US2441769A - Electron lens system - Google Patents

Description

May 18,. 1948. o. E. H. KLEMPERER I ELECTRON LENS SYSTEM Filed March 31, 1944 19 10 f 20 2/ 25 v I F 5 (Ittomeg Patented May 18, 1948 ELECTRON LENS SYSTEM OttoErnst Heinrich Klemperer', Iver, England, assignor to Electric and Musical Industries, Ltd., a British corporation Application March 31, 1944, Serial No. 528,995

In Great Britain March 24, 1942 Section 1, Public Law 690,,August s, 1946 a Patent ex'pires March 24, 1962 11 Claims; (Cl. 250-495) r a l The present invention relates to electron lens systems such as are employed in cathode ray tubes, electron discharge valves and other electron discharge devices for focusing or forming flattened or ribbon-shapedelectron beams.

In electron lens systems of the kind referred to, the/action of the lens is required to be similar to that of a cylindricallens. That isto' say, in the ideal case of an electron lens having straight focal lines, the form of a plane cross-section of the lens field at right angles to the focal line will be invariant as the plane of the cross-sectionis translated or moved'in a direction parallel to the,

focal lines, the lens field thus, being said to have translational symmetry in a direction parallel to the focal lines v v To produce an electron, lens which is approximately translationally symmetrical, electrode systems may be employed in which the lens field mg a cross-section of a given width can be made less than in the case where the lens-forming electrode system is made up of plane electrodes.

Another object of the invention is to provide an improved method of and means for providing cylindrical focusing action substantially over the entire width of an electron lens.v

A further object ofthe present'inventionfis to provide an electrode system for forming an elec-r tron lens which is to'some extent translationally symmetrical and in which the focal length of the lens is substantially constant at least over the path of, the beam to be focused, and wherein thelens-forming electrodes are so formed as to provideshielding for an electron beam passingthrough the lens. a a v According to' the present invention there is provided an electron discharge device. arranged to haveja beam of electrons projected withinit and having an electrode "system arranged about thepath of said beam forfforming an electron lens, said electrode system comprising lens-forming electrodes having, curved surfaces or'edges' facing each other, with said path between them 56 trode system-is'constituted by two pairs of plane thecurvature of said surfaces, or edges being such that electrons passing through the lens field can be formed morenearly as desired than would be the case if said transforming electrodes had parallel plane or straight surfaces or edges facing each other.

In the case where said lens-forming electrodes are'in the form of slotted diaphragms, the slots in said. diaphragms being tapered towards their edges or being rectangular in form, said slots may be limited by having metal strips applied slantwise across the corners of the slots.

Ina preferred form of the invention said lensforming electrodes may have curved surfaces facing each other which wholly or partly embrace the path of said beam and afford electrical shielding therefor in respect of field penetration edgewise of the lens-forming electrodes. 7 v

The invention further includes an electric circuit arrangement including an electron discharge devicehaving a cathode and means for deriving a beam of electrons from said cathode, and electrodes disposed about the path of said beam, said electrodes having curved surfaces or edges facing each other about the path of said beam, and means for applying potentials to said electrodes forfocuslng said beam, the curvature of said surfaces or edges being such that said beam can be focused more nearly as desired than would be the case if said electrodes had parallel plane or straight surfaces or edges facing each other.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be more fully described, reference being made to Figuresl to 3 of the accompanying drawing illustrating schematically a typical prior art device, and further reference being made by way of example to Figures 4, 5, 6' and 7 of the accompanying drawing, of which Figure 4 shows an end view corresponding to Figure 3 of a pair of lens-forming electrodes made in accordance with the invention. r a

Figures is a diagrammatic viewin'sectional side elevation showing an electrode system which may be treated in accordance with the invention.

I Figure fiis a front elevational view of one of the electrodes of Figure 5 treated in accordance with the invention, and a Figure 7 is a. graph's'howing a characteristic curve of the arrangement of Figure 5.

Figure 1 of the accompanying drawing is a diagrammatic sectional view through a simple electrode system for providing an electron. lens having translational symmetry for focusing a ribhon-shaped electronbeam, In this case, the elecat the point F which represents the location of one of the focal lines of the lens. The lens field is produced between the two pairs of plates by charging the two pairs each'toa different potential, the electrodes I and 2 being charged in the case illustrated to a potential V1 which is lower than the potential V2 to which the electrodes 3, 4' are charged. A lens, formed in the manner described with reference to Figure 1, resembles in lens-forming electrodes are combined with shielding means as above referred to.

