US3420977A - Electron beam apparatus - Google Patents

Description

Jan. 7, 1969 C, w, HANKS ETAL ELECTRON BEAM APPARATUS Filed June 18. 1965 Sheet4 l of 4- "$127 """H'Ulll Jan. 7, 1959 C, w, HANKS ET AL 3,420,977

ELECTRON BEAM APPARATU-S Filed June 18, 1965 Sheet g of 4 Jan. 7, 1969 C, W, HANKS ET AL *A 3,420,977

ELECTRON BEAM APPARATUS Filed June 18, 1965 Sheet 3 of 4 Jan. 7, 1969 C, W, HANKS ET AL 3,420,977

ELEcTRoN BEAM APPARATUS Filed June 18, 1965 Sheet 4 of 4 mex;

@www 95 United States atent O 3,420,977 p ELECTRON BEAM APPARATUS Charles W. Hanks, Orinda, Jack D. Merrill, Richmond, and Harold A. Peterson, Concord, Calif., assignors, by mesne assignments, to Air Reduction Company, Incorporated, a corporation of New York Filed June 18, 1965, Ser. No. 464,968 U.S. Cl. 219-121 Int. Cl. B23k 9/08; H05b 7/18 9 Claims ABSTRACT F THE DISCLOSURE This invention relates generally to a heating apparatus, and more particularly it relates to an improved electron beam gun assembly for heating a target.

Electron beam gun assemblies are useful in melting, casting, vaporizing, annealing, and other heat treatment of metals, alloys, compounds, plastics and other mate-- rials. Electron gun assemblies generally comprise an electron source for emitting an electron beam, and suitable means for generating a megentic field in the path of the electron beam for guiding and focusing the electron beam onto the surface of a target to be heated.

The electron source is operated in a region of high vacuum, and accordingly, the target and the gun assembly are disposed within a suitable vacuum enclosure main- -tained at a high vacuum, for example, less than one millimeter of mercury absolute. The gun assembly is generally disposed at a convenient location within the vacuum enlcosure and the electron source, which may be an emissive cathode, is energized to emit an electron beam. The electron .beam is accelerated along an initial path by a suitable accelerating anode into the magnetic ield which guides the electron beam onto the surface. of the target.

Various types of gun assemblies are well known. One type of gun assembly is adapted to be positioned above the target so as to direct a beam of electrons along a generally linear path onto the target. Gun assemblies of this type, where there is a direct line of sight between the electron surface and the target, are susceptible to contamination and shorting due to condensation of volatile materials evaporated from the target on the surface of the electron source.

In order to provide for protection of the electron source from contamination by condensation of volatile materials and ionized atoms evolved from the target during heating thereof, another type of gun assembly is designed to be positioned to one side of the target so that there is no direct line of sight between the electron source and the taIget.- Electron gun assemblies of this type are generally known as transverse field gun assemblies, and are disclosed in Patent No. 3,132,198 and copending application Ser. No. 260,158, filed Feb. 21, 1963. In a transverse iield gun assembly, the electron source directs a beam of electrons into a magnetic field that is transverse to the direction of travel of the electron beam which causes the electron beam to be guided along a curving path onto the surface of the` target. When the Mice electron source is positioned out of the line of sight of the target, the electron source is not directly exposed to materials vaporized from the target, and evolved condensible materials do not readily condense on the source. A substantial decrease in the contamination of the electron source, and a resulting longer life of the electron source is achieved.

However, when the electron source is positioned alongside and just below the surface of the target, as is generally the case when the target iscontained within an open topped upright Crucible, the electron source may be contaminated or shorted due to spalling of condensed materials from the cooled surfaces and shields of the vacuum enclosure above the source. This is particularly true when the gun assembly is employed to vaporize a material which contains a large amount of condensible volatile impurities. The positioning of a shield having a slit therein between the electron source and the target, as shown in Patent No. 3,132,198, partially solves this problem. However, in order for the shield to` serve its purpose, the slit must be of a narrow width which requires very close control of the gun assembly in order to direct the electron beam through the slit. Further, the shield adds expense to the construction of the electron beam furnace and somewhat reduces the uses to which the furnace might be put. Volatile condensibles may also collect on the edges of the slit reducing the width 'of the slit, and spalls of condensate may fall through the slit onto the electron source.

Another problem encountered in electron beam heating apparatus results from the feeding of solid raw materials into a molten target heated by a gun assembly. Generally, splashing and splattering of the molten material will occur when the solid raw materials are delivered onto the molten surface. When the elec-'tron source is disposed to one side and just below the upper surface of the molten target, a significant amount of the molten material that is splashed out of the target may fall upon and contaminate the electron source.

