US2433941A - Television transmitting tube - Google Patents

Description

Jan. 6, 1948. P, K, wElMER TELEVISION TRANSMITTINGTUBE Filed Sept. 16, 1944 3 SheetsSheet l MWF/V702 P14 L K VVE/MEP Jan. 6, 1948. P. K. wElMER 2,433,941

t'ELEVISION TRANSMITTING TUBE Filed Sept. 1 6, 1944 3 Sheets-Sheet 2 TTRNEY Wizz.;

Jan. 6, 1948. P. K. wElMER TELEVISION TRANSMITTING TUBE Filed sept. 16, 1944.v

3 Sheets-Sheet I5 ,4free/Vey Patented Jan. 6, 1948 TELEVISION '.lRANSmTl'lNG TUBE PanlKWelmer,Prlnceton,N.J.,assignnrto BadioCorporationolAmerlca,

ofDelawal-e Application September 16, lad, No. 554,494

My invention relates to television transmitting tubes of the type in which a target is scanned by a low velocity electron beam from an oppositely `disposed electron gun and more particularly to improved :means for amplifying by secondary electron multiplication the part of the main scanning beam which does not reach the target but comes back toward the gun as a modulated return beam and produces the signals.

Prior art tubes of the kind referred to have the dynodes of the multiplier stages arranged symmetrically in front of theelectron gun or asymmetricaliy at one side of the axis of the gun. The symmetrical arrangement with dynodes in front of the electron gun has undesired limitations, due to effects of the stages on the cathode beam and of the succeeding stages upon the ilrst stage, which is the most critical one. The asymmetrical type also has limitations arising from the difilculty of directing the return beam through the entrance area into the multiplier.

Where grids were placed in front of a second dynode arranged coplanar and outside the iirst dynode to accelerate or collect the secondary electrons, part of the electrons did not travel substantially perpendicular to the grids and were intercepted by the'grid wires instead of passing through .the meshes and striking the second dynode. thereby causing the gain of the iirst stage to be non-uniform over the region where the secondaries from that portion oi the first stage quired additional contacts and insulators in the:

tube and has not been an altogether satisfactory solution.

It is an object of this invention to provide a television multiplier tube of the secondary electron multiplier type, having a plurality of axially aligned multiplier stages constructed so as to collect a greater portion of the secondary electrons emitted` by the preceding stage than has heretofore been done.

u cum. (ci. :ss-iso) Vis simple in construction, easy to adjust operate, and whichallows the addition of any number of stages desired without iniluencing the operation of the rst, or most critical, stage.

Another object is todraw the electrons symvmetrically o ut of the complete range of the cathode beam without use of special electrodes or critical adjustments and to permit the addition of as many stages as desired without redesign of the parts.

Another object is to produce a multiplier having a comparatively large secondary electron entrance area with uniform total gain over the entrance.

Another object of the invention is to produce electron. multipliers with reduced shading eiect by the grids or screens, more particularly the one through which the electrons of the rst anode are intended to pass.

Another object is to constructv a television transmittingtube in which only the dynode of the irst multiplier stage is positioned in front of the gun and the dynodes of the additional multiplier stage or stages are placed around the gun back of the irst dynode. p

Another object of my invention is to provide a television transmitting tube of the low velocity electron multiplier type having greater and more uniform secondary electron amplication and minimum distortion vof the ampliiied signal.

Other objects will appear in the following speciication, reference being had'to the drawings. in which:

Figure 1 is a diagrammatic illustration of the tube in longitudinal section. the middle section being broken away to reduce the size of the e'ure.

Figure 2 is an enlarged elevation of the im proved grid-dynode unit of Figure 1. Y

Figure 3 is a sectional elevation taken on the line 3 3, looking in the direction of the arrows.

Figure 4 is an enlarged partial section taken on the line l-I, looking in the direction of the arrows.

Figures 5, 6 and 'i are modications showing diiferent forms of iinal dynodes as applied to a two-stage multiplier, by way of example.

