US2884559A - Electron lens systems - Google Patents

Description

APP!l 1959 H. G. COOPER, JR"; ETAL 2,384,559

ELECTRON LENS SYSTEMS Filed Sept; 7, 1956 2 Sheets-She et 1 L L 5 H6 D-A DH .2 w m 0 v 3 m wt A I I I III I,

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INVENTORS MELHINES Wag-mg ATTORNEY Filed Sept. '7, 1956 April 5 1959} H..G. COOPEISR', JR., ETAL 2,884,559

ELECTRON LENS SYSTEMS v I 2 Sheets-Sheet 2 FIG. 5

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ATTORNEY United States Patent I 2,884,559 ELECTRON LENS SYSTEMS Howard ,G. Cooper, In, Morristown, NJ., and Marion E. Hines, Kellers Church, Pa., assignors to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Application September 7, 1956, Serial No. 608,532 '5 Claims. (Cl. 315-14) The-present invention relates to electron-optical devices and more particularly toelectron discharge devices of the cathode ray type having means for correcting deflection defocusing of the electron beam.

.ln cathode ray tube operation, an electron beam is shaped to. a desired configuration and directed toward a target structure where impinging electrons provide a visual or electrical indication thereof dependent on the particular application. In most instances the electron beam is deflected by suitable electrostatic or electromagnetic means so as to strike the target surface at any one of a plurality of discrete areas, each providing a distinct output signal. Suchdeflection presents serious problems in instances demanding minimum size and .precise uniformity of beam config l'lation at each point of impingement.

' Whereas an undeflected beam emanating from the electron gun is readily focused on a small diameter circular spot at the center of the target surface, an out of round enlargement or defocusing of the beam develops as it is deflected away. from the target center- This effect is more pronounced with electrostatic deflection than with magnetic deflection. A certain measure of such deflection defocusing is tolerable in many applications and may be limited in such applications to the maximum allowable proportions by correction methods available. in the prior Such methods include various electron lens configurations andcombinations of such electron lenses with dynamic correction in which the mean potential of a pair of deflection plates can be varied according to a 'nonlinear-function, of thedeflection voltage and relative to the electron lenses. Such systems, while reducing distortion in one manner, may tend to produce other distortions which offset a portion of the correction attained. Thus in systems demanding uniform control of an electron beam having an extremely small cross section, such as required in electrostatically deflected storage tube applications for example, the degree of deflection distortion correction realized ,by, the correction systems of the prior art may be unacceptable. Q-

It, is ageneral object of this invention to provide an improved deflected beam electron discharge device. More particularly, it is an object of this invention to pro- 2 gun and electrostatic deflection system, the lens system including a series of electrodes having aligned apertures in the beam path and energized to produce electric fields in the apertures tending to converge the electron beam passing therethrough.

It is essential in many storage and related tube applications that overall tube dimensions be kept at a minimum. To achieve such restricted tube dimensions while maintaining uniformity of beam focus at all deflection positions involves manipulation of a number of variables, the cardinal ones being described briefly hereinafter.

. Deflection defocusing is inversely proportional to the length of the deflection field through which the electron beam must travel. Thus it is advantageous to make the deflection system as long as possible commensurate with overall tube length requirements. Also deflection defocusing increases with beam deflection angle so that the maximum required deflection angle should be small. Increasing the length of tube from deflection system to target will assist in maintaining a small deflection angle and is thus advantageous within overall tube dimension requirements. Thus the'deflection defocusing problem may be reduced by increasing the length of deflection system and of beam travel from deflection system to target surface. In order to maintain a restricted overall tube length while satisfying these defocusing improvement factors, the portion of the tube containing the electron gun, lens and deflection system must be made shorter. Unfortunately, shortening the tube between cathode and lens increases the magnification factor and in turn the area of beam impingement at the target surface. Thus, for proper operation of the lens, the distance from the tube cathode to the lens must be greater than one half of the lens length. In view of these factors, it can be seen that optimum performance is obtained by incorporat ing the smallest possible lens length.

An eflective electron lens, such as the einzelflens, serves to converge an electron beam to form a circular spot at the center of the target surface of a cathode ray tube. Such a lens normally comprises a pair of circular apertured electrodes at a potential highly positive with re spect to the cathode and an intermediate circular aperr tured electrode at a potential intermediate the cathode potential and the potential of the outer lens electrodes. This configuration is analogous to the spherical lens of light optics and is eifective to form a pencil shaped beam videfafldmairitain a uniformly small diameter .of the deflection system in an electron dissystem positioned in the beam path between the electron limited in cross section by the outer electrodes and subjected to the converging action of the low potential inner electrode in conjunction with the adjacent high potential outer electrodes. A three electrode einzel lens as described cannot completely control deflection defocusing of a beam deflected in a single coordinate. Such a lens employing slit apertures rather than circular apertures provides better control in a single coordinate, and two such lenses in turn will maintain a measure of uniformity in a beam deflected in two coordinates. However, aberrations are greater in a slit aperture or cylindrical lens than in a spherical lens of the same focal length, and six electrodes comprise the two coordinate lens structure thus requiring a substantially greater tube length to accommodate the lens.

