US3040112A - Electron-beam furnace with beam emission suppressors - Google Patents

Description

June 19,1962 H. R. SMITH, JR 3,040,112

ELECTRON-BEAM FURNACE WITH BEAM EMISSION SUPPRESSORS Filed June 3, 1969 4 Sheets-Sheet 1 4 Sheets-Sheet 2 H. R. SMITH, JR

June 19, 1962 ELECTRON-BEAM FURNACE WITH BEAM EMISSION SUPPRESSORS Filed June 5, 19

June 19, 1962 H. R. SMITH, JR

- ELECTRON-BEAM FURNACE WITH BEAM EMISSION SUPPRESSORS 4 Sheets-Sheet 4 Filed June 3, 1960 INVENTOR. flush P 541/ rag/l.

ilnited States Patent @fhee 3,540,112 Patented June 19, 1962 3,046,112. ELECTRQN-BEAM FURNACE WETH BEAM EMESSIGN SUPPRESSORS Hugh R. Smith, lira, Piedmont, Calii, assignor to Stautfer Chemical Company, New York, N.Y., a corporation of Delaware Filed June 3, 1965 Ser. No. 33,680 Claims. (Cl. 13-31) This invention relates to an improvement in electronbeam furnaces in which materials are heated by concentrated electron beams in a high vacuum and which are particularly useful for producing high purity ingots having an exceptional freedom from contained gases and volatile impurities.

It is well known that electron-beam furnaces operating under a high vacuum are extremely useful formelting and casting expensive materials which are hard to process by ordinary means. These furnaces produce cast ingots which are relatively gas free and of exceptional purity. If the size of this type of furnace and the duration of the uninterrupted furnace runs can be increased, not only can the cost of casting special, expensive materials in the electron furnace be reduced, but these furnaces, with their associated advantages, can be widely utilized for melting and casting a great variety of materials. However, there are a number of problems associated with increasing the size and capacity of electron-beam furnaces. In particular, increasing the size of furnaces involves many problems in focusing and control of the high density electron beams, particularly in the presence of high rates of evolution of gaseous material from the melting metal. Extending the duration of uninterrupted runs of the furnace depends to a great extent on preventing the buildup of deposited material on the electrodes. Obviously, if the electrodes can be removed from the immediate vicinity of the top of the molten ingot, the buildup of deposited material on the electrodes, caused by the condensation of gaseous matters and the collection of splattered material, can be markedly reduced. However, the diverging effect upon the electron beam by its space charge rnilitates against locating the electron beam gun at any appreciable distance from the ingot. A method whereby the divergence effect of the space charge is counteracted and the electron beam can be closely focused in a hollow, converging, conelike beam, so that the electron .gun can be located at some distance from the ingot, is disclosed by this applicant in his copending application Ser. No. 32,215 filed May 27, 1960. That invention utilizes a magnetic field converging in the top of the ingot crucible in combination with a focusing, annular electron gun located some distance vertically above the crucible. The annular electron gun is configured to generate a downwardly projecting beam in the shape of a hollow, converging cone, the sides of which are essentially parallel to the converging magnetic field. The configuration of the electron gun in the referenced invention aligns with the magnetic field those electron paths which have a directional component substantially parallel to the sides of the cone of the beam. However, those electrons in the beam whose paths have a substantial directional component parallel to the base of the cone, i.e., are tangential to the loop of the cathode, are not aligned with the magnetic field and are lost from the beam.

This invention establishes an improvement in the configuration disclosed in the above-identified copending application by adding means for bringing into alignment with the magnetic field those electrons whose paths have a substantial directional component which is tangential to the loop formed by the cathode and its associated electrodes; i.e., has a large directional component parallel to the plane of the cathode. This result is accomplished by installing suppressor plates circumferentially around and transverse to the annular focusing electrode between the cathode and the accelerating electrode. This improvement focuses a larger number of electrons into the hollow, conelike, converging beam and, by increasing the density of the. beam, markedly improves the efliciency of the furnace.

The foregoing and other aspects of the invention may be better understood from the following illustrative description and accompanying drawings.

FIG. 1 is a schematic vertical section of the improved electron-beam furnace through the vertical axis of the crucible.

FIG. 2 is a fragmentary schematic vertical section of the same furnace drawn to a larger scale, showing typical magnetic flux lines and electron trajectories.

FIG. 3 is a view of the bottom side of the focusing electrode as it would appear with the accelerating electrodes removed.

FIG. 4 is a fragmentary vertical view with a part of the focusing electrode cut away along lines 4-4 of FIG. 3.

