US2953707A - Electron beam focusing system - Google Patents

Description

Sept 20, 1960 J. s. COOK ET AL 2,953,707

ELECTRON BEAM FocUsING SYSTEM Filed March 29, 1957 2 Sheets-Sheet l @y M/im Sept 20, 1950 J. s. cooK E-rAL 2,953,707

ELECTRON BEAM FOCUSING SYSTEM Filed March 29, 195'? 2 Sheets-Sheet 2 J. S. COOK /NVENTOICRS` W. H. LOU/SELL ELECTRON BEAM FOCUSIN G SYSTEM John S. Cook, New Providence, and William H. Louisell, Summit, NJ., and Willis H. Yocom, Los Altos, Calif., assignors to Bell Telephone Laboratories, Incorporated,

New York,` N.Y., a corporation of New York Filed Mar. 29, 1957, Ser. No. 649,559 Claims. (Cl. 315-6) This invention relates to an electron beam system and, more particularly, to an electron gun structure for use in electron beam systems which employ electrostatic focusing fields of the kind described in copending application Serial No. 514,423, tiled June 10, 1955, by R. Kompfner and W. H. Yocom, now United States Patent 2,857,548, issued October 2-1, 1958.

The present application is a continuation-in-part of United States application Serial No. 560,546, now abandoned, which was liled on January 23, 1956, by the present applicants.

In the aforementioned patent of R. Kompfner and W. H. Yocom, there is disclosed an electron beam system in which a focusing electrode system is used to establish an electrostatic field having a pair of single equipotential surfaces which are characterized by the fact that electrons injected on either of such surfaces with a correct velocity will iiow along or in the vicinity of such surface. In an illustrative form of an electron beam system' of this kind, a linear array of elements, each of which is maintained at a positive potential with respect to the electron source, comprises the focusing electrode system which sets up a pair of singular equipotential surfaces. Each of these surfaces winds sinuously between the successive elements of the array, the two surfaces being mirror images with the plane of the linear array as the reflection plane. In particular, it is set forth that a correct velocity which the electron beam should have when injected on one such surface for flow therealong may be achieved by associating with the electron source an accelerating anode which is maintained at the potential of the singular equipotential surfaces. In the use of an electron beam system of this kind, it is found relatively diicult to inject by usual expedients an electron beam. of appreciable current onto one of the singular equipotential surfaces with the correct velocity. In particular, it is found that conventional beam injection arrangements result, at the region of injection of the electrostatic field, in a distortion of the field pattern set up by the focusing electrode system optimum for focusing of this kind.

This, in turn, results in the loss of focusing eiiiciency. I.

In a copending application of J. S. Cook, R. Kompfner and W. H. Yocom, Serial No. 514,421, tiled lune l0, 1955, now United States Patent 2,939,034, issued May 31, 1960, there is shown an improved electron beam system of the kind described in which the injection of an electron beam with a correct velocity is facilitated and field distortion is minimized by localizing the injection to a crossover region of the two singular equipotential surfaces. Such a system as therein disclosed provides, in general, improved performance as a result of the minimi'- zation of field distortion.

We have found that, in addition to smooth injection of the beam, there are certain optimum entrance conditions for providing a stable beam in such a focusing system, which hereinafter shall be idenitfied as a slalom focusing system. In particular, it has been found that for certain predetermined conditions of entrance velocity and eld, there are certain maximum limits to the beam dimensions, especially the beam thickness, which, if exceeded, result in beam instability and a deterioration of the focusing effect. It has also been found that where States arent fice the velocity distribution across the beam thicknessk is small, maximum beam perveance is given by the expressiorl 1.,9. i '-GW-i- P"- Vos/z-25X 10 a2 (D where I0 is the total current in amperes; V0 is the beam potential, T is the'beam thickness, W is the beam width, and

a is the spacing of elements in the linear array.

From Equation 1 it can be seen that perveance is directly proportional Vto beam thickness, hence, in general, fora given system, it is desirable to operate at maximum beam thickness. if the electron beam is injected into the focusingsystem at certain specified angles to the equipotential surface which defines the path of flow and with certain displacements from the equipotential surface crossover region of the upper and lower portions of the beam, maximum beam thickness for a stable beam can be achieved.

