US3592955A

US3592955A - Electron beam furnace

Info

Publication number: US3592955A
Application number: US860497A
Authority: US
Inventors: Charles W Hanks
Original assignee: Air Reduction Co Inc
Current assignee: Airco Inc
Priority date: 1969-09-24
Filing date: 1969-09-24
Publication date: 1971-07-13
Anticipated expiration: 1988-07-13

Abstract

An electron beam furnace is described in which at least one electron beam is directed through a curving path to a target. The furnace employs a pair of solenoidal coils positioned to establish a substantially uniform magnetic field in the path of the electron beam, such field having generally straight lines of flux extending transversely of the electron beam path.

Description

United States Patent (72] Inventor Charles W. Hanks [56] References Cited Orinda, Calil. UNITED STATES PATENTS H pp N9 860,497 3,268,648 8/1966 Dietrich 13/31 1 PM 3,390,249 6/1968 Hanks .7 13/31 x [45] Patented July 13, I971 I [7 3) Assignee Air Reduction Company, Incorporated Primary Exammer-semard Gllheani New Yo'rk, y Assistant Examiner-Roy N. Envall, Jr.

Attorney-Anderson, Luedeka, Fitch, Even & Tabin ABSTRACT: An electron beam furnace is described in which [54] 5 FERNACE at least one electron beam is directed through a curving path mg to a target. The furnace employs a pair of solenoidal coils posi- [52] U.S.Cl. 13/31 tioned to establish a substantially uniform magnetic field in [51] Int. Cl "05b 7/00 the path of the electron beam, such field having generally [50] Field of Search 13/31; straight lines of flux extending transversely of the electron beam path.

PATENTED JUL 13 I97! SHEEI 1 [IF 4 INVENTOR CHARLES H. BANKS Adm/smith! m;

ATE'NTEU JULUIQYI 3,592,955

CHARLES W. BANKS PATE-NTEI] JUL 1 3mm 3,592 955 SHEET u 0F 4 F IG.7

m K /A04 INVINTOR OHARLEQ W. BANKS ELECTRON BEAM FURNACE This invention relates generally to electron beam furnaces and,more particularly, to an improved electron beam furnace in which efficient use is made of space and power.

Electron beam furnaces of a variety of designs are useful in the processing of many metals, alloys or other materials, for example where high standards of purity are to be achieved by outgassing or by avoiding reaction with oxygen and nitrogen, or where a substrate is to be coated by vaporization and condensation of the material. Electron beams are a particularly useful form of heating in that it is possible to inject heat into a melt locally. Electron beam furnaces typically include an evacuated enclosure, one or more electron beam guns with associated deflection systems for directing and focusing the beams, and a container for the molten material being processed.

Depending upon the particular type of processing being carried out, the container for the molten material may take a variety of forms. In a situation where it is desired to evaporate the material in the container and subsequently condense the material on a suitably supported substrate to coat the substrate, a typical container consists of an open-topped upright crucible. Electron beam heating enables the crucible itself to be cooled and thereby form a skull of solidified molten materia] between the crucible and the molten material. This protects the purity of the molten material and makes it unnecessary to use high-temperature refractories for the crucible construction.

Another type of processing is the purification of metals and alloys by passing the molten metal or alloy over a shallow hearth. Exposure to the vacuum with coincident electron beam heating of the surface causes many volatile impurities and occluded gases to be drawn off of the molten material and thereby produces a greatly purified product. Other forms of containers which may be utilized include tundishes, launders, and ladles for transferring molten material between various points. Electron beam heating may be utilized to maintain the material in a molten condition while in such containers.

During the processing of molten material in an electron beam furnace, vaporized material may present ionization problems or may coat the various parts of the electron beam gun, impairing its operation. Moreover, spalling of condensed materials from cool surfaces of the vacuum enclosure, and splashing and splattering of the molten material from the crucible, may also impair operation of the electron beam gun. By positioning the electron beam gun underneath the container of the molten material and by deflecting the electron beam through a curving path of 180 or more, contamination and shorting of the electron beam gun is minimized.

In electron beam furnaces in which large target surfaces are to be heated, it is often desirable to sweep the beam in two or more directions in order to minimize the number of electron beam guns required. In order to achieve the desired electron beam sweeping while deflecting the beam through more than 180, transverse magnetic fields which are variable in strength have been utilized. One desirable way of producing an x and y axis sweep pattern is to utilize at least two transverse fields having their flux lines oriented 90 with respect to each other and the electron beam path. If the furnace enclosure is relatively large, this may present nosignificant problem. However, if space is at a premium, the positioning of means for establishing transverse magnetic fields closely adjacent each other may be extremely difiicult due to mutual interference and distortion of the fields. For the same reason, the heating of large surface ares in electron beam furnaces by employing a large number of electron beam guns with separate deflecting fields for each of the beams is also difficult.

