US 3080500 A
Description (OCR text may contain errors)
March 5, 1963 e. w. PRESTON CATHODE RAY SYSTEM Filed June 22, 1959 r N 7 w a an M H 04 s m FF F 3 E a M a w CONTROL VOZI'HGE SOURCE INVENTOR. 64 54/4/ M FRESIOA/ iinited States 3,9895% Patented Mar. 5, 1963 3,089,500 CATl-IGD RAY SYSTEM Glenn W. Preston, Oreland, Pa, assignor, by mesne assignments, to Philco Corporation, Philadelphia, Pa, a corporation or Delaware Filed lane 22, 195% Ser- No. 822,954 4 Claims. (Cl. 315-46) This invention relates to electron discharge devices and in particular to a novel electron gun for use therein.
The invention is particularly adapted for use with cathode ray tubes and will be described in connection therewith, although it should be understood that numerous other applications thereof are possible. In conventional cathode ray tubes there is customarily a cathode or other electron-emitting structure, a beam accelerating electrode to which a high positive potential is applied, a control electrode intermediate the cathode and the accelerating electrode, and an electron-collecting electrode. Modulation of the beam is usually eifected by supplying a negative voltage wave, which may have amp-ntude values in the range 6 to 50 volts for example, to the control electrode. Control voltages of this order of magnitude and polarity are necessary to enable the beam to be cut oil so as to overcome the influence of the highly positive accelerating electrode. The magnitude of this negative cut-off potential is one important factor in limiting attainable values of transconductance.
Conventional electron gun structures are also characterized by uneven utilization of the emitting surfac This is caused by the fact that, as the control signal on the control electrode becomes more positive with respect to the cathode, electrons from a larger area of the emitting surface are employed in increasing the beam current. Conversely, when the control electrode potential becomes less positive with respect to the cathode, electrons from a smaller area of the emitting surface, i.e., from more centrally located areas, are used in forming the beam. This phenomenon results from the fact that the contours of the tips of the equipotential lines from the positive accelerating anode, which extend through the control electrode aperture toward the cathode, become flatter and are parallel to the emitting area at more points when the potential on the control electrode becomes more positive, and become narrower and more pointed toward the center of the emitting surface when the control electrode goes more negative.
It is also a characteristic of conventional electron gun structures that the electric held between the cathode and the control electrode forms a convergence lens which is subject to the electron-optical analogue of the optical aberration known as spherical aberration. The lens act-ion causes electrons from the more peripheral portion of the emitting surface to crossover at points on the electron-optical axis which are different from the points at which electrons from the central portion of the emitting surface crossover. In conventional guns spherical aberration may also occur elsewhere in the electron-optical system and if, as is usually the case, the spherical aberration is positive (i.e. the peripheral electrons from he cathode cross the electron-optical axis ahead of the paraxial electrons), it is difficult to correct or compensate for it by introducing appropriate amounts of negative spherical aberration.
It is therefore an object of the invention to provide an electron gun or use in an electron discharge device which permits attainment of higher values of transc-onductance than are possible with conventional electron guns.
It is a further object of the invention to provide an electron gun for a cathode ray tube in which a novel system of beam-modulation is employed.
Another aim of the invention is to provide a cathode ray tube electron gun whose electron-emitting surface is more uniformly employed in the production of an electron beam.
Another object of the invention is tron gun for a cathode ray aberrations are reduced or Still another object of the invention is to provide an electron gun for a cathode ray tube which can be designed to correct for positive spherical aberration.
In accordance with the invention the foregoing objects are achieved by means of an electron gun in which the intensity of the beam emitted from the cathode emitting surface is maintained substantially constant and in which modulation thereof is produced by absorbing desired numbers of the electrons in the beam and reflecting others to an appropriate electron-collecting body. In one form of the invention I provide a cathode for emitting electrons, an electrode assembly for impelling the emitted electrons along a given path, and an electron mirror electrode to which a relatively low control voltage wave is applied. This electrode absorbs some of the emitted ele trons and reflects others, as a function of the amplitude and polarity of the signal applied thereto, along another path toward a collecting electrode. Thus, as the control voltage goes more negative, more of the emitted electrons will be reflected and fewer will be absorbed than when it goes more positive, and vice versa. The electrons which are reflected constitute an electron beam which is effectively modulated in intensity and which may be directed onto an appropriate electron-collecting body for utilization. According to one feature of the invention the electrons impelled toward the electrode are made to trace essentially parallel rays so that the electron mirror will have substantially the same eifect on all of them and those that are reflected will be reflected in substantially the same way. To obtain this kind of beam a plurality of positive electrodes are disposed in proximity to the cathode which cause the electrons from the emissive surface first to diverge and then to converge slightly, thereby producing substantially parallel electron rays.