Accordingly, a specific object of the present invention is to improve an electron beam system of the kind described, by injecting the electron beam into thev focusing system at the proper angle and position to give maximum beam dimensions which, in general, means maximum stable beam current, thereby greatly enhancing the efficiency of operation of the system. The invention will be better understood from thev foil lowing. more detailed description taken in conjunction with the accompanying drawings, -in which:

Fig.- 1 is a schematicelevation View of a typical slalom focusing system;

Fig. 2A is aschematicview of an electron gun arrangement embodying the principles of the invention;

v Fig. 2B is a schematic View of another gun arrangement embodying the principles of the invention;

Pig.- 2C isa schematic view vof still another gun arrangement embodying the principles'of the invention; and

Fig. 3 is a graph showing the stability characteristics of anelectron beam inthe focusing system vof Fig. 1.

Turning now to Fig. 1in which an electron beam system 10 is schematically depicted, a plurality of conductive elements 11, which typically are wiresrextending transversely normal to the plane of the drawing, are aligned toform alinear array which extends longitudinally inthe direction of desired iiow with successive'wires spaced a distance'a apart. On opposite sides, conduc tive members v12 and 1-3 serveas conductive boundaries: As pointed` out in the aforementioned patent of'R. Kompfner4 and W. H; Yocom, when the conductiveele# ments r11. are maintained vat a positive potential withref spect to the conductive' members 112, 13, as is shown schematically, this electrode system will establish in 'the interspace beuveenconductive members ank electrostatic field which is characterized by a pair of singular equipotential surfaces which-intersect the plane of theI drawing, as shownby the broken lines'r14 andl 15. Each of such equipotential surfaces winds sinuouslyp'ast suc-A cessive elements, the two surfaces being mirror images of one another about the plane of the linear array and crossingover one another along a parallel succession of center lines. As? pointed out in the aforementioned patents,l it has been found that there will iiow in proxV Our analysis has further indicated that electron lsource by an electric ield of strength corresponding to the difference -in potential of the singular equipotential surface Yand the electron source.-` The potential Vof the singular equiptential/ surface, and hence the velocity of the beam, can be adjusted to a desired value by a proper choice of the potentials atrwhich the elements of the focusing electrode system are operated.

The electron beam is injected by an electron gun comprising a cathode =16, a beam focusing electrode 17 and an accelerating anode 18, as schematically shown in Fig. 1. Since the electron .gun advantageously is to provide a strip beam which extends normal to the plane ofthe drawing, the various gun elements alsoA extend normal to the plane of the drawing. It is' to be understood, however, that the features of the present invention are not restricted to lstrip electron beams solely. V'The cathode 16 is of` conventional design and includes a heater compartment 16A in which extends a heating coil and has a portion 16 B of its surface which is electron emissive. 'Ihe electrons emitted from the electron emissive portion are formed into an electron beam( bythe beam forming electrode 17. YAs shown in Fig. 1 and as will be explained morefully hereinafter, ythe cathode 16 and beam lforming electrode 17 are designed advantageusly to convergeV the electrons emitted from the, cathode into a narrow well defined beam, Accelerating anode 18 cooperates with `electrode 17 in forming the electrons into a beam and additionally serves to accelerate the electron in the beam to a desired velocity. In a typical example, the cathode is maintained at a potential slightly positive with respect to the beam forming electrode 17 but considerably negative with respect to accelerating'anode C18. As pointed out in the aforementioned patent of Cook, Kompfner and Yocom, the surface `of the accelerating anode 18 which nis more approximate to the focusing electrode system made up of elements 11, k12. and 13, isv

shaped to coincide substantially with one surface, in this example surface 14, of the two singular equipotential surfaces associatedy with such focusing system, to insure smoothinjection of the beam into the field.

- At the downstream end of the array of elements the electron beam is collected by a ltarget electrode 20 schematically depicted inV Fig. 1, which is maintained at a suitably positive potential with respect to Vthe electron source. r'For instance, the target electrode may be maintained at thepotentialrof the elements 1i1 of the array.