Typically, a magnetic field established between pole pieces has a portion of generally uniform strength near its center, and exhibits considerable variation toward its edges. For maximum conservation of space and good control over the beam position, it is desirable to maximize the size of the uniform portion of the field for a given pole piece spacing. This is because nonuniform portions of a field make beam control difficult, and sometimes require compensation by appropriately varying the overall strength of the magnetic field. Previously known systems have experienced some difficulty in achieving fields with large uniform portions unless the entire field was made large by using large pole pieces and wide spacing therebetween. Not only does this require more furnace space, but larger pole pieces and pole piece spacing require correspondingly greater power for a given field strength.

Accordingly, it is an object of this invention to provide an improved electron beam furnace in which efficient use is made of space and power.

Another object of the invention is to provide an electron beam furnace in which orthogonal control over the deflection of electron beams is provided by means of transverse magnetic fields and in which volume requirements are minimized.

Another object of the invention is to provide an electron beam furnace of minimal complexity in which a plurality of electron beans aredirected and controlled through a curving path to a target.

It is another object of the invention to provide an electron beam furnace which is particularly useful for heating large areas of molten material.

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

FIG. 1 is a perspective schematic view of part of an electron beam furnace constructed in accordance with the invention;

FIG. 2 is a plan view illustrating one of the magnetic fields produced by a deflection system used in the furnace of FIG. 1;

FIG. 3 is a schematic elevational view of the furnace of FIG. 1.

FIG. 4 is a schematic elevational view illustrating a modification of the furnace of FIG. 1;

FIG. 5 is a further schematic elevational view illustrating another modification of the furnace of FIG. 1;

FIG. 6 is a schematic elevational view illustrating a further embodiment of the invention;

FIG. 7 is a top view of the embodiment of FIG. 6; and

FIG. 8 is a sectional view taken along the line 8-8 of FIG. 7.

Very generally, the electron beam furnace of the invention includes a pair of solenoidal coils l1 and 12 positioned substantially parallel with each other on opposite sides of the electron beam path. The coils are of an effective length which is at least 1.2 times the distance separating them. Means 13 are provided for energiZing the coils to produce a substantially uniform magnetic field having generally straight lines of flux extending tranversely of the electron beam path.

Referring now to FIG. 1, one embodiment of the invention is illustrated. The electron beam furnace includes an evacuated enclosure, not shown. The molten material target I4 is contained in an elongated container 16 which is cooled by circulating coolant in passages (FIG. 3) to form a layer or skull 16b (FIG. 3) of solidified material between the molten material and the container walls. The container 16 is illustrated as a hearth into which molten metal flows from a launder 15 at one end. The contents of the hearth are discharged at the other end through a launder 20, and the level of molten metal in the hearth, indicated by the dotted line 140, may be controlled by a weir, not shown. Other means for placing molten material in the hearth and removing it therefrom may include such things as tundishes, siphons, or ladles. Between entry and exit, the material flows slowly along the hearth and thereby has a very high exposure rate to the vacuum environment in which. the illustrated apparatus is disposed.

The hearth type of arrangement illustrated provides a large surface area of molten material with shallow depth for long times of exposure of the molten material to the vacuum. Such an arrangement is particularly useful in the purification of many types of steel and nickel base alloys as well as most of the refractory metals, such as columbium, tantalum, titanium, zirconium and others. Experiments have shown that many purification reactions that involve differential vaporization phenomena or other types of outgassing require residence times of many tens of seconds with the molten surface exposed to very low pressures. ln such instances the illustrated configuration, that of a long linear hearth with the molten material flowing slowly along it, is particularly advantageous. Electron beams are utilized to prevent solidification of the material on the hearth as the material flows along the hearth.

In the illustrated apparatus, a plurality of electron beams are utilized to maintain heating of the target molten material 14 in the hearth 16. The electron beams are produced by a plurality of

electron beam guns

17, 18, 19 and 21 distributed along and underneath the hearth 16 at spaced intervals.

The

electron beam guns

17, 18, 19 and 21 may be of any suitable type. A preferred form, however, is that shown and described in copending U.S. application, Ser. No. 660,024, now U.S. Pat. No. 2,514,656, assigned to the assignee of the present invention. The details of such a gun are shown in F IG. 3 of the present application in connection with the gun 17. It is to be understood that the other guns are of identical construction.

Referring now to FIG. 3, the electron beam gun 17 includes an elongated emitter 22 for producing electrons. The emitter is preferably a tungsten wire and extends between the supporting members 23 and 24. Means, not illustrated, provide a direct current potential across the members 23 and 24, resulting in a flow of direct current through the emitter 22. The current'flow raises the temperature of the emitter causing it to produce free electrons.

