US3132198A - Electron beam furnace - Google Patents

Description

SS1 REFEENCE R gig-LN].

May 5, 1964 A. o. DU BOIS ETAL 3,132,198

ELECTRON BEAM FURNACE 3 Sheets-Sheet 1 Filed Jan. 15, 1962 ANOPia/ 0, 0050/5 (#9255 AA HAM .5

INVENTORS y 1964 A. o. DU BOIS ETAL 3,132,198

ELECTRON BEAM FURNACE 3 Sheets-Sheet 2 Filed Jan. 15, 1962 67/4 455 1d, fln/vzs INVENTORS y 1964 A. o. DU BOIS ETAL 3,132,198

ELECTRON BEAM FURNACE Filed Jan. 15, 1962 3 Sheets-Sheet 3 Awaeia/ 0, 9030/5 Hon 4P0 f/ZEKF? 67/49:!{ &5 F I 6 4 United States Patent Ofitice 3,132,198 Patented May 5, 1964 3,132,198 ELECTRON BEAM FURNACE Andrew 0. Du Bois, El Cerrito, Howard R. Harker, Pleasant Hill, and Charles W. Hanks, Orinda, Calif., assignors, by mesne assignments, to Stauffer Chemical Company, New York, N.Y., a corporation of Delaware Filed Jan. 15, 1962, Ser. No. 166,119 7 Claims. (Cl. 139) This invention relates to an electron beam furnace, and, more particularly, to an electron beam furnace in which materials are heated to melting and in which the electron beam sources are isolated from materials being melted in order to minimize the risk of gases which are frequently released and ionized during the melting, being attracted to the beam sources to establish a short circuiting flow of electrons.

In an electron beam furnace, materials are bombarded by high-energy electrons which are projected in a beam from sources called electron guns. The materials 'may be positioned or fed into the furnace to a target zone overlying the open top of a crucible or mold where the material is melted and caused to drop downwardly into the mold. Preferably, the top of the material in the mold is itself heated in order to maintain a molten pool of metal on top of the ingot so that, as the lower portion of the ingot cools, it is built up uniformly.

As the raw material is melted, quantities of gas are unavoidably released and, under the continuing bombardment of the electron beam, ions of the gas are formed which are susceptible to attraction by the field of electron guns. If the gas ions succeed in reaching the field established between the cathode and the anode of the electron guns, the usual result is a shorting of the electron guns by the resultant flow of electrons across the field. Moreover, heavy ions frequently formed during vaporization of the material might themselves cause electron gun damage if attracted at high velocity to the field of the gun.

The advantage of isolating the guns from the material has been recognized, but it is an advantage not easily realized in practice, particularly because of the difliculty of guiding high-current, high-intensity electron beams from a remote or isolated zone through a small aperture in the shield and from there into the melting zone and onto the target with any degree of accuracy.

It is, therefore, an object of this invention to provide an electron beam furnace including means for isolating the electron source from the gases and vapors released during melting.

It is a further object of this invention to provide means for guiding an electron beam from an isolated source.

These effects are accomplished by providing an elongate electron gun which produces a ribbon-shaped beam and isolating the electron gun by provision of a shield adjacent to the gun with a slit in the shield through which the ribbon-shaped electron beam is projected, the slit being substantially the same size and shape as the cross section of the beam. However, this raises the further problem of reshaping the beam for optimum melting effect.

It is a further object of this invention to provide an electron source isolated by a shield and conditioned to project a beam through a slit in the shield, together with means for guiding and reshaping the beam for bombardment of material.

The last-mentioned object is accomplished by providing a magnetic field with curved lines of force arching across the ribbon-shaped beam, after it passes through the slit, whereby the beam is simultaneously deflected in a direction perpendicular to its width and converged to a narrow width.