Target'electrode 20 is preferably positioned behind an isolating electrode 21 to minimize the elect of the target electrode on the field pattern of the focusing system, as pointedA out in the aforementioned patent of Cook, Kompfner and Yocorn. i' o Recent investigation and mathematical analysis of the behavior of an electron beam in a slalom focusing system has revealed that there are certain optimum conditions of entrance of the beam into the focusing region which should be observed if maximum stability of the beam is foibe realized. which' depicts the stability characteristics of the beam in a slalom focusing system of the type shown in lFig. 1. The ordinate of the graph represents displacement from the crossover of the two equipotential surfaces of the point where an electron in the beam crosses oneyof the equipotential surfaces as it is introduced into the focusing eld. The ordinate is graduated in dimensionless unitse, where e' is equal toV Y Y y E0 o being the displacement from the equipotential surface in the .vicinityv of which the beam travels, measured at Yright-angles to that surface at the crossover point, and

q is the wire spacing. In Fig.-2A, which is an enlarged view of the gun arrangement of Fig. 1, it can be seen that the displacement 4from the intersection Yof the equi- `4 Y potential surfaces 14 and =15 of the point where the center line of the electron beam emitted from cathode 16 crosses the equipotential line 14 is given by e=0. The

abscissa of the graph of Eig. 3 is calibrated in terms of Y the angle Y The angler/iris definedY as Vthe angle Vformed Y between the-direction of one of the equipotential surfaces at the point of intersection and the line along which Yan electron in the beam crosses the other of said equipotential surfaces. Thus, in Fig. 2A, the angle V is the angle between the line BB and the center line of the beam emitted from cathode 1'6 at the point where the center line crosses equipotential line 14. Line BB in turn is tangent to equipotential surface 115 andtpasses through the point of intersection of surfaces 1-4and |15. 'f It'is toV be understood that because of the experimental difliculties in measuring Vthe Various quantities involved accurately and the complexity of the analytical computations,V the specic limits disclosed are merely illustrative of the order of the magnitudes involved. o

stable region defined by the line 22 which represents Y a system where the diameter of a wire inthe array 1s 't approximately one-fifth the spacing between wires in the array and where there ,is a particular value of the parameter where Where v is the velocity at the crossover region and v0 is the velocity an electron must have in order to follow exactly kalong the equipotential path. The stable regions dened by the lines 2,2 and 22 in Fig. 3 are the regions for, respectively, a slightly smaller and a slightly larger value of than that forrline 22. It can be seen that changing parameters results in a slight change in the position and shape of the curve defining the stable region, which`changes are mattersof degree only. Changing the' wire diameter wil-l likewise cause a change in the position and shape of the curves. For example,V decreasing the 'wire diameter increases somewhat the maximum possible stable beam thickness and also increases the absolute value of the optimum Such changes in wire diameter are confined to somewhat narrow limits because of practical considerations such as excessive heating of wires that are too small, and interception of electrons by wires that are too large.

In IFig. 3 thereI is shown a graph Y be :seen that maximum beam current for a converging type beam as shown in Fig. 2A can be achievedV by a proper positioning of the gun structure so that electrons in the beam will enter the focusing region at the optimum be represented by the straight line in the graph of` Fig. 3' which is totally' enclosed by the curved line 22 yand which subtends the greatest range of e. Such "a line for a parallel beam is represented in^Fig.f3 by the Vline: 'MN, which has an angle of -llV and which intersects line 22 at two points, where e-l-.095 and e-.O95. The center of line MN is at ei).V The information thus derived from line MN of` Fig4 3 is'k utilized in` the gun structure of Fig, 2A, asV follows. All of the electrons in the beam emitted by cathode 16 cross the equipotential surface 14 at an angle of =-'11.