The free electrons produced by the emitter 22 are reflected on three sides by a shaping electrode 26. The electrode 26 is insulated from the emitter support members 23 and 24 by insulating

strips

27 and 28, respectively. The shaping electrode 26 is formed with an elongated recess 29 through which the emitter 22 extends. When the shaping electrode is maintained at the emitter potential, by suitable connection not illustrated, the electrons produced by the emitter 22 tend to move out of the open end of the recess 29 and away from the shaping electrode 26.

The electrons leaving the recess 29 in the shaping electrode 26 are accelerated into a beam by an accelerating electrode 31 and pass through an opening 32 therein. The accelerating electrode 31 consists of a'plate with two right angle extensions 33 and 34 thereon which are attached to suitable mounting means, not illustrated. The plate 31 is maintained at a potential which is substantially more positive than the potential of the emitter and the backing electrode to produce an acceleration of the electrons. The result is a ribbon beam, that is, an electron beam having an elongated cross section which is ideally a narrow rectangle but which approximates a narrow oval. The beam has a major axis plane which extends through the emitter.

The electrons in the beam leave the emitter 22 at an acute angle in the major axis plane. The axis of the beam is indicated by the dash-dot line 36, which represents the center of the ribbon beam. A nonnormal orientation of the initial electron path with respect to the emitter 22 is caused by the high intensity circumferential field produced by DC heating current passing through the emitter. After leaving the anode opening 32, the electron beam 36 is deflected about 90 through a curved path by means of a transverse magnetic field. The transverse magnetic field is established in the initial path of the beam between a pair of elongated bar-shaped pole pieces 27, only one of which is illustrated. The pole pieces extend generally parallel with the emitter 22 and each other and are positioned on either side of the beam 36 parallel with its major axis plane. A magnet 38 extends between the upper ends of the pole pieces 37, and a magnet 39 extends between the lower ends of the pole pieces 37. The two magnets are identically oriented with regard to their polarities, and are electromagnets connected to a suitable control circuit and power supply, not shown. The polarities are established with field lines running perpendicularly out of the plane of the paper as illustrated in the drawing, thereby causing an upward deflection of the electron beam 36 as illustrated. The effect of the field on the electron beam also produces a convergence of the opposite edges of the beam toward each other in the plane of the curving path due to a longer path length of electrons toward the lower edge of the beam in the magnetic field established by the pole pieces 37. The details of the deflecting and focusing of the beam are set out more fully in the aforesaid patent application.

The present invention makes use of a substantially uniform magnetic field having generally straight lines offlux extending transversely of the electron beam path in order to produce deflection of the electron beam. Referring to FIG. 2, the manner in which such a field is established may be more clearly observed. The solenoidal coils 11 and 12 are positioned substantially parallel with each other and are of a length which is at least 1.2 times the distance between the coils. Preferably, the distance between the coils is equal to or greater than one third of their effective length. The distance between the coils is indicated in FIG. 2 as the dimension w and the effective length of the coils is indicated by the dimension 1 As mentioned before, the l to w ratio preferably is equal to or less than 3 to 1. It should not be less than L2 to 1 for satisfactory uniformity.

Each of the coils is comprised of one or more helical windings on a ferromagnetic core and may be of a construction similar to those types of coils utilized in solenoids. The coil 11 is provided with a ferromagnetic core 41 and the coil 12 is provided with a ferromagnetic core 42. A pair of pole pieces 43 (see FIG. 1) and 44 join the corresponding ends of the solenoidal coils, respectively. The pole pieces 43 and 44 are of ferromagnetic material and engage the corresponding ends of the cores of the coils to form, with the cores, a closed low reluctance loop. ln order to provide energizing current to the coils 11 and 12, the energizing means or energizing control circuit 13 is suitably connected thereto.

The coils 11 and 12 are energized by passing direct current therethrough from the control circuit 13. The direct current is directed to give the pole pieces 43 and 44 opposite polarities, with a main magnetic field being thereby established having lines of flux directed from the pole piece 44 to the pole piece 43. The main magnetic field which is established has a pronounced rectangular configuration with lines of flux running lengthwise of the rectangular area bounded by the coils and the pole pieces and without marked changes in field intensity over nearly the entire area of the rectangle thus bounded. As illustrated in H0. 2, these flux lines are indicated by the dash-dot lines 46 within the rectangular field area. The unusually high uniformity of field structure occurs because of the leakage flux of the individual wires of the two coils 11 and 12, which reinforces the lines of the main field in appropriate regions. As is known in the art, leakage flux is produced by the current passing through each turn. In a long coil, the leakage flux from each turn merges into that of the adjacent turns, forming long lines of flux running along the coil adjacent its outer surface and parallel with the coil axis.

