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. D¢¢17,_1946. " of; H, KLEMPEREÉ , ELEcTRoN ' ’ .f7 ç_- F5914 , _ » ¿_ . ' ' F j 1v1 Z _ _ »f _ _t_ _- ¿550% l LENS Filed Mamh s1, 3 ` ‘ l . 4 LVZ \.. _1944l - ` ' . _ _ ‘ j" .4g/Q_ ,n _4-Fgr/ß. __ m | 2,412,681» ' _j „ä _y , - M., ‘ ’f 'l (Ittorneg 2,412,687 ‘ Patented Dec. 17, 1946 UNITED STATES PATENT OFFICE ` 2,412,687 ELECTRON LENS Otto Ernst Heinrich Klemperer, Iver, England, as signor to Electric and Musical Industries, Ltd., a British corporation Application March 31, 1944, Serial No. 528,994 In Great Britain April 8, 1942 13 Claims. (Cl. Z50-«1.60) l Z The present invention relates to electrode sys tems for producing electron lenses such as are of the smaller diameter electrode (A in Figure 1B) embraced by the larger diameter electrode employed in cathode ray tubes, electron discharge l (B in Figure 1B), and the mid-surface of the lens is defined as the surface including -the inner valves and other electron discharge devices for focusing electron beams and to electron discharge UI . edge or edges cf the aforesaid end of the smaller v. devices including such systems. diameter electrodes. Thus, in the case shown in ` An electrode system for producing an electron Figure 1B the mid-surface of the lens is consti lens is usually disposed symmetrically about an tuted by the plane of the aforesaid end of the axis and affords a passage through which the smaller` diameter electrode, this plane being beam to be acted on is projected. For example, 10 represented by the line M in Figure 1B. The the electrode system may comprise two axially electrodes A and B may be of circular, rectangualigned plane diaphragm electrodes having cen lar, square, elliptical or other desired cross-sec tion having axial symmetry, or they may each tral apertures defining said passage or tubular comprise a pair of oppositely disposed elements, electrodes embracing said passage with their ends or transverse edges included in planes perpen 15 for example of plate form. In the case of an electron lens system com» dicular to the axis, Thus, Figures 1A and 1B prising a pair oi diaphragm electrodes the mid of the accompanying drawing are each diagram matic side elevational Views» of known systems of surface of the lens is defined, as the surface mid way Ibetween the facing surfaces of the dia tubular electrodes, each comprising two aligned tubular electrodes A and B respectively, through 20 phragm electrodes. Thus, in the case of the hitherto known arrangements including plane which the beam to be acted upon is projected. diaphragms the mid-surface is again also a plane; With such systems, if electrodes A and B are In cases where the electrode system comprises maintained at different potentials V1, V2, respec tively, the electron lens so produced will be con more than two electrodes the field produced is verging whichever electrode is the more positive 25 equivalent to that of several pairs in combination, and the system may be treated as producing a and whether or not an electron beam is projected from left to right or from right to left through combination of separate lenses each constituted the system. The focal length of the lens will between one electrode and the next in succession depend on the ratio of the voltages applied to the along the axis. In the case of a system compris electrodes A and B with respect to the cathode 30 ing three electrodes for forming a saddle ñeld of the beam operated upon. As illustrated in lens, the mid-surface may be deñned as the sur Figure 1A, electrodes A and B may lbe of the same face midway between the surfaces including the inner edges or facing surfaces as the case may diameter so that the edges of the electrodes are be of the outermost electrodes, but the arrange in registry, the electrodes being separated by a gap G having an axial length sumcient to provide ment can also be regarded as composed of a pair of lenses constituted by the electrostatic fields an adequate insulation between the two elec trodes. In this case the electron lens is con in the gaps between the electrodes. stituted by the electrostatic ñeld produced about As far as I am aware, in the prior construction of electron lens systems employing only electrodesthe gap G and the mid~suriace of the lens,‘which is an arbitrarily selected surface of reference 40 which afford an unobstructed >passage for the located in the manner described, for example, in beam to be acted upon, such as tubular or dia the Cambridge Physical Tract “Electron opties” phragm electrodes or combinations of tubular by O. Klemperer, Cambridge University Press, or diaphragm electrodes, the edges of lens-form ing tubular electrodes in the lens i‘leld, herein i939, is constituted by the surface midway be~ tween the surfaces including the inner edges of 45 referred to as the “inner edges,” or the facing surfaces of the lens-forming electrodes have been