From an examination of the graph of Fig. 3 it can Y The 4uppermost electrons, inthe beam. .as viewedrin Fig. 2A cross the equipotential 14. ataV displacement. of e=+.095 `and the lowermost electrons inthe 'beam as viewed in Fig. 2A cross equipotential surface 14 at a displacement of e"=.095. The centerline of the beam crosses surface 14 at 6:0. If the. conditions indicated by line MN are met, maximum stable beam thickness in a parallel flow type beamis realized.

The analysis of Fig. 3 has thus far dealt with maximum stable beam thickness in a. beam which was made to enter the focusing region as a parallelflow beam. Under certain conditions it may be desirable to introducethe beam into the focusing field when it is converging or even diverging so long as the stability conditions are met;` In Fig. 2B there is shown schematically the gun portion of a focusing system such as is depicted in Fig. l which is properly positioned relative to the crossover of equipotential surfaces 14 and 15 so as to produce maximum stable beam current for a converging type beam. For the sake of simplicity only the gun portion of the focusing system has been shown as in Fig. 2A. InV

addition the various angles and displacements bear the same designations in order to avoid confusion.

It can be `seen from Fig. 3 that when the convergence angle, that is, the total included angle of the converging beam, is known, the curve 22 defines the maximum beam thickness, as given by e, at the place where the beam crosses the equipotential line. vergence angle of 6, maximum beam thickness is represented by the line PQ. It can be seen that line PQ intersects the line 22 at two points which are defined on the graph as [3E-14, e+.09 and s-SH fg-.09. corresponds to the centerline of the beam is located at 18E-41 and 5:0. Referring now to Fig. 2B, the beam `as defined by line PQ in Fig. 3 will be formedy if the electrons on the upper outermost edge ofthe beam as viewed in Fig. 2B cross the equipotential surface 14 at an angle =-l4 and at a displacement e=+.09. Electrons in the lower outermost edge of the beam as viewed in Fig. 2B cross equipotential 14 at an angle "=-S and e=-.09. In addition, the centerline of the beam crosses equipotential 14 at an angle "=-ll and e=0. All of the electrons which are introduced into the4 focusing region in accordance with the conditions indicated by the line PQ yofFig. 3

should arrive at collecting electrode 20, thereby achiev-Y ing maximum stable beam thickness at the point of injection for a beam of this given convergence.

In the case where a diverging beam is desired, the curve 22 gives an indication of the maximum permissible beam thickness at the point of injection for a given total in eluded angle of divergence in the same manner as for a. converging beam. It can be seen in Fig. 3 that the maxi.- mum divergence or convergence angle permissible with the curve 22 is approximately 25 as represented by the, yIn such a case, the beam thickness, as given by line XY. e, is zero. The line XY intersects the line 22 at [S-23 and ^={-2. The center of the line XY occurs at -10.5. The information thus obtained is .utilized in the gun arrangement `of Fig. 2C as follows: In order to achieve a diverging beam at the point of intersection of the beam with the equipotential surface 14, various types of electron guns may be utilized. The gun arrangement of Fig. 2C is a schematic representation of one-such `arrangement wherein a sharply converging beam having a focus at approximately the point of intersection .with sur.- face 14 is used. It is to be understood that the gun arrangement so shown in Fig. 2C is for purposes of illustra.- tion only and various other suitable arrangementswell known to those skilled in the art can be utilized. The beam emitted by the cathode 16 in Fig. 2C 'is made to converge 4and to cross the equipotential surface 14 at a displacement e=0. The angle at which the uppermost electrons in the beam as viewed in Fig. 2C leavethe-equi- For a total included con-v In addition the midpoint of line PQ whichl potential surface-after crossing. it is ={2., andthe angle at whichthe lowermost electrons in the beam as viewed in Fig. 2C leave Vthe equipotential surface after crossing it is tm-23. The centerlineof the beam crosses surface 14 at an angle 10.5. In practice,- of course, the beam will have a finite thickness at the point of introduction into the focusing system, .which Will cause the line XY to acquire a linite thickness correspond-y ing to lthe beam thickness, thereby lessening slightly the maximum permissible beam convergence.

