US2295038A - Cathode-ray tube - Google Patents

Description

Sept. 3, 1942- R. CQHERGENRQTHER 2,295,038

CATHODE-RAY TUBE Filed July 25, 1940 "QI-ERGENRO'I'HHR R O m F L mm ATTORN EY Patented Sept. 8, .1942

CATHODE-EAY runs Rudolf C. 'Hclgenrother, mechhurst, N. E, as-

ngnor to time we at Delaware Application at as, raw, Se

cathode-ray tubes employed in television systems generally has as an element thereof a'grid which is used to modulate the electron beam in accordance with videosignals applied thereto. The first anode, grid, and cathode elements, when suitably energized, form a first electron lens of the tube which serves to gather electrons emitted by the cathode and concentrate the electrons within a beam of small cross-secfional area adjacent the grid aperture. There is thus formed a constriction-in the cathode-ray beam known as the"eleetron cross-over whichserves as the "electron object" to be focused on the fluorescent,

screen of the tube by a second electron lensof either the electrostatic or magnetic type. The size of the electron cross-over, and hence the size of the focused electron spot on the fluorescent screen, depends on the focal properties, that is, on the electric field distribution in the first electron lens. r

It has been shown that, in the absence of space-charge efiects, the size of the electron OFFICE Y rporation, a corporation I Ezra (CE. 250--162) within certain limits because the spherical aberration of the second electron lens increases as the square of the eflective lens aperture for any given lens. This aberration must be kept below the value at which it seriously affects the spot size. Aside from spot size, the shape of the spot on the fluorescent screen tends to be distorted by the deflecting fields of the scanning system. This distortion increases as the diameter of the cathode-ray beam in the scanning zone and thereby increases directly with the angle of divergence; The spot. size at the center of the fluorescent screen is thus determined by cross-overfor a given electron gun at given anode potentials depends only on the energy distribu tion of the thermionic electrons leaving the cathode. In practice, however, the size or the electron spot on the fluorescent screen increases quite rapidly with increasing cathode-ray current, due,

at least partly, to the efiect of increasing space charge at the cross-over which eflectively in creases the cross-over size. It is, therefore, de sirable to decrease the space charge to a minimum, thus increasing the maximum permissible value of the cathode-ray currentwhich is limited to a value corresponding to the maximum spot Size which will still permit full resolution of image detail. Any decrease in cross-over 'size is very desirable since it will allow an increase in the picture brightness'without loss of detail, or will allow a decrease in the required value of the anode voltage at-the same picture brightness without loss of detail.

In addition to the efiect of space charge on the cross-over size, increasing values of beam current cause the angle of divergence of the electron beam leaving the electron cross-over to in-' the combined effects of cross-over size and spherical aberration of the second-electron lens, whereas the spot size at the edges of the fluorescent screen is additionally affected by the distortion produced. by the deflecting fields of the scanning system. The scanning distortion may be quite serious in cathode-ray tubes adapted to have the cathode-ray beam deflected over a wide angle and may determine the upper limit on the permissible beam current in these tubes.

As an aid in reducing both the space charge, and consequently the size of the cross-over, and the angle of divergence of the electron beam for any given luminous intensity of the spot, it has been found'desirable to produce the maximum electrostatic field intensity close to the grid aperture of the first electron lens. This has been acforming the first anode as an apertured frustumshaped member of small diameter and by poattempted to reduce the space charge and desiti'oning the apex of the anode in close proximity to, 'andcoaxially aligned with, the grid aperture. In this arrangement, the spacing of the anode from the grid and the size-of theanodeaperture are the only critical factors in reducing the magnitude of the space charge and the angle of divergence of the electron beam. Such tube construction has the disadvantage first, that the alignment of the anode and grid is extremely critical for satisfactory operation of the tube and, secondly the first anode'receives electrons directly from the cathode,-thereby not only wasting power but causing the anode to'heat up to a high temperature and also fogging thecath- I ode-ray beam by secondary electron emission from the anode.

