US2994801A

US2994801A - Electron beam generation

Info

Publication number: US2994801A
Application number: US818306A
Authority: US
Inventors: Charles W Hanks
Original assignee: Stauffer Chemical Co
Current assignee: Stauffer Chemical Co
Priority date: 1959-06-05
Filing date: 1959-06-05
Publication date: 1961-08-01
Anticipated expiration: 1978-08-01
Also published as: FR1251724A; DE1190112B; CH388470A; GB930673A

Description

Aug. 1, 1961 c. w. HANKS 2,994,801

' ELECTRON BEAM GENERATION Filed June 5, 1959 $33 INVENTOR.

(M40455 0. him/(s United States atent cc 2,994,801 Patented Aug. 1, 1961 2,994,801 ELECTRON BEAM GENERATION Charles W. Hanks, Orinda, Califi, assignor to Stautler Chemical Company, New York, N.Y., a corporation of Delaware Filed June 5, 1959, Ser. No. 818,306 8 Claims. (Cl. 315-14) The present invention relates in general to an improved method and means for generating, accelerating, and focusing electron beams, and more particularly to the generation of an elongated electron beam and the focusing of same into a small cross section from widely divergent angles.

. Although a wide variety of electron beam generators have been developed both for general and specific applications, there yet remains substantial and important advancement to be made in the field. This is particularly true as regards certain beam applications. For example, in the field of electron bombardment melting and casting of metals there is found to be a marked lack of well suited electron beam sources. The present invention provides for the generation and acceleration of electron beams particularly adapted for such applications. Although this invention is in no way limited to any one utilization of electron beams generated in accordance therewith, the highly advantageous nature of the present invention in the field of electron beam casting is set out as an example of utility in the following description of the invention.

It is herein contemplated that there shall be generated an electron beam having a materially elongated and preferably continuous line of origin. More specifically, electrons are herein emitted about a loop or circle and are thence focused into a beam. Furthermore, such electron beamis then further focused together at a focal point or plane. Electron beam portions are, in fact, directed generally toward each other at an angle, so that there is produced a hollow cone of electrons emitted about the periphery of the base thereof and directed to converge toward the apex. The term cone is herein employed to include other than right circular conical configurations and to, in fact, include all converging configurations, whatever the cross section. While it is well recognized that various effects, including space charge, operate to materially spread an electron beam of substantial density, such is herein accepted without difliculty for by the converging nature ofthe separate portions of the beam along the length of the origin thereof, these effects only vary the size of the focus and then only in a minor degree as compared to conventional sources.

It is highly advantageous in various applications to direct electrons from a plurality of directions upon a limited area, both from the viewpoint of final obtainable electron energy and for various side effects of the converging trajectory. Not only is there established by the present invention a very intense electron beam bombardment of a desired limited area displaced substantially from the beam origin, but also the converging nature of the bombarding electrons is quite well adapted to the dual function of melting metal and subsequently further bombarding a molten pool thereof. The relative angular trajectory of theportions of the electron beam generated and directed in accordance with the present invention has been found to be highly advantageous in electron beam furnaces, as well as in other unrelated applications.

It is an object of the present invention to provide improved electron beam generating and focusing for direction of an elongated electron beam into a small focus.

, It is another object of the present invention to provide for the generation of an electron beam about a loop and the focusing of same together into a small cross-section.

It is a further object of the present invention to provide an improved elongated electron beam. source directing an intense electron beam into a small focus along the axis of the source.

It is yet another object of the present invention to provide an electron beam source having an elongated hidden cathode together with means focusing electrons therefrom into a converging cone.

It is still another object of the present invention to provide an improved electron beam source generating a rectilinear electron flow from an elongated loop and controllably converging the electron beam so formed into focus at a predetermined distance from the plane of origin thereof.

Various other possible objects and advantages of the present invention will become apparent to those skilled in the art from the following description of the invention. No limitation is intended by the particular terms employed in the following description, but reference is instead made to the appended claims for a precise delineation of the true scope of the present invention.