The contaminati-on of the electron source by the build up of condensate thereon causes erratic operation of the electron gun and may cause complete shorting of -the gun. Heretofore, in order to avoid such erratic operation, it has been necessary to turn olf the heating apparatus and change the electron sources after about 20 hours of operation when vaporizing a material containing a substantial portion of condensible volatile impurities.

It is Ia principal object of the present invention to provide an improved electron beam gun assembly -for heating a target. An additional object is to provide an electron beam gun assembly which has a long life when used to vaporize materials containing substantial amounts of volatile condensible impurities. Another object is to provide an electron beam gun assembly which is adapted to be positioned with respect to the target to be heated so that the electron source will not become contaminated due to spalling of the condensate collected upon the inner surfaces of the electron beam furnace. A still further object is to provide an electron beam gun assembly which may be positioned with respect to the target so that the electron source will not become contaminated by splashing and splattering which may occur when raw materials are fed onto the molten target heated by the electron gun.

These and other objects of the invention are more particularly set forth in the following detailed description and in the drawings, in which:

FIGURE l is a partial schematic pictorial view of a specific embodiment of an electron beam source assembly in accordance with the present invention.

FIGURE 2 is a side view of the assembly shown in FIGURE l.

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FIGURE 3 is a schematic cross-sectional representation ofone form of electron gun Iassembly in accordance with the present invention.

FIGURE 4 is similar to FIGURE 3 and shows a different embodi-ment of an electron gun assembly.

FIGURE 5 is similar to FIGURES 3 land 4 and shows another embodiment of an electron gun assembly.

FIGURE 6 is a partial schematic pictorial view of another embodiment of an electron source assembly of the present invention.

FIGURE 7 is a vertical sectional view of the assembly shown in FIGURE 6.

FIGURE 8 is a partial schematic pictorial view of a rotating crucible having spaced thereabout three electron gun assemblies constructed in accordance with a further embodiment of the invention.

FIGURE 9 is an elevational front view of the apparatus of FIGURE 8.

FIGURE 10 is a sectional View taken along line 10-10 of FIGURE 9.

FIGURE 11 is a partial schematic-elevational view of a further embodiment of a source assembly of the present invention.

FIGURE 12 is a plan view of the apparatus of FIG- URE 11.

Very generally, and having reference to FIGURE 1 of the accompanying drawings, the present invention is directed to an electron beam gu`rr1 1 v comprising a source 13 for emittingan electron beam, generally indicated by numeral 15, and magnetic lens means 17 for generating a transverse magnetic field having a particular configuration in the path of the electron beam 15 which field focuses the electron beam and guides it onto the surface of a tar-get 19. In accordance With the present invention, the magnetic lens means 17 includes a pair of poles 23, 23a for establishing ya first magnetic field of increasing strength in the initial path of the electron beam, and for establishing a second magnetic field of substantially constant strength in the path of the electron beam leaving the first magnetic field. The field of increasing strength comprises lines of flux which increase in curvature in the direction of the electron source to thereby reduce the diverging tendencies of the high intensity electron beam.

In order to avoid contamination of the sourcel by spalling condensate and/or splashing and splattering of molten material, the source 13 is adapted to Ibe positioned with respect to the target 19 so that the source 13 is completely hidden from the target 19, and so that the initial path of the electron beam 15 diverges with respect to a line 27 perpendicular to the surface of the target 19. As used herein, the term completely hidden is defined as the positioning of the source 13 with respect to the surface of the target upon which the electron beam 15 impinges so that spalling condensate from the interior surfaces of the vacuum furnace, and splashing and splattering molten Imaterial from the target are prevented from striking the source. Accordingly, some form of imperiorate barrier or shield is positioned between the source 13 and the target 19. Generally, the source 13 is disposed above or below the -s-urface of the target 19 in some convenient manner so that the source is shielded by the target and its associated support structure, or by the structure of the gun assembly itself. Alternately, the source 13 may be disposed to one side of the target beneath an imperforate shield 29 (FIGURE 7) which positively protects the source from contamination by spalling condensate and/or -splashing molten material.

In a preferred embodiment of the invention, the source 13 and the magnetic lens means 17 are so disposed with respect to one another that the source 13 is located within the fringing portion of the ymagnetic field of increasing strength generated between the pole pieces 23, 23a.

When the electron gun assembly is constructed so that the source 13 will ybe completely hidden from the target, the electron beam is guided over an arc of approximately or more from the source to the surface of the target.