Ourinvntion, as illustrated in the. specific embodiment described herein, satisfies the restricted lens length requirement to achieve reduced overall tube length while providing a, uniformlysmall spot diameter at any point on the target surface.

In accordance-with one aspect of the invention, the lens system. comprises a pair of outer electrodes having circular apertures, which electrodes enclose a second pair of electrodes having'elliptical apertures with their major axes parallel to opposite cartesian coordinates. The aroi'dinatewill tend to weaken the field established by the tion offthe electron stream "and 'a pair or focus electrodes interposed between; tliecont'rol electrodes 'and having apertures ofsubst'antially elliptical configuration aligned with thecontrol electrode apertures.

It'is another feature'of "this 'invention that the major axes oftheellipses formed by the control electrode apertures'be mutually perpendicular.

It 'isjafurther'featureof this invention thatthe control electrodes be connected through a nonlinear circuit to a respective coordinate deflection voltage source.

A complete understanding of this invention and of these andf'other feat'ur'es' thereof may be gained from considerationof the following detailed description and the accompanying drawing in which:

Fig. 1 is adi agrammatic representation'of a barrier grid storage tube incorporating one specific illustrative embodiment 'of this invention;

Fig. '2 is a schematic diagram of one illustrative embodi'ment includinga;'perspective view of the lens system for theemb'o'dirhento'f Fig. 'lgreatly enlarged in relation to adjacent elements;

Figs. '3 andf4 are diagrammatic views of an undeflected and vertically deflected electron beam, respectively,"focused at the target structure;

Figs 5, 6' and 7 are views of a quadrant of the target structure "illustrating respectively the impinging electron beam without defocusing'correction, with defocusing corre'cticfa'n utilizing "an "einzel lens with circular apertured electrodes, and with defocusing correction in accordance theernjbo'dimentof"Fig. 1'; of invention. P

Referring 'nowto'the drawing-Fig. 1 depicts an illustrative "embodiment of this invention utilizing a barrier grid storage 'tube l0. As known in the art, the tube 10 may advantageouslyfcomprise within an evacuated envelope, such as glass, an electron gun including a cathode 11,"hea'ter 12, control grid 13, accelerating anode 14, focusing electrodes 15 defining an electron lens, deflectron plates 16 and 17, "a collector electrode 18, ashield l9, and'a targetassembly 20. The 'target assembly 20 IS a sandwich of three elements including a back plate 22, a dielectrie sheet-23, and a barrier grid 24 positioned directly in front of the dielectric sheet 23.

The dielectric sheetf23 holds an electrostatic charge deposited on its surface by the electron beam for extended periods of time, thereby performing the storage function the tube. 2 2 is insulated from the 1 may be varied to contre fl ijs a??? an fiaie by the electron-beam. Ihe'eharge deposited'atan w cre'te' area of the dielectric sheet 23 is subsequently dtectedby returning the electron beam to the discrete area.

The sizeand' proximityof discrete-"storage areas on a given'ltarget surface"are"dependent in part onthe size, .ntensity .2 and uniformity .of theiin'cident electron beam. The beam" isfnecessarily deflected to reach anyone of die discrete storage areas, but inherent in electrostatic deflection is a certain measure of beam defocusing which tends to restrict the number of possible discrete storage areas.

As shown in Fig. 3 an electron beam converges to a point 37 on a target 36. A cross section of the beam preceding the vertical deflection plates 35 is substantially circular as is indicated by the shaded area. With equal potentials on vertical deflection plates 35, the beam is unafiected thereby and passes to its focal point 37. In Fig. 4 the same electron beam is deflected vertically by a potential difference applied between the plates 35. Rather than converging to a point at the target 36, the beam now is subjected to a focusing eifect by the deflection field tending -to produce 'a-cross over of electrons in the vertical plane prior to reaching the target 36 and resulting in an oblong rather "than circular target impingement area 38 which is appreciably enlarged over the impingement area of Fig. 3. Thus the deflection plates acting as {a cylindrical converging lens tend to restrict the magnitude'of storage in the barrier g'rid tube, since discrete st orage areas must be spaced far enough apart to prevent possible overlap dueto the enlarged beam cross section toward the target extremities. Such overlap would tend to destroy the information stored at adjacent areas of the dielectric surface. One possible solution to the problem is to space the target sufficiently far from the deflection plates as to reduce the deflection angle required to reach the target extremities. Also it is possible to reduce the defocusing'efiect by increasing'the deflection platelength in thedirection of the beam axis. Unfortunately "such expedient's alone would result in a greatly enlargcd tube unsuitable for many applications and would present additional problems of beam magnification atthe target surface.