Referringto Figure 2, it will be seen that one lens-forming electrode comprises upper and lower sides I and 2 respectively corresponding to the electrodes I and 2 of Figure 1 and sides 6 and I which provide shielding for the path of the beam while the second lens-forming electrode comprises upper and lower sides 3 and 4 corresponding to the electrodes 3 and 4 of Figure 4 and sides 8 and 9 for providing the shielding.

its action a lens produced by charging two 00- axial cylindrical tubes as described, for example, in chapter 2 of the Cambridge Physical Tract Electron Optics, published by the Cambridge University press in 1939. That is to say, the lens field'produced inthe manner describedabove has a midplane shownat Min Figure 1 perpendicular to the plane of mirror symmetry of the electrodes represented by OZ between the electrode pairs and has six cardinal points, including two focal points, two principal pointsand two nodal points, asin the case of a cylindrical optical lens, both the principal points being located to the left of the midplane M, for example,'at P and P1. as

shown in the drawing. Both the focal lengths of the lens formed by the fou'r'plates are somewhat shorter than those of the lens formed. by two tubes having diameters equal to the separation of the plates and charged to the same potentials as the plates, but the aperture error or spherical aberrations produced in each case are similar in magnitude. It will be appreciated that for good focusing the angles of divergence of the electron beam to be focused must be confined within reasonable small limits such that the angle at which any ray such as 5 or 5", derived from an electron source E, makes with the plane represented byOZ is so small that'third order effects can be neglected.

Ideally, the parallel plane'electrode employed in forming a cylindrical electronlensshould be of infinite extent in directions of the focal lines of the lens. This is, of course, impossible in prac tical arrangements, but, nevertheless, the width of plates is required to be much greater than that of the beam to be focused. Consequently, in a lens employing parallel plane focusing electrodes the vhousingof the electron discharge device in which the electrodes are enclosed must'have a greater cross-section than that necessary to accommodate the path of the beam, the unused cross-sectional area in this case being much greater than in the case of a field formed by charging cylindrical electrodes. Y

Furthermore, the electrode structure employed should afford shielding for the path of the beam. Cylindrical lens-forming electrodes provide perfect shielding, except possibly at a gap between the electrodes which can readily be covered by an outer annulus, but with plane lens-forming electrodes as above described, shielding means must be provided at the edges of the lens forming electrodes. Usually, the shielding means are combined with the lens-forming electrodes and each pair of plane electrodes is replaced by an open-- ally symmetrical.

The lens systemillustrated in Figures 2 and 3 will be discussedanalytically with reference to the system of co-or'dinate axes :c, y, and z represented to the left of Figure 2, in which the x-axis is parallel to the line of intersection of the midplane and the plane of mirror symmetry of the electrode system represented by OZ in Figure 1; the y-axis is perpendicular to the :c-axis and to the plane sides [,2, 3 and 4 of the lens and the z-axis is perpendicular to the mid-plane and lies in the aforesaid'plane of mirror symmetry. The apertureof the lens is, given by the dimensions 2x3 and 21115 as shown in Figure 3. In an arrangement such as that shown in Figures 2 and. 3, the shielding sides 6, 1, Band 9 are found to have a distorting effect on the field of the lens, such that the lens field ceases to be translation- This distorting effect becomes appreciable when focused electrons approach the side shields 6, I, 8 or 9, within a distance of the order of the aperture dimensions 2gp. Thus, from this point of view it is advantageous to make the distance between the plates I, 2, and .3, 4, as

small as possible so as to keep the, value of ye small. However, in practice the minimum value to which the separation between the sides I, 2 and 3, 4,- c'an be reduced is limited due to thefact vthat if the sides are too, close to ether the lens becomes unsatisfactory in respect to its focusing properties for rays which approach too close to the plates I, 2, 3,4. Thus, if ye represents the thickness of the beam to be focused, then the ratio ye/Z/B must have a value less than a certain fixed small value if satisfactoryfocusing of the beam is to be obtained. Thus, the Value of ye must be made. great in comparison with the thickness of the beam, and the shields 6, I and 8, 9 must be separated to a greater extent than is necessary merely to accommodate the beam. This is especially troublesome where the beam is not many times wider than it is thick as the proportion of waste space to used space is greater the narrower the beam.