Theoretically, a uniform transverse magnetic field is capable of guiding an electron beam through an arc approaching 180 or more. However, it has been found that a uniform transverse magnetic field is not particularly suited for guiding a high power density diverging electron beam, e.g., an electron beam having apower density of 50 kilowatts per square inch, over an arc of about '180 or more, inasmuch as the diverging high power density electron beam L*is not focused, i.e., converged, by the uniform transverse magnetic field. It has been discovered that in order to guide a high kpower density electron beam through an arc approaching 180 or more, such as when the source is completely 'hidden from the target, a particular magnetic field configuration is desirable in order to provide a magnetic lens for preventing divergence of the electron beam and for focusing the electron beam onto the surface of the target.

Referring to FIGUREv 3, there is shown a schematicrepresentation of the source 13 for emitting the beam of electrons 15, the target 19, and the transverse magnetic field generated by Athe magnetic lens means 17 (not shown), in accordance with one embodiment of the present invention. The small circles 31 represent the lines of fiux generated by the magnetic lens means 17, and the distance between the circles illustrates the relative strength of the magnetic field. The transverse magnetic field causes the l electron beam 15 generated by the source 13 to travel along a curving path onto the surface of Ithe target 19.

As shown in FIGURE 3, the source 13 is completely hidden from the target by being disposed beneath the target 19 in alignment therewith. The source 13 emits the electron beam 15 along an initial path which may be generally designated as a linear path, although it is understood that the beam may be diverging and may be traveling in a slightly arcuate path. A first magnetic field of increasing strength, generally indicated by numeral 35, is established adjacent the source in the initial path of the electron beam in the area between "the source and the line indicated by numeral 37. The magnetic field 35 of increasing strength comprises curving ilines of iiux which bow downwardly and which have increasing curvature in the direction of the electron source. Accordingly, the curving lines of linx of the magnetic field 35 of increasing strength cause the electron beam to be compressed or converged in a plane generally perpendicular to the plane of FIGURE 3.

As further shown in FIGURE 3, a second magnetic field, generally indicated by numeral 39, of Substantially constant strength is established in the path of the electron beam leaving the first magnetic lield 35 of increasing strength between the line 37 and a line indicated by numeral 41. The second magnetic field 39 of constant strength further defiects the electron beam 15 toward the target 19, and converges the electron beam in a plane parallel tothe plane of FIGURE 3.

It Ihas been determined thatin order to provide for focusing of the electron beam 1S, the magnetic field 35 of increasing strength should extend over a first portion of the curving path of the electron beam, and the magrie-tic field 39 of constant strength should extend over at least a portion of the remainder of the path of the electron beam. As discussed hereinafter, when the magnetic field 39 of constant strength extends over only an intermediate portion of the curving path of the electron beam, a magnetic field, generally indicated by numeral 43, is established over the remainder of the curving path of the electron beam betwen the line 41 and the surface of the target.

The portion of the path of the electron beam 15 exposed to the magnetic field 35 of increasing strength, and the portion of the path of the electron beam exposed to the magnetic field 39 of constant strength may be varied. It has been generally determined that the magnetic field 35 of increasing strength may extend over the first quarter to about the first one-'half of the curving path of the electron beam, and that the magnetic field`39 of constant strength may extend over at least an intermediate portion of the curving path of the elctron beam from the terminal edge of the magnetic field 35 of increasing strength to at least a point about one-half to about twothirds of the distance along the curving path of the electron beam.

The strength of the magnetic field 43 may be constant, or may weaken, i.e., decrease, toward the surface of the target as may be desired. The magnetic eld 43 illustrated in FIGURE 1 is progressively weaker toward the surface of the target. It lhas been determined that a constant field strength over about the last one-half to two-thirds of the curving path of the electron beam provides for the maximum focusing and concentration of the electron beam on the target. A schematic representation of an electron gun having a magnetic field of this configuration is illustrated in FIGURE 4. Particularly desirable results are obtained when the magnetic field/ofincreasingstrength extends over thjuirstmthiriofliheacurving path of the electron beam, and "the magnetic field 39 of constant strength extends over the remainder ofthpath of the electron beam. l

Desirable results, and good focusing and concentration of the electron beam on the surface of the target are obtained Where the magnetic field 35 of increasing strength is established over a first portion of t e cu path of the electron beam, preferably over about the first onethird of the path, the magnetic field 39 o t strengt blisliedv over an intermediate portion of the curving path of the electron beam, preferably over about the middle one-third of the path, and the magnetic field fllhaiugonstant strength less than the strenth of the magnetic field.39,.is established ovr the remain er of the curving path of the electron beam, preferably over the last one-third of the path. An electron gun assembly cluding means for establishing magnetic fields having hese configurations is schematically illustrated in FIG- RE 5 of the drawings.;