In carrying out this invention, a horizontal shield is provided within the main furnace envelope at or below the level of the target which is to be bombarded and heated, to isolate the electron beam target from the electron guns. The electron guns are disposed below the shield and aimed to project their beams upward through narrow slits in the shield, and for this purpose an elongate gun filament is provided so that the beam is also long and narrow in cross-section, or ribbon-shaped. The beam is then placed under the influence of a magnetic field adapted to reshape it as well as to guide it onto the target. Particularly, the magnetic field includes arching flux lines crossing the beam path above the shield and disposed with respect to the motion of the electrons, so that the diverting forces imparted at right angles to the lines of force include converging components which act in opposition against the width of the beam, i.e. its long cross-sectional dimension, effectively to compress the width while permitting the beam to spread in the other dimension. Under the influence of these arching lines of force, the cross-section of the beam gradually assumes a more nearly circular configuration.

The shield through which the electron beam is projected is at the electric potential of the anode component of the electron gun anode, in order to reduce the tendency for gaseous ions to be attracted past the shield into the field of the electron gun. Hence, after passing through the slit in the shield, the electrons are in an electrically field-free region, and undergo no appreciable change in speed, although their directions are altered by the magnetic field. Moreover, heavier ions including those of the material vaporized during the melting process are not influenced by the magnetic field to the same extent as are the much lighter electrons, and are not accelerated by an electric field, the region above the field being electrically field-free. Hence, there is little tendency for the heavy ions to curve down from the target through the slits in the shield. Any ions that do get through the slits and into the electric field of the guns move in less curved trajectories than the electrons do, and thus do not bombard the cathode. Isolation of the gun from all ions within the melting chamber is enhanced by the extremely limited access to the electron guns afforded by the narrow slit in the shielding enclosure.

Other objects and advantages of this invention will become apparent from the specification following when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is an elevational view in partial section of an electron beam furnace embodying the present invention;

FIG. 2 is a horizontal section view taken along line 2--2 of FIG. 1;

FIG. 3 is a partial horizontal section view showing the influence of the magnetic field 'on the electron beams; and

FIG. 4 is a partial vertical view showing the influence of the magnetic field in a horizontal plane.

Referring now more particularly to the drawings, the electron beam furnace of this invention includes an outer enclosure or envelope 10 containing a crucible or mold 12 which may, if desired, be formed with a water-cooled jacket 14. The mold 12 is open at the top 15, so that raw material 16 above the mold may be melted at 16a to drip into the mold or crucible and form a molten pool 17 on the top of the ingot 18 into which it is ultimately solidified and withdrawn through an opening 20 in the bottom of the furnace 10 by any suitable means (not shown).

The raw material to be melted may be fed by any suitable means, which are here shown merely for purposes of schematic illustration, as feed rollers 21, in order progressively to bring the end portion to a target or melt zone overlying the open top 15 of the mold 12. There, it is subjected to bombardment and progressively melted by a series of electron beams 24 projected from a plurality of electron guns 25, each influencing a filament 26, a cathode 27 and an anode 28. The filament 26 may be an elongated rod or, as shown in FIG. 2, it may be formed of elongated hairpin configuration. Whatever its specific form, the filament 26 is designed to project a thin but wide electron beam.

Straddling each electron gun 25 is a generally U-shaped magnet 30 including a coil 31 and pole pieces 32 which converge progressively toward their outer ends, as shown in FIG. 2, for a purpose hereinafter to be described.

