US2213547A

US2213547A - Electron discharge apparatus

Info

Publication number: US2213547A
Application number: US156618A
Authority: US
Inventors: Harley A Iams
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1937-07-31
Filing date: 1937-07-31
Publication date: 1940-09-03
Anticipated expiration: 1957-09-03

Description

Se t. 3, 1940. H. A. IAMS ELECTRON DISCHARGE APPARATUS Filed July 31.

l l l l l l Z 5 INVENTOR HARLEY A. IAMS ATRNEY Patented Sept. 3, 1940 ELECTRON DISCHARGE APPARATUS Harley A. liams, Berkley Heights, N. 3., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application July 31, 1937, Serial No. 156,618

6 Claims.

My invention relates to electron discharge apparatus of the television transmitting type, and more particularly to improved tubes and methods for operating such tubes in television transmitting systems.

In many television transmitting tubes of the cathode ray type an optical image is focused upon a mosaic electrode which is scanned by an electron beam and which has photoelectrically sensitive elemental areas from which under the influence of the image electrons will be liberated to an electron collecting electrode. The number of electrons liberated from the elemental areas of the mosaic electrode bears a direct relationship to the intensity of light on those areas, and results in the development on the elemental areas of electrostatic charges which are proportional to the incident light. Such charges on the elemental areas of the mosaic form what may be termed an electrostatic image of the projected optical image. In such tubes the electrostatic charge on an elemental area is limited by the effective field available to draw electrons from the elemental areas, and it has been found difiicult to'make this effective field greater than about 3 to 4 volts.

When an optical image is formed on a photoelectric screen or cathode the electron emission from each point on the screen depends on the light at that point, hence the electron streams emitted from the screen correspond in density and distribution to the illumination and distribution of the points on the screen. These electron streams from the screen are arranged in a pattern corresponding to the optical image, and may be said to form an electron image. If all of these electron streams are focused on a target the electron pattern is preserved up to the surface of the target. If the target is in the form of a mosaic comprising a large number of mutually insulated elements, each element acquires and retains a charge proportional to the number of electrons falling upon it. These charges, developed on the insulated elements of the target, either when the electrons reaching the target are collected and produce a negative charge, or when each photoelectron reaching the target releases several secondary electrons and produces a positive charge, correspond to the points on the photoelectric screen which are emitting electrons. As a result, focusing an electron image on a mosaic electrode comprising individually separated electrically conducting particles may produce an electrostatic image on the surface of the mosaic electrode.

The electrostatic image may be intensified by projecting an optical image on a photoelectrically sensitive screen to obtain an electron image and. directing the photoelectrons released from the screen to a target electrode or mosaic electrode having elemental areas with a secondary emission coefficient, or ratio of the number of secondary electrons emitted to the number of impinging primary electrons, which is high and is greater than unity. If in this arrangement the photoelectrons from the photoelectrically sensitive screen are accelerated to a high velocity, each photoelectron liberates several secondary electrons from each elemental area, thus increasing the electrostatic charge of the area and intensifying the electrostatic image. Such an arrangement requires for optimum operation a mosaic electrode having elemental areas with a high sec ondary emission coefiicient. Even with such an electrode a high accelerating potential between 15 the photoelectrically sensitive screen and the electrode is necessary, and the intensification is limited by the ratio of the number of secondary electrons emitted by the elemental areas comprising the surface of the electrode to the number of primary impinging electrons.

One object of my invention is to provide a method of deriving from a tube having a mosaic electrode, television signals which are of greater intensity than are obtainable in the usual way, and which may be obtained at relatively low potentials. Another object of my invention is to provide a mosaic electrode tube having greater sensitivity than has heretofore been obtainable.

In accordance with my invention the electron streams constituting an electron image of a photoelectric screen on which there is an optical image are directed to a mosaic electrode and are also subjected to secondary electron multiplication to produce on the elemental areas of the mosaic electrode charges of an intensity dependcut on the degree of electron multiplication and not primarily on the secondary emission coefiicient of the electrode. In this way the charges on the electrode can be made as high as feasible, and the electrostatic image made much more intense than has heretofore been feasible.

