US2407906A - Low velocity television transmitting apparatus - Google Patents

Description

A. ROSE sept. ,'17, 1946.

l LOW VELOCITY TELEVISION TRANSMITTING APFARATUS FiledAug. 27, 1942 Hf INVENToR aleroe/ ATTORNEY Patented Sept. 17, 1946 LOW VELOCITY TELEVISION TRANSMIT- TING APPARATUS Albert Rose, East Orange, N. J., assignorto Radio Corporation of America, a corporation of Delaware Application August 27, 1942, `Serial No. 456,344

My invention relates to television transmitting tubes and particularly to tubes utilizing low velocity electron scanning beams.` j

In the operation `of a television `transmitting tube of the low velocity electron beam scanning type including a target 'or mosaic electrode with an oppositely disposed electron gun and a combination of magnetic-electrostatic deflection means, the electrons of the scanning beam which do notreach the target because of their low velocity return to a'collecting electrode at the surface of which they ymay produce secondary electrons. It has been customary to utilize a signal plate comprising a portion of the target electrode from which the signal may be taken althoughthe signal may be derived from the co1 lecting electrode. With this latter arrangement, it is advantageous to provide means for develop ing secondary electrons, such as shown by Iams in his U. S. Patent 2,288,402.

The requirements to be met in the design oi a television transmitting tube incorporating an electron multiplier for amplifying the return portion of the scanning beam 'diier in one-important respect from the `requirements for the usual electron multiplier in that in such a television tuJbe the virtual point `of origin varies in accordance with the scanning f the target and consequently in conventional structures, such as shown by Iams, the returning beam strikes at various points on the rst electrode of the multiplier. If exceptional care is not taken in preparing the multiplier electrodes, some distortion may be introduced in this type of structure.

Accordingly it is an object of my invention to provide a structure of the television transmitting llow velocity multiplier type wherein more uniform secondary amplifica-tion may be obtained. It is another object to provide such a structure wherein the multiplier electrodes may be more easily processed. It is another object to provide a structure wherein distortion produced `by nonuniformity of secondary electron emitting :surfaces is minimized. It is still another object of my invention to provide a tube wherein theelectrodes may be made of such form and shape as to minimize introduction of distortion and a structure wherein the overall length of the tube may be decreased.

In accordance with my invention an electrostatic image corresponding in electrostatic energy distribution to an optical image is formed on a target preferably of the photo-sensitive mosaic type which is scanned in two mutually perpen dicular directions Iby wholly magneticmeans, the

electrons of the beam being incident upon the target in accordance with the electrostatic charges thereon so that electrons not reaching the target are returned along substantially the same paths as the scanning `beam to an electron `multiplier made in accordance with my invention. The said electron 'multiplier preferably comprises an electrode or electrodes symmetrically surrounding the initial beam' path immediately following its formation as a `beam and prior to its deflection by wholly magnetic means. More particularly in accordance with my invention I provide a structure comprising a cylinder, truncated cone, annular `electrode or a plurality of such electrodes directly adjacent the point of the `nal beam formation and between said point and the deection means. Such points of beam formation may be'dened as the -point where the cross-sectional areas of the beam is limited and at which the beam has its minimum diameter.

The objects mentionedtabove as well as other objects, features, and advantages of my invention will be apparent when considered in view of the following description taken in connecticn with the accompanying drawing in which:

Figure 1 is a longitudinal cross-sectional View of television transmitting apparatus made in accordance with my invention; and 1 Figures 2 and 3 show modilied forms of certain of the electrodes shown iniFigure l.

