US3483417A - Electron beam deflecting means - Google Patents

Description

C3088 REFERENCE SEARCH Mn.

Dec. 9, 1969 c. w. HANKs ELECTRON BEAM DEFLECTING MEANS 2 Sheets-Sheet l Filed July 26, 1967 n m. f .Ii c. ,1 n..

I N E' N TO I2 CHA/25s h/ HAM/s WM M4 Dec. 9, 1969 c. w. HANKs 3,483,417

ELECTRON BEAM DEFLECTING MEANS Filed July 26, 1967 2 Sheets-Sheet 2 INVENTOR @mais /L/ #4M/s ATTOENEYS 3,483,417 ELECTRON BEAM DEFLECTING MEANS Charles W. Hanks, Orinda, Calif., assignor to Air Reduc tion Company, Incorporated, New York, NSY., a corporation of New York Filed July 26, 1967, Ser. No. 658,579 Int. Cl. H01j 3/20, 3/32, 23/10 U.S. Cl. 313-156 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to mean-s for deecting an electron beam. The invention is particularly applicable to use in a high vacuum, electron beam furnace.

High vacuum electron beam furnaces have been used for some time in the vacuum processing of various materials. Such furnaces are utilized, for example, in the melting and casting of metallic ores to obtain relatively pure metals or alloys. Such furnaces are also used in the melting of materials other than metals, such as ceramics and plastics, and are frequently used to produce vapors of metals and other materials for deposition on a substrate.

Electron beam furnaces utilize one or more electron beam gun assemblies for producing high energy electron beams and directing such beams to a target to be heated. It has been vfpund that the use of transverse magnetic fields for deiiecting the beam through a curving path permits the electifon gun to be placed in a location wherein it is less likely?. to be damaged by direct condensation of evaporant or by ion bombardment. Focusing'action may be achieved in the direction transverse to the plane of the curving electron beam path (lateral focusing) where the lines of flux in the transverse magnetic eld are bowed to produce a barrelshaped magnetic eld.

One way ofproducing bowed ux lines is by disposing the pole piecem'surfaces between which the eld is established at an a'iigle with respect to each other. Such an.

expedient may be satisfactory for larger applications, but where equipment is to be compact or where a large number of adjacent electron beams are to be used, angled pole piece surfaces may occupy too much space. Under some circumstances, it may be desirable to establish very deeply bowed magnetic flux lines to produce a sharp lateral focusing action on the beam. However, it is sometimes not possible to achieve a deeply bowed field without making the magnetic eld gradient along the radius of the curving beam path so large that outer elements of the beam are lost because of a weak field. Because of such losses, the heating process may become too ineicient.

It is therefore an object of the invention to provide compact means for deecting and laterally focusing an electron beam.

Another object of the invention is to provide means for deflecting an electron beam wherein the beam is laterally focused without excessive loss of outer electrons of the beam.

A further object of the invention is to provide means for establishing a transverse magneticeld, for deflecting an electron beam, wherein the lines offorce are deeply bowed.

3,483,417 Patented Dec. 9, 1969 S;ill another object of the invention is to provide an electron beam gunl assembly having improved focusing characteristics and which is low in cost and simple of construction.

Other objects of `the invention will become apparent to those skilled in the art from the following description taken in connection with the accompanying drawings wherein: yI

FIGURE l is aftop view of an electron beam gun assembly constructed in accordance with the invention and illustrating a crucible for containing a target material to be heated;

FIGURE 2 is a sectional view taken along the line 2 2 of FIGURE l;

FIGURE 3 is a schematic end view illustrating the characteristics of av transverse magnetic eld established by means similar 'to those utilized in the electron beam gun assembly of FIGURES 1 and 2; and

FIGURE 4 is a schematic side view of bodiment of the invention. .p

Very generally, 'an electron beam gun assembly in accordance with the invention includes a pair of spaced oppositely polarized main pole pieces adapted to -pro duce a magnetic field having lines of flux extending-between the pole pieces transversely of the path of an electron beam for deecting the beam. An elongated auxiliary pole piece is secured to each main pole piece so as to have the same polarity. The auxiliary pole pieces extend generally transversely of the beam path and are of a. size which causes them to magnetically saturate. The auxiliary pole pieces are of a length such that their free ends are separated by a gap such as` to produce lines of ux which are bowed substantially in the region proximate the gap and in the beam path.