The electron optical effect of the side shields 6, I and 8, 9 is found experimentally to be capable of bein resolvedinto twocomponents which account for practically the whole of the distortion produced, The first component arises from the fact that .the four side shields 6, I, 8, 9 serve as lens-producing electrodes in the same way as the sides I, 2 and 3, 4, andthus produce a, focusing of the electron beam about the y-axis in addition to the focusing about the x-axis which is produced due to the action of the sides I, 2, 3, 4. This effect is not, ingeneral, ver'y'importa'nt, because the focal length of the lens system constituted by the side shields is great in comparison with the focal length of .the lens produced by the electrodes I, 2, 3, 4. For example, if his greater than 2ys the focal length of the lens formed by the side shieldsis morethan ten times as long as the. one produced by the sides I, 2 and 3 4. The second component arises from the fact that the presence of the side shields produces a Referring to Figure 4a of the drawing, the curved lens-forming electrodes employed in accordance with the invention are indicated at II and l2.v These electrodes correspond to the boxshapedelectrode including ,the sides I, 2,15 and 1, shown in Figures 2 and 3 and, are continued until they meet each other attheiredge's l6 and I1 so as to provide complete shielding for a beam passing between them. In order to form a lens field corresponding to that pro'ducedby the arrangement of Figure 2, a further pair of lens-.

forming, electrodes as shown in Figure4b, would be provided, arranged "immediately behind the electrode shown in Figurea. I

It will be readily appreciated that the..effect of employing lens-forming electrodes curved in the manner indicated in Figure 4 such that pairs of lens-forming electrodes are 'more widely, spaced in the center of the lens field than at the edges thereof will be to produce a lens of. which the focallength will be shorterv towards" the edges of the lens fieldrthan would'be the case if pairs of flat lens-forming electrodes such as those represented in Figure 1 were" employed. Thus, it will be seen that by appropriately choosing the curvature of the electrodes II and 12 the focal length of the lens obtained canbe' made to vary in a desired manner in a, direction parallel to the :r-axis, 1 In order to make the invention clear, the method of shaping the lens-forming electrodes in the case where it is desired to make the focal length of the electron lens constant in a direction parallel to the :r-axis will be described by way of example. In this example the lens constructed had an aperture such that xs=2ys and a ratio V2/V1 of four. In making the lens, a lens formed of box-shaped electrodes having the same overall dimensions and of which the. cross-section is indicated in dotted lines at I 9 in Figure 4 was investigated by tracing the paths of rays projected in planes at right angles to the :c-axis and intersecting that axis at $0, $1, x2, x3 respectively,

:ro being on the axis of the lens-forming electrodes, r1 being equal to /2313, m2 being equal to 1/3 and an equal to %ya. In this case the following results were obtained giving the position of the cardinal points F and P with respect to the mid-plane M (see Figure 1) of the lens system, taking 'JB as unity. At the positions we and an it was found that effectively the distance MF was 3.0, MP 2.4 and the focal length f, that is the distance between PF was 5.4; whilst at $2 MF was 4.0, MP 3.8, :and f 7.8; and ate: MF was- 7.1, MP 3.1, and f 10.2. By comparing theresults obtained at positions we and we, it can be seen that the distance MF for the two box lens is increased by a factor r ture has to be shaped so that at any position a iiijice .1: along the c-axis where the aperture dimension is y, the value of y is such that 1:4 5. 113 M F I,

2 where MFo'is the value of the mid-focal length at the center of the lens formed of box-shaped electrodes having the same values of we and ye as the corrected lens, and y is the focal length of the lens formed of box-shaped electrodes at the position :1: along the w-axis. Thus, to'produce a corrected lens the electrodes II and I2- of Figure 4 must be so curved that at the position as the aperture y1=yB; at the position we the aperture yz is 4%; and at the position we the aperture ya is Thus, the required form of the electrodes "H, i2

is determined. In general it is foundthat the forms. of electrodes H and I2 required togive straight line focusing will be substantially the.

same for voltage ratios VZ/Vl between 2 and 6.

. If' the voltage ratio is very different from four,

however, the ratio of the distance MF at a position away from the middle of the lensto the distance ME at $0 is expected to be different from the ratio derived from the results given above and the curvature of the electrodes H and I2 will consequently be different. electron lens is required to focus a converging or diverging beam instead of a parallel beam,-then theiformof the electrodes II and I2 must-be varied in accordance with therconvergency or divergency of thebeam; the distance between the mid-plane M and the image being taken as the basis of calculation instead of as the distance between mid-plane M and the focal point F. It is also possible that the focusing of the, box lens may be further'improved if the spacing of the plate elements II and [2 near their edges l6 and H isevaried from that obtained in the manner described above, as in the region of the edges the mid-focal length obtained is not longer substantially proportional to the spacing between. the plates.

