US3409729A

US3409729A - Electron beam furnace and method for heating a target therein

Info

Publication number: US3409729A
Application number: US602316A
Authority: US
Inventors: Charles W Hanks; Charles D A Hunt
Original assignee: Air Reduction Co Inc
Current assignee: Airco Inc
Priority date: 1966-12-16
Filing date: 1966-12-16
Publication date: 1968-11-05
Anticipated expiration: 1985-11-05

Description

Nov. 5, 1968 c. w. HANKS ET AL ELECTRON BEAM FURNACE AND METHOD FOR HEATING A TARGET THEREIN Filed Dec. 16, 1966 8 mi: NN wmw m. W.A v 6 A R CMW FIE--2- o M: 3 409 729 United States Patent ice 5,

in the wall between the two vacuum chambers to reach 3,409,729 the target.

ELECTRON BEAM FURNACE AND METHOD FOR HEATING A TARGET THEREIN Charles W. Hanks and Charles dA. Hunt, Orinda, Calif.,

assignors, by mesne assignments, to Air Reduction Company, Incorporated, a corporation of New York Filed Dec. 16, 1966, Ser. No. 602,316 9 Claims. (Cl. 13-31) This invention relates to electron beam furnaces for the vacuum processing of various materials. More particularly, the invention relates to an improved electron beam furnace for heating a target for such purposes as melting, vaporizing, or alloying the target material. The invention also relates to a method for heating such a target in an electron beam furnace by means of a novel arrangement for deflecting the electron beams.

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 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 upon a substrate.

Electron beam furnaces utilize one or more electron guns for producing high energy electron beams. These beams are then directed in some manner to a target for heating the same. Electron beam guns generally comprise an electron source or emitter for emitting electrons and suitable means for generating a magnetic field for accelerating and focusing the electrons into a beam.

The generation of an electron beam and the heating of the target in an electron beam furnace is carried on in a region of high vacuum and, accordingly, the target and the electron beam gnn are disposed within a suitable vacuum chamber maintained at a high vacuum. The gun is generally disposed at a convenient location within the vacuum chamber and the electron emitter, which may be an emissive cathode, is energized and heated to emit electrons. The electrons are formed into a beam and accelerated along an initial path by a shaping electrode and a suitable accelerating anode. Magnetic fields are utilized to guide the electron beam onto the surface of the target.

In an electron beam furnace, the heating, melting, or vaporization of many materials produces a large volume of gaseous impurities and vapor particles including ionized and uncharged atoms and molecules in the target area. If the electron gun is positioned in a region of the vacuum chamber where it is exposed to large quantities of the vapor particles and gases, generation of the electron beam may be impaired. For example, wide fluctuation in pressure reduces the efiiciency of electron beam production and makes accurate control difficult. Furthermore, if vapor particles strike the emitter of the gun, shorting and erosion of the emitter may occur. In order to avoid this problem, electron beam guns are sometimes located in a position where exposure of the guns to vapor particles and pressure variation is minimized. One of the ways in which this may be done is to locate the electron beam guns in a separate vacuum chamber provided with a separate pumping system which maintains a relatively even and high vacuum in such chamber. The electron beam produced by the gun is directed through an opening Some vapor particles, including ionized and uncharged atoms and molecules, and some stray electrons will usually be present at the opening between the chambers, and interaction between the electron beam and these items may cause a problem. Negatively charged ions, and stray electrons produced upon ionization of uncharged atoms and molecules by the electron beam, may tend to remain in the electron beam and cause a space charge at the opening which is large enough to hinder direction and focusing of the electron beam through the opening. This problem is further compounded by the desire to minimize the size of the opening and prevent vapor particles from reaching the electron beam gun. Further, during operationof the electron beam furnace, the opening may clog from condensation of vapor and thus become even smaller than its original size.

It is an object of this invention to provide an improved electron beam furnace for heating a target.

Another object of the invention is to provide an improved method for heating a target in an electron beam furnace.

Still another object of the invention is to provide an electron beam furnace wherein the electron beam guns have long operating life and wherein close regulation of power levels and electron beam direction and focus is possible.

A further object is to provide an electron beam furnace and heating method in which the electron gun is positioned in a separate chamber from the target and wherein the tendency for the electron beam to trap stray negatively charged ions and electrons is minimized.

Other objects of the invention and the various advantages thereof will become apparent to those skilled in the art from the following description taken in connection with the accompanying drawing wherein:

FIGURE 1 is a schematic elevational view, with part broken away, of an electron beam furnace constructed in accordance with the invention; and

FIGURE 2 is an enlarged sectional view of the electron beam gun included in the furnace, taken along the line 2-2 of FIGURE 1.