In general, the apparatus of my invention comprises an evacuated envelope having a target `preferably of the photo-sensitive mosaic type at one end thereof and an electron source and an anode oppositely disposed therefrom to develop and project upon the target an electron beam constrained along its path by a longitudinal magnetic eld and deiiected over the target by magnetic means, such a plurality of magnetic coils developing fields normally intersecting one another so that the mosaic electrode may be. scanned in mutually perpendicular directions. 'llhe electron source may be of the conventional type comprising a small area cathode surrounded by a conventional apertured grid for purposes of electron beam intensity'control being followed inthe direction toward the target by an anode having a beam limiting aperture. This aperture defines the minimum electron cross-section and diameter and is necessarily small to prevent returning electrons from approaching the cathode, which electrons might be redirected to the target. Immediately adjacent this defining apertured anode and between this anode and the deflection means I provide symmetrical Vmeans surrounding the kmosaic electrode such as along the path 4 shown by the dashed line. This path will be referred to as the initial electron path. The mosaic electrode is preferably of the type described'in my U. S. Patent No. 2,213,176 and includes a sheet of insulation, s-uch as mica sheet 5, having on its rear surface a semi-transparent electrically conducting condenser plate 6, usually referred to but not used as a signal plate, and on the front surface a mosaic of mutually insulated and separated photo-sensitive particles 1. Various methods of manufacturing such electrodes are well known in the art and obviously photo-conductive or'photovoltaic targets may -be utilized if desired. The

electrode structure 3 at the opposite end of the envelope I comprises a cathode I from which electrons may be drawn, a control electrode II connected to the usual biasing .battery and an anode I2 having a beam deiining aperture I3, this anode being maintained positive with respect to the cathode I0 by a potential source I4 to accelerate the electron beam along the initial electron path 4. The beam defining aperture determines the minimum cross-section of the beam and substantially prevents electrons not reaching the target from entering the space between the cathode and anode as appears more particularly hereinafter. Preferably the cathode III is connected directly to the condenser plate 6 of the mosaic electrode so that electrons approaching the mosaic are decelerated to substantially zero velocity, these electrons being collected or rejected by the mosaic in accordance with theintensity charges thereon developed by light representative of an optical image.

To direct the electron beam in a focused condition between the beam defining apertured anode I2 and target, I provide a focusing coil I6 wholly enclosing Yand preferably extending beyond the space between the anode I2 and the target 2. This focusing coil is so designed as to develop a longitudinal magnetic field strength of approximately -75 gausses which I have found sufficient to maintain the electron beam in a focused condition. Due to the presence of this iield the electrons of the beam passingthrough the beam defining apertured anode with initial transverse velocity follow helical paths having a number of focal points along lthe beam path so that while I have referred to focusing of the beam, it will be appreciated that'the beam has a number of focal points along the beam path, the potential between the anode' and cathode in combination with the strength of the magnetic field being so proportioned as to develop -a beam focal point preferably in the plane of or on the surface of the target 2. In addition I provide, preferably spaced .between the beam limiting apertured anode and the target, magnetic means to deect the beam in two mutually perpendicular directions such as two sets of magnetic deflection coils I'I and I8. The coils I1 are shown displaced from their actual operating position for the sake of clarity in the drawing. However, the coils II and I8 may be superposed, in which case the tube may -be made somewhat shorter.

In operation of the structure so far described the electron beam follows an initial path 4 parallel with the longitudinal magnetic field developed by the coil I6 until under the influence of the deiiection coils I'I and I8 the beam is deflected and scanned over the target 2. Electrons of the beam are collectedV in accordance with the electrostatic charges developed by the optical image light and electrons not being collected by the mosaic then return in the direction of the electrode structure 3 over a return path 4a. Neglecting the transit time of the electrons, inasmuch as their relative velocity is high in comparison with the rate of change ofdeflection, the electrons follow substantially the same path while returning as while approaching the target. I have found that the return path deviates vfrom the initial path and follows paths surrounding the initial path depending on the deflection at any instant of time. Consequently, the great majority of the returning electrons become incident upon the anode I2, very few passing through the beam limiting aperture I3 because of its small diameter and .because of therdivergence between the initial and return paths.