ether cathode-ray tubes of the prior art have crease the angle of divergence of the electron v beam by the use of an apertured, disc-shaped,

first anode closely spaced to, and coaxially crease. 'The angle of divergence must kept aligned with, the d. Arrangements of this nature also have the disadvantage that the. alignment of the anode and grid is extremely critical for satisfactory focusing of the electron beam. Another difllculty. encountered in both the prior art electron gun structures described above is that because of the critical nature of the alignment, the anode must be supported on a common member, usually the reentrant glass stem of the tube, with the grid and cathode. The stray electric fields around the anode supporting wires and the electrical connection to the outside of the bulb may produce cold electron emission or electrical breakdown of the insulation when high anode voltages are used.

w It has also been proposed that the size anddistortion of theluminescent spot on the fluorescent screen be reduced for a given image detail substantially in the same plane. Tubes of this nature have the disadvantage that they require an excessively high second anode potential and require a relatively, complicated and expensive construction by which the first anode is supported from the neck of the cathode-ray tube.

It is an object of the invention, therefore, to

provide a cathode-ray tube of simplified and improved construction which, while ofgeneral application, is especially suitable for use as the image reproducer in television systems and one which 'avoids one or more of the above-mentioned disadvantages of the prior art cathoderay tubes. It is a further object of the invention to provide a cathode-ray tube which requires for its proper operation a value of second anode potential appredably smaller than that required .by similar tubes of the prior art,'yet one in which the reduction of anode voltage does not involve any sacrifice of picture. brightness nor loss of picturedetail. It isan additional object of the invention to provide a cathode-ray tube capable of forming a cathode-ray beam having a minir'num size of cross-over and minimum angle of divergence, whereby, for a given picture detail and given value of anode voltage, a brighter picture is produced and there is little or no distortion of the t spot as the beam is deflected during the scanning operation.

thereof, reference is had to the following description taken in connection with the accompanying drawing and its scope will out in the appended claims.

Referring now to the -drawing, Fig. 1 is a side 7 elevation, partly in section, of a complete cathoderay tube embodying the invention in a preferred form; Fig. 2 is an'enlarged cross-sectional view of the electron gun of the Fig. 1 arrangement; and

- spot size characteristic can be improved by reduction both of the space charge, and consequent reduction of the cross-over area, and angle of divergence of the electron beam. This may be accomplished bythe expedient of increasing in the vicinity of the grid aperture the'electrostatic field intensity of the first electron lens. The present invention makes use of the phenomenon that the electrostatic field intensity at the end In accordance with the invention, a cathodetube comprises athermionic cathode, a first electron lens comprising a substantially cylindrical apertured grid the cathode and a cylindrical first anode, one grid having an external diameter less than 0.15 times that of the internal diameter of the anode, and means for supporting the grid and anode substantially in coaxial alignment.

In a specific form of invention, a cathoderay tube comprises an evacuated tube envelope havinga cylindrical neck of uniform'cross-section along its l ng and a first electron lens comprlaing a substantially cylindrical grid and an electrically conductive first anode coating on the walls of the neck of the tube envelope. The grid and cathode are preferably as small as manufachlrlng .faicilities permit, the cathode having an emissive surface, the permissible electron emission of which is at least as high as milliamperes per square centimeter of emissive surface.

For a better understanding of the present invention, together with other and further objects flquent, high increase in the electrostatic field in-.

of the smaller of two cylindricaLaxially displaced, and coaxially arranged conductors of different diameters is rapidly increased as the diameter of the smaller of the two conductors isreduced to an infinitely. small. value. In accordance with the invention, the larger cylindrical conductor is made the anode of the tube while the smaller conductor is made the grid. The latter, however, necessarily encloses the cathode, the emissive surface of which must be perpendicular to the axis of both the anode and the grid. Thus, the required electron emission for a given beam current constitutes a definite limitation on the minimum physical size of the cathode and thereby on the minimum diameter of the grid.