electrodes A and B, namely the edges of the elec bounded by planes and consequently the mid trodes A and B on either side of the gap G. As surfaces of the lenses have been planes. I am these surfaces are plane the midesurface is a plane and is indicated by the chain dotted aware, however, that electrode systems for form straight line M of Figure 1A. If the electrodes 50 ing electron lenses have been proposed including curved electrically conducting mesh elements A and E are of different diameters they are pref disposed across the path of the beam to be acted erably disposed with the end of one electrode inserted within theV other as electrode illustrated on, and in some such‘constructions the mesh inFigure 1B. In this case the lens is constituted l elements have been `so curved that `the electron l , by the'electrostaticiield producedabout thev end, 55 lenses obtained would have curved ¿mid-surfaces. 2,412,687 4 However, electrode systems employing mesh ele Figures 4A and 4B are plan and elevational> ments have the disadvantages that the mesh ele ments intercept electrons in the beam and it is diflicult to obtain satisfactory focusing. More~ view-s respectively of an electrode system accord ing to the invention including cylindrical elec trodes, over, asl far as I- am aware, the advantages to Ui be obtained by employing lenses with curved inid surfaces have never been appreciated. Figure 4C is a perspective view of one of the electrodes shown in Figures 4A and 4B, Figure 4D is a plane development of the elec trode ofv Figure 4C, The ‘present invention arises out of an investi gation for the purpose of providing improved Figures 5A and 5B are a perspective view and a plane development respectively of a further electrode of cylindrical cross-section for use in a electrode systems for producing electron lenses` for focusing an electron beam in a, line, »for ex ample as required in certain kinds of electron discharge valves for high frequency operation, system according to the invention, Figures 6 and 7 show two electrode systems for example, in devices of the kind known as formed in accordance with the invention, and Klystrons. each of which comprises two tubular electrodes, ' ' According to one feature of the present inven tion an electrode system for formingan electron lens is provided compri-sing two aligned co operating electrodes affording an unobstructed passage for anelectron beam to be acted upon, and having the inner edge or edges or” each elec trode presented to the inner edge or edges of the other, said edge or edges being so curved that the mid-plane of said lens is curved. According to another feature of the present invention an electrode system for forming an one of which is of smaller diameter than the other and has one end inserted within the other elec trode in the manner described with reference to Figure 1B above, Figure 8 illustrates a construction of an elec trode for use in the system in accordance with the invention of which the 1ens-forming elements are of diaphragm form, and Figures 9 and 10 show two electrode systems formed in accordance with the invention and each including more than two lens-forming electron len-s is provided comprising two aligned col-operating electrodes affording an unobstructed electrodes. . In Figure 2 of the drawing an electrode system is shown which is suitable for focusing an elec passage for an electron beam and of which one electrode is of smaller diameter than the other 30 Jtron beam at a line perpendicular to the axis of and'has one end inserted within the other, the edge of said end being so curved that the mid plane of said lens is curved. According to a further feature of the present invention an electrode system for forming an the System. Such a system may be employed, for example, for focusing a, fiat ribbon-shaped beam. The system shown in Figure 2 comprises a pair cf electrodes A and B aligned and having their sides in registry and aifording a passage of uni electron lens is provided comprising. a pair of apertured diaphragm electrodes disposed with form rectangular cross-section throughout their length. The electrodes are separated by a gap G to isolate them electrically from one another. The electrode A has horizontal sides l and 2 and vertical side-s 3 and li, while the electrode B has horizontal sides 5 and E and vertical sides î and 8. The horizontal sides of the electrodes A and ` Bare represented as being wider than the vertical their apertures aligned to afford an unobstructed passage for Van electron' beam, said electrodes having their facing surfaces so curved that the mid-plane of said lens is curved. The invention also includes electron discharge apparatus having an electrode system as above . set forth and means for >projecting an electron beam through said passage and means for main taining the electrodes of said system at such potentials that said beam i-s acted upon in a sides, but for some applications, the sides may all be of the same width so that the cross-section of the electrodes is square or the vertical` sides may be Wider than the horizontal sides. if the electro-des A and B are maintained at desired manner. Among the objects of the invention are to pro vide an improved method of and means for focus- .