As a particular feature of this invention, a floor or shield 40 is provided around the crucible 12 above the level of the electron guns 25 in order to isolate the guns from the target zones, at the end of 16a of the raw material and in the open top of the crucible. From both of the target zones gases may be released during melting and could short out the electron guns if the ions thereof drift into the field of the gun to set up a flow of electrons. The shield 40 is provided to prevent invasion of the electron guns by such gaseous ions. If desired, the shield 40 could be surrounded by a plurality of heat radiating panels which afford a labyrinth path for removal of gases while minimizing heat losses by radiating heat back toward the source at the center of the furnace. Here, there is shown simply a single vertical panel 41 forming a partial heat shield which also further isolates the electron guns from the source of released gases. The floor 40 and panel 41 are formed of non-magnetic material, such as copper, so as to afford no substantial effect upon the magnetic field generated between the pole faces 32. Further, the floor or shield 40 is maintained at the potential of the anode 28, as by means of a mounting strap 43 to form a barrier to gaseous ions that might otherwise be attracted to the field of the electron guns. Both the electron gun and the shield 40 may be energized from a conventional power supply 42.

Provided in the floor or shield 40 above each electron gun 25 is a narrow slit 44 through which the electron bears are aimed, their particular size and location being closely controlled in View of the electron velocities and the influencing magentic field. For maximum isolation of the electron guns 25, it is, of course, desirable to make the slits as narrow as possible and, in conjunction therewith, the filaments 26 of the electron guns are of elongate configuration .so that the electron beam issuing from each gun is of a long and narrow cross-section aimed to pass freely through the appropriate slit 44.

With particular reference to FIG. 3, after the electron beam passes through the slit 44 and arches over toward the mold 12, it moves above the level of the pole faces 32 and the magnetic field through which it traverses comprises progressively higher arching lines of force 33. This is shown particularly in FIG. 3 and becomes more pronounced in the flux flow between the tops and remote sides of the magnet pole pieces 32. In conjunction with this arching magnetic field, there is a substantial and progressively increasing electron velocity vector directed into the plane of the paper in FIG. 3. As to this electron velocity vector, the deflecting forces acting on the electrons under the influence of a magnetic field moving in an arcuate path from right to left, being at right angles to the field, tend to converge as indicated by the arrows F, and approach a state of direct opposition at the extremities of the beam width adjacent to the pole pieces. These converging deflecting forces, acting in opposition, tend to compress the beam and reduce the width thereof. Moreover, since the deflecting forces are at right angles to the electron velocities, as well as the magnetic field direction, the compression deflecting forces are further intensified by the converging paths of the electrons. The strength of the deflecting forces continues through the outward trajectory of the electron beam because the converging pole pieces 32 generate a field between them which is of gradually increasing strength toward their ends to compensate for the reduction in field strength due to vertical distance from the pole pieces.

More over, with reference to the top view of FIG. 4, the converging pole pieces also generate arcuate lines of force between them which, acting on the vertical velocity vectors of the electrons, provide converging deflecting forces further to compress the width of the beam. Outward of the ends of the pole pieces indicated by the plane A the field lines of force arch outwardly of the furnace 10. Consequently, the deflecting forces at right angles to these lines of force, considered with the vertical velocity vectors of electrons emerging from the plane of the paper in FIG. 4 are also converging as shown by the arrows F. Then, if the electron velocity and strength of the magnetic field is adjusted so that the electron beam arches over beyond the ends of the pole pieces 32 and commences its downward trajectory toward the mold 12 inward of the plane A the deflecting forces continue to converge because the lines of force beyond the ends of the pole pieces commence arching inward of the furnace. Therefore, relative to the direction of the electron paths, the magnetic field arches in substantially the same direction. As shown in FIGS. 3 and 4, the magnetic field is barrellike in configuration and the electrons projected from the guns 25 travel within the barrel. Therefore, throughout the electron trajectory upward between the converging pole pieces, across above the pole pieces and downward beyond the ends of the pole pieces the magentic field continuously arches over the paths of the electrons in the same relative direction, with resultant converging deflecting forces. This barrel configuration can be enhanced by rounding the ends of the pole pieces 32 as shown in FIG. 1 so that the flux path emerging perpendicular to the edge of the pole pieces arches both inwardly and upwardly. Since the electrons follow naturally diverging paths with respect to the other cross-sectional axis of the beam, the beam is spread in the plane of FIG. 1 and compressed in directions parallel to the planes of FIGS. 2 and 3. As a result, the cross-sectional dimensions of the beam are substantially equal along both axes when the beam enters the crucible.