Referring now more particularly to FIGURES 1 and 2, molten material 1v1, contained within a crucible 12, is melted by bombarding the material by an electron beam, as explained below. The crucible is supported on a pedestal 13 which rests upon a base plate 14. The crucible is disposed withinI la high vacuum environment and is provided with coolant passages l16 for cooling the crucible during melting operations. Accordingly, a skull of solid a further emmaterial, not illustrated, may form between the molten material 11 and the interior wall of the crucible 12 to prevent interaction between the crucible and the molten material.

An electron beam for heating the molten material 11 in the crucible 12 is produced by an electron beam gun 17. The gun 17 includes an elongated emitter 18 c0mprised of tungsten or a material having similar properties and, when a heating current is passed therethrough, produces free electrons. The emitter 18 is supported, by suitable means not illustrated, in recess 19 formed. in a focusing electrode 21. The unillustrated emitter supports may also provide electricalcontact for passing the heating current through the emitter.

The electrons in the beam are accelerated by the acn celerating electrode or anode 22. The anode 22 consists of a metal plate having an opening 23 therein through which the beam passes. One edge 24 of the metal plate 22 is turned downward and is bolted to a vertical support plate 26. The support plate has two inwardly extending legs 27 and 28 which are bolted to the base plate 14. The focusing electrode 21 and the emitter 18 are maintained at a negative potential with respect to the accelerating electrode 22, and the recess 19 is shaped such that the free electrons produced by the emitter are directed out of the open end of the recess in the shape of a beam. Because of the elongated shape of the emitter 18 and the elongated shape of the anode opening 23, the beam is in the form of a ribbon or a narrow rectangle as it leaves the anode opening. The edges of the ribbon shaped beam are indicated by the dash- dot lines 32 and 33 in FIGURE 1. f

After leaving the anode opening 23, the beam is deflected by a transverse magnetic fieldfto pass through a curving path and make a change in direction of slightly less than 180. In the apparatus of FIGURES l and 2, they main magnetic field is establishedbetween a pair of vertically extending pole pieces 36 and 37. The pole pieces 36 and 37 extend upwardly from the base plate 14, and suitable connection is made to the pole pieces to establish a transverse magnetic field therebetween.

When an electron beam passes through a transverse' field having bowed lines of ux so that the field approximates a barrel shape, lateral focusing of the beam occurs. For a very short emitter which produces a correspondingly small width electron beam, the. flux lines of the transverse magnetic eld cannot be made to bow sharply-venough to focus the small width beam without making the magnetic field gradient so great that the outer parts of the field are relaitvely weakl" The weak part of the field may cause some of the electrons of the electron beam to be lost, and a fall-ofir in heating efficiency results.

The invention provides a deeply bowed magnetic field capable of providing satisfactory lateral focusing in systems wherein the transverse magnetic field is relatively large with respect to the cross sectional area of the electron beam. In accordance with the invention, a magnetic field having strongly or sharply bowed lines of flux is established near the anode hole 23. In this region, the electron beam is slightly divergent but still of small cross sectional area. Such a sharply bowed local field results inI a correction of the beam divergence depending upon the sharpness of the bowing in the lines of liux of the field. The amount of correction can be made to match the divergence of the electron beam and start it gradually to a focal region at a desired distance. Thus, as may be seen in FIGURE 1, the sharply bowed local field represented by the flux lines 31 causes the edges 32 and 33, respectively, of the ribbon-shaped electron beam to converge at a focal region 34.

In order to establish the local strongly bowed field near the opening 23, a pair of auxiliary pole pieces 38 and 39 extend inwardly from the main pole pieces 36 and 37. The auxiliary pole pieces are mounted on mounting blocks 41 and 42, respectively, which mate in grooves formed in the main pole pieces. Because of the partcular geometry of the illustrated system, the auxiliary pole pieces 38 and 39 extend inward angularly toward the emitter 18. In other systems, however, the auxiliary pole pieces may be axially aligned or otherwise disposed according to the particular needs of the system. The auxiliary pole pieces 38 and 39 are of a size which saturate, deriving their magnetic field from the main pole pieces 36 and 3'7, and the strongly bowed local magnetic field is established extending across the gap between the tips of the auxiliary pole pieces.

