US3619706A

US3619706A - Cathode-ray tube in which screening electrodes are provided at the electron gun to produce a beam of uniform density over its cross section along its path to the display screen

Info

Publication number: US3619706A
Application number: US14610A
Authority: US
Inventors: Douglas S Hills
Original assignee: Rank Organization Ltd
Current assignee: Rank Brimar Ltd; Rank Organization Ltd
Priority date: 1966-09-28
Filing date: 1970-02-26
Publication date: 1971-11-09
Anticipated expiration: 1988-11-09
Also published as: GB1205565A; BE704452A; SE337873B; DE1614388A1; NL6713068A

Abstract

A discharge system including a device such as a cathode-ray tube has an electron gun in which an accelerating electrode is placed close to a planar emissive surface of a cathode and is held at a small positive potential to provide an accelerating electric field having unipotential surfaces which are parallel to the electron emissive surface of the cathode. To screen the accelerating field having these unipotential surfaces from the further accelerating electrodes in the gun or in the cathode-ray tube a screening electrode or electrodes is placed in front of the accelerating electrode. In consequence the accelerating electric field close to the electron emissive surface is undistorted by the fields beyond the screening electrode(s), so that the electron beam is devoid of crossover and has a substantially uniform current density over its cross section.

Description

lJnit States 'atent Douglas S. Hills C/t'll'llOlDlI-RAY TUBE IN WHICH SCREENING l-ll,l-lCTl'lOl)ES ARE PROVIDED AT THE liLll lC'llRON GUN TO PRODUCE A BEAM OF lJNll OltM DENSl'lY OVER ITS CROSS SECTION ALONG ITS PATH TO THE DISPLAY SCREEN 5 Claims, 4 Drawing Figs.

US. Cl 315/31, 313/82, 313/85, 315/15 Int. Cl H0lj 29/56 lField ot'Search 315/31, 15,

References Cited Primary Examiner-Roy Lake Assistant Examiner-V. Lafranchi AttorneyGriffin, Branigan and Kindness ABSTRACT: A discharge system including a device such as a cathode-ray tube has an electron gun in which an accelerating electrode is placed close to a planar emissive surface of a cathode and is held at a small positive potential to provide an accelerating electric field having unipotential surfaces which are parallel to the electron emissive surface of the cathode. To screen the accelerating field having these unipotential sur faces from the further accelerating electrodes in the gun or in the cathode-ray tube a screening electrode or electrodes is placed in front of the accelerating electrode. In consequence the accelerating electric field close to the electron emissive surface is undistorted by the fields beyond the screening electrode(s), so that the electron beam is devoid of crossover and has a substantially uniform current density over its cross section.

CATHODE-RAY TUBE IN WHICH SCREENING ELECTRODES ARE PROVIDED AT THE ELECTRON GUN TO PRODUCE A BEAM OF UNIFORM DENSITY OVER TS CROSS SECTION ALONG ITS PATH TO THE DISPLAY SCREEN This invention relates to electron guns for vacuum electric devices such as cathode-ray tubes and to devices including such electron guns.

A conventional cathode-ray tube comprises an evacuated envelope of electrically insulating material having at one end of internally phosphor coated light-transmitting screen and at the other end an electron gun capable of producing a beam of electrons which, when incident upon the phosphor layer, is effective to excite it to an emission of generally visible optical radiation which is transmitted through the screen.

in most applications the excited area of the phosphor upon which the electron beam is incident, is required to have the smallest possible dimension in at least one direction and the electron beam must accordingly be focused onto the screen phosphor to form, say, a spot, by suitable electrostatic or electromagnetic means. The spot must be not only of the smallest possible size for most applications, but should also be sharply defined, that it is to say, the intensity of the spot must be homogeneous and must very sharply fall to substantially zero at the sport periphery.