The foregoing examples Werertaken for agiven set of velocity conditions, that is, for a given value of As was pointed out in the foregoing,.changes in the value of the parameter will result in slight changes in position and shape of the area defining. the stable region, as shown by the lines 22 and 22". In addition, changcslin variou-s others of the operating parameters may causesimilar changes in the area defining the stable region. Such changes, however, are changes in degree only and the principles underlying the invention as discussed for achieving maximum stable beam dimensions remain applicable.

It is to be understood that the `specific arrangements herein describedare merely illustrative of the principles of the invention, and various other embodiments may be devised without departing from the spirit and scope of the invention. In particular, various other types of electron gun structures may be used to produce a beamv of the desired type. Such other gun structures must,v however, in order to achieve maximum stable beam dimensions, be positioned in accordance with the teachings'ofthe present invention. Additionally, the principles of the invention are applicable to other types of electrostatic focusing, such as, for example, a focusing arrangement of the type shown and described in the copending United States application Serial No. 534,090, led September 13, 1955, by C. F. Quate.

What is claimed is:

l. In an electron beam system, a focusing electrode system comprising a plurality of spaced conductive elements forming a longitudinally extending array and a pair `of members on opposite sides of the array for establishing Ia pair of periodically crossing singular equipotential surfaces in the region between said members, and means comprising a source of electrons for introducing an electron beam into the region between said members across one of said equipotential surfaces at an angle to the direction of the other of said equipotential surfaces at the crossover point, the axis of said beam being directed into the region between one of said conductive elements and `one of said singular equipotential surfaces, said beam having a correct velocity for ow along said array in the vicinity of said one equipotential surface.

2. In an electron beam system, the combination as claimed in claim l wherein at least one of said pair of members is of electrically conducting material.

3. In an elec-tron beam system, a focusing electrode system comprising a plurality of spaced conductive elements forming a longitudinally extending array and a pair -of members on opposite sides of the array for establishing a pair ofperiodically crossing singular equipotential surfaces in the region between saidY members, and

means comprising a source of electrons for introducing an electron beam into the region between said members, the axisY of said `beam directed into the region between one of said equipotential surfaces and one of said conduc tive elements in ya direction at an acute angle to one of said surfaces and at an angle to the other of said' surfaces,

said beam having a correct velocity for iiow along said` array inthe vicinity of said one equipotential surface.

for establishing a pair of periodically crossing singular equipotential surfaces Vin' the. region between said" plates;

andmeans comprisingV a source of electrons for introducing an electron' beam into theregion between said 'equi-V .potential surfaces andone of Asaid conductive "elements across Vone ofsaid equipotential surfaces at a correct velocity for flow along said array in the vicinity of one of said equipotential surfaces, the direction of the electrons in the beam as theyV cross 'said equipotential surface beingfsubstantially the same for all the electrons and` forming-an yacute angle with the other lof said equipotential surfaces, the. axis of isaid beam being directed into Ythe region between Ysaid oneequipotential surface and one 'of said conductive elements.

5.' In `au electron beam system theV combination of Y claim v4 characterized in lthat the upper and lower Vsurfaces of said beam are displaced equal distances from the crossover point of said equipotential surfaces, and the acute angle is less thanV twenty-three degrees.

6. In an electronbeam system, a focusing electrode system comprising a plurality of spaced conductive elements forming a longitudinally extending array and a pair of conductive plates on opposite Ysides of the array for establishing a pair of periodically crossing singular equipotential surfaces in the region between said plates vand means comprising a source of lelectrons for introducing an electron beam into the region between said equipotential surfaces and one of said conductive elements Yacross one of said equipotential surfaces at a correct velocity for ow along said array in the vicinity of one of said equipotential surfaces, the direction of the electrons in the uppermost portion of the beam as they cross said equipotential surface being at a first acute angle to the otherfof said surfaces, the direction of the electrons in Ithe lowermost portionl of the beam as they cross said equipotential surface being at a second acute angle to the other of said surfaces, and the Ycenterline' of the beam crossing said equipotential surface at a third acuteV angle velocity for ilow along said array in the vicinity of oneVv of said equipotential surfaces, the direction of the elec-l trons Vin -the'uppermost portion of the beam as Y'they `cross said equipotential surface being at a first acute augleto the other of said surfaces, the direction of the electrons in the lowerrnost portion of the beam'as 'they cross said equipotential surface being at a second acute angle to the Vother of said surfaces, and the centerline of the beam crossing said equipotential surface at a third acute angle to the other of said surfaces, said rst acute angle beingr less than twenty-one degrees less than said second angle in magnitude.