FIG. 5 is a schematic perspective diagram illustrating a typical path of an electron in which there is a directional component substantially tangential to the loop of the cathode and with no suppressor plate installed.

FIG. 6 is a schematic perspective diagram indicating the effect of the suppressor plate on the electron path illustrated in FIG. 5.

Referring to FIG. 1, an annular copper crucible I having a vertical axis and an open upper and lower end is provided with a coolant jacket 2, through which water or other coolant is continually circulated by conventional means (not shown) to cool the crucible and to solidify the molten material therein. The solidified material may be progressively withdrawn through the open bottom end of mold 1 to form a cast ingot 3 of progressively increasing length, which may be cut off from time to time as desired. Progressive withdrawal of the cast ingot is accomplished by any conventional means; e.g., by means of rollers 4 driven by electric motor 5.

An annular electromagnet winding 6 extends coaxially around crucible 1 with its vertical axis concentric with the open top of the crucible. This winding is connected to DC. power supply '7. This electromagnet produces a magnetic field, whose strength may be adjusted by suitable means, having a vertical axis of symmetry and having magnetic fiux lines that converge into the open top of crucible 1 as represented by broken lines 30 of FIG. 2.

An annular electron gun is vertically aligned above the open top end of crucible 1 and is preferably spaced therefrom by a large distance relative to the diameter of the crucible in order to reduce the deleterious effects of gaseous and solid matter deposited upon the gun. In its preferred form the electron gun comprises an annular focusing electrode 15, the center of which is in line with the vertical axis of crucible 1. A trough 16 is cut circumferentially around the underside of the focusing electrode 15 and an annular thermionic cathode 10 is contained within the trough 16 but out of contact with the focusing electrode. Cathode 10 is connected to AC. supply 12 which heats cathode It) to a thermionic emission temperature. An annular accelerating electrode 13 is closely spaced below and coaxial with the focusing electrode 15, and has an annular opening 14 aligned below the circumference of trough 16. Suppressor plates 17 are spaced around the circumference of the focusing electrode 15 and transverse to the trough 16, extending downward between the cathode it) and the accelerating electrode 13. The accelerating electrode 13 is maintained at substantially the same electric potential as crucible 1; and cathode 1t) and focusing electrode 15' are maintained at substantial negative potentials by a high voltage D.C. supply 18.

tangentially traveling electrons.

Other than the suppressor plates 17, the overall design of the furnace and its electron gun may be similar to that described in the copending patent application identi fied above. The cathode, accelerating electrode, and focusing electrode along with its transverse suppressor plates are shaped and aligned to direct electrons downwardly and inwardly, and parallel to the converging magnetic lines of force, thereby forming a hollow, conelilre, converging beam which is focused on the top of the ingot.

A horizontal feed trough 19 extends inwardly above crucible 1 for feeding melt stock 26, in any convenient form, into one side of the electron beam converging into crucible 1. As the melt stock emerges from the discharge end of the feed trough 19, it is bombarded and melted away by the electron beam. The melted material falls into the open top of the crucible 1 for continually replacing the pool of molten material on the top of ingot 3.

It is essential that the space between the electron gun and the open top of crucible 1 be maintained in a high vacuum, preferably one micron of mercury absolute pressure or less, which requires continuous evacuation by high capacity pumps to remove gaseous matters evolved during large scale operations. 'For this purpose the essential elements of the furnace are included within vacuum tank 21 which is connected to high capacity vacuum pumps 22.

In so far as the paths of electrons which are traveling essentially Within the plane of FIG. 2, the configuration of the focusing and accelerating electrodes will align these electron paths closely parallel to the magnetic field as the beam emerges from the opening between the accelerating electrode. Referring to FIG. 2, those electrons emerging from opening 14 between the accelerating electrodes and which are traveling substantially to the plane of this figure are represented by path 31, which is substantially parallel to the magnetic flux lines 30. The magnetic field counteracts the diverging effect of space charge in the beam by bending the path of any electrons which tend to move nonparallel to the flux lines into a downward spiral path following the magnetic fiux lines 30.