According to one feature of my invention the image of an object to be transmitted is focused on a photoelectrically sensitive screen, the primary electron streams liberated from the sur face of the screen in the form of a primary electron image are directed to the elements or" a mosaic target electrode and to the screen in succession to liberate secondary electrons and obtain secondary electron multiplication, thereby producing an intensified electrostatic image on the target electrode, which is periodically scanned by an auxiliary cathode ray beam to discharge the elements and produce a useful output current. 55

According to a further feature of my invention the primary electron streams liberated from a photoelectrically sensitive screen in the form of a primary electron image are subject to secondary electron multiplication between the screen and a secondary electron emitter in the form of a wire mesh electrode having a multiplicity of apertures, the electrons which pass through the apertures or interstices of the wire mesh electrode or emitter producing an intensified electrostatic image upon the elements of a mosaic target electrode which is periodically scanned by an auxiliary cathode ray beam to discharge the elements and produce a useful output current.

A better understanding of my invention will,

be obtained and other objects, features, and advantages of my invention will appear from. the following description taken in connection with the accompanying drawing in which,

Figure is a diagrammatic view illustrating one form of my television device, and

Figure 2 is a diagrammatic view illustrating, in part, a preferred embodiment of my invention.

Referring to Figure l the tube of the type in which a mosaic electrode is scanned by an electron beam, comprises a highly evacuated envelope or bulb i, with an elongated cylindrical or bocy section, and a tubular arm or neck section enclosing devices for producing and controlling a scanning beam. A conventional electron gun, comprising a cathode 2, control electrode 3 connected to the usual biasing battery, and a first anode t maintained positive with respect to the cathode by a battery 5, produces an electron stream which leaves the cathode 2 under control of the electrode 3, is projected through the anode l, and is accelerated and concentrated into an electron scanning beam by a second anode 6 which is preferably a conductive coating on the inner surface of the envelope l near the neck of the bulb. Conventional beam deflection means, such as the deflection coils H and H, or conventional electrostatic deflection plates, sweep the beam horizontally and vertically. As in the conventional tubes there is mounted a short distance beyond the second anode ii an electron collecting electrode 9 which is connected through the impedance ill to ground. In operation the current collected by the electrode 9 produces a voltage drop across the impedance Ill which may be impressed upon the input circuit of a translating device 2!, further amplified, and applied to a transmitting network in a manner well known in the art. The target or mosaic electrode 52 which is scanned by the beam, is preferably of the double sided mosaic type and comprises a grounded mesh of fine insulated wire with individual metal plugs secured in the interstices of the mesh and coated with caesium to provide a mosaic surface which exhibits high secondary electron emission when bombarded with high velocity electrons. The construction of the double sided mosaic electrode 52 and .the operation of a tube using such an electrode is more fully disclosed in U. S. Patent 2,047,369.

In accordance with my invention I produce from the optical image to be transmitted an electron image by projecting the optical image on a phctoclectrically sensitive cathode or screen 13 in plane parallel relation to the mosaic surface of the double sided electrode I2 and axially aligned with that electrode and the electron gun.

The photoelectrically sensitive cathode or screen it comprises a. transparent foundation sheet such as mica which has been coated with a very thin transparent silver film upon the surface facing the mosaic electrode 3. The silver film is oxidized and photoelectrically sensitized in the usual Way by depositing caesium on the silver oxide. Such a film has photoelectric sensitivity and also a high secondary emission coefiicient. Electrons liberated from the screen it under the influence of light are focused upon the mosaic electrode H! by a focusing coil i l, and produce on the mosaic surface of the electrode E2 an electrostatic image corresponding to the optical image on the screen E3. 'The electrons from the screen l3 are accelerated toward the mosaic electrode l2 by an accelerating electrode 55, which is preferably a grid-like electrode such as a relatively coarse wire mesh in a plane parallel to and between the screen and mosaic electrode 52, but which may be an annular band of metal on the wall of the tube.