.l For most operations, it is not necessary that the mag- ;netic field strength remain constant over the terminal vportion of the curving path of the electron beam. In fact,

in some instances, as when a rotating crucible is employed, the structure of the target is such that it is not possible to establish a magnetic field which has a constant field strength over the terminal portion of the path of the electron beam. In such instances it has been found that equally desirable results may be attained when the strength of the magnetic field weakens progressively along the terminal portion of the electron beam path as shown in FIGURE 3 of the drawings.

Referring now to FIGURES 1 and 2 of the drawings, there is shown a schematic pictorial view of a specic embodiment of an electron gun assembly 11 for focusing an electron beam. 15 onto the surface of a target 19, which may, for example, be a pool of molten material disposed in an upright open ytopped crucible 47. The electron gun assembly 11 includes the magnetic lens means 17 for generating magnetic fields having the configuration illustrated in FIGURE 5,. A

The electron gun assembly 11 includes a generally horizontal ferromagnetic base plate 49 which forms the return path for the magnetic flux as discussed below. The base plate 49 is provided with a notch 50 adjacent one edge thereof, which for convenience may be designated the front edge. Four upstanding electromagneticmoils 51, 535/and-i74or-generating a magnetic fiux as later descri e are mounted on the base plate adjacent the corners thereof and are suitably connected to a source of direct current (not shown).

L-shaped pole pieces 61, 61a are supported atop the coils 51 and 53, and 55 and 57, respectively, so that one leg 63, 63a thereof is generally upstanding. The upstand- 75 ing legs 63, 63a are disposed in generally parallel relationship and are spaced apart a distance sufficient to receive the crucible 47 containing the target material 19 therebetween. Diamagnetic support means 65 may be provided on the base plate 49 .for supporting the crucible 47. The L-shaped pole pieces 6l, 61a are of a Asufficient length so as to extend beyond and overhang the base plate 49 adjacent the front edge thereof.

Ferromagnetic support members 67, 67a are attached at right angles to the forward'end offeach of the upstanding legs 63, 63a respectively, extending inwardly of the space defined between the L-shaped pole pieces 61, 61a. The pole pieces 23, 23a, formed of a ferromagnetic material depend from the inward end of each of the support members 67, 67a. rPhe dependingy pole pieces 23, 23a are generally parallel to one another and extend outwardly and downwardly of the L-shaped `pole pieces 61, 61a and support members 67, 67a to a point below the the base plate 49. As seen in FIGURE l, the depending pole pieces 23, 23a are disposed adjacent the notch 50 in the plate 49, and are attached to support members 67, 67a over approximately one-half of their length. The lower portions of the depending pole pieces 23, 23a that are not attached to the support members 67, 67a define first pole sections 69, 69a, and the upper portions of the depending pole pieces 23, 23a, attached in iiux-conducting relationship to the support members 67, 67a, define second pole sections 70, 70a. A magnetic circuit is thus defined by the base plate 49, coils 51, 53, 55 and 57, I.-shaped pole pieces 61, 61a, support plates 67, 67a and the air gap between the upstanding legs 63, 63a of the L-shaped pole pieces 61, 61a and between the depending pole pieces 23, 23a. The location of the depending pole pieces 23, 23a in the region of the notch 50 prevents a magnetic circuit from being established between the depending pole pieces .23, 23a and the base plate 49.

As best seen in FIGURE 2, an electron source 13 is supported by a suitable support means (not shown) at a position `beneath the crucible 47 so that the source 13 will be completely hidden from the target material 19 maintained within the crucible. The electron source 13 may be selected from several commercially available sources. One form of commercially available electron source generally comprises an elongated rod-shaped lament cathode 73 disposed within a backing electrode 75, and an accelerating anode 77. The electron source for emitting the electron beam 1 5 from the cathode 73 may be connected to a suitable power source 79 in accordance with conventional procedures.

As discussed above, the electron source 13 is positioned with respect to the target so that it will be completely hidden therefrom. Accordingly, the source is oriented so as to emit the electron beam 15 along an initial path which is divergent with respect to the line 27 perpendicular to the surface of the target material in the crucible.