Accordingly to one aspect of the present invention, an electron gun suitable fore use in a vacuum electric device such as a cathode-ray tube, comprises a cathode, accelerating means for producing adjacent the electron emissive surface of the cathode an accelerating electric field having unipotential surfaces which are substantially parallel to that surface, together with electric screening means effective to prevent distortion of those unipotential surfaces as a result of electric fields produced by further electrodes in the device.

According to a further aspect of the present invention, an electron gun suitable for use in a vacuum electric device such as a cathode ray tube, comprises in sequence a cathode, a first apertured electrode being an accelerating electrode uniformly spaced apart from the electron emissive surface of the cathode and effective when positively biassed with respect to the cathode, to produce an accelerating electric field having unipotential surfaces which are substantially parallel to the cathode emissive surface and to the first electrode, together with a second electrode being a control electrode spaced apart from and having an aperture aligned with that of the first electrode, the second electrode being arranged to electrically screen the space extending to the cathode so as to prevent distortion of those unipotential surfaces.

By unipotential surface is meant a surface over which there are no potential differences.

Suitably the electron emissive surface of the cathode and the adjacent unipotential surfaces are substantially planar.

The specified electric field configuration must be maintained at least over the area of the apertures or of the utilized beam area as the case may be to ensure operation of the gun according to the invention. This may be achieved by uniformly spacing the first electrode inclose proximity to the adjacent cathode surface area overlaps the area of the aperture to avoid electrostatic edge effects.

Electrons leaving the emissive surface of the cathode will tend normally to cross the unipotential surfaces of an impressed accelerating electric field. Accordingly, the present invention ensures that substantially all electrons leaving the cathode surface are accelerated to tend to travel along paths normally of this emissive surface to produce a beam in which lateral velocity components are reduced to a minimum. Subsequent focusing and scanning of a beam in which substantially the only velocity components are those along the beam axis is thus considerably simplified and in particular requires a minimum energy input to produce the required focusing convergence of the beam,

In addition, the electron beam launched into the tube will be derived only from substantially that area of the cathode surface underlying the aperture in the first and accelerating electrode. Considered together with the velocity distribution of electrons in the beam produced by the field, the beam launched into the tube by the electrode structure of the invention will be of substantially homogeneous density and will also have an accurately and sharply defined periphery. This beam can accordingly be focused to an image or sport having a luminous intensity which is both homogeneous and which falls sharply to substantially zero at the spot periphery.

The beam having the characteristics hereinabove set forth will accordingly also uniformly illuminate an aperture in a further electrode, for example a final anode along the tube axis, enabling this aperture to be sharply imaged by suitable focusing means and enabling a variably resolution to be obtained in a cathode-ray tube including the gun. The ability of such a tube to produce a sport or the like, which is an image of an aperture is a of considerable significance, since it enables both the ultimate size and shape of the spot to be controlled irrespectively of beam current and therefore electrode potentials in the tube including modulation depth. Such control is unobtainable in most conventional guns, wherein the spot on the tube screen is an image of a beam crossover which varies considerably with beam current, modulation depth and various other factors.

Accordingly the beam current from the electron gun of the invention can be increased considerably in excess of those values usable in conventional electron guns and the aperture size and thereby the spot size can be reduced to well below those values hitherto not possible without excessive loss of spot intensity.

In, for example, a cathode-ray tube using a conventional electron gun the maximum beam current for a given spot size say 0.5 mm. in diameter is normally no more than 250 microamperes whereas in the gun of the present invention this beam current can be increased to well above l.5 ma. for the same spot size. This increased current density enables the spot size to be reduced to 0.1 mm. without reduction in the overall spot intensity.

in most applications the electric field configuration that is to say a field having substantially planar unipotential surfaces will be maintained up to the second control electrode as a result of the ability of this electrode to screen the space extending to the cathode, from external electric fields generated by potentials applied to further electrodes, i.e., anodes disposed further up the tube.

