US3330901A - Electron bombardment melting furnace - Google Patents

Description

July 11, 1967 VON ADRIENNE ETAL 3,330,901

ELECTRON BOMBARDMENT MELTING FURNACE Filed March 25, 1964 INVENTORS MANFRED VON ARDENNE SIEGFRIED SCHILLER United States Patent ELECTRON BOMBARDMENT MELTING FURNACE Manfred von Ardeune and Siegfried Schiller, both of Dresden-Weisser Hirsch, Germany, assignors to VEB Lokomotivbau-Elektrotechnische Werke Hans Beimler,

Hennigsdorf, Kreis Oranienburg, Germany Filed Mar. 25, 1964, Ser. No. 355,999 1 Claim. (Cl. 13-31) The present invention relates to electron beam melting furnaces and in particular to improvements in the control of the electron beam.

In general, two methods have been developed for employing electron bombardment for melting materials such as metals. In one method the cathode is situated in the same vacuum chamber as the material to be melted, i.e., the melt. In the other method the electron gun chamber, in which the cathode is situated, is separated from the melting chamber by one or more vacuum separating chambers. The advantage which the first method presents lies in its structural simplicity. However, the gases which evolve from the melting materials tend to produce undesirable discharges which may damage or destroy elements of the electron beam generator. The discharges result because the gas is generated within the chamber which also includes the high intensity electric field necessary to generate the electron beam. Thus, additional pumping apparatus must be provided during the melting operation to keep the pressure below the discharge threshold.

The second method obviates the disadvantage noted above by isolating the beam developing apparatus from the melting section by providing a plurality of chambers between the gun chamber and the melting chamber. Thus, the melting zone may be situated in a field free region.

However, the second method also presents operating disadvantages. Thus, in the second method it has been found diffic-ult to guide high intensity electron beams of, e.g., 7 to 60 a. at 30 kv. beam voltage. The inherent space charge of the beam, i.e. the fields caused by the charge in the beam itself, produces a significant increase in the diameter of the beam. The diverging efiect of the space charge tends to make focusing of the beam more difficult. It was therefore necessary to limit the amount of current directed to the melting chamber to reduce the diverging effect of the space charge. Thus the space charge necessitated a reduction in the intensity of the electron beam and thereby reduced the effectiveness of the unit.

In the instance where the originally parallel beam has a circular cross-section the following formula is applicable:

wherein: X=space charge spread, r=radius of the cross section, r =initial radius of the beam cross-section, r final radius of the beam cross-Section, j initial density of the electron current in ascmr a.=ampere, cm.=centimeter, U accelerating voltage, v.=bolt and L=length of the beam in m.=meters. Under normal conditions, in electron bombardment melting furnaces, the value i 2 a. cmf wherein a. cm:- =amperes/cm. and U 30 kv., X-3;8 when L-0.2 meter. Similar results were obtained for 'bandfor-m beams. Such a beam spread is 3,33%,9hl Patented July 11, 1967 inadmissible in practice because of resulting excessively high flow resistance which would demand inordinately large vacuum pumps. (In common practice the beam length exceeds 0.2 m. where no focusing means are used.) These problems may be solved by providing magnetic lenses at suitably short intervals. However, in electron bombardment furnaces where magnetic lenses must be employed at short intervals insufficient room is left for the evacuation flange. In addition, the extra magnetic lenses add considerable cost to the device.

Auxiliary focusing may be obtained with thread beam formations, e.g., by over compensation of the space charge. This process may be utilized in the pressure range of approximately 10 millimeters of mercury and with currents not in excess of 1 ma. However, in electron bombardment furnaces where the current rating is approximately 7 to 60 a. and with accelerating voltages of, e.g., 20 to 30 kv., it is impossible to apply such a method since at those high pressures high tension flashovers would take place and gas dispersion would become undesirably high.

In electron gun bombardment devices having limited output electron beams, a space charge compensating effect may be achieved by ion formation in the always present residual gas. However, with electron guns having kw. outputs and higher, utilization of the space charge compensation on residual gas does not provide satisfactory and dependable results. Furthermore, where ions are utilized to neutraliZe the electron space charge a means must be provided to prevent the entry of ions into the electron gun chamber, because the presence of ions in the gun chamber tends to cause high voltage flashovers therein.