Depending upon the emitter length and other considerations, the focusing effect can be varied by a change in the spacing between the ends of the auxiliary pole pieces 38 and 39. In the particular illustrated system, the focal region 34 may be attained to coincide with the surface of the target where a spacing between the tips of the anxiliry pole pieces 38 and 39 is about one to two times the width of the beam at that point and where the beam path length in the main magnetic field is about two to four times the length of the gap between the pole pieces 38 and 39. By providing sets of interchangeable pole pieces 38 and 39 with various lengths, or by making the effective length of the pole pieces variable by a threaded type ad justment (such as illustrated in FIGURE 3 by the threaded opening 43 and the threaded pole piece end 44) an adjustable beam size is possible.

It may be noted from FIGURE 3 that, in addition to the production of a local strongly bowed magnetic field, the use of auxiliary pole pieces as described causes a general increase in the bowing of all field lines in the main magnetic field. In FIGURE 3, a transverse magnetic field is established between the po'e pieces 46 and 47. Were the auxiliary pole pieces 48 and 49 not present, the lines of ux between the main poe pieces would appear as the dotted lines 51. As a result of the placement of pole pieces, however, the lines of flux in the main magnetic'field are more deeply bowed as indicated by the solid lines S2. The local strongy bowed field is indicated in FIGURE 3 by the dotted lines 53.

A further advantage accruing from the use of auxiliary pole pieces as described is in the ability to achieve bowed flux lines with compact apparatus. It is unnecessary to make the main pole pieces of irregular shape, or to dispose them at an angle with respect to each other, in order to produce bowed flux lines. Thus, for example, several electron gun assemblies may be placed immediately adjacent each other and occupy a smaller space. Moreover, structural aspects are more simple when the pole pieces may be a pair of parallel plates.

The use of two sets of auxiliary pole pieces is shown in FIGURE 4. The main pole pieces, one of which is indicated at 54, may be similar to those in FIGURES l and 2. The pole pieces are used to deflect an electron beam produced by a suitable electron gun 56 to impinge upon the surface of material in a crucible 57. Two pairs of axially aligned pole pieces 58 and S9 are utilized. The auxiliary pole pieces 58 and 59 extend inwardly from the main pole pieces and terminate to leav'e a gap of the desired size. A control circuit or similar arrangement 61 is connected to the main pole pieces in order to vary the strength of the magnetic field established therebetween. By increasing the strength of the magnetic eld, the total amount of deflection of the electron beam may be increased, and the two extreme positions of the beam are indicated in FIGURE 4 by the solid lines 62 and by the dotted lines 63, respectively. By suitably varying the strength of the magnetic field, the beam may be swept between the two extremes across the surface lof the molten material in the Crucible 57 to melt the material in a desired manner.

In order to insure that the beam will be suitably laterally focused in all its variable positions, the use of the two sets of auxiliary pole pieces 58 and S9 is made. Lateral focusing of the outer beam 62 is accomplished by the pair of auxiliary pole pieces 59, whereas lateral focusing of the beam in the position 63 is accomplished by the pair of auxiliary pole pieces 58. Naturally, more than two pairs of pole pieces, suitably positioned, may be utilized, however, in the illustrated embodiment, the size of the local fields established by the pole pieces 58 and 59 may be sufficient to cover the entire amount of deflection of the beam between the two positions 62 and 63. Therefore, 'only two pairs may be satisfactory.

Because of the production of local strongly bowed magnetic fields, and because of the general increase in the bowing of all field. lines in the main magnetic field as the result of the use of auxiliary pole pieces as shown, considerable flexibility in the establishment of transverse magnetic fields for deflecting electron beams is provided. For example, if auxiliary poles are placed near the center of mutually parallel main pole pieces each of which is generally in the shape of three-quarters of a circular disc, the converging effect on an electron beam passing through a field established between such pole pieces is almost identical to that provided by a pair of conically-shaped pole pieces. Thus, the use of saturating auxiliary pole pieces in connection with various shapes of `main pole pieces makes possible the production of complex magnetic fields without the necessity for complex configuration in the pole pieces. It also facilitates experimentation, since complex magnetic fields with local variation may be obtained and varied empirically rather than by the design and machining of more costly shaped pole pieces for each type of field.