It will be appreciated that if it is required to focus a rectangularbeam on a curved line ratherthan a; straight line the mid-focal distance of the lens will be required to vary across the lens; and the shape of the plate element in a system: according to the invention can be adjusted as: desired to give the required form of the focal' line.

In some cases instead of making the electrodes; I I "and I2 meet each other as illustrated in. Figure 4, they may be terminated before they meet and the shielding of the beam path through:

, the lens may be completed by providin side;

plates such as 5, 6, l and 8, in Figures 2 and 3.. However, the lens will not be thoroughly corrected in this case.

i This invention is also applicable to electron:

, lenses in which the lens-forming electrodes are:

in the form of slotted diaphragms. In such an. arrangement where the diaphragms are provided with simple rectangular slots, it is found that thelens field is such as to cause undesired curva- 4 ture' of the focal line of the lens similar to that ing edges of the slots are curved in the same manner as the plates 1 I and I2 of'Figuree.

Moreover, if the A Simple d h m e s s epr ented i Figure 5,'the diaphragm elements being indicated therein at 20 and 2|, these elements being mounted on the ends of :bpxeshaped or tubular electrodes 22 and 23 respectively, which shield the path of the beam to or from the elements 20 and 2|. A front view of the element 130 is shown in Figure 6, wherein the rectangular slot in the element is shown at 24. In order to produce an electron lens, a suitable potential difference is established between the electrodes 2!! and 2|. The arrangement of Figure will be dis; cussed analytically with respect to co-ordinate axes at, y and z disposed in the same way as the correspondingly referenced axes in Figures -2 to 4 of the drawing.

A lens consisting in a system formed in the manner described with reference to Figure 5 will focus a flat beam at a line in an we plane, which is straight in the vicinity of the central yz plane, but becomes increasingly curved outside this region, thus, for example, in the case of a lens in which the diaphragms 2D and 2| v have semiapertures are mm. and ya 3 mm. and are spaced from each other by a gap of 1 mm., if the ratio V/Vl between the potentials applied to electrodes 26 and 2| respectively is 1.7, then a beam passing first through electrode and then through electrode 2! will be focused at a line which is straight to distances equal to 0.4 $3 on either side of the z-axis, but then begins to become curved so that while at the center of the afield the mid-focal length is 29 ye at a:=:0.8 ms the focal length is 59 1/13. In applyin the invention to the correction of such a lens it is sufficient for most purposes to limit the apertures in the diaphragms 2B and 21 by means of inclined metal strips 25 (see Figure 6) these inclined strips being applied across the corners of the apertures. The strips 25 may be welded on the diaphragms 2i! and 2| and are arranged so as to stop out the triangular portions 26 at the corners of the slot having horizontal and vertical sides 0.5 we and 0.31 ye respectively. With a lens so corrected the focal line is straight up to distances equal to 0.8 me from the z-axis.

In any given case the best position of strips such as 25 (Figure 6) should be found experimentally. For this purpose it is first necessary to ascertain the degree of correction necessary at two or more regions away from the middle parts of the slots. For example, in Figure '7 the semiaperture gm of the slot at the point x=0.8 ye is plotted against the measured mid-focal length MB, The semi-aperture yi is measured in terms of the semi-aperture ye. Typical results .obtained for the mid-focal lengths are indicated by the dots 3! to 34 in Figure 7 of which the dot 3! shows the mid-focal length in the plane a:=0.8 me when there is no restriction of the slot width by strips 25, (i. e., 111 ye), Whilst the points 32, 33 and 34 show results obtained for the mid-focal length as the slot width is progressively restricted. The dotted line 35 in Figure '7 indicates the value of the mid-focal length in the central ye plane of the lens The curve A intersects the line 35 at a point 36 corresponding to a semi-aperture of 0.7 yrs. Thus the strips 25 should be applied in such a manner as to reduce the semi-aperture of the slot in the plane :r=0.8 we to 0.7 yrs. If the reduction of the semi-aperture required to beeffectedbyithe strips 25 is determined for another valueof 3: along the slot, theneach strip can be positioned ,to give "1 pm gi p s d- 9: n ce s y l by the rule employed in obtaining the curvature,

t e d m ts ,1 as 1 pf i e 4, h a q for this ,difierence being that in the immediate neighborhood of the edges of the slot the mid for calden th obtained is far from proportionality with tte' lsi i th an Pa ticula size of sl t in the diapnragnis gn' nd 2! of Fi ure 5 the curvature of the focal line obtained will depend on the ratio of the voltages applied totheelectrbdfi Z9 and 2! n th orm i he s a required to Produce a iven d re 9 Qor e iisn will dep n on this ratios M Havin go w articular y d cribe a d a rame th nat re f my sai n ntion a in hat me n r he s m istp b P r e de- Clare t, whatlsleiieis;