The sole FIGURE illustrates a form of the invention as used in a cathode ray tube.
Within an evacuated envelope 10 a cathode 12 of the thermionic-emissive type is located which is heated by the filament 14 when current is applied thereto. Electrons emitted from the cathode 12 are attracted by an electrode 16 to which a constant positive potential of about +5 volts is applied. The electrode 16 may be in the form of a hollow cylinder having a closed end containing an aperture 18 through which the emitted electrons pass. They are then attracted by the electrode 20 which is at a positive potential of about 500 volts and which makes the electrons diverge somewhat and pass through the aperture 22 therein. A similar electrode 24 is at a lower positive potential of about 200 volts which causes the electrons to be converged somewhat as they are attracted thereto. The electrons pass through the aperture 26 in the electrode 24 and are then attracted by the high positive potential on electrode 30 which is at about the same potential as the electrode 26. Again the electrons are caused to diverge somewhat so that as they pass through the aperture 28 therein they emerge therefrom as the beam 34 containing substantially parallel electron rays. The electrons then pass through the first aperture 37 of an electrode 43 which may consist of two cylindrical members having respective apertures 37 and 39 and having respecto provide an electube in which electron-optical essentially eliminated.
tive flat end portions 41 and 42. Both parts of this structure are connected electrically to a voltage of about +5000 v. This electrode tends to minimize the variation of the electrostatic field in the retroflexed portion of the tube neck thereby minimizing any deflection of the beam resulting from the changein voltage that is applied to electrode 40 whose operation will now be explained.
It will be noted that constant potentials have been applied to all the electrodes mentioned thus far so that there is no modulation of the beam produced thereby. Consequently the beam current density as it leaves the aperture 37 will remain constant and the emission over the emissive surface of the cathode will be maintained constant. i
In accordance with my invention a mirror electrode 40 is located in the path of the beam of electrons 34. It is connected to a source 38 of a control voltage wave which may comprise, for example, a source of a signal modulated at video frequencies .if the cathode ray tube is to be employed for reproducing television images. The control wave may typically have a voltage swing of about to volts which is much less than is requiredin a conventional electron gun. This is due to the fact that the modulating electrode does not have .to act as a screen with respect to the field created by the high positive voltages on the subsequent accelerating electrodes (as the control grid does in a conventionalcathode ray tube) and therefore does not require the application thereto of a relatively large negative voltage to cut-off or reduce the current in the beam to-a minimum.
As the potential on the electrode 40-goes more positive, more of the electrons in the incident beam 34 will be attracted thereto as shown at the numeral 50 and will be absorbed thereby,.and fewer will be reflected to form the reflected beam 35. Conversely, when the signal goes more negative (less positive), more of the electrons in the original beam 34 will be reflected to form the beam 35 .and'fewer will be absorbed by the electrode 40.
The electrons in the reflected beam 35 then pass through the aperture 39 of electrode 43 and traverse the neck of the tube untilthey are subjected to the focussing field of a conventional focussing coil 33 (if magnetic focussing is employed), forexample, and then are deflected by a conventional yoke 32 and swept over the screen 36 to which a high positive voltage is applied.