« ing an electron beam. Another object is ’to provide an improved method of and means for reducing aberration in an electron lens. ` A further object is to provide an improved i method of and means for deriving a line image of anelectron source. An additional object of the invention is to pro v »vi-de an improved electron lens having a curved mid-plane to reduce spherical aberration. Another object is to provide an improved method of and means for focusing an electron beam by employing an electron lens system in which the component electron lenses have curved mid-planes. . ' v ‘In order that the said invention may be clearly understood and readily carried into effect the same will now be described with reference to `Figures 2 to i0 of the accompanying drawing, of which Figure 2 is a perspective View of an electrode system according to the invention, Figure 3 -isa perspective view of an electrode for use in a further electrode system according to the invention, > > different potentials, the electron lens produced is analogous to twov crossed cylindrical optical lenses, one lens corresponding in effect to the ñeld due to the sides E, 2 and 5, S of the electrodes A and E and serving to focus a projected electron beam at a line in the horizontal plane through the axis of the system, and the other lens corre sponding in effect to the field due to the narrow sides 3, 4 and "l, 8 of the electrodes A and B and tending to focus the beam at a line in the vertical plane through the aforesaid axis. `In accordancewith the present invention, :the inner ends of the vertical sides 3, ¿i and l, 8 of the electrodes A and B are formed with curved edges to constitute an electron lens having a curved mid-surface. Thus, in the example il lustrated in Figure 2 the curved edges are cut to conform to cylindrical surfaces of radius R having their axes perpendicular to the axis of the electrode system, the curved edges on’ elec trode A being recessed and the curved edges on electrode B being convex and complementary to those of electrode A. The mid-surface of the electron lens will then be a cylindrical surface of radius R located in the middle of the gap G. >,Such a system is found to give better focusing 75 of a» beam of given cross-section than a system 5 42,412,687 “having au the innereedges straight ‘and win handle satisfactorily a beam filling a greater part of the cross `section of the electrode sys tem. ‘ For best results, the radius R is pref erably made small, for example, it may be made - 6 l*lens forming electrode system than it is on the edges‘thereof. ï » Figures 4A to 4D of the drawing illustrate the applicationV of the invention to a system of elec trodes in which the electrodes are cylindrical and 'half the width ofthe vertical sides of the elec afford a passage of uniform circular cross-sec trodes A and B so that the edges thereof are tion throughout. This system, like the systems Besides improving the focusing described with reference to Figures 2 and 3 is intended `for focusing a ribbon shaped beam to . semicircular. ' properties of the lens, the curvature of the mid -surface also controls to some extent the focal 10 Vlength of the lens. If theV electrode B is maintained at a higher _positive potential than the electrode A, a parallel electron beam `passing through the lens field will a line. ` Referring to Figures 4A and 4B it will be Iseen that the electrode system shown comprises a pair of electrodes A” and B” having their edges formed with diametrically opposed projecting be caused to converge whatever the ratio of the 15 portions II, I 2 or I3, I4 as the case may be, the edges being complementary. The electrodes A” voltages on the electrodes. However, if, in the arrangement of Figure 2, the electrode A is main and B" are identical in form, the single elec trode B" being shown in Figure 4C and a plane tained at a more positive potential, the lens field development of this electrode is shown in Fig will cause a beam projected through it in either 20 ure 4D. From the latter figure it will be Iseen direction to diverge. that the projecting portions I3 and I4 are formed As already pointed out, the vertical sides 3, 4 by cutting the edge of the electrode to conform and l, 8 of the electrodes A and B of Figure 2 passing through the electrodes. 