By using a pair of conventionally magnetized spaced pole pieces similar to the pole pieces 43 and 44 but without the solenoidal coils 11 and 12, it is usually very difficult to produce a relatively uniform magnetic field over a narrow rectangular area where the magnetic field lines are travelling the long axis of the rectangle. Typically, the field lines leaving the poles at the ends of the rectangles spread out and the field intensity drops markedly in the center area. In addition, such a system is very susceptible to excessive field distortion in the presence of adjacent ferromagnetic structures. The use of a pair of solenoidal coils on each side of the long field in accordance with the invention, produces a field strength which can be maintained uniform to about i 10 percent. The leakage flux of the coils resupplies the field strength over the rectangular area and the system can be placed in quite close proximity to adjacent magnetic structures without detrimental effect With an I to w ratio of greater than I to l. the field becomes so narrow as to pennit very little variation in the placement of the beam as injected into the field. Moreover, as a practical matter, the excessive length of such a field would be wasted. At the other end. an I to w ratio of L2 to l is about as low as it is practical to go without encountering unsatisfactory field uniformity.

it may be seen from FlGS. l and 3 that the solenoidal coils ll and I2 are disposed about even with the top level of the hearth 16. This disposes the hearth and region immediately above the hearth in a magnetic field having the desired rectangular configuration as above described. Although field strength diminishes with the distance above the hearth, sufficient strength and uniformity are achievable with proper spacing of the pole pieces and adequate current in the coils. This may be determined empirically and depends on the configuration of the particular system. The fact that the contents of the hearth may be ferromagnetic material (the hearth itself is typically not) does not significantly affect the field warping during operation because the temperature of the hearth contents typically exceeds the Curie point. Moreover, the affect of the hearth contents during startup, when such contents are cool and below the Curie point, does not seem to present a major problem in controlling the beam. The electron beams are injected into the main field above the hearth such that they remain within the main field throughout their entire curving path length, curving about 180 in the field before striking the target 14.

in H6. 3, the dash-dot line 36, representing the axis of the electron beam produced by the gun i7, is shown in three positions, 36a, 36b, and 36c. The three dash-dot lines indicate the middle position and inner and outer extreme positions to which the beam may be longitudinally swept, that is moved in the direction transversely of the long dimensions of the hearth (not longitudinally with respect to or along the hearth, but within the plane of the curving beam path). Such sweeping may be accomplished by varying the strength of the main magnetic deflecting field (the field between the pole pieces 43 and 44), but is preferably efiected by use of a trim coil as subsequently described.

In order to provide lateral beam sweep, that is, movement of the beam in the direction of the long dimension of the hearth 16, a further pair of solenoidal coils is provided for each of the electron beam guns l7, l8, l9 and 21 (see FIG. 1). As these pairs of solenoidal coils are identical for each gun, only one will be described. The electron beam produced by the gun 17 is deflected upwardly, as above described, by the pole pieces 37 (FIG. 3), and a pair of mutually parallel

solenoidal coils

48 and 49 are positioned on the opposite sides of the path of the electron beam. As was the case with the coils ll and 12, the

coils

48 and 49 have their cores joined at the corresponding ends of the coils by

pole pieces

51 and 52, respectively The energizing control circuit 13 is connected to the

coils

48 and 49 for energizing them with a suitable current. The current is such that a field similar to that illustrated in FIG. 2 is produced upon energization of the coils, such field being substantially uniform and having generally straight lines of flux extending transversely of the electron beam path. The

coils

48 and 49 are oriented, relative to the path of the electron beam, 90 from the coils 11 and 12. Accordingly, variation of the strength and direction of the field produced by the

coils

48 and 49 produces sweep of the electron beam in the direction of the long dimension of the hearth l6 (lateral sweep).

In FIG. 1, the axis of the electron beam is indicated at 36 in each of three positions denoted by the subscripts x, y and z, respectively. The lines of flux in the field established by the solenoidal coils 48 and 49 extend from the pole piece Sll to the pole piece 52. The position of highest energization of the field in one direction causes the beam to assume the position at x, the absence of energization causes the beam to assume the position at y, and the highest energization in the opposite direction causes the beam to assume the position at z. in this manner the beam may be swept laterally (longitudinally along the hearth) back and forth over a given segment of the total hearth length To do this. a wave form of convenient shape may be used, eg sinusoidal By appropriate positioning of the other guns and their corresponding deflecting systems, the entire surface area of the hearth may be covered by appropriate sweeping of the electron beams in the longitudinal and transverse directions. Moreover, guns not shown may be provided for sweeping over the launders l5 and 20 as well, to enhance flow by maintaining superheat.