In accordance with my invention secondary electron multiplication of the electron streams constituting the electron image may be obtained by applying a high frequency alternating potential between the photo-electrically sensitive screen it and the target or mosaic electrode l2 by connecting them to the opposite terminals of the secondary of the transformer 45 through the conductors ll and it. It has also been found desirable to connect the electrode 55 through such a source of potential as. the battery l9 to ground so that its potential with respect to the mosaic electrode l2 may be maintained at a positive potential which is greater than the maximum peak negative potential supplied by the transformer l6. An image of an object to be transmitted is focused upon the photoelectrically sensitive screen 53 by the lens system 29. An alternating high frequency potential, the frequency of which depends upon the spacing between the mosaic electrode !2 and the screen it, is impressed upon the primary circuit of the transformer l6 which changes the potential of the screen it with respect to the mosaic electrode 62 at a rate corresponding to this alternating potential. At the instant the screen It is negative with respect to the mosaic electrode E2, th accelerating electrode i is positive with respect to the mosaic electrode by an amount determined by the potential supplied by the battery Ill. The electron. streams constituting the electron image of the photoelectrically sensitive screen are accelerated toward and focused upon the mosaic electrode by the accelerating electrode 55 and the focusing coil Ml, and upon impinging on the surface of the mosaic electrode liberate from the points of impingement on the surface of the mosaic electrode secondary electrons which exceed in number the electrons the electron streams. The greater the number of secondary electrons released from the surface of the mosaic electrode the greater the intensity of the electrostatic charges acquired by the individual metal plugs and as a result an electrostatic image of the optical image is formed on the mosaic electrode. The distance between the photoelectrically sensitive screen it and the mosaic electrode l2 determines the time period of flight of the electrons between these electrodes, and the frequency of the voltage supplied by the transformer ta is so chosen that this time period is roughly equivalent to one half cycle of the alternating potential applied between these electrodes. Thus secondary electrons leave the surface of the metal plugs on the mosaic electrode i2 at a time when the metal mesh of this electrode is at a negative potential with respect to the screen it. Likewise at this time the electrode l5 and the screen it are positive with respect to the foundation of the mosaic electrode, which causes the streams of secondary electrons emitted from the mosaic electrode to impinge on the photoelectrically sensitive screen I3, thereby liberating additional secondary electrons at substantially those points on the surface of the screen at which the primary electron streams originated. These secondary electron streams are then accelerated toward and focused upon the mosaic electrode to liberate more secondary electrons, which action greatly intensifies the electrostatic image already formed on the mosaic electrode. This action is repeated in accordance with the alternations in the potential applied by the transformer 56, additional photoelectrons being emitted from the photoelectrically sensitive screen l3 whenever its potential is negative with respect to the mosaic electrode. The focusing coil 14 is adjusted to focus the electron streams'between the screen [3 and the mosaic electrode E2 on corresponding points of the screen and the mosaic electrode, so that during successive flights of electrons between these two electrodes the electron streams will be maintained in a sharply focused electron replica of the optical image, and the electrostatic image on the mosaic electrode will be intensified each time secondary electrons are liberated from the surface of this electrode.

From the foregoing it will be seen that there is formed an electron image of the optical image projected on the photoelectrically sensitive screen [3 which electron image is focused on the mosaic electrode 12. This electron image may be termed a primary electron image because it is formed by the emission of primary photoelectrons. likewise the electron image formed upon bombardment of the mosaic electrode 82 by the electrons in the primary electron image may be termed a secondary electron image which is also focused upon the screen [3 to form a tertiary electron image, the formation of such a progression of electron images being continued by reason of the oscillation of the electrons between the screen and the mosaic electrode.

As a result it is possible with the use of my device to obtain on the mosaic electrode an electrostatic image of the optical image to be transmitted whichis of much greater intensity than heretofore obtainable and in consequence of this intensified electrostatic image the sensitivity and output of the device is greatly increased.

After the electrostatic charges constituting the electrostatic image on the mosaic electrode have been intensified to the desired degree, the surface of the mosaic electrode opposite that facing the screen is is scanned in a manner well known in the art by a. high intensity electron beam from the cathode 2. As the electron beam scans the mosaic electrode and impinges on the plugs of the electrode the intensified electrostatic charges on the plugs are neutralized, the secondary electrons produced by the impinging high intensity electron beam being collected in part by the electrode 13. As a result of the intensified electrostatic image on the mosaic electrode a greater signal output than heretofore obtainable is produced when the mosaic electrode is scanned by the electron beam, and consequently the sensitivity of the device, especially under low light intensity of the optical image, is greatly increased.