The present invention is illustrated both as to the method and apparatus thereof, in the accompanying drawings, wherein:

FIG. 1 is a projected view of an electron source in accordance with the present invention, and including an illustration of the converging electron beam trajectory herein attained.

FIG. 2 is an elevational view of an electron source and converging electron beam as may beemployed in an electron bombardment furnace.

FIG. 3 is a sectional view taken in the plane 3'-3 of ducing a hollow conical electron beam 12. The electron beam generated by the present invention is focused upon an area of reduced cross-section relative to the lateral dimensions of the generating equipment and displaced axially from the electron source or generator. One preferred embodiment of the apparatus of the present invention is illustrated in FIGS. 2 and 3 of the drawings wherein one possible application of the present invention is also indicated. As shown in FIGS. 2 and 3, the generator includes a first or backing electrode 13 formed as a loop which may have the configuration of an annulus or, alternatively, may have an elliptical shape in plan view;

Additionally, the first electrode 13 may, as a loop, not be entirely closed at one point. A relatively wide variety of configurations are herein possible for the shape in plan view of the electron beam generator of the present invention, and consequently the term loop is hereinafter employed as encompassing the variety of shapes in which the generator may be formed and yet conform to the principles of thepresent invention. The first or backing clectrode 13, which will hereinafter for convenience be considered to be annular, has, a trough or recess 14 formed about the undersurface thereof. This trough is adapted to contain an electron-emissive cathode 16 disposed about the length of the electrode 13 and out of contact'therewith. Suitable insulating means, notshown, are employed to mount the electron-emissive cathode 16 within the trough of the first electrode 13, and in the illustrated.

embodiment of the invention, wherein it is contemplated that a hidden cathode shall be provided, the electrode trough 14- is made somewhat reentrant. This is herein accomplished by the provision of a protruding lip 17 about the radially inner edge of the trough 14- and extending outwardly to thereby conceal or hide a certain portion of the trough from the underside of the electrode 13. It is within this relatively hidden portion of the trough 14 that the cathode 16 is mounted, for reasons set forth in detail below.

Again, in connection only with this particular embodiment of the invention, the electrode trough 14 is formed at somewhat of an angle into the electrode about the radially outer and lower edge of the electrode. A focusing ring 18 is mounted upon the electrode 13 in axially movable position relative thereto, as by means of bolts 19 extending through slots formed in the focusing ring and threaded into the outer periphery of the electrode 13. This focusing ring 18 depends from the electrode 13 about the outer periphery thereof to thereby define with the electrode lip 17 a throat portion 20 communicating between the electrode trough 14 and the exterior of the electrode 13. Immediately beneath the electrode 13 there is provided an accelerating electrode 21 which is preferably provided with a smoothly curving outer edge and is formed as a ring disposed beneath the electrode 13 and in extension radially outward of the electrode lip 17. It will be appreciated that with the foregoing structure, the width of the throat 20 defined between the focusing ring 18 and the electrode lip 17 is actually reduced beneath the electrode 13 by the accelerating electrode 21, inasmuch as same is displaced radially inward of the focusing ring 18 a lesser amount than is the electrode lip 17. Suitable mounting means may be provided for the accelerating electrode 21 beneath the electrode 13, and to this end there may be provided a cylindrical flange about the inner periphery of the accelerating electrode 21 and extending upward within the annular electrode 13, as illustrated. Suitable cooling means are provided for the individual elements of the generator as required, and the accelerating electrode 21 is preferably provided with a cooling tube 22 secured beneath same and adjacent the outer edge thereof in order to remove such heat as may be imparted thereto by stray electrons bombarding the accelerating electrode. Similar cooling means may be provided for the focusing ring if same should become necessary, and likewise for the electrode 13, although in normal practice it is only the accelerating electrode which is sufficiently heated during operation of the generator to require removal of substantial amounts of heat therefrom.