The electron source 13 is preferably disposed in a fringing magnetic field which is convex with respect to the source, that is, a magnetic field in which the lines of force bow from the poles toward the source. Accordingly, as seen in FIGURE 2, the source 13 is positioned at a point spaced from the edges of the poles 23, 23a and is oriented with respect thereto so as to emit the electron beam 15 into the fringing field established between the lower edges of the pole sections 69, 69a.

It can be seen that the path of least resistance for the,

magnetic fiux generated by the coils 51, 53, 55 and 57 is alongthe L-shaped pole pieces 63, 63a, through the support members 67, 67a, and across the air gap between the.

pole sections 70, 70a of the ldepending pole pieces 23, 23a. Accordingly, the magnetic field will have a maximum strength in the gap between the pole sections 70, 70a. The magnetic field between the depending pole pieces 23, 23a will have the least strength adjacent the lower edge of the pole sections 69, 69a since this point is the furthest distance from the path of least resistance between the pole pieces 70, 70a. The strength of the magnetic iield between the pole sections 69, 69a increases along the surface thereof to the point at which the pole sections 69, 69a merge into the pole sections 70, 70a, i.e., at the point that the depending pole pieces 23, 23a are connected to the support members 67, 67a.

A magnetic field of constant strength is also established between the upstanding legs 63, 63a of the L-shaped pole pieces 61, 61a. However, since the air gap between the upstanding legs 63, 63a is greater than the air gap between the depending pole pieces 23, 23a, the magnetic field established between the L-shaped pole pieces 61, 61a will be of lesser strength than the magnetic iield established between t-he pole sections 70, 70a. Accordingly, in the electron gun assembly depicted in FIGURES l and 2, and as schematically represented in FIGURE 5, the electron beam 15 is emitted along an initial path that is divergent with respect to the line 27 perpendicular to the surface of the target 19 into a magnetic field 35 of increasing strength established between the pole sections 69, 69a, which magnetic eld extends over approximately t-he first third of the curving -path of the electron beam. The electron beam passes out of the magnetic eld 35 of increasing strength into a magnetic eld of constant strength established between the pole sections 70, 70a, which magnetic eld extends over approximately the intermediate `onethird of the curving path of the electron beam. The electron beam passes out of the magnetic eld 39 into a constant magnetic eld 43 of lesser strength established between the L-shaped pole pieces 61, 61a. The constant magnetic tield 43 extends over approximately the final one-third of the path of the beam.

In operation, the electron source may be operated at a potential of 15,000 volts to emit an electron beam having a density of one amp per cm?. When an electron source operated at these conditions is placed about six inches beneath an open topped crucible, the coils 51, 53, 55, 57 may be connected to a suitable source of direct current for generating a constant magnetic field 39 having a strength between about 50 gauss and about 100 gauss between the pole sections 70, 70a. With a magnetic field of about 70 gauss established between the pole sections 7 0, 70a, the magnetic tield 35 established between the pole sections 69,*6N9a increas`e`f'm about 35 gauss to about 70 gauss, i.e., Ey a factor of about 2, over the length of pole sections 69, 69a, and the constant eld between the L-sha-ped poles 61, 61a has a strength of about 30 gauss. Alternately, other magnetic field strengths may be employed for controlling Iother electron beams having other power densities.

Referring to FIGURES 6 and 7, there is shown an embodiment of an electron gun assembly 11 in accordance with the present invention, wherein a Iuniform air gap is maintained between the pole pieces 77, 77a. The electron gun generally includes a plurality of coils 79, 79a, 81, 81a, 83 83a, and 85, 85a mounted in flux conduction relationship on a generally U-s-haped ferromagnetic support member 89 which forms the return path for the ux. The pole pieces 77, 77a are connected to the coils and define a space therebetween for receiving a generally upstanding open topped crucible 91 which contains the target material 19 to be heated. The pole pieces 77, 77a comprise generally horizontally extending pole sections 93, 93a extending on opposite sides of the crucible 91 at least partially above the surface thereof, intermediate pole sections 95, 95a which extend angularly downwardly and outwardly of the horizontal pole sections 93, 93a, and depending pole sections 97, 97a which extend downwardly from the intermediate pole sections 95, 95a. The respective pole sections may be connected together, as shown o r may be spaced apart, as desired.

