US2831144A

US2831144A - Electron discharge device

Info

Publication number: US2831144A
Application number: US439684A
Authority: US
Inventors: Walter G Gibson
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1954-06-28
Filing date: 1954-06-28
Publication date: 1958-04-15
Anticipated expiration: 1975-04-15

Description

April 15, 1958 w. G. GIBSON `ELEcTRoN DISCHARGE DEvIE;

Filed June 28. 1954 mw .A

AA AA IN VEN TOR. WHL ri a'. @asa/v BY n mum-iii.`

2,831,144 itinerante orsenanon Dnvicn waa-ai o. gametes, N. J., assigpbrjo Radio Corporation of America, acorporation of Delaware Applicaties im@ 2s, 1954, se'ra'iNu. 439,684 9 claims. (ci. srs-L11) This invention relates to electron multiplier systems and particularly to systems of electron multiplicatiin incorporated in television camera tubes of the orthicon or image orthicon types.

The orthicon tube contains: within its envelope, a'target upon which an electrostatic charge image is produced from alight image through apliotoelectricproeess;an electron gun for generating a beamof electrons "having a low velocity at the targetand which beam is'caused to scan back and forth across the target by appropriate ields; and an electron multiplier arranged symmetrically around the electron gun fer receiving and multiplying a portion of the electron beam reilected the target. As the electron beam isscarined across the target enough electrons are subtracted from thefbeam to neutralize the charge at each point ofV the:electr ostatic lcharge image. The electrons remaining in the beam do not strike lthe target but are reflected back toward the electron gun forming-a modulated return beam. Due to the operating characteristics of thesel tubes, the lreturn beam scans :across the surface of the first dynode, or electrodenearest the target, `of 'the electron multiplier inaccordance :with the scanning of the original beam on the target. Secondary electrons are given oi in a shower from lthe first dynode, varying in density in accordance with the vdensity of the return beam, and are directed into succeeding stages of the electron multiplier. The output of "the laststage of the electron multiplier is collected Vto constitute Vthevideo signal output of the tube.

In tubes of the type described, the electron multiplier was found to cause shading of the pictureproduced from the video signals (i. e. one section of the picture was lighter than another when the relative brightness of the entire picture should have been uniform). Such shading, which is often referred to as multiplier shading, results from the non-uniform sensitivity of the secondaryelectron-emitting surface of the Erst dynode of the electron multiplier. As the return beam scans across the surface Vof the tirst dynode any portion of that surface which is less sensitive than the rest will consistently at fect a corresponding portion of the received picture making that portion appear to have a different shade thanother portions of the picture which should have the, sameshade.

Succeeding stages of electron multiplication arenot critical with regard to multiplier shading because there is no scanning action by the electrons which reach them from proceeding stages of multiplication.

Accordingly, it is one object of this invention to provide Brieily, these objects are accomplished lnveni tion through the provision of a non-'symmetric electro'- 2,831,144 Patented Apr. 15, 1958 ,e lCC static field across the secondary-electron-emitting 'surface of the first dynode of the electron multiplier. The configuration of this electrostatic field may he adjusted in accordance with the non-uniformsensitivity of the surface of theV first electrode of the electron multiplier yso that collection of the secondary electrons emitted by Vthe more sensitive portions of the surface will be inhibited while collection of secondary electrons emitted by the less sensitive portions will be enhanced.

In one embodiment of this invention, the non-sym# metric electrostatic fleldat the secondary-electronemit ting surface of the first electrode of the electron multiplieris obtained by independently adjusting the potentials impressed on a plurality (e. g. four), of curved electrodes positioned adjacent to such surface. The curved electrodes are arranged to form a segmented tube with one end surrounding the secondary-electron-emitting surface of the rst electrode of the electron multiplier, the remainder of the tube extending axially away "from that surface.

This, invention will be more clearly understood from theA following detailed description when read inconjunction with the appended single sheet of drawing in which like numerals denote like parts and wherein:

Fig. l is a longitudinal Vsectional Vview of a television camera tube of the orthicon type incorporating-the details of this invention.

Figure 2 is anA enlarged View of a portion 'of the camera tube shown in Fig. 1 and Fig. 3 is an end view of the structure shown in Fig. 2 that embodies this invention.

Structure The glass envelope 1i of the orthicon type of' television camera `tube shown in Fig. 1 is an elongated hollow cylinder having an end portion 13 of enlarged diameter to house a target 15. The opposite end of the envelope 11 is provided with a base 17. The electron gun i9 and the electron multiplier 21 are mounted in the envelope. il adjacent the base 17.