It may therefore be seen that the invention provides an electron beam gun assembly including means for deecting an electron beam which achieve superior focusing at low cost and with relatively simple construction. The invention is of particular advantage in focusing and deflecting electron beams of relatively small cross section.

Various embodiments of the invention other than those shown and described herein will beco-me apparent to those skilled in the art from the foregoing description and accompanying drawings. Such other embodiments, and modifications thereof, are intended to fall within the scope of the appended claims What is claimed is:

1. Means for deliecting an electron beam, including a pair of spaced oppositely polarized main pole pieces adapted to produce a main magnetic field having lines of flux extending ybetween said pole pieces transversely of the beam path, and a pair of elongated auxiliary pole pieces for producing a local magnetic iield, each being secured to a respective one of said main pole pieces to have the same polarity, said auxiliary pole pieces projecting inwardly from said main pole pieces toward each other to extend generally transversely of the beam path, said auxiliary pole pieces being of a size with respect to said main pole pieces so as to saturate magnetically and produce a general bowing of the flux lines of the main magnetic field, said auxiliary pole pieces being of a length such that their free ends are separated by a gap to produce lines of flux in the local iield which are bowed substantially more than those of the main ield in the region proximate said gap and in the beam path.

2. Means in accordance with claim 1 wherein the distance each of said auxiliary pole pieces projects from said main pole piece to which it is attached is adjustable.

3. Means according to claim 2 wherein said auxiliary pole pieces are threaded into correspondingly threaded openings in said main pole pieces and are adjustable therein.

4. Means in accordance with claim 1 wherein at least one further pair of elongated auxiliary pole pieces are provided for producing a second local magnetic field, each being secured to a respective one of said main pole pieces to have the same polarity, said further pair of auxiliary pole pieces projecting inwardly from said main pole pieces toward each other to extend generally transversely of the beam path, said further auxiliary pole pieces being of a size with respect to said main pole pieces so as to saturate magnetically and produce a general bowing of the iiux lines of the main magnetic field, said further pair of auxil= iary pole pieces being of a length such that their free ends are separated by a gap to produce lines of flux in the local field which are bowed substantially more than those of the main eld in the region proximate said gap, means for varying main lield strength between said main pole pieces to change the beam path between at least two positions, said further pair of auxiliary pole pieces being positioned such that the region proximate said gap is in one of said beam path positions,

5. An electron beam gun assembly for producing and deflecting an electron beam, including an electron beam source, a pair of spaced4 oppositely polarized main pole pieces adapted to produce a main magnetic iield having lines of ilux extending between said pole pieces transversely of the beam path, and a pair of elongated auxiliary pole pieces for producing a local magnetic ield, each being secured to a respective one of said main pole pieces to have the same polarity, said auxiliary pole pieces projecting inwardly from said main pole pieces toward each other to extend generally transversely of the beam path, said auxiliary pole pieces being of a size with respect to said main pole pieces so as to saturate magnetically and produce a general bowing of the iiux lines of the main magnetic field, said pole pieces being of a length such that their free ends are separated Iby a gap to produce lines of iiux in the local'iield which are bowed substantially more than those of the main `iield in the region proximate said gap and in the beam path.

6. An electron beam gun assembly in accordance with claim S wherein said auxiliary pole pieces are positioned proximate the electron beam source and wherein said gap is larger than the corresponding cross sectional dimension of the beam thereat.

7. An electron beam gun assembly in accordance with claim 6 wherein said gap is about one to two times the corresponding cross sectional dimension of the beam thereat.

References Cited UNITED STATES PATENTS 3,132,198 5/1964 Du Bois et al 13-31 X 3,202,794 8/1965 Shrader et al 13-31 X 3,235,647 2/1966 Hanks 13-31 3,390,222 6/1968 Anderson 13--31 I AMES W. LAWRENCE, Primary Examiner R. F. HOSSFELD, .Assistant Examiner U.S. C1. X.R.

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