in the embodiments of FIGS. 1 and 3, there is illustrated two ways in which fields established in accordance with the invention may be employed. The use of two large solenoidal coils on either side of the rectangular area to be heated makes it possible to deflect and control a plurality of electron beams with a single uniform field. This is particularly useful in processing where large areas of molten material are to be heated by electron beams, such as in the hearth purification of metals or alloys. The use of smaller coils such as the

coils

48 and 49 in accordance with the invention produces a uniform magnetic field which is particularly adapted to accurate deflection of the electron beam with simple control. in the latter situation, the beam is injected into the field and passes completely therethrough, emerging on the other side of the field. in the case of the field established by the coils 11 and 12, the electron beam curves to remain substantially in the field throughout its entire path length.

When sweeping the beam in the lateral manner, that is, along the length of the hearth, the injection angle of the beam into the magnetic field established by the coils 11 and 12 changes. The injection angle is the angle of the electron beam path with respect to the magnetic flux lines of the magnetic field. The radius of curvature of the electron beam within the magnetic field is a function of the injection angle of the beam into that field. The radius of curvature of the beam in the magnetic field may be described by the following mathematical formula:

3.37X 10- V sin 0 R where R=the radius of curvature in meters; V the voltage in volts of the electron beam potential; 9=the angle of the electron beam path with respect to the magnetic flux lines; and 3=the flux density of the magnetic field in Weber's per square meter.

The variation in the radius of curvature with injection angle, if uncorrected, causes the beam to sweep in an arc. This complicates the sweep pattern for uniform coverage of the surface area by the beam, typically necessitating the use of a complicated programmed wave shape for the sweep current supplied to the

coils

48 and 49.

in accordance with the invention, such complexity is avoided by the utilization of a solenoidal radial trim coil 53 for each of the guns l7, l8, l9 and 21, positioned adjacent the respective beam paths. in H6. 3, only one of such coils is visible. The coil 53 is appropriately energized by a suitable energizing circuit 54 (not shown in H6. 1), which may be part of the energizing control 13. As is known in the art, the magnetic field intensity adjacent a single solenoidal coil is strongest in the center and weakest near the end. By selecting a solenoidal coil 53 of proper length and by proper placement of the coil with respect to the electron beam path, the effects of the injection angle on changes in radius of curvature of the beam can be cancelled out and a linear sweep achieved. As the lateral sweep coils 48 and 49 move the electron beam across the area in front of the radial trim coil 53, the electrons enter a weaker part of the trim field as the injection angle becomes more parallel to the flux lines of the main field established by the coils. 11 and 12. Consequently, the electrons are directed into a higher path in the main field above the target, where the main field is weaker and the radius of curvature IS therefore longer.

The radial trim coils 53 may also be used to effect longitudinal sweep of the beams (across the hearth), rather than by varying the strength of the magnetic field established by the coils ll and 12. By using the trim coils 53 for longitudinal sweep, more precise adjustment of the individual beams is possible. When current in a trim coil is decreased, the electrons are directed into a higher path (e.g. 36a in FIG. 3) where the main field above the target is weaker and the radius of curvature is therefore longer. When current in a trim coil is increased, the electrons are more sharply deflected, staying lower (36c in FIG. 3) and moving through a stronger region of the main field so that their radius of curvature is shorter. A sinusoidal wave form, or a step function wave form or other convenient wave form may be applied to the trim coil to effect sweeping. By properly coordinating the current wave form applied to the trim coils with the wave form applied to the lateral sweep coils 48 and 49, the impact point of each beam may be swept over the surface in a predetermined desired pattern.

By way of example, a satisfactory operating system may be constructed for heating the surface of molten metal flowing on a hearth of about 20 inches width and 74 inches length. The

solenoidal coils Ill and 12 may be made of 2"X5 core cross section and 84 inches long wound with 43 turns of wire per inch of length. The coils may be spaced 54 inches apart center to center with the pole pieces 43 and 44 consisting of steel plates of 2"X5" cross section. The hearth I6 is positioned about 6 inches from the coil 12 and the electron guns 17 etc., placed 40 inches from the same coil and 12 inches below the plane of the coils. The solenoidal coils 48 and 49 for the gun l7 and the corresponding coils for the other guns 18, I9 and 21 may be placed in the beam path about 8 inches below the plane of the main coils. The cores for the

coils

48 and 49 may each be made of ;"Xl" cross section and 6 inches long assembled with /"

Xl steel poles

51 and 52.