Accordingly, the gun of the invention permits continuous modulation of the beam without causing any significant degradation of its properties. Conveniently the ratio of diameter of the aperture in the second electrode to the thickness of the electrode is arranged to produce this screening effect. However, in the case where the diameter of the aperture is so large as to produce an undesirably electrode thickness, the aperture is conveniently covered with an electrically conducting and electron permeable mesh effective to produce the screening effect. Alternatively a screening effect can be produced by two closely spaced-apart thin electrodes, in which the spacing is not less than the aperture diameters. This combination is equivalent to a single thick electrode.

Suitably the electron emissive surface and the accelerating electrode are planar, and are spaced apart by a distance of about 0.005 inches, the potential on the accelerating electrode being commonly less than .+l0 volts with respect to the cathode. This accelerating electrode also each be made effective when negatively biased with respect to the cathode to produce cutoff of the electrode beam through the aperture therein. This facility is particularly useful when the electron gun is used in a cathode-ray tube intended for television application, as it enables flyback suppression to be easily obtained.

The control electrode which is effective to modulate the electron beam passing through the aligned apertures, conveniently is maintained at a potential within the range 0 to volts with respect to the cathode.

In the case where the beam control signal applied to the second electrode is of such magnitude as to produce thereon an appreciable potential swing, which may produce undesirable distortion of the unipotential surfaces, the first electrode is also conveniently arranged to produce a screening effect on the space extending to the cathode. A suitable screening effect is produced by arranging the first electrode to have a penetration factor of less than about 0.70, penetration factor p being defined as P=(E,/E where E is the potential difference between the second control electrode and the cathode, and E is the potential on the first electrode required to suppress the beam.

Suitably the electron gun includes a third electrode being an accelerating electrode spaced apart from the control electrode and having an aperture aligned with those of the first and second electrodes. This third electrode is arranged to be more positively biased with respect to the cathode than the first or the second electrode and is effective to produce that cohesion in the electron beam which might otherwise be lost by scattering due to the relatively low accelerating potential on the first and/or the second electrode. Conveniently the third electrode is supplemented in this function by at least a fourth apertured electrode spaced apart but electrically connected thereto.

Accordingly to a yet further aspect of the present invention, therefore, a cathode-ray tube comprises an evacuated envelope having at one end an internally phosphor coated lighttransmitting screen and at the other end an electron gun capable of producing a beam of electrons onto the screen, the electron gun comprising a cathode, a first apertured accelerating electrode uniformly spaced apart from the electron emissive surface of the cathode and effective when positively biased with respect to the cathode, to produce an accelerating electric field having unipotential surfaces which are substantially parallel to this electron emissive surface and to the first electrode, a second control electrode spaced apart from and having an aperture aligned with that of the first electrode, the second electrode being arranged to screen so as to prevent distortion of any unipotential surfaces between the second electrode and the cathode as a result of electric fields produced by further electrodes in the tube, together with beam-focusing means effective to produce an electric field intersecting the constriction free electron beam of substantially constant area which is produced by the gun and having unipotential surfaces which are effective to focus the beam onto any part of the screen area.

Preferably and in order to assist to control the focusing of the beam, the cathode-ray tube includes, means producing an electric field which if effective to control the divergence or the convergence of the electron beam. Such means are well known in the art.

Suitably the focusing means comprise any two or more substantially circumferentially continuous electrically conducting elements spaced apart along the axis of and surrounding the electron beam and preferably are in the form of a graphite or other electrically conductive coating on the inner surface of the tube envelope. Alternatively the focusing electric field is produced by spaced-apart apertured electrodes adapted to be energized at different potentials or by coaxial cylinders, both well known in the art.

In the case where a graphite coating is provided in the form of a high-resistance helix extending along the neck of the tube and effective when suitably energized to accelerate the electron beam onto the screen, the electrically conducting elements conveniently comprise those adjacent end turns of the helix facing the cathode. If the helix is of a particularly high resistance per turn, the resistance between these adjacent first end turns may be adequate to produce therebetween a potential difference effective to provide unipotential surfaces at the end of the helix nearest the gun which are for example sufficiently convex to produce focusing of the beam. Alternatively the first turns of the helix may be electrically isolated from the adjacent turns in order to produce a sufficient potential difference to produce the required convexity of the unipotential surface.