Energization of the electron-beam generator of the present invention is accomplished by raising the cathode 16 thereof to electron-emissive temperature, as by passing a current through the cathode 16 from external power supply, not shown. Establishment of the desired electron accelerating and deflecting fields is accomplished by the application of a potential between the electrode 13 and accelerating means 21. In this connection there is illustrated a battery 23 connected by leads 24 with the negative terminal thereof to the electrode 13 and the positive terminal thereof to the accelerating electrode 21. Inasmuch as the focusing ring 18 is mounted upon the electrode 13 in electrical contact therewith, the potential of this focusing ring is maintained the same as that of the electrode 13. If desired, the cathode 16 may be connected to a source of potential intermediate that of the electrode 13 and accelerating electrode 21, so that the electrode 13 is maintained at a negative potential thereto for repelling electrons therefrom. By the establishment of a substantial potential difference between the accelerating electrode 21 and backing electrode 13, there is produced an electric field having lines of force, particularly between the outer end of the accelerating electrode 21 and the lip 17 of the electrode 13, as indicated by the light lines 26 in FIG. 3 of the drawings. This field 26, as indicated by the illustrated lines of force thereof, will be seen to curve between the accelerating electrode 21 and the lip 17, so as to thereby constrain electrons emitted from the cathode 16 to follow a curved trajectory in general conformity to the curvature of the field lines. Electrons emitted from the cathode 16 are attracted by the relatively positive potential applied to the accelerating electrode 21, so that there is formed a beam 27 of electrons which flows outwardly from the trough or cavity 14 in the electrode 13. This electron beam 27 leaves the cathode 16 in a direction generally radially outward therefrom and is curved by the field 26 back downward from the electrode 13 through the throat 20 formed therein and thence further back toward an axis 28 through the electron-beam generator.

The degree of curvature or deflection imparted to the electron beam 27 will be seen to be determined by the magnitude and curvature of the accelerating electric field. Thus by controlling the accelerating potential it is possible to also control the point of intersection of the electron beam 27 with the axis 28 of the electron-beam generator. As a further control over the electron beam trajectory, the focusing ring 18 is mounted in adjustable position upon the electrode 13 so as to be movable axially thereof. With the focusing ring 18 electrically connected to the electrode 13, movement of the focusing ring downward from the electrode 13 will thereby provide a negative electric field at a lower point along the electron beam trajectory and radially outward therefrom. Thus,-by lowering the focusing ring 18 it is possible to increase the radial deflection of the electron beam inwardly of the generator.

It will be appreciated that the illustration of FIG. 3 only embraces the production of an electron beam at a single point along the length of the cathode 16. In actuality this cathode 16 is formed as a loop, as are the other elements of the generator, and thus the illustration of FIG. 3 is infact repeated at an infinite number of points along the cathode about the circumference As a consequence of the foregoing, there is tion of electron beam density at any particular point separated from the source of the beam. In the present invention this spreading of the electron beam, as may be noted from FIGURE 3 of the drawing,-does not pro-' duce a material disadvantage. Thus, in the present inven-- tion the electron beam generated about a loop is directed to a focal point at a distance from the point of generation, as may be seen, for example, in FIG. 2 of the drawings' Although any individual cross-section of the electron beam will be appreciated to materially spreadat increasing distances from the point of generation thereof, the overall beam itself is focused together so that the spreading beam converges into a single high density entity.

The total electron beam from the generator of this invention produces a converging hollow cone of electrons. Thus, even though the cross-section of any particular portion of the beam is at all times divergent, owing to effects such as space charge, the overall beam is in fact converging. This is best illustrated in FIG. 2 of the drawing, wherein there is illustrated in dashed lines a thin layer of the electron beam from opposite sides of the generator and, wherein it will be seen that the electron beam density at a focal plane separated axially from the generator itself is materially maximized over the electron beam density at any other point. Depending upon various factors including the electron velocity, the converging electron beam may actually cross over itself as shown in FIG. 2, or, alternatively, a more laminar flow situation may actual-1y exist in the vicinity of thebeam focus. Innumerable applications of an electron beam generated in accordance with the present invention may be realized and thus for example at 'a focal area 31 wherein the beam attains a substantially minimized diameter and thus maximized density, there may be provided such as an electron lens whereby the resultant beam may be directed into apparatus wherein it is utilized. In connection with such an electron lens there may also be provided a vapor trap, for example, in order that no gases or vapors from the utilizing apparatus may pass into the electron beam generation area.