An oxide-coated cathode operates at electron saturation of the order of 700 milliamperes per ening in life is' disproportionately smaller with values of cathode cun'ent'only slightly below saturation and' it is thus possible to have a cathode of sufilciently long life as to be y practicable when the cathode is operated at an electron-emission density greater than 50 milliamperes per square centimeter of cathode-emissive area and, in fact, of the order of several hundred milli-amperes per square centimeter of emissive 'area. It is obvious that the smaller the cathodeemissive area, the smaller the diameter ot the surrounding grid may be, whereby the grid and its enclosed cathode approach an electrical conductor of relatively small dimensions with consetensity in the vicinity of the grid aperture. The lower limit of the cathode-emissive area in square centimeters is equal to the maximum required beam current in miliiamperes divided by the maximum allowable emission current density which, as has already been stated, may vary from 50 to several hundred milliamperes per square centimeter depending on the specific nature of the oxide emitter. The limit in this regard is, in

neck portion ll preferably of uniform crossc section along its length and a flared bulb portion i2. One end of the neck ii is formed into a reentrant stem 9 through which 'aresealed, and a from which are supported, a plurality of relatively.

rigid electrical conductors it. These conductors support a substantially cylindrical apertured grid It substantially in coaxial relation or alignment with respect to the tube neck I l and also complete electrical circuits from the grid [4, and from the thermionic cathode and its heater, which are not shown but which are enclosed within the grid, to prongs i5 provided on a tube base It fixedly secured to the closed end of the tube neck.

The inside. walls of the neck H and the flared bulb [2 have an electrically-conductive coating i1 formed thereon to provide a cylindrical first anode. This coating, which may be formed by deposit from a eommercial product merchandised under the trade name Aquadag, a commercial form of an aqueous colloidal suspension of graphite, is a continuous electrically-conductive anode coating which extends from a fluorescent screen i8, formed on the flared endof the tube envelope, along the inner walls of the flared bulb and neck portions of the envelope to a plane which is perpendicular to the axis of the neck ii and which preferably includes the apertured end of the grid it. The distance between the plane defining the end of the anode cylinder and the grid aperture is not critical. The apertured end of the grid may be placed far within the anode cylinder or may be withdrawn sufilciently that the distance from the gi'id aperture to the plane defining the end of the anode is of the order of one-half'the anode radius, either extreme position of the grid having no significant change in the first electron lens action as long as the axial alignmentof the grid and anode is maintained. The eifect of any irregularities in the trim of the anode cylinder on the electric field at the grid aperture would increase as the grid is withdrawn from the anode and decrease as the grid is placed deeper within the anode.

Electrical connection is made to the coating l'l through a lead-in conductor i9 sealed through the side of the tube envelope i2. The grid l4 and anode II, when suitably energized, provide a; first electron lens, the function of which is to produce a cathode-ray beam of elemental cross-section at its electron cross-over and directed toward the fluorescent screen It of the tube.

A second electron lens of the magnetic type is provided by a focusing 'coil 20, comprising a large number of turns of wire disposed coaxially around the tube neck H and displaced axially from the first electron lens toward the screen 18. The purpose of focusing coil 20 is, as well known in the art, to reconverge or focus the cathoderay beam, which diverges beyond its cross-over, to a relatively small spot on the fluorescent screen it. Instead of a magnetic focusing coil 20, an electrostatic type second electron lens can be provided simply by removing a ring of the coating H at about the mid-plane of the focusing coil 2t, thereby to provide a first anode in the vicinity of the grid it, the first anode being provided with a separate electrical connection, and a second anode which extends along the neck of the tube and into the fiared bulb portion, these two anodes being'suitably energized to accomplish on the fluorescent screen l8.

The position of the scanning coils or plates along the neck of the cathode-ray tube is generally indicated at 2!. These scanning coils or plates operate in well-known fashionto deflectthe cathode-ray beam in a predetermined pattern of scanning lines on the" fluorescent screen of the tube.