'In general. a television transmitting tube embodying my invention is of the type in which a Another object is to provide a multiplier which 50 relatively low velocity electron beam is directed and decelerated to a photosensitive mosaic target, on whichan optical image is formed, and a portion of the beam modulated by the electrostatic charges on the elements of the mosaic target is accelerated towards and returns to the vicinity of the gun to impinge on an electrode having a beam-forming aperture. This electrode constitutes an anode of the gun with the surface facing the target, constituting the rst secondary emitting electrode or dynode of an electron multiplier. In such a tube the paths of the accelerated return beam from the target, where some of the electrons are removed from the beam by the electrostatic charges developed directly or in-r directly on the target by the optical image, are substantially, but not exactly, the same as the paths of the outgoing beam decelerated to the target. The return paths deviate slightly from the outgoing paths and may be at different points on circles around the outgoing paths, depending upon the deflection at any instant of time. The majority of the returning electrons impingeon the dynode o the first multiplier stage and very few pass through the beam-limiting aperture because of its small diameter and the divergence between the outgoing and returning paths.

The apertured dynode has a surface of high secondary electron emissivity facing the target and the secondary electrons produced by impact of the return beam are collected by a secondary electron emitter symmetrically positioned in respect to the beam path and producing further secondary electrons, which pass into the next stage of the multiplier. More secondary electron emission may be obtained by interposing another dynode in the paths of the secondary electrons from the second dynode.

The secondary electrons emitted by the dynodes have relatively low initial velocity and are readily aifected by the electrostatic field developed between them and the collector or the preceding dynode. as the case may be. The returning electrons of the beam follow non-symmetrical paths surrounding the outgoing path and are more subject to small potential gradients developed by the collecting electrode; consequently, these secondary electrons may be collected readily without introducing high potenitall gradients in the electron beam path.

While my invention is not restricted to use in any particular form of tube, I have shown in Fig. 1, by way of example, how the invention may be applied to a. form of tube in which reference character I indicates the envelope of glass or other suitable material, having the usual metal base 2 enclosing suitable insulation through which the terminals of the electrodes pass to the outside circuit. However, for convenience of illustration, the various connections normally passing through this base are shown as passing through the side of the tube, but it will be understood that this is for simplification of the illustration. A deflecting yoke 3 rits over the envelope' I and may be adjusted thereon by means of push rod 4. This deecting yoke vcontains a coil which produces an electromagnetic eld. say, perpendicular to the plane of the drawing, for horizontal scanning, and another coil which produces a field at right angles thereto for vertical scanning. A eld coil 5 surrounds the deiiecting yoke and extends from a point beyond the target end of the tube towards the gun at such distance as to produce an axial focusing eld in which the deiiecting elds are entirely immersed. Another field coil 6. may extend from a, point within the axial field of coil 5 to a point substantially adjacent the end of the gun. This is wound to produce a ileld perpendicular to the axial field of coil 5, which by rotational adjustment may be made to compensate for mechanical misalignment of the gun in the tube.

The envelope I has an enlarged diameter at the target end and encloses a semi-transparent photocathode I of the conventional type. A photocathode anode ring I0 adjacent the photocathode serves as an accelerating electrostatic field which, in combination with the axial magnetic eld of the coil 5, focuses the electrons liberated from the photocathode 1 upon the target II through the target screen I2. The target II may be of the type having an exceedingly thin glass or ceramic nlm of sufllcient conductivity to permit conduction perpendicular to the target, but due to this thinness, constituting an insulator to current flow at right angles thereto. Metal frame I2 supports the target II and screen I2 and is conductively connected to the latter. In front of the target II and closely adjacent thereto is a decelerating ring or lens I3, having substantially the same applied potential as the target.

While electrodes in the target end of the transmitting tube, as just described, may have various potentials, by way of example I will say that the terminal I4 of the photocathode may be connected to 350.volts, the terminal I5 0f the photocathode ring I0 may be connected to the -100 volt terminal and the terminal I6 for the target II and the terminal I1 of the decelerating ring I3 may be connected to zero potential. These terminals ordinarily pass out through the seals at I8 through the insulation ring base I8', but for simplicity of illustration I have indicated them as passing through the side wall of the enlarged end of envelope I. The electrodes I0 and I3 and the frame I2 are suitably supported and properly spaced, but these are not necessary for the understanding of the invention and are not indicated in the drawing.