The'beam-forming electrodes shown in the sole FIG- URE may alternatively be of other types. In certain applications where raher large beamcross-sections can .be tolerated, the beam-generating and formingelectrodes may be supplanted by a so-called Pierce gun which is described in lectron Optics, by O..Klemperer (Cambridge University Press, Second Edition, 1953) at page 270, et seq. This type of electron gun has found utility in ,microwave devices where large -beam cross-sections can The tolerated. .If a small beam cross-section is-desired, '-as in television-typecathode ray tubes, manufacture of an appropriatePierce gun-for-this purpose would pose a considerable problem from the point of view of the tolerances involved.
his a fact known to workers in electron-optics that electron lenses formed by static fields invariably introduce positive spherical aberration. Inordinary optical applications such aberration may be compensated by a corrective lens which produces a negative spherical abernation. In electron-optical applications there is no way of producing an electron-optical lens/having a negative spherical aberration characteristic. It has been known,
however, that spherical aberration in electron-optical systemsmay be corrected somewhat by the use of an electron mirror. It is therefore possible by employing an electron mirror as the modulating electrode to correct for spherical aberration that is introduced either by an electron lens in the cathode region or by a lens located in the region in which electrons are focussed, i,e.,'in the focus field ofa conventional cathode ray tube. Thus, if the cathode-grid lens produces a positive spherical aberration I the modulating mirror electrode 40 andthe field produced thereby can be constructed to compensate therefor by introducing a certain amount of negative aberration.
' Reference is made'to "Electron Optics, supra, at pages 152, 153, and the Ramberg article (1949) listed on page 455 thereof. It is even conceivable that the mirror 40 may be so constructed as to over-compensate for the spherical aberration introduced in the electron-gun region so as simultaneously to correct for spherical aberration introduced into the reflected beam 35 by subsequent focus fields or other lenses.
With the use of an electron mirror it is also possible to obtain corrections of the electron-optical analogue of the optical aberration known as chromatic aberration." Electrons emitted from the cathode possess different velocities and therefore react differently to the eiicct of the electron lenses thereupon. In some cases it may be advisable to shape the electron mirror either physically or by the application of appropriate voltages thereto in order to compensate for this type of aberration. See Electron Optics, supra, page 174.
While the present invention has been shown in a form in which electrons emanating from the beam portion of the electron-gun trace essentially parallel electron rays this is not an indispensable requirement. Hence, conventional types of beam-generating and accelerating structures may be employed, but it is believed that optimal operation of an electron-gun constructed according to the present invention is more likely to be achieved when the electron rays approaching the electron mirror are substantially parallel since the effect of the electron mirror is more likely to be uniform thereupon than if they are not.
1. An electron gun system for an electron discharge device comprising: means including a cathode for producing an electron beam of substantially constant intensity, means for causing said electrons of said constant-intensity beam to travel in substantially parallel paths extending in a first direction, an electron mirror electrode disposed in said electron paths, means for applyinga relatively low and varying potential positive with respect to the potential of said cathode uniformly to said electron mirror electrode, thereby to cause said mirror electrode to intensity-modulate said beam by absorbing some of 'said electrons and reflecting others in a second direction as a function of said varying potential, electrode means near said mirror electrode, said electrode means having first and second apertures and being positioned to permit electrons approaching said mirror electrode to pass through said first aperture and electrons reflected from v:said mirror electrode to pass through said second aperture, and means for applying to said electrode means a high potential positive with respect to said cathode potential thereby to establish an electrostatic field between said electrode means and said mirror electrode which substantially prevents deflection of said electrons in response tosaid varying voltage.
2. A cathode-tray system comprising means including a cathode for producing an electron beam having a substantially constant intensity, a single electrode positioned in the path of said constant-intensity beam, means for applying uniformly to said electrode a control potential positive with respect to the potential of said cathode and having a value such as to cause said single electrode to modify the intensity of said beam by absorbing some of its electrons and reflecting others in respective amounts dependent on said value, and an anode for collecting those electrons of said beam reflected by said single electrode.
3. A cathode-ray system according to claim 2, wherein said single electrode is substantially planar and is positioned in said beam path at an acute angle with respect thereto, thereby to cause the path of said electrons reflected by said electrode to differ from said path of said constant-intensity beam.
4. Acathode-ray system according to claim 2, wherein said means for applying said control potential comprise means for applying a control potential having variations in a range suchas to cause said single electrode to inten- References Cited in the file of this patent UNITED STATES PATENTS Knoll May 2, 1939 Orthuber Nov. 26, 1940 6 Morton Feb. 3, 1942 Smith Oct. 26, 1948 Sziklai Aug. 7, 1951 Giacoletto July 15, 1952 Dichter Jan. 24, 1956 FOREIGN PATENTS France Dec. 1, 1943