However, with regard to the electrons whose paths have a directional component substantially tangential to the loop of the cathode; unless the suppressor plates 17 are installed, these electrons would follow a path which intersects the magnetic fiux lines at relatively large angles. FIG. 5 illustrates the condition where no suppressor plate is installed. The dotted line 41 represents the path which an electron having a substantial directional component tangential to the loop of the cathode would follow upon intersecting the magnetic flux lines 30. The electromagnetic force induced by the motion of the electron across the magnetic field will bend the electron path into a wide spiral about the flux line indicated by the dotted path 41. This spiral path is of such large radius that electrons in such paths would not strike the top of the ingot. FIG. 6 illustrates the effect of the suppressor plates 17 on these Dotted line 40 illustrates the effect of the suppressor plate 17 on the path of an electron which has a large initial velocity component tangential to the cathode loop. The repelling eifect of the negatively charged suppressor plate 17 will bend the electron path 40 so that it more nearly parallels the magnetic flux lines 30. This will result in the electron path 40 following a tight spiral of small radius down the fiux lines and impinging on the top of the ingot.

FIG. 4 illustrates the improvement gained by installing suppressor plate 17 transverse to the annular trough of the focusing electrode. Paths 40 represent those electrons which leave the cathode 10 with a directional component substantially tangential to the loop of the cathode 10. As the electron approaches the suppressor plate 17, its path is bent downward so that the path is parallel to or crosses the magnetic flux lines at a small angle and the path then follows down the flux lines 30 in a tight spiraling curve so that the electrons impinge on the top of the ingot 3. If it were not for the bending effect of suppressor plate 17 4 spaced around the circumference of the focusing electrode, the electrons having paths with a large directional component tangential to the loop of the cathode would be lost from the beam being focused at the top of the ingot.

Whereas the prime utility of the suppressor plates, disclosed by this invention, is in connection with an electron furnace having a magnetic field converging at the top of the crucible, this invention offers a distinct improvement for an electron-beam furnace operating without the magnetic field. The suppressor plates add another degree of focusing for the electron beam which is generated by an electron gun such as disclosed in copending application of Charles Hanks, Serial No. 813,306 filed June 5, 1959 and assigned to the same assignee as the present application. Therefore, this disclosure should not be construed to mean that this invention must be utilized exclusively in an electron furnace having an imposed magnetic field. It is conceived that the scope of this invention covers the improvement established by the suppressor plates in any electron furnace having an annular gun which produces a hollow converging conelike electron beam focusing on the top of the ingot.

It should be understood that the specific embodiment illustrated is merely one example of how this invention may be practiced, and that numerous changes and modifications are possible without departing from the inventive principles herein disclosed.

What is claimed is:

1. An electron-beam furnace for casting molten material under a high vacuum comprising an open top crucible containing said material, a negatively charged focusing electrode vertically aligned above said crucible having an annular shaped recess in the underside thereof, a thermionic cathode at the same potential as said focusing electrode disposed within said recess, a positively charged accelerating electrode at the same potential as said crucible closely spaced below said cathode and having an annular opening therethrough aligned coincident with said cathode to produce a hollow, conelike electron beam converging downwardly and focusing at the top of said crucible, said electron beam melting said material thereby producing copious quantities of gases which are ionized by said beam, and a plurality of negatively charged suppressor elements separately spaced about the circumference of and above said annular opening between said opening and said cathode to direct the path of those electrons having a directional component along the longitudinal axis of said cathode to a more nearly vertical direction, whereby substantially all electrons passing through said annular opening are retained within said conical beam.

2. An electron-beam furnace for casting molten materials under a high vacuum comprising an open-top crucible containing said material, means providing a magnetic field having flux lines converging into the top of. said crucible, an annular thermionic cathode vertically aligned above said crucible, a negatively-charged focusing electrode closely spaced above said cathode and at the same potential, a positively-charged accelerating electrode at the same potential as said crucible closely spaced below said cathode and having an annular opening therethrough aligned coincident with said cathode to produce a hollow, conelike electron beam converging downwardly and focusing on the material within said crucible with the sides of the cone being substantially parallel to said flux lines, said electron beam melting said material thereby producing copious quantities of gases which are ionized by said beam, and a plurality of suppressor elements at said focusing electrode potential circumferentially spaced transverse to said annular opening between said cathode and accelerating electrode to direct the paths of those electrons having a directional component along the longitudinal axis of said cathode downwardly and substantially parallel to said converging flux lines, whereby substantially all electrons passing through said annular opening are accelerated in a direction substantially parallel to said flux lines so that said magnetic field is highly effective in retaining said electrons Within the confines of said converging beam and counteracts the space effect between electrons and the distortion effect created by said ionized gases.