'I'hey also cause to a series of semicircular arcs I 5, I6, I‘I and I8 each of radius equal to an eighth of the circum the focal line obtained when a ñat, ribbon ference'of the tube, the arcs being alternatively shaped beam is projected through the system to be curved especially in the vicinity of the sides. convex and recessed. have a slight focusing action on an electron beam The electrodes may be formed by severing a single flat sheet of mate rial along the line of the arcs I5 to I8 and roll These difficulties can be avoided by curving the ing the two severed portions of the sheet to pro inner edges of the horizontal sides I, 2 and 5, 6 of electrodes A and B. For example, Figure 3 30 vide the electrodes. The system of Figures 4A and 4B will focus a shows a perspective View of an electrode B' which beam having a diameter equal to the radius Rr corresponds to the electrode B of Figures 1 and of the tubular electrodes at a line without giving 2, but which has the edges of its wide sides 5, 6 rise to any appreciable aberration and with quite cut away or recessed to‘conforrn to a cylinder small ratios between the voltages on the elec of radius RI. If the electrode B’ is employed trodes. For example, to produce a line focus at with a co-operating electrode similar to the eleca distance of ñve `tirnes the tube radius from the trode A of Figure 2 but having the .inner edges principal plane of the lens system the voltage of its horizontal sides such as I, 2 convexly curved ratio is only about 1.4 whereas a straight edge to conform to a cylinder of radius RI, then if the value of RI is correctly chosen an electron lens 40 two-tube lens requires a voltage ratio of about , may be produced which is effectively the equiv alent of a 'single pair of wide plate form ele ments corresponding to the sides I, 2 and 5, 6 of electrodes A and B without the vertical sides 3, 4 and '1, B, though the vertical sides are still present to shield the path of the beam through the electrodes. The radius RI may be made quite small, for example, a simple, but quite ef fective, construction is afforded by making the , edges of the horizontal sides semicircular, the curvature of the edges of the horizontal sides being determined by the width thereof. In the systems described with reference to Figures 2 and 3 of the drawing the curvature of the mid-surface of the lens is identical with that of the surface in which the curved inner edges of the lens-forming electrodes A and B , or A’ and B’ are located, but it is not essential six to give the s_arne focal length. However, the length of the focal line may be greater than the original diameter of the beam and the beam may be divergent in directions par allel to the plane, and, in fact, with the construc tion of electrodes described with reference to Fig ures 4C and 4D, the divergency of the focused beam is such that the focal line will have a length of the order of twice the diameter of the original beam. If it is desired that the beam shall not be divergent, or should be of different divergency the inner edges of electrodes A" and B" can be shaped accordingly. For example, if, instead of cutting the inner edges of the electrodes to con form to semícircular arcs of radius (or one eighth the circumference of the tube) as that this should -be so. Thus the curvature of 60 described with reference to Figure 4D, the arcs the inner edgesof the electrodes may be made are not made semi-circular, but are given a radius to diiîer so that the curvature of the mid-surface » 21rRr approximately, a system which produces is different from that of the edges of either elec ' practically no divergency of the beam will be pro trode. For example, if the radii of the register~ duced. The focal length of the system will be in ing edges of the electrodes differ by an amount 65 creased however, and also the aberration pro equal to the width of the gap G, the radius of duced. Such behaviour is of course to be expected the mid-surface will be equal to the mean of from the fact that as the radius of the arcs I5 to the radii of the adjacent edges. Furthermore, vI8 is increased, the electrodes A” and B” become the inner edges of the electrodes A and B need more nearly straight edged. Thus, if the radius not conform-Ito cylindrical surfaces. For ex ¿is increased indefinitely a system producing a ample, in order to correct for aperture error cor~ » «point focus would‘be obtained. From this it will . responding to' _spherical aberration in an optical ¿be appreciated that if the radii of the aforesaid lens, it may be- desirable to cause the radius of - arcs are greater than 21rRT the beam will con `curvature of the mid-surfaceto be greater at the VergeV after passing through the lens systems center of the lens neld about' the> axislof the.