Of particular utility is the application of the electron beam of this invention in connection with electron beam furnaces. There may thus, for example, be provided a melt stock 32 which is adapted to be fed into the beam,

as for example along the axis of the beam generator. In such circumstance, the melt stock would be progressively liquified by electron beam bombardment from all sides thereof adjacent the lower end of the melt stock so as to stream downward into such a metal mold 33, wherein the liquified metal would be further bombarded and heated by the converging electron beamth'ereat. Thus, the element 33 disposed at the focal plane of the electron beam of this invention may comprise a variety of different elements or apparatus depending upon the particular application of the electron beam. Furthermore, the angle at which the electron beam converges at the focal point 31 thereof may be readily varied by movement of the focusing ring 18 of the generator. Particular advantage is found in this feature, for various applications call for various degrees of beam convergence. Thus, for example, in electron beam furnaces it is highly desirable to provide electron beam bombardment of a molten metal pool from a large plurality of angles and directions such that complete 'and uniform heating of the upper surface of the pool may be realized. The electron-beam generator of this invention is well suited to such application for, even though a melt stock or the like may be fed into the beam at any desired point thereof for initial melting of the material to be cast in the furnace, yet the converging nature of the electron beam herein produced insures a bombardment of the entire surface of the resultant molten metal pool. A further substantial advantage of the source hereof in electron beam' furnaces is the relative inaccessibility of the cathode to condensing metal vapor formed in the furnace. With the hidden cathode feature described above, metal vapor cannot readily reach the cathode to coat same and thus one of the major causes of short cathode life is herein removed. With this source configuration, the cathode life is increased by a factor of ten to a thousand to thereby provide a materially improved performance.

'It will be appreciated from the foregoing description of a preferred embodiment of the present invention that there is provided in addition to a converging conical electron beam, an initial electron beam trajectory which is directed radially outward and is subsequently redirected into the desired conical trajectory. As may be seen from FIG. 3, the initial electron trajectory provides for the traverse of'electrons through the throat 20 and is required for theelectrons to clear the accelerating means 21. Thus, the structure of the above-described embodiment of the invention provides for the positioning of the electron accelerating means intermediate the electronemissive cathode and the focal area of the conical beam. As a consequence of this structure, the cathode 16 of the generator is wholly hidden from the ultimate focal point or area of the converging beam. The cathode 16 will be'seen to be in actuality disposed in such position within the cavity or trough 14 of the electrode 13 as to be substantially behind the electrode lip 17 and to be further hidden by the accelerating electrode 21 which extends into the trough throat .20. This electron-beam generator structure will thus be seen to provide a hidden cathode configuration. Inasmuch as only a very limited throat area is provided for the egress of electrons from the emitting surface, and furthermore by the fact that this emitting surface is in fact materially ofiset from the resultant electron beam trajectory, maximized protection for the emitting ctahode is provided. Thus in those instances wherein gases or vapors may be present in the volume thorugh which the generated electron beam traverses, and as a result of which ions are formed by interaction of the electron beam and such gases and vapors, the cathode hereof is fully protected from bombardment by such ions. In the event that ions are in fact formed in the volume beneath the electron beam generator, such ions would be normally attracted toward the electrode 13, and although repelled by the accelerating means 21, same may in part travel back along the electron beam path. In the present instance, these ions, because of the greater mass thereof, are unable to curve sharply back to the cathode and would at most impinge upon the focusing ring 18 or electrode 13.

The field configurations in the area of electron emission are such that ions approaching the generator and passing through the throat 20 will be attracted to the electrode 13 rather than to the cathode 16. The substantial structure of the electrode 13 makes it possible for same to be bombarded by such ions as may in fact pass through the throat 20 without material damage to the electrode 13. One of the difficulties in connection with high density electron sources is the material bombardment of the cathode thereof by ions produced from the electron beam generated. As applied to electron beam furnaces, an even greater difliculty lies in the coating of the cathode with metal to thereby significantly reduce the useful life thereof. In the present invention these difiiculties are almost entirely overcome for the likelihood of bombardment or coating of the cathode 16 is extremely slight.