3. An electron-beam furnace for casting molten materials under a high vacuum comprising an open-top crucible containing said material, means providing a magnetic field havin flux lines converging into the top of said crucible, a negatively-charged focusing electrode vertically aligned above said crucible and having an annular recess in the underside thereof, a thermionic cathode at the same potential as said focusing electrode disposed within said recess, a positively-charged accelerating electrode at the same potential as said crucible closely spaced below said cathode and having an annular opening therethrough aligned coincident with said cathode to produce a hollow, cone-like electron beam converging downwardly and focusing on the material within said crucible with the sides of the cone being substantially parallel to said flux lines, said electron beam melting said material thereby producing copious quantities of gases which are ionized by said beam, and a plurality of suppressor plates at said focusing electrode potential separately spaced about the periphery of said recess and transverse thereto between said cathode and accelerating electrode to divert the path of those electrodes having a directional component along the longitudinal axis of said cathode downwardly and substantially parallel to said converging flux lines, whereby substantially all electrons passing through said annular openings are accelerated in a direction substantially parallel to said converging flux lines so that said magnetic field is highly effective in retaining said electrons within the confines of said converging beam and counteracts the space effect between electrons and the distortion effect of said ionized gases.

4. An electron beam furnace for casting molten materials under a high vacum comprising an open-top crucible containing said material, means providing a magnetic field having a vertical axis of symmetry and flux lines converging into the top of said crucible, a negatively-charged focusing electrode vertically aligned above said crucible and having an annular recess in the underside thereof, a thermionic cathode at the same potential as said focusing electrode disposed within said recess, a positively-charged accelerating electrode at the same potential as said crucible closely spaced below said cathode and having an annular opening therethrough aligned coincident with said cathode to produce a hollow, cone-like electron beam converging downwardly and focusing on the material within said crucible with the sides of the cone being substantially parallel to said flux lines, said electron beam melting said material thereby producing copious quantities of gases whch are ionized by said beam, and suppressor plates attaching to the lower face of said focusing electrode and separately spaced about the periphery of and transverse to said recess between said cathode and said accelerating electrode to divert the path of those electrons having a directional component along the longitudinal axis of said cathode downwardly and substantially parallel to said converging flux lines, whereby substantially all electrons passing through said annular opening are accelerated in a direction substantially parallel to said flux lines so that said magnetic field is highly effective in retaining said electrons within the confines of said converging beam and counteracts the space effect between electrons and the distortion effect of said ionized gases.

5. An electron-beam furnace operating under highvacuum conditions for casting molten material containing matter which evolves as a gas at the melting point of said material, comprising an anode in the form of an open-top crucible containing said gas-containing material, an annular electromagnetic winding extending coaxially around said crucible and energized by direct current to produce a magnetic field having a vertical axis of symmetry and flux lines converging into the top of said crucible, a negatively-charged focusing electrode spaced vertically above said anode at a distance substantially greater than the diameter of said crucible and having an annular recess in the underside thereof substantially coaxial to the vertical axis of said crucible, a thermionic cathode at the same potential as said focusing electrode and disposed within said recess, an accelerating electrode at the anode potential closely spaced below said focusing electrode and having an annular slot therethrough aligned coincident with the peripheral axis of said annular recess, said focusing electrode, cathode, and accelerating electrode being shaped and aligned to direct the electrons emitted by said cathode into a hollow, cone-like electron beam directed downward and converging onto the top of the material within said crucible so that the sides of said conical beam are substantially parallel to said converging magnetic field, said electron beam melting said material thereby providing copious quantities of gases which are ionized to 'form copious quantities of ionic plasma in the space above said anode, and a plurality of suppressor plates attaching the lower face of said focusing electrode and separately spaced around the periphery of and transverse to said annular recess to lie between said cathode and said accelerating electrode, thereby diverting the path of those electrons having a directional component along the longitudinal axis of said cathode to a more nearly vertical direction to be substantially parallel to said flux lines, whereby substantially all the electrons passing through said slot to form said conical beam are accelerated in a direction substantially parallel to said flux lines so that said magnetic field is highly effective in retaining said electrons within the confines of said conical beam against th scattering effect of the electron space charge and the distortion of the electric field created by said ionic plasma.

References Cited in the file of this patent UNITED STATES PATENTS 2,321,886 Anderson June 15, 1943 2,423,729 Ruhle July 8, 1947 2,801,361 Pierce July 30, 1957 2,817,033 Brewer Dec. 17, 1957 2,880,483 Hanks et al Apr. 7, 1959 2,888,605 Brewer May 26, 1959 2,921,214 Broad Jan. '12, 1960 2,934,666 Shrader Apr. 26, 1960

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