In accordance with my invention, I provide the beam limiting apertured anode I2 of secondary electron emitting material sovthat as the returning electrons are reaccelerated by the anode I2,

they impinge thereon and develop secondaryV electrons, and I provide means symmetrically surrounding the electron beam path to collect these secondary electrons or produce further secondaries which are then collected. Referringagain to Figure 1, I have shown a cylindrical electrode 20 immediately between the beam limiting apertured anode I2 and the first s et of deection coils I'I. The electrode 20 is preferably maintained slightly positive with respect to the anode I2 to provide a iield capable of collecting secondary electrons from the anode I2. Any secondary electrons which may be emitted on the surface of the electrode 20 are likewise collected. Thus, the electrode 20 may be supplied from the potential source I4 through an output impedance 2l so that the signal developed across this impedance by electrons being collected may be ap-V plied to a translating device such las the amplifier 22. From this showing of Figure 1, it will be seen that the cylindrical electrode 20 is positioned symmetrically with respect to the going electron beam and likewise with respect to the returning electrons so that electrons returning'from any portion of the target are amplified in accordance with the secondary emitting properties of the anode I2 and uniformly collected by the electrode 20. The material of the anode, at least over the surface exposed to the target 2, preferably has high secondary electron emitting properties. Thus, the anode I2, as well as other electrodes provided for-secondary electron multiplication may be such as described by LeverenZ et al., U. S. Patent 2,233,276.

A further advantage results from the use of my improved construction in that further secondary electron emission may be obtained by interposing an electron permeable perforated electrode between the secondary electron paths and the electrode 20. Thus, I have shown a cylindrical Wire mesh electrode 23 which may be operated at a higher potential than the electrode 20 if asecond ondary Yelectron multiplication.

adomos stage -of `electron multiplication is desired, .in which "event, vthe youtput Vimpedance is connected in 'the AAmesh `electrode circuit to the potential source I4. However, improved operation is obtained even for single electron multiplicationfutilizingl-the mesh-electrode `23 connected to the electrode as shownat24.

'I have Afound in operating apparatus of the type described that secondary electrons liberated by the anode '|2 orother electrodes adjacent thereto may 'flow :in the direction ofthe mosaic `electrode and escape the eld developed by the electrodes -20 or'23, and`I therefore providea trap for 4these electrons comprising'an-electrode *25 preferably of annular-form having a relatively large aperture alignedwith the 'axis of the .tube so that theelectrons over the initial and returning paths may be unimpeded. The electrode is operated at apotential slightly negative with respect to the electrode -I 2 -sothat electrons in 'the space between the anode I2 and the electrode "'25 are driven to the collector 20.

Referring to Figure `2 which shows a modification of my invention, the cylindrical electrode -20 is replaced #by a truste-conical electrode which is `likewise operated at a positive potential with respect to the anode I2 to collect electrons :thereon. Immediately adjacent the conical collecting electrode, I'prefer to-provide a frusto-conicalelectron permeable yapertured electrode, such as a Vformed `Wire mesh screen '3| which may be operated 'at a positive -potential with respect to the 'electrode'SIL While I have shown in Figure 1 the output impedance vbeing located in the circuit of the electrode 20, additional advantages may be yobtained either in the structure of Figure l `or Figure 2 by connecting the output impedance 21| `directly to the cylindrical lscreen 23 or conical 'screen 3| whereupon the -screen maybe operated at a slightly higher potential 'than the adjacent electrode and as the conical electrode 30 which in turn is at `a slightly higher potential than the Yanode 4`I 2. In the structure of Figure 2, secondary electrons emitted by the anode -I2 flow through the lapertures of the screen 3| and impinge the Velectrode 3U liberating further secondary electrons which are then collected by the screen electrodeSI and utilized in developing theoutput signal across the impedance 2fI. The yconical form of this electrode is found lto be conducive to more enicient emission of secondary electrons and lmore uniform collection of these electrons. 'The inner surface of the cylindrical or conical electrodes may be roughened such as by sand blasting Vor etching to increase the uniformity of secondary `electronemission.

I-have shown in FigurelS afurthermodiication wherein the electrode I2 is of smaller diameter :but vperforms the same function as described, eX- cept I provide an initial electron `collecting orsece fond multiplying electrode of annular form surrounding the anode |2as` shown at 35. Secondary electrons emitted from the anode I2 are made incident thereon for collection or for further .sec-

Obviously, `in view .of .the structures of Figures l and 2, a Wire mesh electrode 36 may be .provided immediately infront of the electrode for final electron col-- lection when the annular electrode 35 is operated as a .second secondary electron emitter.