With magnetic induction of 88 ampere turns per inch on the coil 12, 25 ampere turns per inch on the coil 11, and 75 ampere turns per inch on the trim coil 53, the electron beam may be directed onto the rear edge of the hearth, that is, the edge closest the coil 12. Each coil may be wound with 60 turns of wire per inch. The

coils

48 and 49 are positioned so that, when unenergized, the electron beam produced by the corresponding gun 17 passes through the center of the rectangular space,

between the coils and the pole pieces. The same is true of the other pairs of lateral deflection coils. When the

coils

48 and 49 are energized with 78 turns per inch, a deflection from center on the target surface of about 9 inches may be achieved. By reversing the current direction in such coils, a deflection in the other direction from center of 9 inches may be achieved, providing a total beam sweep length of 18 inches. This is adequate to allow four guns to cover the entire hearth length.

The trim coil 53 may be wound with about 100 turns per inch on a 2-inch diameter iron core. When it is placed such that it is an inch or so outside the beam path and 6 inches above the

coils

48 and 49, energization of the trim coil to values of IO ampere turns per inch to 200 ampere turns per inch will move the electron beam impact point from the far side of the hearth to the near side or side closest to the electron beam gun.

In FIG. 4, a modification of the furnace illustrated in FIGS. 1 and 3 is shown. In the furnace of FIG. 4, the trim coil is dropped beneath the level of the target surface 14 and the coils 11 and 12 a distance of about 4 to 6 inches. The affect of the change in trim coil elevation is to require a slightly shorter coil for a given amount of lateral sweep, since the excursion of the beam becomes less as the lateral deflection coils 48 and 49 are approached. Other than this factor, the trim coils may be placed wherever mechanically convenient. The coils 11 and 12 are each comprised of mild steel cylindrical cores 6 inches in diameter and 60 inches long, wound with wire to a value of 64 turns per inch. The coils 11 and 12 are placed 50 inches apart center to center. The trim coil 53 in the embodiment of FIG 4 is provided with a core diameter of 2 inches wound with about I00 turns per inch. The lateral deflection coils 48 and 49 may be constructed identical to that described in connection with FIG. 3 and are operated in a like manner. The

electron gun I7 is positioned about 22 inches below the level of the molten material in the hearth and is encased in a steel box 56 which prevents penetration of magnetic fields. The box 56 is provided with an opening or slit 57 therein through which the electron beam passes. The

coils

48 and 49 are positioned about 2 inches above the exit slit 57. The trim coil 53 is positioned about 6 inches above the lateral deflection coils 48 and t 49.

In the arrangement of FIG. 4, the coil 12 is operated at ampere turns per inch, the coil 11 is operated with 15 ampere turns per inch and the trim coil 53 may be operated with 70 ampere turns per inch. Under such conditions the electron beam bends to an impact point in the middle of a 15-inch wide hearth placed with its centerline 4 inches from the coil 12. A 3 change in trim coil current from 30 ampere turns per inch to I40 ampere turns per inch moves the beam from the side of f the hearth nearest the coil 11 to the side of the hearth nearest the coil 12. Energizing the lateral deflection coils 48 and 49 to values of i30 ampere turns per inch provides a lateral sweep along the target over a path length of about 10 in a direction parallel to the coils 11 and 12.

Referring now to FIG. 5, a further modification of the furnace of FIGS. 1, 3 and 4 is illustrated. In this furnace, the electron guns are positioned so that the beams go outside of the main coils l1 and 12 over the top of one of the coils before coming down into the rectangular magnetic field between the main coils. In this instance, the trim coil 53 is placed about even with the level of the coils 11 and 12 to operate as previi ously described. The arrangement shown in FIG. 5 may be of advantage where the heat input requires more dense electron beams than are achievable in the other illustrated modifications. v

The use of the invention in the furnace described in FIGS. 1-5 is of particular advantage in simplifying the control of a plurality of electron beams which are to be swept over a generally large area target surface. The invention is, however, E applicable to situations where only a single beam is to be controlled and swept, and is particularly useful where the availa- T ble volume in the evacuated enclosure is minimal. Under such circumstances, the positioning of heretofore known types of i deflection systems close to each other has typically created considerable complexity in the magnetic fields due to mutual interference. The use of solenoidal coils for at least one of the deflecting fields enables magnetic elements to be placed in close proximity to each other without creating unduly complex field distortion problems. i

I in FIGS 6 to 8,55 electron beam furnace is shown for heat ing the surface of molten material 114 contained in a crucible 116. As was the case in connection with the hearth of the previously described embodiment, the crucible is provided with a plurality of coolant passages 116a therein for cooling the crucible walls and thereby causing the formation of a skull 116b between the molten material and thewall of the crucible. The electron beam gun and associated deflection magnetics for causing a 90 deflection of the beam immediately after leaving the gun are identical with that described in connection with FIG. 3. Accordingly, no further description will be given, and the elements of the gun 17 have been given identical reference numerals as those of the gun 17 in FIG. 3, preceded by a l. The beam, the envelope of which is indicated by the dotted lines 136, is deflected upwardly through an opening 161 in a vapor shield 162. The shield I62 aids in preventing contamination of the various parts of the gun 117 by condensation of vapor thereon. The beam is focused to reach a nodal point as it passes through the opening 161 in order to minimize the required size of the opening. A more specific description of this may be found in the aforementioned US Pat. No. 3,514,656.

