to transmit a Well-defined television picture with adequate signal strength and without spurious effects. In the first place, the period of discharge between the two sides of the target sheet should be equivalent to the frame time. Secondly, the capacitance between the two surfaces of the semi-insulator should be as great as and preferably several times greater than the capacitance between the picture side of the target and the signal screen.

The capacitance between the areas A and B should be several times the capacitance between B and the signal screen 20. These relationships of capacity are to insure that a reasonably large fraction of the picture charge is converted to useful -picture signal. This fraction of picture charge so used is actually CAB CAB 't C1320 where Caszcapacitance between areas A and B and Cszo=capacitance between area B and the signal screen 20. Due to the exceptionally small spacing desired between the signal screen 2e and the semi-insulating sheet I9, the capacitance between B and any surrounding conductors other than the signal screen 2i! is negligibly small. The third condition is that the thickness of the semiinsulator should be less than one-half the diameter` of a picture element, and the signal screen spaced less than a, picture element from the nearest adjacent surface of the target sheet I9 in order to confine the electrostatic influence of a picture element on the picture side to only the corresponding element on the scanned side. The fourth condition requires that the time constant for the lateral diffusion of a picture charge should be equal to or greater than the period of frame scanning. This condition may be expressed and satised by the following relationship:

Where p :volume resistivity of target (ohm-cm).

E=e1emental picture area (square centimeters).

C='capacitance between picture side of target and signal screen (farads/cm-2).

D=thickness of the target sheet (cms).

t :frame time (seconds).

A reasonable set of values for these parameters satisfying all of the four conditions above is:

While in the example given above I have described a tube making use of electron image projection to form the electrostatic Ipicture image on the rear surface of the semi-insulating target, I have also mentioned that this electrostatic image may be formed by light projected directly on the rear surface, provided the surface is sensitized with caesium. However, the described electron image projection method is preferred because of the larger obtainable signal. My electrode may be distinguished from the conventional double-sided type of mosaic electrode, such as the type described in my U. S. Patent 2,213,173, in that I provide no conductive plugs extending through andinsulated from a wiremesh screen embeddedinhigh resistance insulation such as vitreous enamel. In this type of construction it is necessary to yprovide exceptionally high resistance between the plugs and the embedded screen for the express purpose of preventing electrical leakage between the plugs and the screen. In my improved structure I avoid the diiiiculties attendant upon the yprior art structure by wholly eliminating the embedded screen. I have found it disadvantageous to provide any good metalI contact between the semiconducting target sheet I9, which must be' homogeneous, and any other electrode in the4 tube such as the signal screen 20. The signal screenl 20 must therefore be and is substantially insulated from the sheet I9 so that electrical contacts between the signal screen and the sheet I9 are a minimum. My invention is further distinguished over the prior art which utilizes a mica foundation for a mosaic electrode or any other insulation having high electrical resistance such as mica which is of the order of 1020 ohm-centimeters. My invention should therefore not be confused with tubes using these types of electrodes. In fact, I have attempted to operate tubes of this type wherein the target consisted of a mica foundation scanned on one side by a low velocity electron beam, the electrostatic image being formed on the opposite side and have found that such tubes will not operate with a low velocity electron beam because after a few scansions of the beam, elemental areas of the target become charged up to the secondary electron collector potential, thereby preventing any further operation. With my new and'improved tube, however, I have obtained from 3 to 10 times the sensitivity usually obtained in tubes of the yOrthicon type as described in U. S. Patents 2,213,174-6 which, in turn, are from 10 to 50 times more sensitive than the conventional Iconoscope which utilizes a high velocity electron beam. Consequently, the electrodes made in accordance with my invention when utilized in conjunction with low velocity electron beam scanning have. produced sensitivities of the order of 30 to 500 times the sesitvity of the 'well-knownconoscope type of tu e.

While I have indicated the preferred embodiments of my invention and have indicated the specific application as directed to cathode ray television transmitting tubes, it will be apparent that my invention is by no means limited. to the purpose of television transmission and that many Variations may be made in the particular structure disclosed without departing from the scope of the invention as set forth in the appended claims.

I claim:

1. A target electrode for an electron discharge device comprising a thin imperforate sheet of insulating material sealed to a foundation including a Woven wire mesh screen, the said sheet being sealed only to the. crimp promontories of said foundation.

2. A target electrode for an electron discharge device comprising a thin imperforate sheet of insulating material sealed to a woven wire mesh screen, the said sheet contacting the wire mesh screen only over the crimp promontories thereof.

3. A target electrode for an electron discharge device including a woven wire mesh screen, a thin coating of insulation on said woven wire mesh and a contiguous thin sheet of insulating material sealed. only to the crimp promontories of said insulated Wire mesh screen.

4. A target electrode adapted to be scanned by an electron beam comprising an insulated Woven wire mesh screen, a contiguous sheet of material having a specic resistance of from 109 to 1012 ohm-centimeters, said sheet of material being sealed only to the crimp promontories of said insulated wire mesh screen.

5. A target electrode adapted to be scanned by Van electron beam comprising a woven wire mesh screen having crimp promontories extending from the surface thereof, a thin coating of insulation on said wire mesh screen, an imperforate sheet of insulating material contiguous with the insulated crimp promontories of said screen, said sheet having a specific resistance lower than the specific resistance of said coating of insulation.

6. A target electrode comprising a woven wire mesh screen having crimp promontories extending` above the surface thereof, a coating of insulation covering one side only of said wire mesh screen and a thin imperforate sheet of glass contiguous with the coating of insulation only over the crimp promontories of said wire mesh screen.

7. A target electrode comprising a woven Wire mesh screen having crimp promontories extending above the surface thereof, a coating of insulating material having a specic resistance of at least 5 x 1012 ohm-centimeters over at least one side of said screen and a thin sheet of insulating material having a specic resistance of 109 to 1012 ohm-centimeters contiguous only with the insulated crimp promontories of said screen.

8. A target electrode comprising a Woven wire mesh screen having crimp promontories extending above the surface of said screen, a coating of insulation on said screen of insufficient thickness to prevent a light transmission of 50% of incident light through the interstices of said screen, and a thin planar sheet of insulation contiguous with and tangent to the insulated crimp promontories of said screen.

9. A target electrode comprising a perforated metal screen having small sections extending above the surface thereof, a coating of insulation over said small sections, and a thin sheet oi' insulating materia1 contiguous with and sealed only to the small sections of the insulated screen.

ALBERT ROSE.

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