In each of the modifications of my invention `described above, the electrons returning from the target are accelerated to a velocity determined by the potential of the anode I2 and incident thereon Yat that velocity. Secondary electrons emitted i bytheianode |2, showever, have relatively `ilovv 1intiali'velocity andere more read-ily affected 'by Ethe electrostatic felddeveloped between thelanode I2 andishield 125 bythe collecting electrode.. In addition, `returning electrons follow non-symmetrical-paths surrounding the initial path and are more subject to small potential gradients developed by .the collecting electrode. Consequently, these secondary electrons may be collected readilywithout introducing high potential gradients in `theelectron beam path.

From the foregoing description relating to each of Figures1,2,and i3, it will he noted lthat the collecti-ng electrode or the second'secondary Velectron emitter symmetrically surrounds the initial electron beam path. In effect the conical structure shown -in Figure2 is a modification intermediate of those of Figures l and 3, the cylindrical structure 2|) being expanded at the end nearer the target to obtain theconical structure ^3|J, .this in Lturn being lfurther expanded to form the annular structure 35. Consequentlmeach of these 'electrodes whether in the form `of a cylinder, frustum of `a cone or annular zhasfa surface Jwhich -is formed las Aa surfacenf revolution `about the initial` electron beam path. The imaging aperture I3 must be of exceedingly small diameter suchas itw'o-to-three thousandths of an inch to prevent `returning electrons from penetrating this aperture. Furthermore, the minimum diameter of the electrode formed as a surface of revolution must be suflicient in accordance with my invention to allow the returning electrons to reach'the anode I2. I have found thata `minimum diameter of one-quarter of an inch will 'allow substantially all of `thereturning electrons to reach the anode.

Furthermore this limitingfdimension is controlled "in part by the diameteriof the imaging aperture |13. Consequently the minimum diameter of .the surface of revolution maybe specified as a function of the limiting `aperture diameter. For example, I have found this minimum diameter t0 -lbe preferably at least '75 to 100 times the beam limiting aperture diameter. Consequently, the effective diameter yof the .anode I2 utilized "for secondary electron emission is likewise of similar -value and lconsequently with the surface of revo- 1-lution'electrode surrounding the `anode as shown -in Figure 3, the anode `I2 must be at least '75 `to times the diameter of the limiting aperture therein.

I claim:

-1. Television transmitting apparatus comprising :an evacuated envelope, an electron Vsource within said envelope, a target oppositely disposed -from said source adapted to receive electrons lfrom said source, magnetic means to develop a magnetic eld extending from said cathode to said target, an anode having an electron beam limiting aperture between said source and said target, magnetic means between said anode and -said target to deect electrons Vpassing through `said limiting aperture over the surface of said target wherebyelectrons'not reaching said target are returned to said anode along paths substantially coincident with but surrounding the initial path of electrons `iiowing toward said target, and 'means including `an electrode positioned about -said 'initial path and between 'said anode fand said `magnetic` means to intercept secondary electrons developed by electrons returning from said target 'incident on said anode. Y

2. Television transmitting apparatus including an evacuated renvelope, a cathode ,for liberating electrons within .saidaenvelope .a target oppositelydispo'sed fromV said cathode,= `a, .magnetic coil surrounding the space between said cathode and said target, an Aanode having an electron beam forming and limiting aperture in the path of electrons issuing from said cathode to direct electrons along an initial path toward-said target, wholly magnetic means to deflect the electron beam in two mutually perpendicular directions over the surface of said target and cause the electrons not reaching said target to return to said anode along paths substantially parallel and surrounding the initial electron `bearn path, and electrode means wholly removed fromV the paths of returning electrons and surrounding said paths and lying between said anode .and said magnetic means to intercept secondary electrons developed by said returning electrons without impingement of said returning electrons on said electrode means.