It is therefore an object of the present invention to provide improved space charge compensation means in an electron gun apparatus.

It is another object of this invention to provide space charge compensation of electron beams having a relatively high output.

It is still another object of the present invention to provide an improved electron beam melting furnace.

The apparatus as disclosed herein includes an electron gun chamber for producing a high output electron beam. The beam is directed through a cylinder having a passage down into a melting chamber where the electron beam impinges upon the material to be melted. A magnetic lens at each end of the cylinder is employed to provide the requisite beam focusing. An ion trap electrode is positioned proximate the anode of the electron gun at the top of the passage within the cylinder. A separating chamber and -a high pressure chamber are provided intermediate the cylinder ends and are connected to the passage which runs along the longitudinal axis of the cylinder. A Valve provides control of the quantity of fluid transmitted to the high pressure chamber, through a suitable conduit. A pair of electrodes are positioned within the passage on either side of the high pressure chamber. The electrodes serve to pull ions, which have been formed in the high pressure chamber, into the electron beam thereby diminishing the effect of the space charge and limiting the divergence of the electron beam. Suitable pumps and fluid connecting lines are provided to draw the requisite amount of fluid from within the gun, separating, high pressure and melting chambers.

Other and further objects of this invention will be apparent from the following description and claim and may be understood by reference to the accompanying drawing, which by way of illustration shows preferred embodiments of the invention and What is now considered to be the best mode of applying the principles thereof.

In the drawing,

The single figure is a diagrammatic elevational crosssectional view of a heavy duty electron bombardment melting furnace.

The furnace includes an upper cylindrical housing 19 having a flange 12 and radial exit ducts 13 and 14 which may be secured to a vacuum pump (e.g. a diffusion pump) to evacuate the housing. A cylindrical insulating member 16 is mounted vertically on the upper horizontal flange 12. A flat ring 17 rests on the upper surface of insulating member 16 and supports an inverted hat shaped focusing electrode 18. Focusing electrode 18 includes a bottom conically shaped opening 20 through which the beam of electrons passes. A round cover plate 22 is secured to the upper surface of the flange of electrode 18.

A supporting disc 24 is conventionally secured to the center of the lower surface of cover 22 and supports a pair of cylindrical members 26 and 28 between which a second horizontal disc 30 is secured. A beam generating cathode 32 is held in the lower surface of cylinder 28 and concentrically disposed with respect to opening 20.

A generally conically shaped anode 33 is concentrically disposed with respect to cathode 32 beneath focusing electrode 18. Anode 33 is physically supported above a magnetic lens which comprises coil 34 and core member 36 secured to housing in any desired manner. A high voltage may be applied to a pair of terminals 38 and 40 coupled to cathode 32 and anode 33, respectively, to control the flow of electrons through the anode.

A concentric electrode 42 is placed in the lower portion of anode 33 within the upper portion of core 36. Electrode 42 is supported by means of an insulating cylinder 44- situated between appropriate flanges on the electrode and an internal ledge of core 36. The potential on electrode 44 is such as to prevent ions from entering the electron gun chamber 11. The space 46 beneath electrode 42 is a part of the electron path and leads into the separating chamber 48 at the bottom of housing 10.

A vertical cylindrical support member 52 is positioned between a lower housing 53 and upper housing 10. An electrode 50 is contained within cylinder 52 and supported thereon by means of an insulating cylinder 54. Electrons pass through electrode 50 and aperture 55 into a high pressure chamber 56 in the upper portion of housing 53. The purpose of electrode 50 is to draw ions away from chamber 56. Chamber 56 includes a flange 58 for connection to a vacuum pump and a gas inlet valve 60.

A second magnetic lens is located in the bottom portion of chamber 56 and comprises a coil 62 and core 64 concentrically arranged with respect to the path of the electron flow and including an electron path 66 and a lower electrode 68 supported as in the case of electrodes 50 by means of an insulating cylinder 70. The purpose of electrode 68 is to draw ions away from chamber 56.