In the case where a continuous graphite coating is provided on the inner of the surface of the tube envelope, the two conducting elements are conveniently produced by electrically isolating an annular end region of the coating facing the cathode.

The cathode-ray tube of the invention may, according to yet a further aspect, to be included in display apparatus the first electrode with respect to the cathode and applying a more positive bias as well as a modulating potential onto the control electrode.

An embodiment of the invention will now be particularly described with respect to the accompanying drawings in which:

FIG. ll. Is an exploded sectional side view of an electron gun according to the present invention; and

FIG. 2 ls a sectional side view of a cathode ray tube including the electron gun shown in FIG. 1.

FIG. 3 Is a sectional view of the accelerating electrode 14.

FIG. 4 Shows a further embodiment of accelerating electrode 14. Referring now to the drawings and in particular to FIG. R thereof, the cathode-ray tube comprises an evacuated electrically insulating envelope 2 having at one end a lighttransmitting screen 4 internally coated with a phosphor layer 6 capable of transmitting through the screen, radiation of a selective wavelength when excited by an incident electron beam produced by a gun indicated generally at 8 and shown in exploded detail in FIG. l.

The electron gun 8 which is supported within the second end of the envelope 2 by means well known in the art, includes a cathode 9 having an electron emissive surface 10 which is electrically heated to emission by an element 12.

Supported about 0.004 inches from the electron emissive surface H0 is an accelerating electrode 114 comprising a solid metal plate about 0.005 inches thick and provided with a central aperture 16 of about 0.020 inches diameter. This accelerating electrode M is connected to a suitable terminal point outside the tube envelope and is positively biased 5 volts above the cathode potential. This electrode arrangement produces in the space between the electrode 14 and the electron emissive surface of the cathode 9, an electric filed which has substantially planar unipotential surfaces parallel to the electrode 14 and the surface 10. This electric field produces from the emissive surface 10, an electron current which traverses this space substantially normally of the emissive surface 10 over the whole of the area thereof and in which the current density also is substantially uniform. The utilized electron beam current passing through the aperture 16 of the electrode M accordingly is derived only from that corresponding area of the emissive surface it) which underlies the aperture 116 and is therefore both of substantially constant area and current density. Since the electrode 14 during beam propagation is not used for modulation purposes and is maintained at a constant accelerating potential, the current density from the cathode 9 and particularly the current density of the beam emerging from the aperture l6 also remains not only substantially constant in spite of subsequent modulation or deflection of this electron beam, but also homogeneous.

In order to modulate the electron beam emerging from the aperture 16, a control electrode 18 in the form of a metal plate provided with a central aperture 119 is disposed about 0.005 inches from the electrode 114. The thickness of this control electrode 18 in relation to the diameter of the aperture 19 is arranged to ensure electric screening of the space adjacent to the cathode 9 from any electric field produced by further electrodes in the tube and thereby to prevent distortion of the unipotential surfaces of the electric field in this space. In one embodiment of the invention an aperture of 0.020 inches is provided in a metal plate 0.030 inches thick.

In the case where the modulation applied to electrode 18 is substantial, some distortion of the electric field in the space extending to the cathode 9 is likely to occur due to the corresponding electric field excursions. This distortion is particularly undesirably in the space between the electrode 14 and the cathode 9 and is to a large extent avoided by arranging the electrode M, for example by selecting the diameter to depth ration of the aperture therein, to have a penetration factor which is preferably less than 0.70. Alternatively the aperture in the electrode M, if thin, can be covered with a suitable screening mesh.