As the beam leaves the opening 161 and passes upwardly, it enters a field established between a pair of vertical plate-

type pole pieces

163 and 164. Appropriate magnetization of the

pole pieces

163 and 164 produces a magnetic field having lines of flux 165 extending in accordance with the dash-dot lines in FIG. 7. The lines of flux are directed from the pole piece plate 163 to the pole piece plate 164, thereby causing a deflection of the electron beam as shown. Moreover, since the length of time which the electrons toward the outer edge of the beam are in the magnetic field as compared with the length of time those electrons toward the inner edge of the beam are in the field, a correction of the divergence of the beam as it leaves the opening 161 is effected, as well as a slight focusing of the beam to a desired spot size as it impinges on the surface of the material 114. By varying the strength of the magnetic field between the

pole plates

163 and 164, the beam may be longitudinally swept, that is, swept in the plane of the curving beam path, from left to right as viewed in FIG. 6.

As the electron beam exists the field established between the

pole plates

163 and 166, a long rectangular magnetic field which is substantially uniform and which has generally straight lines of flux is established by means of a pair of

solenoidal coils

166 and 167. As may be viewed in FIG. 7, the

coils

166 and 167 are positioned in alignment with the

pole pieces

163 and 164, respectively, parallel with the vertical edges thereof. A pair of

pole pieces

168 and 169 connect the

ferromagnetic cores

171 and 172 of the

coils

166 and 167 to thereby form a low reluctance rectangular loop.

The field which is established by energization of the

coils

166 and 167 is of the same nature as that illustrated in FIG. 2, and the current passed through the coils may be reversed to produce lateral deflection or sweep of the beam. In FIG. 7, the field is shown having lines of flux extending downwardly into the paper tothereby produce a deflection as illustrated by the dotted lines. The beam may be swept in a path as indicated by the arrow 173 by causing the strength of the field established by the

coils

166 and 167 to first decline to zero and then to reverse so that the flux lines extend upwardly out of the paper in FIG. 7.

By way of example, a satisfactory furnace of the type shown in FIGS. 6, 7 and 8 may be constructed with a separation between

pole pieces

163 and 164 of about b 6 inches and a total length along their vertical edges of about inches. By utilizing two

solenoidal coils

166 and 167 each having cores of a %"X%" cross section by 24 inches long, and with connecting 'k" square

ferromagnetic poles

168 and 169 of about 7 inches length, placement of the structure at about 1 inch in front of the vertical edges of the

pole pieces

163 and 164 may provide a beam sweep arc of about 65.

In some situations, longitudinal sweep, that is, sweep in the plane of the curving beam path, may be more readily provided by maintaining the strength of the field between the

plates

163 and 164 constant and by providing a further pair of solenoidal coils, not shown, immediately following the

solenoidal coils

166 and 167. The further pair of solenoidal coils are placed at 90 with respect to the

coils

166 and 167 and are spaced about lW'therefrom. By energizing the further coils in an appropriate manner sweep in a direction perpendicular to that of the sweep produced by the first set of solenoidal coils may be achieved.

The illustrated furnace has the unique advantage of being markedly unaffected by the close proximity of magnetic systems to other such systems or magnetic substances. Although spurious magnetic fields are set up between a set of solenoidal coils and the fields in adjacent magnetic systems and between a set of solenoidal coils and other magnetic substances, such spurious fields apparently use lines of force from the remainder of the coil leakage flux to a great degree, and do not markedly affect the main field. The leakage flux in the rectangular aperture bounded by the two coils and the end poles is affected to a much smaller degree. Thus, placement of deflection systems in close proximity to each other provides a great saving in the volume required. Moreover, the ability to produce uniform magnetic fields of substantial size without using very large pole pieces effects a saving in power requirements.

It may therefore be seen that the invention provides an improved electron beam furnace in which at least one electron beam is directed through a curving path to a target. The invention is particularly useful in connection with a plurality of electron beams where a relatively large surface area is to be heated and where the beams are to be swept over the surface area. The invention is of further advantage where volume requirements are minimal and magnetic elements must therefore be placed in close proximity to each other. The extreme uniformity and linearity of field established in accordance with the invention provide much greater control over the beams and greatly simplify deflection magnetics, particularly in instances where beam sweeping over a large surface area is desirable.