Vapor ions formed during vaporization such as those of the metal itself, are of course not influenced by the magnetic field to nearly the same extent as are the much lighter electrons. Consequently, there is little likelihood that such relatively heavy particles would be influenced by the magnetic field to traverse the reverse path of the bombarding electron beams and move from the melt zones into the electron guns. Further, the extremely limited access to the zone of the guns provided by the narrow slits sharply reduces the possibility of vapor molecule deposition of the electron gun components.

In direct communcation with the main furnace envelope 10 through large windows or ducts 48 are evacuation chambers 50 through which the envelope 10 is continuously evacuated by vacuum pumps 52 to a pressure in the order of one micron of mercury. Similarly, large evacuation windows 56 are formed in the heat radiating panels 41 and they are arranged in substantial alignment with the main furnace evacuation windows 48 so that direct paths of gas and vapor evacuation are provided to the evacuation chambers 50. As a result, the possibility that gas or vapors above the shield 40 will drift around and below the shield to the electron guns 25 is extremely remote. Preferably, the windows 56 of the inner panels 41 are provided with a series of vanes or baflles 58 to provide vapor condensation surfaces and heat conserving radiation members.

If desired, the shield or floor 40 and the pole faces 32 are themselves water-cooled to protect them against the intense heat within the furnace enclosure 10 resulting from electron bombardment. This water-cooling may be accomplished by securing copper tubing or the like (not shown) to these elements.

Other modifications and changes in and to the preferred embodiment shown may be made by those skilled in the art, without departing from the spirit and scope of this invention which is defined by the claims appended hereto.

What is claimed as invention is:

1. An electron beam furnace comprising a furnace enclosure,

an electron gun including an elongate thin filament for projecting a high-energy electron beam having a long and narrow cross section upward in said enclosure,

means for presenting material to be treated to a melt zone within said enclosure,

a horizontal shield disposed adjacent to and above the gun between said electron gun and said melt zone, said shield including a long, narrow slit therethrough through which said electron beam is aimed, and

a generally U-shaped magnet having converging pole pieces on opposite sides of said electron gun so that the rearwardly and upwardly arching lines of force tend to exert converging deflecting forces against the width of an electron beam moving upward and forward therethrough after it passes through said slit and to guide the beam in a curved path onto the material to be treated.

2. The electron beam furnace defined in claim 1 wherein said pole pieces terminate below the point in said electron beam at which downward travel thereof commences so that lines of force arching outwardly and forwardly from the ends of said pole pieces exert converging deflecting forces against the width of said electron beams.

3. An electron beam furnace comprising a furnace enclosure,

a holder for material to be bombarded in the upper portion of said furnace enclosure,

an open top crucible in said furnace enclosure for melting material,

an electron gun in the lower portion of said furnace enclosure generating an electron beam of long and narrow cross-section,

a horizontal wall surrounding the open top of said crucible and isolating the lower portion of said furnace enclosure including said electron gun from said upper portion,

said horizontal wall having a narrow slit extending therethrough of substantially the same cross-sectional dimension as that of said electron beam,

means generating a magnetic field for focusing said beam through said slit and additionally compressing said beam above said wall in the long cross-sectional dimension thereof while guiding the electron beam in curved paths to bombard said material, whereby the beam cross-section at bombardment is substantially circular, and

high vacuum evacuating means in direct communication with said furnace enclosure both above and below said horizontal wall for maintaining the furnace enclosure at a high vacuum.

4. The electron beam furnace defined in claim 3 including bafiles in said furnace enclosure between said crucible and said evacuating means.