3. Television transmitting apparatus comprist ing an evacuated envelope enclosing an electron emitting cathode, an oppositely disposed target to intercept a portion of the electrons emitted byv said cathode, a beam limiting'apertured anode of secondary electron emitting material interposed between said cathode and target to limit the quantity of electrons flowing toward said target and to` intercept electrons not reaching said target, means to develop a longitudinal magnetic field wholly Aenveloping-the space between said anode and said target, magnetic means to deflect electrons approaching said target over the extended surface thereof, an electrode having a surface of revolution surrounding said beam positionedbetween said anode and said magnetic deflection means to intercept secondary electrons deflected on said anode, said 'electrode being `wholly removed from the path of electrons flowing between said anode and said target and between said target and said anode.

4, Television transmitting apparatus comprising a tube having an electron emitting cathode todevelop a stream of electrons, an oppositely disposed target electrode, a secondary electron emitting anode having a beam limiting aperture to form said electrons into a beam having a minimum diameter at said aperture, magnetic means to develop a longitudinal magnetic field with lines of force extending from said cathode Yto said target, whollymagnetic means to. deflect said beam over the surface of said target for collection in accordance with electrostatic image developed thereon and to direct electronsV not reaching said target to said anode along paths surroundingthe initial path of said beam to develop secondary electrons thereon, an electrode having a surface of revolution about said initial beam path wholly removed from said surrounding paths and between said anode and said magnetic deflection means to intercept said secondary electrons andvdevelop additional secondary electrons, and perforated electrode means substantially coextensive with said electrode having a surface of revolution and between said electrode and said surrounding paths to collect said additional secondary electrons.

5. Apparatus for television transmission as defined in claim 4 including an apertured secondary electron intercepting electrode positioned between said electrode having a surface of revolution and said magnetic deflection means, said intercepting electrode extending transversely to said initial beam path. Y

6. Apparatus for television transmission comprising anevacuated envelope, an oppositely disposedv cathode and target electrode within said envelope, a magnetic coilv surrounding4 said renvelope to develop a magnetic field having longitudinal lines of force extending substantially, from said cathode to said target to direct an electron beam along an initial path toward said target, an anode having an aperture in alignment between said cathode and target'to formand direct an electron beam toward said target, wholly magnetic 'means `between said anode and target to scan said beam over the surface of saidtarget and to direct electrons not reaching said target along returning paths surrounding said initial path to said anode, a cylindrical electrode wholly removed from said paths and surrounding .said paths between said anode and said magnetic means to receive secondary electronsA developed by said returning electrons on vthe surface of said anode and means comprising an aperturedelectrode between said cylindrical electrodeand said Y magnetic means to prevent said developed trons from reaching said target; f

7. Television transmitting apparatus'comprising an evacuated envelope yenclosing a cathode and target electrode, an apertured anode of secondary electron emitting material adjacent said cathode to developand direct an electron beam toward said target, a secondary electron emitting `cylindrical electrode immediately adjacent vsaid vanode and exposed to the surface thereof nearer said target, an electron collecting electrode substantially coextensive with and within said cylindrical electrode, magnetic meansv to scan electrons passing through said apertured anode lover the surface of said target in the presenceof a longitudinal magnetic eld whereby electrons not reaching -said target are.returned along paths substantially parallel with and surrounding said electron beam without impinging upon said cylindrical and coextensive electrodes. Y

8. vTelevision transmitting apparatus comprising an elongated envelope having a cathode and target adjacent opposite ends thereof, an intermediate electrode structure comprising an apertured anode and an apertured secondary electron shielding electrode inthe order named between said cathode and target, a frusto-conical electrode between said electrodes with its apex ad- -jacent said anode to receive electrons from said cathode passing through said apertured electrodes but not reaching said target and a perfoelecvrated electron collecting electrode substantially predetermined area aligned with its apertureV in the path of said beam, an annular electrode surrounding said anode having a minimum diameterat least times the diameter of said anode aperture to collect secondary electrons developed by the portion of said beam not reaching said target and impinging on said anode.

10. Apparatus as claimed in claim 9 including a perforated electron collecting electrode in a plane substantially parallel with anddirectly adjacent said annular electrode.

ALBERT ROSE.v

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