Referring now to the electron gun end of the tube with which my invention is more particularly associated, the gun cathode I9 may be of the indirectly heated type. but the heating means is not illustrated. It also may be of any other desired type. This cathode may be grounded. Surrounding the cathode is the control electrode 20, which has the usual opening adjacent the electron emissive end of the cathode. Surrounding the control electrode generally called a grid and spaced therefrom is the rst anode, having the outer surface of its end adapted to act as the first dynode 2I of a multiplier stage. The end of the first anode consists of a disc having a minute perforation 22, through which the electrons are projected and decelerated toward the target II. This opening, of course, is shown greatly enlarged. Ordinarily it is only two or three thousandths of an inch in diameter. The

' disc of the dynode 2I is preferably integral with constitute the second, third, fourth and fth multiplier stages, but a lesser or greater number of these may be used, the illustration being by way of example only. The particular construction of dynodeunits 25. 26. 21 and 28 will be referred to later. All the dynodes may have surfaces of highemissivity for efficient production of secondary electrons by electron impact.

A collecting anode 29 of mesh or screen type is placed in front of the last multiplier dynode 28 to collect the secondary electrons emitted thereby. This collecting electrode is connected to one end of resistance 30, the other end of the resistance being connected to a high potential source. The grid of amplifier or signal tube 3| is connected to the collector end of resistance 30 through a suitable condenser 32 and-the usual leak resistance 33 may connect the grid andcathode of this tube. The plate of the tube 3| may be connected to the B supply and to additional ampliers for transmission to the receiving end of the television circuit or to. any other device for receiving the signals.

Ring anode 34 extends from adjacent the rear ofdynode 2| towards the target end of the tube for a sufiicient distance to shield the multiplier stages from the wall-coating voltage and in my improvement it may be given the same potential as the rst dynode.

The usual wall-coating electrode 36 may extend from the pOint on the envelope adjacent the ring 34 to a point adjacent the accelerating lens I3 and it may extend into the enlarged portion of the envelope at the target end of the tube, as shown. This wall-coating electrode may be a metallic film deposited on the envelope, as usual.

While the wall-coating and the `various electrodes and dynodes associated with the gun, as just described, may have various desired potentials, by way of example I will say that the resistance 3|)l may be connected to such point in the potentiometer or bleeder resistance R as to have a voltage of 1500 volts. be connected to a potential of 1450 volts, the dynode 21 to 1100 volts, the dynode 26 to 800 vol" the dynode 25 to 500 volts, the dynode 2| and the ring 34 to 220 volts, the wall-coating electrode 36 to 127 volts and the control electrode resistance 30 to a point -10 volts. The vzero voltagepoint of the bleeder resistance is grounded.

The terminals I4, I5, I6 and` I1 for the electrodesA at the front end of the tube may also be connected to bleeder resistance R.

Having described the general construction of the transmitting tube, I will now refer to the particular construction of the dynode units 25, 26 and 21,

The dynode units 25, 26 and 21 are preferably of the same construction and each consists of a metal plate having a secondary electron emissive surface and each is so constructed that the electrons emitted thereby may pass therethrough towards the rear of the tube (left in Figs. 1 and 4). Directly connected thereto is a screen element for controlling the field distribution adjacent the dynode. The details cannot be properly understood by using the scale of dimensions shown inFig. 1, so Fig. 2 illustrating the dynode units has been greatly enlarged, being substantially four times the scale of that used in Fig.` 1.