Fig. 2 is an enlarged cross-sectionalview of the electron gun of the tube illustrated in Fig. 1. Elements corresponding, to like elements of Fig. l are designated by like reference characters. The grid H projects through and is supported in an orifice or aperture 22 in theap'ex of a hollow conical member 23 of insulating material, for

- example, steatite. The hollow conical member 23 is open at its enlarged end and comprises a means for supporting the grid I4 and the anode il subl5 stantially in coaxial alignment. The member 28 has an annular rim 24 around the circumference of which are a plurality of apertures 25 through which the relatively rigid conductors is are looped. The conductors, as previously stated,

support this assembly from the reentrant stem s 9, of the tube with grid I in axial alignment with the neck ii of the tube envelope. Therelatively rigid'electrical conductors i3, each oi which extends through one of the apertures in the 5 rim of the hollow member 23, are sealed through cludes an indirectly-heated cylindrical cathode' sleeve26 is fixedly'disposed within, and in spaced relation to, the inner walls of the grid M by an insulating bushing 21 and spacer nings 28. One end of the cathode sleeve 26 is provided with an electron-emissive surface 29 in opposing relation to an aperture 30 in the end of the grid it. Within the cathode sleeve 26 is a heater 3i.

A suitable electrically-fired getter 32 is a positioned within the member 23 and near the apex thereof. Two of the conductors iii are 'used to complete an electrical circuit to the getter material and comprise a means for supportin the electrically-fired. getter within the member 23 and near the closed end thereof. The member 23 thus acts as a shield to prevent overheating of the getter material during seal in of the reentrant stem of the tube and also acts as a shield when the getter is fired to prevent the getter material from co'ndensing'on the inner walls of the neck i l near the first anode H. The getter ma- .terial is conductive and, for that reason, would 1mpair the symmetryof the electrostatic field between the anode i7 and grid M, thereby impairing th e first electron lens action, if it were allowed to'be deposited on the inner walls of the neck H in contact with the first anode ill.

The area of the electron-emissive surface 29 is sufiiciently small that the maximum required electron emission is not less than milliamperes per square centimeter of emissive surface and is preferably as much higher than this value as is consistent with the required length of life of the cathode-ray tube. The grid aperture 30 is preferably made between about, 0.5 and 0.75 times the diameter of the cathode-emissive surface 29, the diameter of the surface 29 being substantially that of the cathode sleeve 26. The gridis preferably spaced \from the electron-emissive surthe required focusing of the cathode-ray beam I there shall be developed adjacent the grid aper- 'ance with 'ditionally,

nation of electron lens action. theelectron-amis thepricr external diameteroi the grid II should be not greater than, but preierablyless than, 0.15 times that o! the internal diameter of the first anode ll. As an aid in attaining the desired high intensity field gradient, the closed end of the grid II is rounded ,to have a substantially hemispherical contour.

It will be evident from Fig. 2" that the end 01.

the first anode l1 lies in a-plane-Derpendicular to r the axis of the neck II at substantially the apertured'end oi the grid It. It is this arrangement of the cylindrical first anode ll, substantially cylindrical apertured grid I4, and cathode 26,. 29

which comprises the first electron lens of the tube. I

Fig. 3 represents a modified gridconfiguration suitable for use in a tube constructed in accordthe invention. One 'end of the grid ll is closed by an apertured cup-shaped cap 33 whereby the closed end of the rid is flatorblunt. Similarly, the cathode sleeve 26 is' closed at one end by a cap 34 upon the bottomoi which is formed the electron-emissive area 20. The

* cathode sleeve 25 is maintained 'coaxially aligned withimthe grid i l by insulating discs, 35 which are held in position by split rings 3,1, and 30. The grid I4 is provided with a slot 30 "to permit the closed chamber 40 thus formed to be suitably exhausted when the tube itself is .ex-'

hausted' during the iorming period. This grid 2,295,088 ture as an electrostatic field 0f high intensity, the

constructed in accordance with the invention requires only about 0.8 watt. This means not only reduced power drain, from the heater supply,'but also a decrease in the temperature of the metal parts of the tube, a decrease in the temperature of the glasswall of the tube neck and the reentrant stem and its seal, and a decrease in the.

, ly decreased by the relatively small dimensions of the electron gun elements. Thus, while tubes of the prior art generally have a grid-cathode capacitance of from 10-20- micro-microfarads, a

tube constructed in accordance with the invention has agrid-cathode capacitance inthe order of one or two'micro-microfarads.