.The method of the invention is for heating a target in an electron beam furnace which has a first vacuum chamber 11 containing a target 10, a second vacuum chamber 12 of higher vacuum and an opening 13 communicating between the first and second vacuum chambers. The method comprises producing an electron beam 14 in the second vacuum chamber at a position offset from the opening. The beam of electrons is deflected and focused in the second vacuum chamber to pass through the opening and into the first vacuum chamber. After passing through the opening, the beam is deflected and focused in the first vacuum chamber to impinge upon the target in a desired manner.

In the electron beam furnace of the invention, a first vacuum chamber 11 contains the target 10 and an opening 13 connects the first vacuum chamber with a second vacuum chamber 12 of higher vacuum. The furnace includes at least one electron beam gun 16 having an emitter 31 positioned in the second vacuum chamber such that it is offset from the opening. A magnetic deflecting and focusing device 17 is positioned in the second vacuum chamber for deflecting and focusing the beam 14 of electrons emanating from the emitter so that it passes through the opening into the first vacuum chamber. A further magnetic deflecting and fousing device 18 is positioned in the first vacuum chamber and operates to deflect and focus the beam of electrons in a desired manner against the target.

Referring now more particularly to the drawing, the details of the method of the invention will be described. The method of the invention is practised in connection with an electron beam furnace including an enclosed tank having walls 21, partially shown. The tank is divided into two vacuum chambers 11 and 12 (both partially shown) by a transverse Wall 22. The target to be heated is contained in the first vacuum chamber 11. The vacuum tank may be a cylinder, or may be rectangular or of other suitable shape. The vacuum chamber 11 is evacuated by a suitable vacuum pump, not shown, to a vacuum which is preferably less than 10 torr.

The target 10 is positioned in the vacuum chamber 11 and, in the drawings, is shown as a crucible 23 of copper or similar material which is provided with a plurality of coolant passages 24 therein. A coolant such as water may be circulated through the coolant passages to maintain the crucible Well below its melting temperature. In the drawings, the melting of a material 27 is illustrated. Such material may be metallic or may be a ceramic or plastic substance. Because the crucible 23 is cooled and because heat is applied to the central portion of the upper surface of the target, a skull 26 of the solid target material will form adjacent the crucible surface. This skull enables a high degree of purity to be obtained in the molten material in the crucible due to the fact that reaction between the molten material and the crucible material is avoided. Of course, the target configuration and material may be other than that shown,and the heating may be for purposes other than melting.

The molten material 27 in the crucible 23 is, as will be subsequently described, heated by electron beams. In such an operation, which is carried out in substantial vacuum, the pressure in the vacuum chamber 11 may vary considerably due to the vaporization of certain impurities in the molten material and due to vaporization of the molten material itself. Because of the continuing nature of the heating operation, the resulting vapor particles, some of which may be ionized, will be'always present despite continuous operation of the vacuum pump. The number of vapor particles present will vary depending upon such factors as the constitution of the molten material itself and the power of the electron beam.

In order to minimize the effect of pressure variations and to avoid exposure to large quantities of vapor particles, the electron beam 14 is produced at a location which is less affected by pressure variation and vapor particles. In the invention, the electron beam is produced in a separate vacuum chamber 12. The vacuum chamber 12 is defined by a portion of the vacuum tank and by the wall 22. A separate vacuum pumping system, not illustrated, evacuates the vacuum chamber 12 to maintain a high vacuum, preferably less than 10* torr. a

As will be explained subsequently, the electron beam is directed into the vacuum chamber 11 and against the target 10 through the opening 13 in the wall 22. The opening 13 and the target 10 are maintained in relative positions such that the opening is out of a line of sight from the molten material 27. Because there is no direct line between the surface of the molten material and the opening, the number of vapor particles passing through the opening and into the vacuum chamber 12 is substantially reduced.

Although the opening 13 is out of the line of sight with the surface of the molten material 27, the higher pressure in the vacuum chamber 11, coupled with the fact that there are numerous vapor particles dispersed throughout the chamber, will cause vapor particles to flow through the opening 13 into the vacuum chamber 12. As mentioned before, the more the vapor particles which reach the electron gun, the greater the detrimental effect on electron beam generation. For example, when vapor particles 4 strike the emitter of an electron gun, shorting or erosion may result in a reduction in the life of the gun.

In order to avoid the vapor particles passing through the opening 13, the electron beam 14 is produced at a region displaced from alignment with the axis of the opening 13. The beam indicated by the dash-dot lines in the drawing is shown to be of the ribbon type. However, it is to be understood that other types of electron beam configurations may also be utilized. The ribbon beam produced is deflected in the high vacuum chamber 12 through a generally arcuate path with constant or varying radius. The magnetic fields used for deflection are also designed to focus the beam into a spot of minimum cross sectional area. This helps to pass the maximum number of electrons through the opening. In the illustrated embodiment, the deflection of the beam is about degrees, however, it is to be understood that greater or lesser deflection may also be used in accordance with the invention.