A generally horizontally extending imperforate shield 29 is disposed between the pole sections 97, 97a. The electron source 13 is positioned beneath the imperforate 8 shield 29 in a manner so that it is completely hidden from spalling condensate and/ or splashing molten material from the target 19, and is oriented so that the electron beam 15 is emitted into the fringing magnetic field, which is concave toward the source 13, established between the lower edges of the pole sections 97, 97a.

rl`he coils may be connected in suitablerpairs, i.e., 79 and 79a, S1 and 81a, to sources of direct current for generating magnetic lields of different strengths between the pole sections 93 and 93a, 95 and 95a, and 97 and 97a. The number of turns on the respective coils and/ or the direct current sources may be varied in order to establish a magnetic field 35 of increasing strength in the direction of travel of the electron beam between the depending pole sections 97, 97a, and a magnetic field 39 of substantially constant strength between the intermediate Ipole sections 95, a and horizontal pole sections 93, 93a. An electron gun assembly embodying a magnetic eld configuration of this type is schematically represented in FIGURE 4.

The electron gun assembly illustrated in FIGURES 6 and 7 provides a magnetic field which has an increasing strength over approximately the irst one-third of the curving path of the electron beam, and a constant strength over the remainder of the path of the electron beam, and is particularly suitable for loperations where maximum concentration of the electron beam on the surface of the target and large flow rates of evaporated material are desired.

A further embodiment of the invention is illustrated in FIGURES 8 to 10 of the drawings. As seen in FIGURES 8 and 9, three electron gun assemblies 11 embodying the principles of the present invention are disposed about the open mouth of a generally horizontally extending rotating crucible 101. The rotating crucible 101 contains a molten target 19 (FIGURE 10) which due to centrifugal force is urged into a slot 103 in the inside wall of the crucible 101. At rotating crucible similar to that illustrated is more fully disclosed in copending application Ser. No. 287,386, filed June l2, 1963. Each of the electron gun assemblies 11 includes a pair of parallel spaced apart coils 105, 10Sa which are radially'disposed from the side wall of the crucible andI which dene a space there-"- between within which is positioned the electron source 13. The coils 105, 105a extend slightly beyond the open mouth of the crucible (FIGURE 10) and are connected adjacent their rearward ends to a ferromagnetic 107 which forms a ux return path.

Angularly disposed pole pieces 109, 109a are attached in ux conducting relationship to the forward ends of the coils 105, 105a. As seen in FIGURE 9, the angular pole pieces 109, 109a depend from the ends of the coils 105, 105:1 and extend inwardly thereof at an angle. Pole faces 111, 111a are attached to the ends of the angular pole pieces 109, 109a, defining an air gap therebetween.

As seen in FIGURE 9, the angular pole pieces 109, 109a dene an air gap of decreasing width over the length thereof toward the pole faces 111,111a, and the pole faces 111, 111a deline an air gap of constant width. The electron source 13 is mounted between the coils 105, 105a adjacent the side wall ofthe crucible 101 in a position so that the source is completely hidden from the target, and so that the electron beam 15 is emitted along an initial path which diverges with respect to a line 113 perpendicular to the surface ot` the target material 19 within the rotating crucible.

The magnetic lens shown in FIGURES 8 to 10 establishes a magnetic field similar to that schematically represented in FIGURE 3. As best seen in FIGURE 10, the electron gun is located rearwardly of the angular pole pieces 109, 109a and accordingly, the electron beam is emitted into the fringing portion of the magnetic iield established between the upper ends of the angular pole pieces 109, 109a. The electron beam travels through a magnetic eld 35 of increasing strength, due to the demember 9 creasing air gap between the angular pole pieces 109, 1090: and thence into a magnetic field 39 of constant strength between the pole faces 111, Illa. The electron beam then enters a magnetic field 43 of progressively weakening strength which is provided by the fringing portions of the magnetic field 39 established between the pole faces 111, 111a.

It can be seen that when the electron gun is employed to heat a rotary crucible such as shown in FIGURES 8 to 10, it is not possible to have pole pieces extending alongside the target, and accordingly, the eld will be weakening in the region of the target. As pointed out above, this is not particularly undesirable and close control over the direction of the electron beam may be achieved utilizing the apparatus illustrated in FIGURES 8 to 10.

Another embodiment of an electron gun assembly including a magnetic lens 17 which provides a magnetic field configuration similar to that shown schematically in FIGURE 5, is shown in FIGURES l1 and 12. The electron gun assembly includes an electron source 121 which may be located beneath an open topped crucible 123 as illustrated in FIGURE 1l or may be located beneath an imperforate shield similar to that illustrated in FIGURE 7. A pair of pole pieces 125, 12511 is mounted adjacent to the crucible above the electron source 121 in the initial path of the electron beam emitted by the electron source 121. The pole pieces are connected to suitable coils 127, 127a and a ferromagnetic return path 129 is provided between the coils. The positioning of the pole pieces 125, 125a' above the electron source 121 provides a fringing magnetic field in the region of the electron source, which magnetic field increases in strength in the direction of travel of the electron beam. A constant magnetic field is provided in the region between the pole pieces 125, 12Sa and a magnetic field of decreasing strength is provided in the region adjacent the surface of the target within the crucible 123.