We have found that ancle'ctro'n lens arrangement in accordance with this invention will satisfy the requirements of uniformly small spot diameter at any deflection angle and will permit a reduction in overall tube length without increasing the deflection angle in comparison with other arrangements known in the art.

In Fig.2 there is shown the principal elements of the embodiment of Fig. l whichprovide the desired deflection defocu's'ing'correction in'a twocoordinate deflection system. The electron lens there depicted comprises a first limiting electrode 25 having a'circula'r aperture therein and placed at a high positive potential with respect to the cathode. "Electrodes-2'6 and 27 have elliptical apertures and each obtaina variable potential from respective coordinate "deflection voltage "sources through nonlinear circuits whichpotential-is advantageously between that of the cathode "a'rid'th'at :of electrode 25. Thus electrode 26, having the minoraiiis'ofits elliptical aperture in a vertical'plane, is connected through nonlinear circuit 31 to the vertical d'e'flection system input circuit 32. Similarly, electrode 27 has its minor elliptical axis in 'a horizontal plane and is connected through nonlinear circuit 33 to the horizontal deflection system input circuit 34. Electrode 28 completes theelcctron lens and resembles electrode 25 in configuration and applied potential.

In the absence of deflection fields, electrodes 25, 26, 27 and 28 coact to'focus the'electron beam, formed from electrons emitted by thetubcs thermionic cathode, in a spot of minimum size at the center of the target screen. The focusing action'of these electrodes in this instance is comparable to that of one or more spherical-surfaced opticallenscs and to the well known einzel electrostatic lens The focusing action 'is "effected by maintaining a potentialdifferen'cebetween adjacent electrodes. It is also'infiue'nced tosome' extent by the mutual separation -of tlie electrodes. The lens electrodes are adjustable without changing their physical arrangement, just by changing the potentials and relative potentials of the constituent electrodes.

Each elliptical apertured lens electrode zs and 26 bears similarity in operation to the cylindrical optical lens 5. in which equal convergence or divergence may be eflected between-parallel planes, the major axis of the ellipse being perpendicular to the parallel planes. A pure cylin drical electron lens is one dimensional and is formed by a pair of coplanar plates of infinite length separated to form a slit of infinite length across which the electrostatic focusing field is developed. Obviously the slit length is dictated by maximum permissible tube proportions so that in operation the ends are closed to form a compromise rectangular aperture. *Such a compromise, however, results in interaction between focus control in the desired and perpendicular planes. It is possible in accordance with this invention to reduce this interaction by shaping theaperture in the form of an ellipse so as to remove any sharp comers about which such interaction occurs. Any interaction due to the proximity of the two elliptical focus electrodes 26 and 27 may be minimized by proper selection of the major to minor axis ratio of each elliptical aperture, the electrode spacing Y 1 Upon application of a signal to the deflection plates 16 and '17,"the electron beam will be deflected to impinge the dielectric target surface at a spot other than the target center. As shown in Fig. 4 andagain in Fig. 5, the cylindrical focusing efiect of the deflection plates 16 and 17 will distort the beam such that a circular spot on the target becomes elliptical with deflection of the beam away from the center of the target. The distortion due to the deflection system alone, as seen in Fig. 5, increases proportionate to the deflection angle and, in fact, proportionate to the square of the mean deflection angle.

In Fig. 6 the deflected beam distortion is shown which results from the employment of an einzcl lens with circular apertured electrodes preceding the deflection plates in the tube structure. The spherical converging action of the electrostatic field developed in such a lens prefocuses the beam'to compensate for the cylindrical lens eflect of the deflection plates. By adjusting the lens field to compensate for changes in the deflection field, the distortion at the target is corrected to some extent.

' There"are two major disadvantages to einzcl lens araperture dimensions and lens rangements. A spherical lens arrangement introduces a pencil shaped beam to the deflection system and cannot completely correct for the one-dimensional deflection defocusing eflects on such a beam configuration at all points on the target. Correction will be least when the beam is deflected to maximum in one coordinate and undeflected in the other coordinate as at points 60 and 61 in Fig. 6.

Provision of an einzcl lens comprising three slit apertured electrodes for each deflection coordinate may overcome the first difliculty when coupled with dynamic correction circuitry, but the length of tube required to accommodate six spaced electrodes is prohibitive in the limited tube dimensions of many storage tube applications. Compensating for this lens space requirement by shortening the deflection plates and their distance from the target would further aggravate the deflection defocusing problem as described hereinbefore. Additionally, aberrations are greater in the slit electrode cylindrical lens making it less attractive in this respect than the circularly apertured spherical lens.