In a modification of my improved television transmitting tube, shown in Figure 2, I intensify the electrostatic image on the mosaic electrode by liberating primary electron streams from the photoelectrically sensitive screen in the form of a primary electron image of the optical image on the screen, and. multiply these electron streams by successive secondary electron multiplication between the screen and a secondary electron emitter having a multiplicity of apertures, such as a wire mesh electrode of high secondary electron emissivity. Electrons which pass through the apertures produce on the mosaic electrode an electrostatic image which is intensified with each successive step of electron multiplication occurring at the surface of the wire mesh electrode.

More specifically, in the tube shown in Figure 2 wherein equivalent parts thereof are similarly referenced as in Figure 1, the secondary electron emitter is a wire mesh electrode 25, preferably of fine closely spaced wires, which may if desired be treated to have a high secondary emission coefficient, and which is located between and in a plane parallel to the mosaic electrode l2 and the photoelectrically sensitive screen l3. The wire mesh electrode 25 may be referred to as an apertured cathode to differentiate it from the photoelectrically sensitive cathode or screen l3 previously referred to. The emitter electrode 25 and the screen l3 cooperate to multiply the photoelectrons liberated from the surface of the screen [3. A wire mesh electrode 26 of relatively coarse mesh between the fine wire mesh electrode 25 and the screen I3 acts as an accelerating elec trode or anode for the electrons between the photoelectrically sensitive screen l3 and the fine wire mesh emitter electrode 25. I prefer to use another wire mesh electrode 27 between the fine wire mesh electrode 25 and the mosaic electrode l2 to accelerate the electrons which pass through the apertures of the fine wire mesh secondary emitter 25 toward the mosaic electrode l2. Both the electrodes 26 and 2! are of coarser mesh than the emitter electrode 25, and have little distortional effect upon the distribution of electrons flowing to or from the electrodes between which they are interposed.

The photoelectrically sensitive screen I3 and the fine wire mesh electrode or secondary emitter 25 are connected to the terminals of the secondary winding of the transformer [6, the midpoint of which is connected through a potential source, such as a battery 28 to the anode electrode 26, and is also connected through ground to the foundation mesh of the mosaic electrode E2. The accelerating electrode 27 is maintained positive with reference to the anode 26 and also to the mosaic electrode I2 by a battery 29 having the negative terminal connected to the midpoint of the transformer I6. Otherwise the connections are as shown in Figure 1.

In this modification of my secondary electron multiplying transmitting tube the electron multiplication occurs between the photoelectrically sensitive screen 53 and the mesh electrode or emitter 25. The electron streams constituting the electron image of the screen I3 with the focused optical image thereon are caused to oscillate between and impinge upon the wire mesh electrode 25 and the screen I3 in rapid succession at a frequency determined by the frequency of the alternating potential applied to these electrodes by the transformer I 6. Each time the electrons constituting the electron streams impinge on the wire mesh electrode and the screen l3 secondary electrons are liberated to obtain secondary electron multiplication. The electrons which do not strike the wires of the electrode 25 pass through the apertures or interstices of this mesh, are accelerated by the wire mesh electrode 21, impinge on the mosaic electrode l2 and liberate secondary electrons of a greater quantity than the electrons passing through the electrode 25 thereby producing an intensified electrostatic image on the metal plugs of the mosaic electrode. This action is repeated at a rate corresponding to the frequency applied to the transformer it, so that each time electrons pass through the interstices of the electrode 25 and impinge on the mosaic electrode the electrostatic image thereon is intensified to a greater degree. After the electrostatic charges constituting the electrostatic image on the metal plugs of the mosaic electrode have been intensified to the desired degree, the charges are neutralized by scanning the surface of the mosaic electrode opposite that facing the mesh electrode 25 with an auxiliary cathode ray beam, and a television signal is generated as described in connection with Figure 1.