TheV target 15 comprises a thin sheet 23 of transparent material (e. g. mica) Vpositioned transversf-:lyA to 'the axis; of the envelope 11. The surface of the target 15 facing the Velectron gun 19 is provided with a photoemissive mosaic coating 25. The opposite surface of the target 1S is provided with a transparent conductive coating 2X7 which `acts as a signal plate. A light image 29 focused by a lens system 31 upon the surface of the target 1:5 will passv through the conductive coating 27 and the mica sheet 23 and impinge upon the photoemissive mosaic coating 25.

The electron gun i9, positioned in the opposite end of the envelope 11,is adapted to lform a beam 33 of elec'- trons which are directed `toward the target i5. The electron gun i9 consists of a ilam'ent .35 for indirectly heating a tubular cathode 37, A iirst tubular beam for-ming electrode 39 and a last `tubular beam forming electrode 41a`re both axially aligned with the tubular cathode 37 and have closed ends facing the target i5. Apertures 43 are formed in the closed ends of 'the beam vforming electrodes 39 and 41 to allow the electron beam to passthrough. The aperture 43 in the lastbeam forming electrode 4l is very small `(e. g. .CQZ inch in diameter) to provide a beamof minimulndivergence.

Referring to Figs. 2., the outside surface 4 5 of the closed end of the last beam forming electrode 41 is prog ,dienst/aad thwllscfiag decussa@ symmetrically surrounding the electron gun 19 and being progressively closer to the base 17 end of the envelope 11 and having impressed thereon progressively high positive potentials.

A persuader electrode 51 is positioned adjacent to the first dynode surface 45 of the electron multiplier 19. The persuader electrode 51 is a tubular electrode axially aligned with the electron multiplier 21 and the electron gun 19. One end of the persuader electrode 51 surrounds the first dynode surface 45 of the electron multiplier 21, the remainder of the electrode 51 extending toward the target 15.

According to one embodiment of this invention, the above described persuader electrode 51 is divided into longitudinal curved segments 53 (e. g. four) which are electrically insulated from each other (e. g. byvspacing the segments). Each segment 53 is supported by two hollow ceramic supporting rods 55 which serve also asy supports for the electron multiplier 21 and electron gun 19. Each segment 53 is electrically connected to a separate and independently variable source of potential 57 as by a pin 59 in the base 21 and a lead 61 which extends through one of the ceramic supporting rods 55 associated with each segment 53.

Operation In operation, the filament 35 is energized to heat the cathode 37 and cause the emission of electrons. The emitted electrons are formed into a beam 33 by the beam forming electrodes 39 and 41 of the electron gun 19. The cathode 37 is maintained at zero potential and the last beam forming electrode 41 is maintained at positive potential (e. g. 300 volts) to aid in the beam forming action and to provide a certain amount of acceleration of the electrons.

The beam of electrons 33 emerges from the aperture 43 in the last beam forming electrode 41 and passes through the segmented persuader electrode 51 which has little effect on the beam since the potential thereon approximatcs that on the last beam forming electrode 41 (e. g. 300 v. |50 v.). A positive potential (e. g. 200 v.) placed on a conductive coating 65 and a lesser positive potential on the target 15 and a decelerating ring 63, along with the initial velocity of the electrons, causes the electron beam to proceed toward the target. The decelerating ring 63 has a lower positive potential placed thereon (e. g. l v.) to lower the velocity of the electron beam 33 as it approaches the target. A still lower positive potential (e. g. 2 v.) is placed on the metallic coating 27 or signal plate of the target 15. In the dark, the small positive potential on the target causes it to collect elec trons from the beam 33, driving the mosaic surface 25 of the mica target sheet 15 to cathode potential, the unused portions of the beam being reflected toward the electron gun 19.

An alignment coil 67 surrounds the envelope 11 at a point adjacent the segmented persuader electrode 51 to provide a corrective magnetic field whereby the electron beam 33 may be centered in the envelope 11 prior to deection. The beam is caused to scan the target by a deection yoke 69 which surrounds the remainder of the envelope 11 between the alignment coil 67 and the enlarged end portion 13 housing the target 15. A focusing coil 71 surrounds the alignment coil 67, the detiection yoke 69 and the envelope 11, including the enlarged end portion 13 thereof, to provide an axial magnetic field between the electron gun 19 and the target 15.