After deflection and focusing, the electron beam 14 is passed through the opening 13 into the vacuum chamber 11. There, the electron beam is once again deflected and focused onto the surface of the molten material 27. This deflection may be controlled by an operator observing the process visually through suitable view ports (not illustrated) such as are common in the art.

It has been found that transverse deflection of the electron beam in the manner described reduces interaction of the beam with vapor particles and stray electrons, making it easier to direct and focus the electron beam through the opening 13. It is "believed that this results from the fact that stray electrons are not readily trapped in the electron beam to build up the space charge, but rather tend to more easily pass to other regions of the vacuum chamber 12. This is of considerable advantage where the vapor particle concentration is high, as around the opening 13, and thus where the production of stray electrons and negative ions is high. The source of the electron beam may thereby be virtually isolated from the target 10 and yet the desired direction and focus of the electron beam as it strikes the target are easily obtained by transversely deflecting and focusing the beam in the vacuum chamber 11.

The method of the invention may be more fully understood when considered in connection with the apparatus of the invention, namely, the improved electron beam furnace. The general configuration of the walls of the furnace 21 and the dividing wall 22 have been described above. The electron gun 16 for producing the electron beam may be of any suitable type, however, the particular type of electron gun illustrated has been found especial ly advantageous in the invention. The electron beam gun 16 comprises two (although only one may be used) elongated emitters or filaments 31 each suitably supported by a pair of conductors 35. The emitters 31 are disposed in a pair of recesses 32 in an electron beam former 33. The electron beam former 33, and particularly the recesses 32 therein, are shaped to focus and direct the beam of electrons produced by the gun and to maximize its density. The emitters 31, the beam former 33 and the recesses 32 therein are shaped to produce two closely spaced ribbon beams which converge into a single beam at a focal point 34 located in the deflecting magnetic field subsequently described. This convergence allows both of the beams produced by the gun to form a strong composite beam for passing through the aperture 13 in the pressure barrier 22. If desired, a plurality of electron beam guns may be disposed in the vacuum chamber 12 and deflecting means provided for directing the electron beams through a corresponding plurality of openings arrayed about the target. Such an arrangement provides very high energy transfer to the target. In the drawings, however, only a single gun and associated deflecting means are shown for the purpose of clarity.

Heating current and accelerating potentials are applied to the various electrodes of the gun 16 through suitable electrical connections, not shown. The electron beam gun 16 is preferably mounted at a slight angle which will com pensate for any magnetic deflection of the electrons by the magnetic field created around the emitters 31 by the heating current (preferably direct current). The beam former 33 is mounted to a gun mounting plate 36, which is at ground potential, by means of an insulated support 37. The three accelerating electrodes 38 are mounted upon two suitable support and insulator bars 39, and are maintained at suflicient positive potential to produce the desired electron beam strength. The insulator bars 39 are attached to the mounting plate 36. As shown in FIGURE 2, the electron beams produced by the emitters 31 pass between the elongated accelerating electrodes 38 to the focal point 34. The mounting plate 36 supports the gun 16 and is suitably mounted, by means not illustrated, in the vacuum chamber 12.

The first magnetic deflecting device 17 preferably comprises a pair of pole pieces 41 coupled at either end of a coil 42. Energization of the coil 42 causes the pole pieces 41 to produce a magnetic field. This magnetic field is shaped by suitably positioning and shaping the pole pieces 41 in order to produce the desired deflection and focusing characteristics. As shown in the drawing, the deflecting device 17 operates to deflect the composite electron beam 14 through about 90 in a generally arcuate path, After being deflected, the electron beam is passing in the direction of the axis of the opening 13, and is aligned with the opening. Vapor particles passing through the opening, however, do not readily strike the emitter of the electron beam gun 16 because of the position of the gun. Furthermore, the deflecting of the electron beam as described markedly reduces the tendency for space charge buildup at the opening 13.

It will be noted that the emitters of the electron gun 16 are oriented such that they lie approximately in (actually, on opposite sides of) the plane containing the arcuate path of the electron beam 14 as it is deflected by the magnetic deflecting device 17. It has been found that thisparticular orientation of the emitters of the electron gun provides an economical construction, excellent control of the electron beam, and high electron density and power in the beam. In addition, this particular orientation affords certain advantages in deflection of the beam.

The deflecting device 17 is also designed to minimize the cross sectional area of the beam 14 as the beam passes through the opening 13. Naturally, operating considerations may make it preferable to not reduce the beam to its smallest possible cross section. Thus, the term minimum cross sectional area, as used herein, is meant to refer to the smallest obtainable beam consistent with other operating factors. In order to determine the cross section of the beam as it passes through the opening 13, metal plates or carbon plates may be placed surrounding the periphery of the opening. Such plates will burn through in the event of excessive divergence or inaccurate deflection of the beam.