In accordance with the present invention there will be seen to be produced an electron beam of a converging nature. This is herein accomplished by the steps of emitting electrons from a cathode formed in a loop. The acceleration of such electrons is then accomplished by the establishment of appropriate electrical fields which attract emitted electrons from the emissive cathode and furthermore direct same into a converging conical trajectory. Thus, about the loop of electron emission there is established electron accelerating fields which withdraw emitted electrons from the emitter and direct same away from the emitter. This electron acceleration by the established electric fields is carried out over the length of the electron emitter to constrain emitted electrons to travel axially of the emitter and radially inward thereof, whereby the resultant electron beam has a particular desired configuration of a converging hollow cone. In this manner the necessarily divergent nature of any particular portion of the electron beam is precluded from militating against the resultant densification of the focused electron beam.

Various other embodiments of the present invention including substantially all of the advantages thereof are possible, andone such additional embodiment is illustrated in FIG. 4 of the drawing. A schematic crosssectional view of a single portion of the loop of the ion beam generator is shown in FIG. 4 wherein there will be seen/to be provided a first electrode 43 having a generally concave undersurface. The electrode 43 is also for-med in a loop, as for example an annulus, and has the concave undersurface thereof tilted so that the outer edge of the electrode 43 is considerably thicker than the inner edge. A V-shaped trough 44 is formed in the electrode. 43 at the undersurface thereof about the circumference of the electrode and directed toward the focalpoint of the generator. Within the trough 44 there is disposed an electron-emissive cathode 46 extending about the circumference of the electrode 43 within the trough and preferably located interiorly of the trough.

In front of the under electrode surface there is disposed anelectron accelerating electrode 47 having an opening therethrough about the circumference thereof in alinement with the trough 44- between same and the focal point of the generator. Suitable cooling means, such as for example cooling tubes 48, are disposed heat exchange relationship with the accelerating electrode 47, to prevent overheating of same. Electrical connections are made to the electron-beam generator to provide an accelerating electric field for attracting electrons from the cathode 46 through the accelerating electrode 47. To this end, there is provided a power supply, illustrated as a simple battery 49, and having the negative terminal thereof connected to the electrode 43 and the positive terminal thereof connected to the accelerating'electrode 47. An intermediate potential is applied to the cathode 46 by connection of the latter to the power supply 49. The cathode 46 is maintained in an electron-emissive state as by the passage of an electrical current therethrough to raise the cathode temperature to that of electron emission.

With electrons being emitted from the cathode 46, same are repelled by the relatively negative potential of the electrode 43 and additionally are accelerated by the relatively positive potential of the accelerating electrode 47. There is consequently formed an electron beam emanating from the cathode 46 and directed through the opening in the accelerating electrode 47. This particular source configuration is generally one of the type known as a line source known in the art and particular focusing advantages are possible therewith. It isthus possible by the proper location of the cathode 46 within the trough 44- to produce a substantially rectilinear electron beam extending from the cathode 46 through the accelerating electrode 47. Beyond the accelerating electrode 47 conventional electron beam spreading is encountered so that, as in the above-described embodiment of the invention, the resultant electron beam at any one particular cross-section thereof tends to diverge. As explained above, the present invention provides an electron beam emanating from the circumference of a loop or circle, and directed into focus upon an axis through the loop at a distance from the plane of electron generation. Consequently, the beam divergence does not materially limit the resultant electron beam intensity at the focal point of the beam. In this embodiment, as in the one above described, there is produced a hollow conical electron beam converging toward a focal area which may be made quite small. Insofar as the resultant electron beam is concerned, both of the described embodiments produce substantially the same electron beam intensity at the focal area thereof, however, the first described embodiment has the particular feature of a hidden cathode which is not available with the latterdescribed embodiment. It will of course be appreciated that variation in the electron beam trajectory may be accomplished in the second embodiment of the invention by the provision for relative movement of the accelerating electrode or separate portions thereof, or alternatively, by the application of different accelerating potentials to diiferent portions of the accelerating electrode so as to thereby attain a particular desiredc'onfiguration of electron accelerating fields.