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

What I Claim is:

1. In an electron beam furnace, a heating system comprising, an electron beam gun for producing an electron beam, a pair of solenoidal coils positioned substantially parallel with each other and each being of an effective length which is at least 1.2 times the distance between said solenoidal coils, a pair of pole pieces joining the ends of said solenoidal coils to define a generally rectangular space located in the path of the electron beam, and means for energizing said coils to produce a substantially uniform magnetic field having generally straight lines of flux extending between the pole pieces transversely of the electron beam path.

2. A system according to claim 1 wherein said energizing means include means for providing different levels of energization of said coils to control the amount of deflection of the electron beam.

3. A systemaccordingto claim below the level of the target and on opposite sides thereof.

4. A system according to claim 1 wherein further deflection means are provided in the path of the electron beam for I deflecting the beam in a direction parallel with the axes of said i sg l a dalsqi s .H.. -...,.a

5. A system according to claim 4 wherein a pressure barrier f is provided in the path 0 the beam, said pressure barrier having a slit therein for permitting the beam to pass therethrough, and wherein said further deflection means are positioned adjacent said slit.

6. A system according to claim 4 wherein said further deflection means comprise a pair of spaced pole pieces having facing surfaces which are generally parallel with the axes of i said solenoidal coils.

7. A system according to claim 4 wherein said further deflection means comprise a second pair of solenoidal coils positioned substantially parallel with each other on opposite sides of the electron beam path and being of a length which is greater than the distance between said solenoidal coils, means for energizing said second pair of solenoidal coils to produce a substantially uniform magnetic field having generally straight lines of flux extending transversely of the electron beam path, said second pair of solenoidal coils having their axes oriented to provide deflection of the impingement area of the electron beam on the target in a direction perpendicular to the direction of deflection provided by said first pair of solenoidal coils.

8. A system according to claim 7 wherein a solenoidal trimming coil is positioned adjacent the beam path between and wherein said second pair of solenoidal coils are positioned below said first pair of coils.

10. A system according to claim 9 wherein said second pair of solenoidal coils are below the space between one of said first pairs of solenoidal coils and the target.

11. A system according to claim 9 wherein said second pair of solenoidal coils are below the space on the opposite side of one of said first pairs of solenoidal coils from the target.

12. In an electron beam furnace having an elongated container for molten material, a system for heating the molten material comprising a plurality of electron beam guns distributed at spaced intervals along the container and below the level of the molten material therein, a pair of solenoidal coils positioned substantially parallel with each other on opposite sides of said elongated container and being of a length which is greater than the distance between said solenoidal coils, a pair of pole pieces joining the proximate ends of said solenoidal coils and forming, with the cores of said solenoidal coils, a generally rectangular closed low reluctance loop, means for energizing said coils to produce a substantially uniform magnetic field having generally straight lines of flux extending along the length of said container, and means for directing each of the electron beams produced by said electron beam guns into said uniform magnetic field to be deflected thereby onto the surface of the molten material.

13. Apparatus according to claim 12 wherein said electron beam directing means comprise a plurality of pairs of solenoidal coils said coils in each pair being positioned substantially parallel with each other on opposite sides of a respective one of the paths of the electron beam produced by said electron beam guns, each of said solenoidal coils being of a length which is at least 1.2 times the distance therebetween, and means for energizing said coils to produce a substantially uniform magnetic field having generally straight lines of flux extending transversely of the electron beam path, and for varying the strength of said uniform fields to produce a lateral sweep of the electron beams.

Claims

3. A system according to claim 1 wherein said coils are both below the level of the target and on opposite sides thereof.

4. A system according to claim 1 wherein further deflection means are provided in the path of the electron beam for deflecting the beam in a direction parallel with the axes of said solenoidal coils.

5. A system according to claim 4 wherein a pressure barrier is provided in the path o the beam, said pressure barrier having a slit therein for permitting the beam to pass therethrough, and wherein said further deflection means are positioned adjacent said slit.

6. A system according to claim 4 wherein said further deflection means comprise a pair of spaced pole pieces having facing surfaces which are generally parallel with the axes of said solenoidal coils.

8. A system according to claim 7 wherein a solenoidal trimming coil is positioned adjacent the beam path between said first and second pairs of solenoidal coils, and wherein means for energizing said trimming coil is provided.

9. A system according to claim 7 wherein said first pair of solenoidal coils are positioned on opposite sides of the target, and wherein said second pair of solenoidal coils are positioned below said first pair of coils.