` 75 "there Ybeing two real focal lines for the lensdis '2141256.87 - While in the Vdetailed description above, the In some cases a satisfactory focusing of a rib- application of the invention to lens forming sys tems .including tubular electrodes has been- .de scribed, the invention may also be appliedto sys bon shaped beam may be obtained with comple mentary cylindrical tubular electrodes formed in the manner illustrated at A’" in Figures 5A and tems-in which diaphragms are employed as lens 5B with straight edge portions l2l and 22 alter nately with arcuate portions either convex as shown at 23 and 24 in Figures 5A and 5B or con cave. 8 _the construction of Figure 6 being particularly valuable in this respect. ¿posed at different distances fromthemid-surface and in planes at right angles to each other. forming elements. For example, in Figure 8 of the drawing an electrode D is shown constituting of a rectangular tube 21 constituting a shield However, a lens system formed of elec- ` portion, the shield portion having one end closed .by a diaphragm 28, constituting the lens-forming element, the diaphragm being curved to conform trodes constructed in this manner is no-t- thought to have a very wide application. It is believed that having regard to the description already given with regard to the system of Figures 4A to a cylindrical surface and havingv a slot 29 eX tending parallel to its curved edges. This elec and 4B, the Inode of operation of a system em ploying electrodes formed in the manner indi cated with reference to Figures 5A and 5B will be trode may be arranged to co-operate with a sim ilarly formed electrode but with the end carry ing the diaphragm recessed and curved in a man ner complementary to the curvature of the end of the electrode D. With such an arrangement, if a iiat ribbon-shaped beam is projected axially through the system with its longer dimension in the direction parallel to the edges of the slots such as 29 in the diaphragm, and the electrodes are maintained at different potentials, the beam understood without difficulty. Figures 6 and 7 of the drawing show theappli cation of the invention to lens systems of the kind described above with reference to Figure 1B, in which the lens ñeld is formed at the end of a tubular electrode inserted within‘another elec trode‘of larger diameter. ’Thus, in the example shown in Figure 6, the system is constituted by co-operating cylindrical electrodes A” and C, the will be focused at a short line. In'some applications, diaphragms'such as 28 might be constituted simply by elements in the form of strips separated by a gap corresponding above, and having this end inserted within the 30 to the slot 2:23, and the shield portion 2‘! might electrode A" having one end formed with pro jecting portions Il and l2 in the manner de scribed with reference to Figures 4C and 4D larger diameter electrode C, shown partly broken away to show the `disposition ’of parts within its be omitted. Figures 9 and 10 show the electrode systems for interior. a conducting coating on the neckof a cathode ray tube for example. Preferably the electrode A" .i producing a plurality of lenses having mid-sur faces curved in accordance with the invention. The arrangement of Figure 7 includes three of Figure 6 carries a conducting flange 25 for ef electrodes 3E?, 3|, 32, separated by gaps 33, 34, rî‘heelectrode-C may be constituted by the two mid-surfaces of which are oppositely fectively closing the endI of `electrode C so as to shield the lens field from field penetration through the gap between the electrodes. A suit curved. The potentials V1, V2, V3, respectively, able gap must be maintained between the flange i 25 and the electrode C for insulating purposes. With the arrangement o-f 'Figure A6 the lens field to increase or decrease successively, or the elec trode 3i may be at a more positive or less posi tive potential than either of the electrodes 30 and obtained is similar in effect to «that produced by the system of Figures 4A `and 4B, which has been described in detail above. 32. If the potential of the electrode 3| is lower of the electrodes Bü, 3l and 32 may be arranged than both of the electrodes 3i) and 32, a saddle u... Si field is produced.l In Figure 8'an electrode sys Figure 7 shows an arrangement similar to that tem is shown comprising five electrodes 35, 36, shown in Figure 6 but in which the cylindrical 3?, 33, 39, separated by gaps 4U, 4l, 42, 43, the electrodes being formed with curved edges in ac electrode A" is replaced by an electrode A’ of cordance with the invention. In operation the square cross-section, and the inner end of which is formed with semi-circular edges in the manner 50 potentials V1, V2, Va, V4, V5, respectively, of these electrodes may be arranged to increase or de described with reference to Figure 3 above. The crease successively to provide a series of similar effect of the square section electrode A’ is sub lenses. stantially the same as that of a tubular electrode Having now particularly described and ascer such as A" of ÍFigure 6 of a diameter equal to the tained the nature of my said invention and in length of the side of the square cross-section, and i it will be appreciated that the curvatures of the what manner the same is to be performed, I de clare that what I claim is: inner ends of the electrodes A” and A’ of Figures l. .An electrode system for forming an electron 6 and 7 can be varied 'in the manner indicated lens comprising two aligned co-operating elec with-.reference to Figures 4A and'4B_above, the lens being diverging or converging or substantial 60 trodes "affording an unobstructed passage for an ly ineifective in planes parallel vto the plane of electron beam to be acted upon, and having the » symmetry between the projecting portions of the inner edges of each electrode presented to the adjacent inner edges of the other of said elec electrode according to the curvature of the edges trodes, said edges being differently lcurved with of electrode A’ or A" as the case may be. The construction shown in Figures 6' and ’7 are particularly free from errors due to misalignment of the lens forming electrodes yand are eminently Asuitable for use in cathode Yray guns. The choice Y respect to each other to provide a curved lens mid-plane, the curvature of said mid-plane being related to the shape of said electrodes to mini mize spherical aberration in said lens. «, as to whether a cylindrical electrode such as A” 2. An electrode system of the type described in lof Figure 6 or a square section electrode such as 170 claim l characterized in thatsaid adjacent inner ,'A’ of „Figure 7 is employed will depend on which edges of said electrodesare -concentrically curved. _- electrode is the more suitable for incorporation in 3. An electrode system of the type described >in the particular device in which it is employed. claim l characterized in that said adjacent inner edges of said‘ electrodes are complementarily Both constructions give lens fields with excellent qualitiesin respect'of freedom from aberration, 75 curved and in registry. , . , 2,412,687 9 4. An electrode system of the type described in claim 1 wherein each of said electrodes comprises a pair of parallel disposed plane elements dis posed symmetrically on opposite sides of said pas sage, said adjacent inner edges of each of said pairs of said elements being curved t0 determine said mid-plane curvature. 5. An electrode system for forming an electron lens comprising two aligned co-operating elec 10 potentials to said electrodes, and wherein said lens mid-plane curvature provides an electron image forming a line transverse to the axis of said electron beam. 10. An electrode system of the type described in claim 5 including means for applying focusing potentials to said electrodes, and wherein said lens mid-plane curvature provides an electron image forming a line transverse to the axis of said trodes affording an unobstructed passage for an li) electron beam. electron beam and of which one electrode is of l1. An electrode system of the type described in smaller diameter than the other and has one end claim 8 including means for applying focusing inserted within the other, the edge of said in potentials to said electrodes, and wherein said serted end of said smaller diameter electrode be lens mid»plane curvature provides an electron ing curved to provide a curved lens mid-plane, image forming a line transverse to the axis of said the curvature of said mid-plane being related to electron beam. the shape of said electrodes to minimize spherical 12. An electrode system for forming a corn aberration in said lens. pound electron lens system comprising a plurality 6. An electrode system of the type described in of aligned cooperating electrodes affording an un.. claim 1 wherein said electrodes are of cylindrical obstructed passage for an electron beam to be tubular form. acted upon and having the inner edges of each 7. An electrode system of the type described in electrode presented to the adjacent inner edges claim 1 wherein said electrodes are of tubular of another of said electrodes, said edges being form, differently curved with respect to each other to 8. An electrode system for forming an electron provide a curved lens mid-plane between each lens comprising a pair of apertured diaphragm electrodes disposed with their apertures aligned to afford an unobstructed passage for an electron of said adjacent electrodes, the curvature of said mid-plane being related to the shape of said elec trodes to minimize spherical aberration in said beam, said electrodes having their facing surfaces lens. diiîerently curved with respect to each other to - provide a curved lens mid-plane, the curvature of said mid-plane being related to the shape of said electrodes to minimize spherical aberration in said lens. 9. An electrode system of the type described in claim 1 including means for applying focusing i 13. An electrode system of the type described in claim 12 including means for applying focusing potentials to said electrodes, and wherein said lens mid-plane curvature provides an electron image forming a line transverse to the axis of said electron beam. OTTO ERNST HEINRICH KLEMPERER.