The resultant electron beam configuration attainable with the embodiment of the invention illustrated in FIG. 4'of the drawings is substantially the same asthat shown in FIG. 1 and is in fact identical to that obtained from the embodiment of FIG. 3, except for the' initial portion of the electron trajectory, as noted above.

'With the present invention it is possible to produce a very intense electron bombardment of a relatively small area. The problem of beam spreading in high density electron beams is herein materially reduced if not overcome. By the provision of a line source folded into a loop or the like, the disadvantages of electron beam divergence are hereby overcome. Even though" any individual portion of the electron beam hereof diverges in the normal manner, in accordance with the wellltnown rules of electron beam behavior, the resultant focused electron beam has a materially increased density over that possible of attainment with conventional prior art devices.

As an example of electron beam generators constructed in accordance with the present invention there may be provided a generator as illustrated in FIG. 3 of the drawings wherein the backing electrode 13 has an inner diameter of 5 /2 inches and an outer diameter, including the focusing ring, of about 8%. inches. The internal cavity or trough formed about the undersurface of this backing electrode 13 may have a generally curved inner configuration with a major radius of about of an inch. Immediately interiorly of the electrode 13 from the lip 17 thereof the cavity curvature may be formed upon a radius of of an inch with the cathode 16 disposed at the center of such curvature. In this example the focusing ring may depend about 1% inches below the backing electrode 13 and the total depth of the trough or cavity 14 about the backing electrode may be made about of one inch. In this instance the accelerating electrode may be electrically grounded and there may be applied a negative potential of some thousands of volts to the backing electrode and focusing ring. In this instance it is possible to produce an electron beam of substantial density having a high electron velocity. Although the electron beam is divergent in thickness, it is convergent in diameter so as to thereby cross over in the vicinity of the focal plane thereof to produce an electron beam of very high density thereat. The focal plane in this instance may be disposed about one foot below the electron-beam generator and there may, if desired, be provided with a vapor blocking stage at this focal point with an electron lens disposed below same to provide a second electron beam focus further displaced from the generator.

An electron-beam generator constructed in accordance with the embodiment of FIG. 4 of the drawingsmay, for example, be formed with the backing electrode hav ing an outside diameter of about 9.inches and an inside diameter of about 5% inches. The concave base of this electrode may, in fact, be formed with the surface radially outward from the trough thereof formed as a straight surface in cross-section inclined at an angle of about 40 degrees to horizontal. The undersurface of this electrode radially inward from the trough may be inclined in the opposite direction at an angle of about 7 degrees to horizontal. The trough itself may be formed with the sides thereof inclined at an angle between 40 and 50 degrees and the inner side thereof inclined, for example, at an angle of 40 degrees to vertical. The depth of the trough is dependent in part upon the particular focusing action desired and likewise the relative disposition of the electron-emissive cathode within the trough is dictated by the same considerations. The accelerating electrode in this embodiment may have the outer portion thereof inclined at an angle of about 25 degrees to horizontal and the inner portion thereof inclined at an angle of 12 degrees to horizontal in the opposite direction, with the two sides or portions of the accelerating electrode, however, being disposed slightly out of alinement with each other. The electrode 43 may, for example, be formed of graphite and the accelerating electrode formed of copper with the accelerating electrode being electrically grounded and the backing electrode 43 being maintained at a negative potential of some thousands of vol-ts. With this particular construction and dimensions there is accelerated from the generator an electron beam at an angle of about 16 degrees to vertical, in which case the focal plane of the electron beam is disposed about 12 inches beneath the generator.

What is claimed is: 1. An electron-beam source comprising an electron emissive member of small cross-section formed in a loop about an axis and adapted for energization to emit electrons over the length thereof, electron accelerating means disposed along said loop and displaced from said electronemissive member axially of said loop, and means intermediate said electron-emissive member and said electronaccelerating means curving the electron-accelerating field between same for accelerating electrons toward said axis at an angle thereto whereby electrons are focused in a small area in axial displacement from said loop from a plurality of directions.