In ct n separate including m a for gnerating an electron beam, an electron lens structure'fdisposed' in the nether said beam compris n at least two cooperative tubular elect odes each defining lens apertures for saidbeam, said aperture defining electrodes haying" intersecting concaye surfaees symmetrically disposed about and facing the transverse axis of said lens to provide cylindrical focusing action substantially over h e t e w dths? anew; 1 i

ln d si es a a a u in ludi me s f generating an electron beam, an electron lens structure disposed in the pathof said beam com,- i ins a sa ttyo plqoper ativle tiibularelectrodes each defining lens apertures for ,said beam, said aperture defining electrqdes being shaped to inf s ted si ed edges vi h sdssrm gt i e ith re pe t i h trans s axis of gel electron beam to proyide cylindrical fo cus'ng action substantially Q, er the enti e width fsa ii I H a ,elegtrpn apparatus n ludin means for ene atin an le een an electron l n tr re q ss ed n the athai b am mi as at least tw s e tiiz t ula ele r ach defining le s ape ur s ic said be m sai se' ere's fieine sl s sfie be n Sl flRQ- t in.- ed 9 91 .9 'i l i' i qs e urrd su -a es ha i 99 91 .sr s eet' i qa wit res t to t e iansv rse yes seie e e t on beam t Yid cy in r a M ,re qveih eme each defining lens apertures for said beam, said aperture defining electrodes being shaped to provide rectangular apertures symmetrical with respect to the transverse axis of said electron beam and having predetermined corner portions thereof masked out symmetrically with respect to said transverse axis to provide cylindrical focusin action substantially over the entire width of said lens.

6. Apparatus of the type described in claim 1 including means for applying potentials to each of said electrodes for focusing said electron beam.

7. Apparatus of the type described in claim 3 including means for applying potentials to each of said electrodes for focusing said electron beam.

8. In electron apparatus including means for generating an electron beam, an electron lens structure disposed in the path of said beam comprising cooperative concave electrodes symmetrical with respect to the transverse axis of said electron beam, said electrodes having intersecting arcuate surfaces having common chords on said transverse axis of said lens to provide cylindrical focusing action substantially over the entire width of said lens.

9. Apparatus of the type described in claim 8 including means for shielding said electron beam intermediate predetermined adjacent edges of said electrodes.

10. An electron-discharge device arranged to have a beam of electrons projected within it and having an electrode system arranged about the path of said beam for forming an electron lens, said electrode system comprising lens-forming electrodes having intersecting arcuate edges facing each other symmetrically disposed about said path, the curvature of said edges being such that electrons passing through the lens field can be focused more nearly as desired than would be the case if said lens-forming electrodes had parallel plane edges facing each other.

'11. In electron apparatus including means for generating an electron beam, an electron lens structure disposed in the path of said beam comprising .at least two cooperative tubular electrodes each defining lens apertures for said beam, the cross-sections of said aperture defining electrodes being in the form of concave elements symmetrically disposed about and facing the transverseaxis of said lens to provide cylindrical focusing action substantially over the entire width of said lens.

OTTO ERNST HEINRICH KLEMPERER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,605,781 lEtogowslri et a1 Nov, 2, 1926 2,072,957 McGee Mar. 9, 1937 2,080,449 Von Ardenne May 18, 1937 2,124,270 Broadway July 19, 1938 2,202,631 Headrick May 28, 1940 2,268,194 Glass Dec. 30, 1941 2,277,414 Ramo Mar. 24, 1942 2,289,071 Ramo July 7, 1942 2,293,539 Gray Aug. 18, 1942 2,303,166 Laico Nov. 24, 1942 2,312,723 Llewellyn Mar. 2, 1943 2,318,423 Samuel May 4, '1943 2,351,501 Gray June 13, 1944 2,351,757 Gray June 20, 1944 FOREIGN PATENTS Number Country Date 525,000 Great Britain Aug. 20, 1940

Images