The last dynode 28 may' The dynode designated as 25 in' Fig. 2 is stamped or vanes 31 'separated by slots 38. These vanes` are punched up with the slots 38 (see also F18. 4) at the counter-clockwise side 39, the clockwise side 40 being left uncut by the stamping die. These vanes are inclinedat an angle to the plane of the disc and the direction of inclination is preferably reversed for alternate dynodes to form a tortuous pathfor the electrons passing through the slots between the blades. Preferably this die is so constructed that the outside or peripheral end 4| and the inside end 42 of the vane are left integral with the outside ring 43 and the inside ring 44. While it is not a limiting feature, I prefer to make the angle. a between the vanes 31 and the plane of the ring's 43, 44 about 30 degrees. Alternate discs, as above indicated, have the vanes slanted in opposite direction, so that there will not be a straight open path through adjacent dynodes for electron travel. This insures that the electrons, after passing through a slot, will strike the vane of the succeeding anode.

To shield a dynode from a preceding dynode, a

screen rigidity, it is preferable to mount them onV wire rings 41 and 48, respectively, to which they may be secured by light spot-welding or by any other means. These rings are of such diameter that they yfit snugly within the metal band or ring 46 and the three parts may be fastened together by spot-welding or by other means.

By Way of example I will say that I have secured good results in transmitting tubes by using metal of 5 mils thickness from which the dynode is stamped and crimping the peripheral edge around a ring of Wire 41 of 30 mils diameter. The ring 48, to which the screen is attached, may also be made of similar wire and the screen 45 may be made by knitting together wires of 1 mil diameter, having openings between the wires totaling about of the screen area, so that they do not intercept an appreciable number of the electrons. This is a standard form of screen obtainable on the market.

The last multiplier dynode 28 is a plane disc of metal and does not have the vanes, as shown in Fig. 2. This is because the secondary electrons emitted` from the last stage /are attracted back to the adjacent screen 29, which in this case is insulated from the dynode 28. This collecting screen may be mounted on a wire ring, as shown in Fig. 3, and supported in any way in front of the dynode 28 to space and insulate it therefrom.

When two multiplier stages, only, are used, the last stage 28 may take the' place of the second stage, 25, as shown in Fig. 1.

In the modification of Fig. 5, concentric rings 49, 50 are employed. By maintaining these at a potential equal to, or slightly less than that of the iinal dynode 5I, collection of the secondaries may be gotten by use of a coarser collector grid 52 than the screen 29` of the embodiment of Fig. 1. This also permits use of a lower voltage on the collector screen.

In the modification of Fig. 6, the dynode 53 of the last stage has been formed so as to have the outer and inner rings of Fig. 5 integral therewith and a ne screen 54 at or near the potential of the dynode 53 is placed in front of the collector screen 56. This permits use of a still coarser collector screen.

In the embodiment of Fig. 7, the rings 49 and 50 of Fig. 5 have been electrically connected and given the collector potential, so that they serve as the collecting anode. This dispenses with a collector screen altogether.

In the operation of the transmitting tube, an optical image of the object to be televised is pro- Jected upon the photocathode 'I by well-known means (not shown) and the electrons emitted by the cathode are focused on the target Il at high velocity due to the action of the focusing electromagnetic field of coil and the electrostatic eld of anode 2l. The target emits secondary electrons, due to this impact of the electrons, and a positive electrostatic image vis formed on the right-hand side of the target in Fig. 1. The positive charges on this surface of the target vary with the detail of the optical picture on photocathode 1, as well known. The secondary electrons emitted by the target aregfcollected by the screen I2 and pass to ground. The electrostatic image builds up continuously during the frame time.

Due to the extreme thinness of the target, the positive electrostatic image on the right-hand side of target Il in Fig. l sets up the same image potentials on the left-hand side. Electrons from the cathode beam of the gun are attracted to the left-hand side of the target and reduce it to gun cathode potential. During the time of scanning one frame, the electrons leak across the thin target film and discharge the positive charges on the right-hand side. The target fllm is thus conditioned for receiving an electrostatic image for the succeeding frame. During the scanning time of one frame, the charges on one elemental area cannot leak in the plane of the target film to adjoining elemental areas, due to the resistance of the thin film in this direction. The distance between the centers of the elemental areas is much greater than the distance between the two sides of the target, so the target film is a conductor transversely of the target and an insulator in the plane of the lm insofar as the time of scanning of one frame is concerned.