2. An electron source comprising a line cathode curved about an axis and adapted to emit copious quantities of electrons from a substantial portion of the length thereof, an electrode disposed adjacent said cathode on at least one side thereof, and an accelerating electrode disposed along said cathode in separation from said first electrode and maintained at a positive potential relative thereto for accelerating electrones from said cathode and directing same in diverging beams from each point of the cathode into overall convergence toward said axis at a distance from said cathode.

3. Electron beam generating means producing a very high current beam for the bombardment of metal to heat and melt same in quantity comprising an elongated slotted electrode, an elongated electron-emissive cathode disposed in said slot out of contact therewith for emitting a large quantity of electrons, and electron accelerating means adjacent said electrode slot along the length thereof and establishing an electron accelerating field with said electrode to accelerate electrons from said cathode, said slotted electrode being formed as a loop about an axis whereby electrons are emitted substantially about such loop, and said accelerating electrode directing said emitted electrons into a hollow beam in which each longitudinal segment diverges and which is accelerated radially inward so that the overall beam diameter decreases with distance to focus an intense beam upon a limited area displaced axially from said cathode.

4. An electron-beam furnace comprising a backing electrode formed in a loop about an axis and having a slot along the length thereof, a casting mold having an open top and disposed on said axis in spaced relation below said backing electrode, an electron-emissive cathode disposed in said electrode slot, an accelerating electrode disposed along said backing electrode adjacent the slot therein and adapted to be maintained at a positive potential relative thereto for establishing an electric field accelerating electrons from said cathode about said loop, and means directing said accelerating electric field both axially and radially inward to direct electrons into a converging conical trajectory for focusing at a controlled distance from said loop in the open top of said casting mold.

5. An electron-beam generator comprising an annular electrode having an elongated cavity extending about the length thereof with a radially extending lip in part closing the under side of same, an annular electron-emissive cathode disposed in said cavity behind said lip out of contact with said electrode, and a concentric ring-shaped accelerating electrode displaced axially of said annular electrode and adjacent said radially extending lip for energization to establish with said annular electrode an electron accelerating field attracting electrons radially outward from said cathode and thence back about said lip through an angle of at least ninety degrees and axially as well as radially inward to focus the electrons in an intense beam at the axis of said electrode at a distance therefrom to establish a hollow converging electron beam.

6. An electron-beam generator comprising a loop electrode having a recess formed in the undersurface about same with a curved inner periphery and a throat opening limited by a rounded lip extending radially outward across a part of the recess, an electron-emissive cathode disposed about said electrode loop within said recess and radially inward of the lip edge, and accelerating means adapted for impression of a positive potential relative to said electrode and disposed beneath said electrode in close proximity therewith about the loop thereof with a curved outer edge extending slightly radially outward from said electrode lip to accelerate electrons radially outward from said cathode in trajectory curving back toward the loop axis into focus thereat.

7. An electron-beam generator as set forth in claim 6 further defined by a focusing ring disposed about the outer periphery of said loop electrode in electrical contact therewith, said focusing ring being mounted upon said electrode for movement axially with respect thereto whereby axial displacement of said focusing ring with respect to said electrode radially varies the inward component of acceleration applied to said electron beam to thereby control the axial separation of the beam focus from the electrode and included cathode.

8. An electron beam generator comprising a backing electrode formed as a loop about an axis and having a reentrant circumferential slot therein opening in part to the outer periphery of the electrode and in part to the undersurface thereof, an electron-emissive cathode disposed within said electron slot longitudinally thereof, an accelerating electrode in the form of a loop displaced axially from said backing electrode and having an outer edge alined with the slot edge of said electrode along the undersurface thereof, and a further electrode secured to said backing electrode in electrical contact therewith about the outer periphery thereof and mounted to slide axially of said backing electrode to overlay a controllable portion of the slot in said backing electrode, whereby the impression of a positive potential upon said accelerating electrode relative to said backing electrode and further electrode attached thereto establishes electron accelerating fields withdrawing electrons from said cathode generally outward thereof and directs the electrons axially of the electrodes into focus as a hollow, conical beam.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Steinlcamp: Electron Beam Welding-A New Development for Industry, Welding World, April 1959, pp. 38 to 40.