5. An electron beam furnace comprising a furnace enclosure,

a means for delivering material to be bombarded to a melt zone in the upper portion of said furnace enclosure,

an open top crucible for melted material in the lower portion of said furnace enclosure,

a plurality of electron guns in the lower portion of said furnace enclosure,

a horizontal wall surrounding the open top of said crucible and dividing the main furnace enclosure above said electron guns,

said horizontal wall defining a plurality of long, narrow slits therethrough,

each of said electron guns being aimed to project a high-energy beam of long and narrow cross-section upward through a separate one of said slits,

magnetic field generating means establishing a magnetic field exerting laterally inward deflecting forces against the long cross-sectional dimension of said beam while guiding said beam in a curved path to bombard the material, and

high vacuum evacuating means in direct communica tion with said furnace enclosure both above and below said horizontal wall.

6. The electron beam furnace as set forth in claim 3, additionally defined by the means generating said magnetic field comprising,

a pair of magnet pole pieces disposed below said horizontal wall with one pole piece at each end of said electron gun and establishing a substantially uniform magnetic field in the immediate vicinity of said guns,

and said pole pieces having ends converging toward each other toward said crucible for maximizing the focusing of electrons therein.

7. An electron beam furnace comprising,

a furnace enclosure,

means disposing a material for treatment in a melting zone within said furnace,

a horizontal wall separating a portion of said enclosure from the remainder of the furnace including said melting zone,

said horizontal wall having at least one narrow elongated slit therethrough,

at least one electron gun disposed beneath and on the opposite side of said wall from said melting zone and having an elongate filament for generating a ribbon-shaped beam having substantially the same cross-sectional dimensions as said wall slit,

said electron gun being laterally displaced with respect to said wall slit,

a magnet field generator establishing a magnetic field having lines of force substantially parallel to said electron gun filament in the vicinity thereof for guiding said electron beam in a curved trajectory through said slit for maximized protection of the electron gun from ion bombardment,

said magnetic field generator further establishing said magnetic field with lines of force in a barrel shape curving over at least a part of said melting zone and concave with respect to said melting zone to laterally converge and longitudinally extend the beam cross-section so that the beam impact pattern is substantially circular for maximized beam heating efiiciency,

and evacuation means disposed in direct communication with said furnace enclosure both above and below said horizontal wall for continuously maintaining the furnace enclosure at a high vacuum.

References Cited in the file of this patent UNITED STATES PATENTS 2,252,052 Van Embden Aug. 12, 1941 2,291,948 Cassen Aug. 4, 1942 2,715,693 MacNeille et a1 Aug. 16, 1955 3,046,936 Simons July 31, 1962 3,068,309 Hanks Dec. 11, 1962

Claims (1)

1. AN ELECTRON BEAM FURNACE COMPRISING A FURNACE ENCLOSURE, AN ELECTRON GUN INCLUDING AN ELONGATE THIN FILAMENT FOR PROJECTING A HIGH-ENERGY ELECTRON BEAM HAVING A LONG AND NARROW CROSS SECTION UPWARD IN SAID ENCLOSURE, MEANS FOR PRESENTING MATERIAL TO BE TREATED TO A MELT ZONE WITHIN SAID ENCLOSURE, A HORIZONTAL SHIELD DISPOSED ADJACENT TO AND ABOVE THE GUN BETWEEN AND ELECTRON GUN AND SAID MELT ZONE, SAID SHIELD INCLUDING A LONG, NARROW SLIT THERETHROUGH THROUGH WHICH SAID ELECTRON BEAM IS AIMED, AND A GENERALLY U-SHAPED MAGNET HAVING CONVERGING POLE PIECES ON OPPOSITE SIDES OF SAID ELECTRON GUN SO THAT THE REARWARDLY AND UPWARDLY ARCHING LINES OF FORCE TEND TO EXERT CONVERGING DEFLECTING FORCES AGAINST THE WIDTH OF AN ELECTRON BEAM MOVING UPWARD AND FORWARD THERETHROUGH AFTER IT PASSES THROUGH SAID SLIT

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