Fig. 7 illustrates the results obtained utilizing the arrangement in accordance with the embodiment of this invention illustrated in Fig. 2. A spot size compatible with the rigid requirements of the barrier grid tube for large scale storage is obtained at the target center, and the identical spot size is maintained at every deflected beam position about the target surface. The unique crossed elliptical lens arrangement utilizes a single elliptical lens electrode in each coordinate serving to converge the beam only in that coordinate and coacting on an undeflected beam to converge the beam at the center of the target.

An electrical circuit provided between the lens and deflection systems weakens the electrostatic field at each elliptical electrode as its corresponding deflection plates 6. are energized. To a first approximation the dynamic voltage applied to each lens is proportional to the square of the voltage difierence between the deflection plates which act in that plane. One manner of accomplishing this dynamic field variation is shown in Fig. 2 wherein balanced push-pull deflection is utilized. A sample of the vertical deflection voltage from source 32 is applied to the grids of parallel connected vacuum tubes 40 and 41 of nonlinear circuit 31. The grid voltages vary in a balanced manner, but the sum of the plate currents should, to a first approximation, vary as the square of the deflection voltage. The combined plate current of tubes 40 and 41 produces a voltage across resistance 42 which varies in the desired manner and is applied to the electrode 26. A similar circuit is provided to control electrode 27 in accordance with the horizontal deflection voltage. As shown in Fig. 2 the focus electrodes 26 and 27 are operated at a positive potential to obtain the basic focusing. If negative voltages are employed, a phase inverter stage may be utilized to'weaken the electrostatic fields at electrodes 26 and 27 when deflection voltages are applied.

The crossed elliptical lens with electrostatic fields compensated dynamically as described serves to converge the beam to a uniform small spot diameter at any deflected position. Also, with only four electrodes comprising the lens structure a minimum tube diameter and length is achieved.

In 'one specific illustrative embodiment of this invention wherein the lens structure is as shown in Fig. 2, the various lens elements had the following dimensions and spacing:

Aperture diameter: Inch Electrode 25 .048 Electrode 28 .075 Electrodes 26, 27-

Major axis .287 Minor axis .217 Thickness of electrodes .012 Spacing between adjacent electrodes .210

Electrodes 25 and 28 were placed at a potential 1000 volts above that of the cathode. Electrodes 26 and 27 are placed at 420 volts and 470 volts above that of the cathode, respectively, to obtain initial undeflected focusing in both horizontal and vertical coordinates. With these values, of the electron beam will pass through a square .007 inch on a side. Thus a beam diameter at the target of approximately .007 inch was obtained, which diameter remained constant and uniform at every deflected position of the beam on the target.

Accordingly we have found it advantageous to have the dimensions of the various components of the electrooptical system specifically related to each other. More specifically we have found it advantageous to position the electrodes 25, 26, 27, and 28 so as to be spaced apart by equal distances and to have the minor axis diameter of the elliptical apertures of the electrodes 26 and 27 substantially equal to this spacing between adjacent electrodes of the lens. Further we have found it advantageous to have the ratio of the major axis diameter to minor axis diameter of the elliptical apertures of electrodes 26 and 27 be of the order of 1.3.

It is to be understood that the above-described examples and arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of this invention.

What is claimed is:

1. A cathode ray tube comprising means for projecting; a beam of electrons along a path, means for deflecting the projected beam in mutually perpendicular directions, and means for focusing said beam prior to deflection comprising a pair of equipotential control electrodes having circular apertures and a pair of focus electrodes having apentures of uniformly varying, width elongated in mutually perpendicular directionsparallel to respective deflection directions, eachof said focus electrodes being positioned between one of said control electrodes and the other of said focus electrodes and coupled through a nonlinear circuit to the source of deflection voltage.

2. An electron discharge device comprising target means, meansfor providing an electron beam and means for deflecting said electron beam over said target means including means for producing a pair of mutually perpendicular deflection fields with separate coordinate deflection voltage source's, an electron optical system comprising a pair of control electrodes having circular apertures aligned. to pass a portion of said electron beam,v a first focus electrode having an elliptical aperuire elongated perpendicular to one of said deflection'fields and a second focus. electrode having an elliptical aperture elongatedpen pendicular to the other of. said deflection fields, said focus electrodes being positioned between said control electrodes. p v r 3. A cathode ray device in accordance with claim 1 wherein said focus electrode apertures are substantially References inthe file of this patent UNITED STATES PATENTS 2,103,645

Schlesinger err-Dee. '28-, 1937 2;4 4'9,s-24 Wither-by er a1 Sept. 1-4, 1943- 2 ,s7' 2,ss s Harrison r c Oct; so, 195-1 2,572,861 Hatter abet. a0, 1951 259x400 S'c'li'i eiher Dec 28', 1954

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