The electron gun assembly shown in FIGURES ll and 12 thereby provides a magnetic field of increasing strength over about the first one-third of the path of the electron beam, a magnetic field of constant strength over the intermediate one-third of the path of the electron beam, and a magnetic field of decreasing strength over the last one-third of the path of the electron beam. As previously described, an electron gun assembly similar to that set forth in FIGURES ll and 12 is capable of providing close control over the electron beam and provides a simple and convenient apparatus for heating of a target material while insuring that the electron source will not become contaminated due to spalling of contaminate from the interior of the electron furnace,r or splashing of molten material from the target.

It is to be understood that various other configurations of pole pieces and magnetic field generating means within the skill of the art may be employed to achieve the desired magnetic lens. The pole pieces establishing the vari-- ous magnetic fields of different strength need not be connected in flux conducting relationship, and the air gaps between the various magnetic field generating means need not be of different widths.

Various of the features of the invention are set forth in the following claims.

What is claimed is:

1. An electron beam gun assembly. for heating a target comprising, a source for emitting a high power density diverging electron beam in a direction away from the target along an initial path which diverges from a line perpendicular to Ithe surface of the target, said source positioned rearwardly from the surface of t-he tar-get and isolated therefrom, and magnetic lens means between said source and the target for deflecting and focusing the electron beam from its initial path along a curving path onto the surface of the target, said magnetic lens means including means outwardly spaced from the target for establishing a magnetic field of increasing strength adjacent said source in the initial path of the electron beam, said magnetic field of increasing strength having lines of flux of increasing curvature toward said source, said magnetio lens including means outwardly spaced from the target for establishing a magnetic field of substantially constant strength in at least a portion of the curving path of the electron beam between the. magnetic field of increasing strength and the target.- :Y

2. An electron beam'v gun assembly for heating a target comprising, a source for emitting a high power density diverging electrorrbeam in a directionaway from the target along an initial path which diverges from a line perpendicular to the surface of the target, said source positioned rearwardly from the surface of the target and isolated therefrom, and magnetic lens means between said source and the target for deflecting and focusing the electron beam from its initial path along a curving path onto the surface of the target, said magnetic lens means including means outwardly spaced from the target for establishing a first magnetic field of increasing strength in the direction of travel of the electron beam adjacent said source in -the initial path of the electron beam, said first magnetic field of increasing strength having lines of flux of increasing curvature extending in a direction that is generally rearwardly from the surface of the target toward said source, said magnetic lens including means outwardly spaced from the target for establishing a second magnetic field of substantially constant strength in at least a portion of the curving path of the electron beam between the magnetic field of increasing strength and the target.

3. An electron beam gun assembly for heating a target comprising, a source for emitting a high power density diverging electron beam in a direction away from the target along an initial path which diverges from a line perpendicular to the surface of the target, said source positioned rearwardly from the surface of the target and isolated therefrom, and magnetic lens means between said source and the target for deecting and focusing the electron beam from its initial path along a curving path onto the surface of the target, said magnetic lens means including means outwardly spaced from the target for establishing a rst magnetic field adjacent said source and in the initial path of the electron beam, and means outwardly spaced from the target for establishing a second magnetic field in the curving path of the electron beam between said first magnetic. field and the target, said first magnetic field having increasing strength inthe direction of travel of the electron beam and extending over about the first one-fourth to about one-half of the curving path of the electron beam, said second magnetic field having substantially constant strength and extending over at least a portion of the remainder of the curving path of the electron beam, whereby an electron beam generated by the said source is guided from its intial path along a curving path and is focused onto the surfaceof the target.

4. An electron beam gun assembly for heating a target comprising, a source for emitting a high power density diverging electron beam in a direction away from the target along an initial path which diverges from a line perpendicular to the surface of the target, said source positioned rearwardly from the surface of the target and isolated therefrom, and magnetic lens means between said source and the target for defiecting and focusing the electron beam from its initial path along a curving path onto the surface of the target, said magnetic lens means including means outwardly spaced from the target for establising a first magnetic field of increasing strength in the direction of travel of the electron beam adjacent said source and in the initial path of the electron beam, and means outwardly spaced from the target for establishing a second magnetic field of substantially constant strength in the curving path of the electron beam between said first magnetic field and the target, said first magnetic field having lines offiux of increasing curvature extending in a direction generally rearwardly from the surface of the target, said first magnetic field extending over about the first 'one-fourth to about one-half of the distance of the curving path of the electron beam, said second magnetic field extending between said first magnetic field and a point between about one-half to about twothirds of the distance of the curving path of the electron beam, whereby an electron beam generated by said source is guided from its initial path along a curving path and is focused onto the surface ofthe target.