The deflecting device 18, which is located in the vacuum chamber 11, is shown as being comprised of two

separate deflection electromagnets

43 and 44. The

electromagnets

43 and 44 may be of similar construction to that of the deflection device 17. Such devices are oriented to produce magnetic fields at generally right angles to each other and therefore provide for deflection of the beam in two mutually perpendicular directions. Thus the direction of the beam 14 can be controlled by varying the strength of the two deflecting fields to impinge upon any spot over the entrie surface of the molten material 27. The electromagnet 43, as shown in FIGURE 1, is for deflecting the beam in a direction perpendicular with the plane of the paper, whereas the electromagnet 44 is for deflecting the beam in a direction parallel with the plane of the paper. Control of the beam may be exercised by an operator observing the melting operation by providing suitable electrical controls, not illustrated, for the electromagnet power supply.

The invention provides a method and furnace construction wherein the number of vapor particles striking the emitter of the electron gun or guns is minimized. This effects a substantial increase in the gun operating life. Satisfactory control over the electron beam as to its density, strength and direction is facilitated.

Various other embodiments and modifications in addition to those shown and described herein will be apparent to those skilled in the art from the foregoing description and accompanying drawings. Such other embodiments and modifications are intended to fall within the scope of the appended claims.

What is claimed is:

1. A method for heating a target in an electron beam furnace having a first vacuum chamber for containing the target, a second vacuum chamber adjacent said first vacuum chamber, and an opening communicating between the first and second vacuum chambers, said method comprising producing an electron beam in the second vacuum chamber at a position offset from said opening, deflecting and focusing the beam of electrons in the second vacuum chamber so as to cause said beam to pass through the opening and into the first vacuum chamber, and deflecting and focusing the beam of electrons in the first vacuum chamber so as to direct the electron beam against the target.

2. A method in accordance with claim 1 wherein the electron beam is focused in the second vacuum chamber to a reduced cross sectional area prior to passage through the opening.

3. A method in accordance with claim 1 wherein the electron beam produced is of the ribbon type lying generally in a plane and wherein the deflection of the beam in the second chamber causes the beam to follow a generally arcuate path which lies approximately in the plane of the ribbon beam.

4. An electron beam furnace for heating a target, including in combination, a first vacuum chamber for containing the target, a second vacuum chamber adjacent said first vacuum chamber, an electron beam gun including an emitter positioned in said second vacuum chamber, an opening communicating between said first and second vacuum chambers, said emitter being positioned to be olfset from said opening, first magnetic deflecting and focusing means positioned in said second vacuum chamber for deflecting and focusing a beam of electrons emanating from said emitter through said opening and into said first vacuum chamber, and second magnetic deflecting means positioned in said first vacuum chamber for deflecting the beam of electrons so as to direct the electron beam against the target.

5. An electron beam furnace in accordance with claim 4 wherein said opening is positioned to be out of a line of sight of the heated surface of the target.

6. An electron beam furnace in accordance with claim 4 wherein said second magnetic deflecting means include means for deflecting the beam of electrons in two mutually perpendicular directions to thereby enable deflection of the beam to any point on the target.

7. An electron beam furnace in accordance with claim 4 wherein said first magnetic deflecting means operate to focus the electron beam to a reduced cross sectional area prior to passage through said opening.

8. An electron beam furnace in accordance with claim 4 wherein said emitter comprises an elongated cathode producing a ribbon-type beam, and wherein said first magnetic deflecting means cause the electron beam to follow a generally arcuate path which is coplanar With said cathode.

9. An electron beam furnace for heating a target, including in combination, a furnace housing defining a vacuum tank, a wall dividing said vacuum tank into first and second vacuum chambers, said first vacuum chamber being adapted to contain the target, said wall having an opening therein communicating between said first and sec- 0nd vacuum chambers, said opening being positioned to be out of a line of sight of the heated surface of the target, an electron beam gun positioned in said second vacuum chamber, said electron beam gun having a pair of adjacent elongated emitters displaced a substantial distance from the axis of said opening and extending generally parallel therewith, first magnetic deflecting means positioned in said second vacuum chamber for deflecting the beam of electrons emanating from said emitters through a generally arcuate path which is approximately coplanar with said adjacent emitters and through said opening into said first vacuum chamber, and second magnetic deflecting means positioned in said first vacuum chamber for 15 deflecting the beam of electrons in a desired manner against the target.

- References Cited UNITED STATES PATENTS 2/1968 Anderson 21'9 121 BERNARD A. GILHEANY, Primary Examiner. H. B. GILSON, Assistant Examiner.