While the spacing between the photoelectrically sensitive screen l3 and the emitter electrode 25, and the frequency applied to the transformer l6 are not critical, I have obtained satisfactory results with a spacing of 5.2 cm., and a frequency of l l by 10 cycles per second. The potentials applied to the accelerating electrodes 26 and 21 are adjusted to give optimum operation, but I have found that the potentials supplied to these electrodes may be varied over a fairly wide range without materially affecting the operation of the device.

From the foregoing description it will be apparent that various other modifications may he made in my invention without departing from the spirit and scope thereof and I desire, therefore, that only such limitations shall be placed thereon as are necessitated by the prior art and set forth in the appended claims.

I claim:

1. The combination with a television transmitting tube having in plane parallel relation a photoelectrically sensitive screen and a mosaic electrode, an accelerating electrode between said screen and said mosaic electrode scanning means including an electron gun for producng an electron beam to scan said mosaic electrode, and an electron collecting electrode between said mosaic electrode and said electron gun, a source of alternating potential connected between said screen and said mosaic electrode, and a source of uni-directional potential greater than the peak negative potential of said alternating potential with its positive terminal connected to said accelerating electrode and its negative terminal connected to said mosaic electrode.

2. The combination with a television transmitting tube having in plane parallel relation a photoelectrically sensitive screen, a multiply apertured secondary electron emitting electrode, and a mosaic electrode, an accelerating electrode between said sensitive screen and said apertured electrode scanning means including an electron gun for producing an electron beam to scan said mosaic electrode and an electron collecting electrode between said mosaic electrode and said electron gun, a source of alternating potential connected between said sensitive screen and said apertured electrode, and means to maintain said accelerating electrode positive with re spect to the midpoint of said source.

3. The combination with a television transmitting tube having in plane parallel relation a photoelectrically sensitive screen, a multiply apertured secondary electron emitting electrode,

and a mosaic electrode, an accelerating electrode between said sensitive screen and said aperture electrode, a second accelerating electrode between said sensitive screen and said mosaic electrode, scanning means including an electron gun for producing. an electron beam to scan said mosaic electrode and an electron collecting electrode between said mosaic electrode and said electron gun, a source of alternating potential connected between said sensitive screen and said apertured electrode, a direct current source having its positive terminal connected to said firstmentioned accelerating electrode and its negative terminal connected to the midpoint of said source of alternating potential, and a direct current source having its positive terminal connected to said second accelerating electrode and. the negative terminal connected to said emitting electrode.

l. Apparatus for transmitting an optical image which includes means for producing an electron image of the optical image to be transmitted, means against which to direct said electron image to produce an intensified electron image and an electrostatic image of said electron image, means to oscillate electrons between said two mentioned means to intensify said electrostatic image, and scanning means including an electron gun for producing an electron beam to scan said second mentioned means and generate signalling energy impulses from the intensified electrostatic image.

5. In combination an envelope containing a photolectric cathode and an opposed target to receive electrons from said photoelectric cathode, an apertured cathode capable of emitting secondary electrons at a ratio greater than unity positioned between said photoelectric cathode and said target, a multi-apertured electrode between said photoelectric cathode and said apertured cathode to control electrons flowing between said photoelectric cathode and said apertured cathode, means including a source of alternating potential connected between said photoelectric cathode and said apertured cathode for oscillating electrons between said photoelectric cathode and said apertured cathode to create electron multiplication in the space therebetween and means for directing electrons passing through said apertured cathode against said target.

6. In combination an envelope containing a photoelectric cathode and an opposed target to receive an electron stream from said cathode, an apertured cathode capable of emitting secondary electrons at a ratio greater than unity positioned between said photoelectric cathode and said target, a multi-apertured electrode between said photoelectric cathode and said apertured cathode to control electrons flowing between said. photoelectric cathode and said apertured cathode, means including a source of alternating potential connected between said photoelectric cathode and said apertured cathode for oscillating electrons between said photoelectric cathode and said apertured cathode to create electron multiplication in the space therebetween, means for directing electrons passing through said apertured cathode against said target and means for maintaining substantially the same relative electron densities in the elemental cross-sectional areas of said stream at all times throughout the envelope between the photoelectric cathode and said target.

HARLEY A. IAMS.