If a light image 29 is focused on the target 15 through the lens system 31, various points on the photoemissive coating 25 on the target 15 will be caused to give up electrons corresponding in numbers to the amount of light which reaches each point on such coating 25. Thus, an electrical charge image is formed on the target 15 corresponding to the light image 29 focused thereon, lthe degree of positive potential representing the degree of light.

As the electron beam v433 scans each point of the mosaic surface 25 of the target 15, enough electrons are subtracted from the beam 33 to neutralize the charge image at that point. The unused portions of the beam 33, after neutralization of each point of the mosaic surface 25, are reflected back toward the electron gun 19 thus forming a return beam 33 which is modulated in density by the electrical charge pattern on such mosaic surface 25 as the electron beam 33 is scanned thereacross.

The return beam 33 will follow substantially the same path as that taken by the electron beam 33, being affected by the focusing coil 71 the deliection yoke 69 and the alignment coil 67 in reverse. However, the return beam 33' will not re-enter the aperture 43 in the last beam forming electrode due to the extremely small size of such aperture 43 and to other phenomena such as non-uniformity of focusing, deflection, and accelerating fields. Instead, the return beam 33 will scan a small portion of the first dynode surface 45 in accordance with the scanning of the electron beam 33 on the target 15. Y

When the return beam 33' impinges on the first dynode surface 45 secondary electrons are liberated therefrom. Such secondary electrons follow devious paths 73 under the infiuence of the electrostatic elds created by the potentials on the segmented persuader electrode 51, the second dynode 47 and the first dynode surface 45, finally traveling back past the edges of the first dynode surface 45 and into the second dynode 47 due to the higher positive potential placed thereon (e. g. 500 v.). The secondary electrons which reach the second dynode 47 induce further secondary emission from such dynode 47', resulting in a further multiplication of electrons which proceed to succeeding dynodes 47 due to progressively higher positive potentials (c. g. as shown in Fig. l). Finally, the electrons are collected to constitute the video output of the camera tube. For a description of an orthicon television camera tube, exclusive of the subject matter of this invention see U. S. Patent 2,498,082.

The exact coaction of the electrostatic field of the persuader electrode 51 with the electrostatic fields of the first and second dynodes 45 and 47 is not clear. In theory, if the potential on the persuader electrode 51 is slightly negative with respect to that' on the first dynode 45 it will tend to repel the secondary electrons emitted by the first dynode 45 back into the second dynode 47. However, if the potential placed on the persuader electrode 51 is made more negative with respect to first dynode 45 it will tend to raise the work function of secondary emission from the first dynode 45. Conversely, if the potential placed on the persuader electrode 51 is slightly positive with respect tothe first dynode 45 it will tend to attract secondary electrons from the first dynode 45 and will inhibit vthe reception of such electrons by the second dynode 47. However, if the potential placed on the persuader electrode 51 is made more positive with respect to the first'dynode 45 it will tend to lower the work function of secondary emission from the first dynode l45. Thus, it is seen that making the potential placed on the persuader electrode 51 negative with respect to that of the first dynode tends to inhibit the emission of secondary electrons but aids in the reception of secondary electrons by the second dynode 47'; while making the potential of the persuader electrode 51 positive with respect to that of the first dynode 45 tends to enhance the emission of secondary electrons but hinders their collection by the second dynode 47.

In accordance with this invention the persuader electrode 51 is divided into segments 53 and means 57 are provided whereby the potential on each of the segments 53 may be varied independently of the others, thereby producing a controllably non-symmetric electrostatic field and allowing any combination of the above phenomena to be obtained across the surface of the first dynode 45. Thus, since it is difiicult to apply a secondary electron emitting coating to the surface of the first dynode 45 of the electron multiplier 21 which has uniform sensitivity throughout, the potentials impressed on the segments 53 of the persuader electrode 51 may be independently adjusted to cause the secondary electrons collected from the less sensitive portions thereof to equal the secondary electrons collected from the more sensitive portions thereof. In addition, the return beam 33 may be deflected slightly by the unequal potentials placed on the segments 53, making it possible to cause the return beam 33 to scan a portion of the first dynode 45 having more even sensitivity than the portion it would otherwise scan.

It is obvious that careful adjustment of the potentials impressed on each segment 53 of the persuader electrode S1 should compensate all but the most extreme unevenness of sensitivity present on the surface of the first dynode 45 of most electron multipliers. Thus, an improved electron multiplier is provided in which uneven sensitivity is effectively controlled and which, when used in a television camera tube, will greatly reduce the problem of shading. It should be observed that this invention is not limited to the embodiment here illustrated and described in detail, but is equally applicable to any system of electron multiplication in which even sensitivity is required of the first stage of electron multiplication.