While the grid and the first anode have been described as cylindrical or substantially cylindrical,- -it isto be understood that these terms are to be construed according to their generally accepted usage to define and describe hollow generally cylindrical elements or elements of similar configuration.

While there. havebeen described what are at present considered, to be the preferred embodiconstmction is slightly more complex than that of the Fig. 2 construction, but has-substantially, 1 similar characteristics and is g muchbetter irom the standpoint or mechanicalconstruction; Ad-

rate alignment oi the cathode and grid.

ments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing ii om the invention, and it is, therefore, aimed in the appended claims to cover all such changes .35 and modifications as fall within the true spirit the-construction permits more accuand scope of the invention.

It will now be apparent from the foregoing de- :7

scription "of theinvention that the invention has a number of-outstanding advantages in addition to the improvement of the spot characteristic. The size of the oxide-coated electron-emissive area of the cathode has been greatly reduced,

released from the cathode during the breakdownduring the operating life of the tube by nega- What is claimed is: 1. Acathode-ray tube comprising, a thermionic cathode, a first electron lens comprising a substantially cylindrical apertured grid surrounding said'cathode and a cylindrical first anode, said grid having an external diameter less than 0.15

. times that of the internal diameter 01' said anode, thus reducing the amount of material which is g a cathode, a first electron lens comprising a subtive ion bombardment of the emissive surface is correspondingly reduced. Similarly, the amount of metal in the electron gun structure is'reduced. thl'j minimizing the amount of gas which must .be released bybombardment during the forming process and which is released during normal operatlon of the tube. v

A tube constructed in accordance with the invention has the advantage of simplicity of con struction, the first anode being merely a coating the inner wall or the neck of the tube envelope, or being a cylindrical conductor closely fitting the inner walls or the neck and supported directly therefrom. Where the anode is comprisedby a coating, the problem of properly trimming the end of the anode adjacent the grid is not critical, since almost the entire first electron lens action is localized inthe vicinity of the grid aperture. Furthermore, the problem of properly the first anode and grid is alsoless critical since the size of-the grid is decreased to the order of relation The decreasein the area of sive surface of'the cathode results in a substanstantially cylindrical apertured grid surrounding said cathode and a cylindrical first anode axially displaced from said grid, said grid having an external diameter less than 0.15 times that of the internal diameter or said anodepa'nd means for v supporting said grid and anode substantially in fcoaxial alignment.

3. A cathode-ray tube comprising, a thermionic cathode having a predetermined minimum permissible size, a first electron lens comprising a the grid aperturesize and the coaxial is not critical, also because oi the localisubstantially cylindrical apertured grid sur rounding said cathode and a cylindrical first anode, the external diameter of said grid being not greater than 0.15 times that of the internal diameter of said anode and of a smallness limited only by the size of said cathode, and means for supporting said grid and anode substantially in coaxial alignment.

'4. A cathode-ray tube comprising, an evacuated tube envelope having a cylindrical neck, a thermionic cathode, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and an electrically-conductive first anode coating on the'inner walls of the neck of said envelope, said grid having an external diameter less than 0.15 times that 01 the internal diameter of said conductive first anode coating, and meansror supporting said grid substantially in coaxial alignment with said anode coating.

5. A cathode-ray tube comprising-an evacuated tube envelope having a cylindrical neck, a

thermionic cathode, a first electron lens co'mprising a substantially cylindrical apertured grid SU rounding said cathode and a cylindrical first anode closely fitting the inner walls of said neck,

said grid having an external diameter -lessthan 0.15 times that of the internal diameter of said anode, and means for supporting said grid and anode substantially in coaxial alignment.

6. A cathode-ray tube comprising, an evacuated tube envelope havin cylindrical neck and flared bulb portions, athermionic cathode, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a continuous electrically-conductive anode coating on the inner walls of the neck and flared bulb portions of said envelope, said grid having an external diameter less than 0.15 times that, of the internal diameter of said anode'coating, and means for supporting said grid substantially'in I coaxial alignment with said anode coating.