When the decelerating electrons from the cathode beam reduce the left-hand side of the target to cathode potential, say along path 56, the remaining electrons of the beam cannot land and return as a modulated beam accelerated to the gun along path 51, where they strike the first dynode 2|. The impact of the returning electrons knocks out secondaries from the surface of the dynode and these are accelerated towards the rear of the gun by the potential oi the screen and dynode of the second multiplier stage 25. Due to the large percentage of open area of the screen and also to the perpendicular travel of the secondary electrons, very few hit the wires of the screen and the remaining ones strike the vanes of the dynode, as at 59 (see Fig. 4) The impact knocks out more secondary electrons, as 62. The secondaries leaving the vane are considerably greater in number than'the primaries hitting the vane and this is called emission at a greater than unity ratio.

The secondaries, such as 62, are not attracted back to the vane by the potential of the preceding dynode 2| in this case, as the screen 45, which has the same potential as the dynode vane 31, shields the vane from that potential. The electrons are'thus subject to attraction through the slots 38 by the higher potential of the screen 76 and dynode of the third multiplier unit 26. The process is repeated for this unit and the secondary electrons pass out through the openings I8 to the fourth multiplier unit 21, where the multiplication proceeds in the same way. In this case, the secondaries emitted by the dynode 28 are attracted directly to the collector screen 28 and pass through the input resistance 30 of the signal tube 3|, which transmits the signal in greatly multiplied form to the amplifier. f

The construction and location of the improved multiplier units are particularly eflicient, as they direct the electrons toward the rear of the gun away from the neighborhood of the cathode beam shot out by the gun towards the target. They therefore do not deleteriously affect the action of the beam and the second and succeeding stages do not affect the critical first stage.

The construction of the units makes it practical to readily construct mulltpliers of any desired number of stages, as more or less units can be readily mounted into position over the gun.

Instead of using the wheel or vane type of dynode, I may use other forms of perforated discs, or mesh screens. ,Y

I have described my improvement in connection with a particular form of tube, but this is by way of example only.v The invention can be utilized in various kinds of tubes without limiting the usefulness of the improvement.

Various modifications may be made in my improvement without departing from the vspirit of the invention.

Having described my invention, what I claim is:

l. A television tube comprising an envelope containing a, cathode beam gun, a target. said gun having an anode with an aperture through which electrons of said beam are adapted to be projected from the gun toward said target and returned toward the gun with suitable velocity for producing secondary electron emission, said anode having a surface facing said target constituting a rst dynode for returning electrons, a second dynode adapted to be bombarded by the secondary electrons emitted by the flrst dynode, and a collector electrode for the secondary electrons emitted by the second dynode, both said second dynode and collector electrode being symmetrically disposed about said anode and both being farther away from said target than said first dynode.

2. A television tube comprising an envelope containing a cathode beam gun, a target, said gun having an anode with an aperture through which electrons of said beam are adapted to be projected from the gun toward said target and returned toward the gun with suitable velocity for producing secondary electron emission, said anode having a` surface facing said target constituting a first dynode for returning electrons, and a plurality of additional dynodes and a grid-like collector disposed coaxial with and behind the rst dynode and farther from said target than said rst dynode. Y

3. A television tube comprising an envelope containing a cathode beam gun, a target, said gun having an anode with an aperture through which electrons of said beam are adapted to be projected from the gun toward said target and returned toward the gun with suitable velocity for producing secondary electron emission, said anode having a surface facing said target constituting a rst dynode for returning electrons, a series of spaced perforated dynodes around said gun behind the rst dynode. each dynode adapted to be bombarded by the secondary electrons emitted by the preceding adjacent dynode, and a grid between i adjacent dynodes, each grid except the last one being conductively attached to the succeeding dynode, andthe last grid being adapted to collect I l the secondary electrons emitted by the last dynode of said series. f

4. A television tube comprising an envelope con- Y taining a' cathode beam gun,.a target, said gun producing Vsecondary electron emission, said anode having a surface facing said target constituting a first dynode for electrons, a second dynode around said gun behind the first dynode and having transversely inclined radial vanes separated by slots, said vanes adapted to be bombarded by the secondary electrons emitted by the first dynode, and a grid electrode between the first and second dynodes.