What is claimed is:

l. An electron discharge device comprising an electrode having a secondary-electron-emitting surface, and means directly adjacent to said surface for providing a non-symmetric electrostatic field across said secondaryelectron-emitting surface including a plurality of electrodes arranged to form a segmented tube directly adjacent to said surface.

2. An electron discharge device comprising an electrode having a secondary-electron-emitting surface, means for directing electrons onto said surface, means adjacent to said surface for collecting secondary electrons emitted from said surface, and means directly adjacent to said surface for providing a controllably non-symmetric electrostatic field across said secondary-electron-emitting surface, said last named means including a plurality of electrodes arranged to form a segmented tube adjacent to said surface.

3. An electron discharge device comprising an electrode having a secondary-electron-emitting surface, means for scanning said surface with a beam of electrons, means adjacent to said surface for collecting secondary electrons emitted from said surface, and means Vdirectly adjacent to said surface for providing a controllably nonsymmetric electrostatic field across said secondary-electron-emitting surface and including a plurality of electrodes arranged to form a segmented tube adjacent said surface.

4. An electron discharge device having an envelope, a target at one end of said envelope adapted to have a charge image generated thereon, an electron gun Within said envelope opposite said target for directing a beam of electrons toward said target to neutralize said charge image, meansfor returning the unused portions of said beam toward said gun, a secondary-electron-emitting electrode positioned adjacent said electron gun for receiving said return portions of said electron beam, and means directly adjacent to said secondary-electron-emitting electrode for providing a controllably non-symmetric electrostatic eld across the surface of said secondary-electronemitting electrode.

5. A television camera tube having an elongated envelope, -a target at one end of said envelope, means for converting a light image into a charge image on said target, an electron gun in the opposite end of said envelope from said target for directing a beam of electrons toward said target to neutralize said charge image on said target, means for returning the unused portions of said beam along a path toward said gun, a secondaryelectron-emitting electrode positioned in said return beam path for receiving said reflected portion of said electron beam, and means directly adjacent to :said secondaryelectron-emitting electrode for collecting secondary electrons emitted by said secondary-electron-emitting electrode, said means for collecting secondary electrons including a means for providing a non-symmetric electrostatic field across the surface of said secondary-electronemitting electrode.

6. An electron discharge device including an electron multiplier system comprising a surface coated with a secondary-electron-emitting substance, means for directing electrons onto said surface, means for collecting secondary electrons emitted from said surface, and a plurality of electrodes arranged directly .adjacent to said surface in the form of a segmented tube to provide a non-symmetric electrostatic field.

7. An electron discharge device comprising an electrode having a surface coated with secondary-electronemitting substance, means for directing electrons along paths intercepting said surface, means for collecting secondary electrons emitted from said surface, a tubular electrode surrounding said paths and positioned adjacent said surface whereby said electrons pass axially through said electrode prior to reaching said surface, said electrode comprising a plurality of segments spaced from each other, and means for applying potentials to each of said plurality of segments independently.

8. An electron discharge device comprising an electrode having a surface coated with secondary-electronemitting substance, means for directing electrons onto said surface, means for collecting secondary electrons emitted from said surface, a plurality of curved electrodes positioned symmetrically about an axis normal to said surface to form a segmented hollow cylinder, and means for applying potentials to each of said plurality of electrodes independently.

9. A television camera tube comprising an electron gun for generating a beam of electrons; a target positioned in front of said electron gun; means for converting a light image into an electrostatic image on said target; means for causing said electron beam to scan said target for neutralizing said electrostatic image, means for returning portions of said beam as a return beam which scans a surface of said electron gun facing said target in accordance with the scanning of the electron beam on said target; an electron multiplier including said surface as the first stage of electron multiplication and a collector electrode spaced therefrom; a plurality of curved electrodes positioned adjacent to said surface and arranged to form a segmented cylinder with its axis normal to said surface, said segmented cylinder being located between said electron gun and said target so that it surrounds said electron beam and said return beam; and means for varying the potential on said curved electrodes independently of each other.

References Cited in the file of this patent UNITED STATES PATENTS 2,560,585 McMillan JulyV 17, 1951 2,667,599 Rajchman Jan. 26, 1954 2,675,499 Sears Apr, 13, 1954