Two further electrodes and 22 both 0.012 inches thick and provided with central aligned apertures 243 and 2b of 0.020 inches diameter respectively are supported 0.040 inches and 0.140 inches from the central electrode M5. These electrodes which are each maintained at a positive potential of about 1,030 volts with respect to that of the cathode, are effective further to accelerate the beam emergent from the control electrode 110 and to produce that cohesion of the beam which might otherwise be lost by scattering as a result of the low accelerating potential on electrode M.

In order to more exactly control the divergence of the beam emerging from the apertures 11d and 19, further spaced-apart electrodes 29 to 36 each 0.012 inches thick and having central aligned apertures of 0.020 inches diameter are sequentially spaced 0.040 inches from each other along the axis of the tube. The

further electrodes

29, 30, 3E and 33 are electrically connected together and to the

further electrodes

20 and 22, whereby to be at the positive potential of 1030 v, whereas the

further electrodes

32, 3d, 35 and 3:6 only are connected together and are maintained at a positive potential of about 320 v. This potential difference on the electrodes produce an electric field at the region of the aligned apertures which is effective to control the divergence of the electron beam passing through and emerging from the apertures in these electrodes.

Electrode 1% is connected to a continuous graphite coating 30 within the neck 40 of the tube envelope which coating ter minates in and is connected to, a graphite high resistance helix 42. The end of this helix, which is nearest the gun is maintained at the potential of the electrode as namely 320 volts, and is effective to produce further acceleration of the electron beam onto the screen 1 of the tube. The other end of the helix is at a higher potential.

By this arrangement the beam can be brought to a force at the phosphor 6 on the screen 4, without the use of any other internal or external focusing means.

Since the beam is of substantially constant cross section and undergoing no crossover or constriction, the focused beam area will be an image only of the aligned apertures in the electron gun, rather than of the constricted area of the beam as in conventional hitherto used electron guns. Accordingly the focused beam area is limited only by that size and shape of the apertures which is compatible with the lowest permissible beam current for a required screen luminance. The apertures may be circular if a spot is required, or may be in the form of an elongate slot if a focused line is required. Since in this invention the focused beam area can be made accurately to reflect the aperture shape rather than the beam constriction of conventional electron guns, an accurately rectangular or square spot can be produced upon the tube screen. This rectangular spot if dimensioned in accordance with the picture point size in the line and in particular by the frame dimension of a television picture scan, can substantially reduce the visible effect of the scan on an image produced on the screen without significant loss of definition.

At the screen end of the envelope 2, the helix is connected to a further continuous conducting graphite coating Ml, for example of graphite or aluminum, which terminates at the screen ll.

The tube may be used in any suitable display apparatus including means, well known in the art, for applying the potentials hereinabove recited, necessary to operate the tube.

The cathode-ray tube of the invention can simply provide a focused beam area of extremely small size and high current density which size remains substantially constant over wide variations in the beam current and position on the tube screen and current density remains homogeneous throughout the beam. In addition, due to the nature of the beam, the power input necessary to scan the beam between different screen positions and also to focus the beam is extremely low compared with conventional cathode-ray tubes. in for example the typical tube hereinabove described, a spot size of 0.010 inches diameter, the diameter of which remains substantially constant over the whole of the area of the circular screen of 16 inches in diameter, can also be maintained constant up to a beam current of 1 ma.

it will be appreciated that while the electron gun of the invention has been described as used in a cathode-ray tube, it is also eminently suitable for use in any vacuum electric device such as a vidicon orthicon, travelling wave tube storage tube or even in devices merely using an electron beam in which a beam of the character described is of advantage.

It will be appreciated that while the invention has been described with respect to a gun producing a single electron beam, it is equally applicable to guns producing a plurality of beams. A further advantage of the gun of the invention is that the spot size can be varied without defocusing to produce a variable magnification. in use, a cathode-ray tube incorporating the electron gun of the invention, will be operated with the electrode M permanently positively biased at a potential with respect to the cathode and with the electrode 118 also permanently positively biased with respect to the electrode 14.