5.v Avtelevision tube comprising an envelope containing a. cathode beam gun, a targetsaid gun having an anode with an aperture through which electrons of said beam are adapted to be projected from the gun toward said target andreturned toward the gun with suitable velocity for producing secondary electron emission, said anode having a surface facing said target constituting a first dynode for returning electrons, a second dynodeV around said gun behind the first dynode and having transversely inclined radial vanes separated by slots,said vanes adapted to be bombarded by the secondary electrons emitted by the rst dynode, and a screen between the rst and second dynodes conductively secured to the first dynode.

6. A television tube comprising an envelope con-4 taining a cathode beam gun, a target, said gun having an anode with an aperture through which electrons of said beam are adapted to be projected from the gun toward said target and returned -toward the gun with suitable velocity f or producing secondary electron emission, said anode having a, surface facing said target constituting a first dynode for returning electrons, a series of spaced additional dynodes around said gun behind the first dynode, each dynode adapted to be bombarded by the secondary electrons emitted by 4the preceding dynode, all but the last one of said additional dynodes having transversely inclined radial vanes separated by slots, a grid preceding each additional dynode, a ring conductively attaching each additional dynode except the last one to the preceding grid, the grid preceding said rearmost dynode being adapted to collect the secondary electrons emitted thereby.

'7. A-television transmitting tube comprising a cathode beam gun, a target in the tube spaced from said gun, said gun having an anode with a surface constituting a first dynode and having an aperture through which an outgoing beam ondary electrons emitted by the first dynode, and

an electrode impinged by the secondary electrons an aperture through which an outgoing beam from said gun is adapted to be decelerated to zero velocity at said target and a return beam accelerated from said target to the surface of said rst dynode, electric field producing means for producing a focusing eld between said anode and said target` for the outgoing and the return beam, electric eld producing means forV scanning theoutgoing beam over said target and the return beam over the surface of the first dynode to bombard secondary electrons from-said surface, a second dynode adapted to be bombarded by the secondary electrons emitted by the rst dynode, and an electrode impinged by the secondary electrons emitted by the second dynode, both said second dynode and said electrode being symmetrically disposed about and behindv said rst dynode. Y Y

9. A television transmitting tube comprising a cathode beam'gun, a target in the tube spaced from said gun, said gun having an anode with a surface constituting a first dynode and having an aperture through which an outgoing beam from said gun is adapted to be decelerated to zero v'elocity at said target and a return beam accelerated from said target to the surface of said first dynode,.electric field producing means for y turn beam over the surface of the rst dynode to bombard secondary electrons from said dynode,

a second dynode adapted to be bombarded by the secondary electrons emitted by the first dynode, and a grid between the rst and second dynodes, both said second dynode and said grid being symmetrically disposed about and behind said first dynode.

10. A television transmitting tube comprising a cathode beam gun, a target in the tube spaced from said gun, said gun having an anode with a surface constituting a first dynode and having an aperture through which an outgoing beam from said gun is adapted to be decelerated to zero velocity at said target and a return beam accelerated from said target to the surface of said lirst dynode, electric field producing means for producing a focusing eld between said anode and said target for the outgoing and the return beam, electric field producing means for scanning-the outgoing beam over said target and the return beam over the surface of the rst dynode to bom- `bard secondary electrons from said dynode, a

series of perforated additional dynodes adapted to be bombarded by the secondary electrons emitted by the preceding dynode, a grid between adjacent dynodes, said additional dynodes and said grids being symmetrically disposed about and behind said rst dynode.

11. A television tube comprising an envelope containing a cathode, a grid, an anode having a. beam aperture with a multiplier dynode surface therearound, a target spaced from said aperture adapted to have substantially the potential of said cathode, said anode adapted to have a.

positive Apotential relative to said cathode. a' multiplier dynode encircling said anode behind said aperture adapted to have a greater positive potential than said iixst dynode and a collector electrode encircling said anode behind said aperture adapted to have a. greater positive potential than the last mentioned dynode.

' PAUL K. WEIMER.

REFERENCES CITED The following references are of record in the file of this patent:

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