9. In an electron beam system, a focusing electrode system comprising a plurality of spaced conductive elements forming a longitudinally extending array anda pair of conductive plates on opposite sides of the array Y for establishinga pair of periodically crossing singular to the other -of said surfaces, said centerline being directed into Ithe region between said one equip-otental sur'- Yface and one of said conductive elements.

7. In an electronV beamV system, a focusing electrode system comprising a plurality of spaced conductive elements forming Va longitudinally extending Aarray and a pair of conductive plates on opposite sides of the array for establishing a pair klof periodically crossing Vsingular equipotential surfaces in the region between said plates and means comprising 'a source of electrons for introducl ing an electron beam into vthe region betweenV said equipotential surfaces and one of said conductive elements across one'of said equipotential surfaces at a correct velocity for `iiow `along said -array in the vicinity of one of said equipotential surfaces, the direction ofthe electrons in the uppermost portion of the beam as they cross said Vequipotential surface being at a rst acute angle to Vthe other of said'surfaces', the direction of the electrons in the lowermost portion of the beam as they cross said equipotential surface being at'a second acute angle to the other of said surfaces, and the centerline of the beam crossing said equipotential surface at a third acute angle to the other of said surfaces, said first acute angle being of lesser magnitude than the second acute angle,land the third acute angle being of greater magnitude than said first acute angle and of lesser magnitude than the second acute angle. v

8. In an electron beam system, Aa focusing electrode system comprising a plurality of spaced conductive ele ments forming va'longitudinally extending array and a pair of conductive plates on opposite sides of the array for establishingk a pair of periodically crossing singular equipotential surfaces in the region between said plates and means ,comprising a source of electrons for introducingan'electron beam into the region between said equipotentialsurfaces and vvone ofvsaid conductive elements across one of said equipotential surfaces at a correct equipotential surfaces in the region between said plates and means comprising a source of electrons for introducing an electron beam into the region between said equipotential surfaces and one of said conductive elements across one of said equipotential surfaces at a correct velocity for flow along said array in the vicinity of-one of said equipotential surfaces, the direction of the electrons inthe uppermost portion of the beam as they cross said equipotential surface being at a first acute angle to lthe other of saidY surfaces, the direction of the electrons in the lowermost portion of the beam as they cross said equipotential surfacek being at a second acute angle to the other of said surfaces, and the centerline of the beamV crossing said equipotential surface at a third acute angle to the other of said surfaces, said rst acute angle being of greater magnitude than said second acute angle and said third acute angle being of lesser magnitude than said iirst acute angle and of greater magnitude than saidsecond acute tangle.

' 10. In an'electron beam system, a focusing electrode system comprising aplurality'of spaced conductive ele ments forming a longitudinally extending array and a pair of conductive plates on opposite sides of the array for establishing a pair Vof periodically crossing singular equipotential surfaces in the region between lsaid plates and means comprising a source of electrons for introducing an electron beam into the region between said equipotential surfaces and one yof said conductive elements across oneof said equipotential surfaces at a correct velocity for flow along said array vin the vicinity of one of said equipotential surfaces, the direction of lthe elec-Y trons in the uppermost portion of the beam as they cross said equipotential surface being at a first acute angle to lthey other of said surfaces, the direction of the electrons in the lowermost portion of the beam as they cross said equipotential surface being at a second acute angle to the other of said surfaces, and the centerline of the beamv Vcrossing said equipotential surface at a third acute angle to the other of said surfaces, said first acute angle being approximately six degrees greater than said secondv acute angle in magnitude.

References Cited in the file of this patent UNITED STATES PATENTS 2,059,863 Hansell Nov. 3,

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