Either of these electrodes M and M can be connected to a terminal to which modulation can be applied. Apparatus for or including such a cathode-ray tube will accordingly include circuit means, well ltnown in the art, to which the electrodes M and are convectible or connected and which are capable of maintaining these electrodes positively biased.

It will be noted that in order to ensure correct operation of the tube the

electrodes

20 and 22 must be capable of operation at voltages lower than that of the final anode voltage. These electrodes, 20 and 22 must therefore be isolated within the tube from electrodes operating at or near final anode voltage.

lclaim: ll. Electron beam producing system including a vacuum electronic device such as a cathode-ray tube, comprising:

an envelope; an electron gun mounted in said envelope, said gun comprising in succession a cathode having a planar electron emissive surface, a first accelerating electrode immediately adjacent and parallel to the said emissive surface, said first accelerating electrode having therein a single aperture of smaller area than said electron emissive surface, at least one apertured screening electrode, and at least one further electrode electrically separate from the screening electrode(s); means applying to the first accelerating electrode a positive potential relative to the cathode to produce adjacent the electron emissive surface an accelerating electric field the unipotential surface of which are substantially parallel to that surface in the space defined between said electron emissive surface and the accelerating electrode; means applying potentials, higher than that applied to said first accelerating electrode, to the said at least one further electrode to provide a further accelerating electric field beyond the said at least one screening electrode(s);

means applying a variable beam modulation potential to the screening electrode(s); the latter being effective to prevent penetration of said space by said further accelerating electric field, and the first accelerating electrode having a penetration factor as herein defined less than 0.7 to shield the said space from the effects of the variable beam modulation potential applied to the screening electrode(s), whereby the electron beam produced by the gun is devoid of crossover and of substantially uniform current density over its cross section.

2. A system as claimed in claim ll wherein the apertured accelerating electrode is spaced from the adjacent cathode surface by a distance of about 0.005 inches.

the electron beam and includes a metallic mesh covering said aperture to produce screening over the area of the aperture.

5. A system as claimed in claim 3, in which the ration of the diameter to the depth of the bore in the screening electrode is substantially 2:3.

Claims

1. Electron beam producing system including a vacuum electronic device such as a cathode-ray tube, comprising: an envelope; an electron gun mounted in said envelope, said gun comprising in succession a cathode having a planar electron emissive surface, a first accelerating electrode immediately adjacent and parallel to the said emissive surface, said first accelerating electrode having therein a single aperture of smaller area than said electron emissive surface, at least one apertured screening electrode, and at least one further electrode electrically separate from the screening electrode(s); means applying to the first accelerating electrode a positive potential relative to the cathode to produce adjacent the electron emissive surface an accelerating electric field the unipotential surface of which are substantially parallel to that surface in the space defined between said electron emissive surface and the accelerating electrode; means applying potentials, higher than that applied to said first accelerating electrode, to the said at least one further electrode to provide a further accelerating electric field beyond the said at least one screening electrode(s); means applying a variable beam modulation potential to the screening electrode(s); the latter being effective to prevent penetration of said space by said further accelerating electric field, and the first accelerating electrode having a penetration factor as herein defined less than 0.7 to shield the said space from the effects of the variable beam modulation potential applied to the screening electrode(s), whereby the electron beam produced by the gun is devoid of crossover and of substantially uniform current density over its cross section.

2. A system as claimed in claim 1 wherein the apertured accelerating electrode is spaced from the adjacent cathode surface by a distance of about 0.005 inches.

3. A system as claimed in claim 1 wherein the apertured screening electrode comprises an electrode having a bore to permit passage of the electron beam, the ratio of the diameter to the depth of the bore being effective to produce screening.

4. A system as claimed in claim 1, wherein the apertured screening electrode has an aperture which permits passage of the electron beam and includes a metallic mesh covering said aperture to produce screening over the area of the aperture.

5. A system as claimed in claim 3, in which the ratio of the diameter to the depth of the bore in the screening electrode is substantially 2:3.