5. An electron beam gun assembly for heating a target comprising, a source for emitting a high power density diverging electron beam in a direction away from the target along an initial path which diverges from a line perpendicular to the surface of the target, said source positioned rearwardly from the surface of the target and isolated therefrom, and magnetic lens means between said source and the target for deflecting and focusing the electron beam from its initial path along a curving path onto the surface of the target, said magnetic lens means including means outwardly spaced from the target for establishing a first magnetic field adjacent said source and in the initial path of the electron beam, and means outwardly spaced from the target for establishing a second magnetic field in the curving path of the electron beam between said first magnetic field and the target said first magnetic field having increasing strength in the direction of travel of the electron beam and extending over about the first one-fourth to about one-half of the curving path of the electron beam, said second magnetic eld having substantially constant strength and extending over the remainder of the curving path of the electron beam, whereby an electron beam generated by said source is guided from its initial path along a curving path and is focused onto the surface of the target.

6. An electron beam gun assembly for heating a target comprising, a source for emitting a high power density diverging electron beam in a direction. away from the target alongthe initial path which diverges from a line perpendicular to the surface of the target, said source positioned rearwardly from the surface of the target and isolated therefrom, and magnetic lens means between said source and the target for defiecting and focusing the electron beam from its initial path along a curving path onto the surface of the target, said magnetic lens means including means outwardly spaced fro-m the target for establishing a first magnetic field of increasing strength in the direction of travel of the electron beam adjacent said source and in the initial path f the electron beam, means outwardly spaced from the target for establishing a second magnetic field of substantially constant strength in the curving path of the electron'beam between said first magnetic field and the target, and means for establishing a. 5

third magnetic field having a strength less than the strength target, said first magnetic field having lines of fiux of iucreasing curvature extending in a direction generally rearwardly from the surface of the target, said first magnetic field extending-over about the first one-fourth to about one-half of the distance of the curving path of the electron beam, said secondmagnetic field extending between said first magnetic field and a point between about one-half to about two-thirds Kof the distance of the curving path of the electron beam, whereby an electron beam generated by said source is guided from its initial path along a curving path and is focusedv onto the surface of the target.

7. An electron beam gun assembly in accordance with claim 6 wherein the third magnetic field decreases in the direction of travel of the electron beam.

' 8. An electron beam g-un assembly comprisingy a target, a source for emitting fa high power density electron beam in a direction away from the target along a'n initial path which diverges from a line perpendicular to the surface of the target, said source being positioned beneath the surface of the target and isolated therefrom, a pair of pole pieces spaced outwardly from the target and above said source for magnetically defiecting the yelectron beam from its initial path along a curving path onto the target, means for energizing said pole pieces to establish a magnetic field of increasing strength in the initial path of the electron beam, and a magnetic field of substantially constant strength over a portion of -the curving path of the electron beam between the magnetic field of increasing strength an-d the target.

9. An electron ibeam gun assembly comprising, a target, a shield below and to the side of the target,'a source disposed beneath the shield for emitting a high density diverging electron beam in a direction away from the target along an initial path which diverges from a line perpendicular to the surface of the target, a pair. of spaced pole pieces spaced outwardly Afrom the target and said source, and means for energizing said pole pieces to establish a first magnetic eld lof increasing strength in the initial path of the electron beam', a second magnetic field of constant strength in the path of the electron beam leaving said first magnetic field of increasing strength, and a third magnetic field of decreasing strength in the path of the electron beam leaving said lsecond magnetic field, said third-4 magnetic field extending nto the region above the target.

References Cited UNITED STATES PATENTS 3,068,309 12/1962 Hanks 219-121 3,132,198 5/1964 Du Bois et al. 219-121 3,177,535 4/1965 Hanks 13-31 3,202,794 8/1965 Shrader et al. 219-121 3,204,096 8/1965 Anderson et fal. 219-121 5 RICHARD M. wooD, Primary Examiner.

W. D.v BROOKS, Assistant Examiner.

U.S. Cl. X.R. 13-11

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