7. A cathode-ray tube comprising, a thermionic cathode, afirst electron lens comprising a substantially cylindricalapertured grid surrounding said cathode and a cylindrical first anode, said grid having an external diameter less than 0.15 times that of the internal diameter of said anode, means for supporting said grid and anode'substantially in coaxial alignment, and a second 'elec-' tron .lens substantially coaxial with and axially displaced from said first electron lens away from said cathode.

8. A cathode-ray tube comprising, a thermionic cathode having an electron-emissive surface, the permissible electron emission of which isat least as high as 50 milliamperes per square centimeter of emissive surface, a first electron lenscomprising a substantially cylindrical apertured grid surrounding said cathode and a cylindrical first anode, said grid having an external. diameter less than 0.15 times that of the internal diameter of said anode, and means for supporting said grid and anode substantially in coaxial alignment.

9. A cathode-ray tube comprising, an indirect- 1y .heated cathode having a cylindrical sleeve and an electron-emissive surface at the end thereof, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a cylindrical first anode, said gridhaving an external diameter less than 0.15 times that of the internal diameter of said anode and an internal diameter not less than 1.5 times that of the external diameter of said cathode sleeve, and means for supporting said grid, said cathode sleeve, and said anode substantially in coaxial alignment.

first electron lens comprising a substantially ,cy-'

lindrical apertured grid surrounding said cathode and a cylindrical first anode, the aperture of said grid having a diameter substantially between 0.5 and 0.75 timesthe external diameter'of said cathode sleeve and said grid having an external diameter less than 0.15 times that of the internal diameter of said anode, and means for supporting said grid, said cathode sleeve, and said anode substantially-in coaxial alignment.

12. A'cathode-ray tube comprising, an evacuated tube envelope having a cylindricalneck, a

thermionic cathode having a cylindrical sleeve and an electron-emissive surface on one end thereof, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a cylindrical first anode disposed on the inner surfaces of the neck of said envelope, the electron emission of said emissive surface being not less than milliamperes per a square centimeter of said emissive surface and 3 eter of said .anode, means for supporting saidgrid and anode substantially in coaxial alignment said grid having a diameter substantially between 1.5 and 3 times ;the' diameter of said cathode sleeve and lessthan. 0.15 times thatof-qthe internal diameter or said anode, and means for comprising a hollow member of insulating material open at one end and closed at the other end except for an orifice through which said grid pro ects, and means for supporting an electri- Cally-fired getter within said member near the closed end thereof.

'14. A cathode-ray tube comprising, a thermionic cathode, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a cylindrical first anode,

,said grid having an external diameter less than 0.15 times that of the internal diameter of said anode, means for supporting said grid and anode substantially in coaxial alignment comprising a hollow conical member of insulating material having an open base end and an apertured apex through which said grid projects, and means for supporting an electrically-fired getter within said member near the apex thereof.

15. A cathode-ray tub comprising, an evacuated tube envelope, a thermionic cathode, a first electron lens comprising a substantially cylindri- 10. A cathode-ray tube comprising, a thermionic cathode having a cylindrical sleeve and an electron-emissive surface at the endthereof,

a first electron lens comprising a substantially cylindrical'apertured grid surrounding said cathode in spaced relation thereto and a cylindrical first anode, the aperture of said grid having a diameter substantially equal to the spacing of said grid and cathode and said grid having an external diameter, less than 0.15 times that of the internal diameter of said anode,-and means for supporting said grid and anode substantially in coaxial alignment.

11. A cathode-ray tube comprising, a therm-' ionic cathode having a cylindrical sleeve and an electron-emissive surface at the end thereof, a

cal apertured gridsurrounding said cathode and 1 a. cylindrical first anode, said grid having an external diameter-less than 0.15 times that of the internal diameter of said anode, and means for supporting said grid and-anode substantially in coaxial alignment comprising a hollow conical member of insulating material open at the base and supporting said grid and cathode at the apex thereof, and relatively rigid electrical conductors each extending through apertures in the rim of said member and sealed through said. tube envelope for supporting said member within said tube and for completing electrical circuits from said grid and cathode externally of said tube.

RUDOLF C.

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