The assembly, of course, includes a melting chamber 72 in which the bar to be melted 74 is located above a liquid material 76 in a water cooled copper crystalizer 78.

Electrical means (not shown) serve to provide the electromagnetic fields necessary for the proper functioning of the electrodes.

Turning now to the operation of the invention, the cathode and anode cooperate to provide an electron beam which is directed down through the center of the furnace to finally impinge upon bar 74. High pressure chamber 56, through which the beam passes, serves as an ion source. The pressure in chamber 56 is normally in the vicinity of 310* to 10- millimeters of mercury column, depending upon the quantity of ions to be produced. In low energy electron beams, the potential drop produced by the electron beam charge and the diffusion produced by the passage of the beam through the fluid of the passage suflices to draw off a suflicient quantity of ions to limit the diverging effect of the space charge on the electron beams. However, as noted above, in high output heavy duty electron bombardment furnaces, towards which this invention is directed, the quantity of ions drawn will not sufficiently neutralize the space charge. Electrodes 50 and 68 are provided to pull additional ions from chamber 56 into the electron beam. Since the electrodes are positioned one on each side of the chamber the ions are pulled both in the direction of the beam as well as in the opposite direction.

If additional ions are required the ion stream may be intensified by auxiliary fields generated by electrode 56 in combination with separating chamber 48. The slow electrons generated at the same time in chamber 56 travel toward the walls of passage 55. The ions are drawn towards the beam axis as a result of the potential drop between the beam boundary and the beam axis. Thus, relatively few recombinations occur on the walls of passage 55. Similar conditions prevail in the case of rectangular beam cross-sections.

The pressure in chamber 56 may be controlled by adjusting the gas inlet valve 60. If the vacuum in melting chamber 72 is very high (e.g., 10 to l() millimeters of mercury column) more gas must be admitted into high pressure chamber 56 to thereby raise the pressure in the chamber higher than the pressure in the melting chamber, e.g., 10- millimeters of mercury column. In practice the pressure in the melting chamber is usually between 10 and 10* millimeters of mercury. Since the flow resistance of a tube is proportional to the root of the molecular weight, it is desirable to use a heavy gas, e.g.,

argon.

The ions penetrated to the gun chamber would cause flashovers and considerable damage to the cathode would result. To obviate this drawback, ion trap electrode 42 is connected to a power source (not shown), at a potential which will repel the ions penetrated to cathode chamber through passage 46. It should be noted that electrode 42 may have positive as well as negative potential relative to the anode. Thus, the ions in the gun chamber may be repelled or pulled out of the gun chamber. Because the spacing between the cathode and magnetic lens 34 or electrode 42 is small (e.g., 50 to millimeters) the space charge spread is not critical within the gun chamber. Thus, the removal of ions from the gun chamber will not effect eflicient operation of the device. However, the distance between magnetic lenses 34 and 62 in electron bombardment melting furnaces such as embodied herein is in the order of 700 millimeters. Therefore, in this instance, the space charge spread between the magnetic lenses does become critical. Hence, it is necessary to produce additional ions to counter-act the diverging effect which the space charge has on the electron beam.

We have described what we believe to be the best embodiments of our invention. We do not wish, however, to be confined to the embodiments shown, but what we desire to cover by Letters Patent is set forth in the following claim.

What is claimed:

An electron bombardment melting furnace comprising an elongated cylinder having a passage extending along the longitudinal axis thereof, an upper housing encompassing the upper portion of said cylinder, said housing including an electron gun chamber, a cathode positioned within said electron gun chamber above said cylinder and in alignment with said cylinder passage, an anode positioned directly above said cylinder and in concentric relation thereto, a focusing electrode positioned between said anode and said cathode, an ion trap electrode concentrically mounted 'within said passage in close proximity to said anode, a separating chamber, at the lower end of said upper housing, in connection with said passage, means for removing fluid from said separating chamber,

a lower housing encompassing the lower portion of said cylinder, a high pressure chamber in the top of said lower housing, means for introducing fluid into said high pressure chamber, means for removing fluid from said high pressure chamber, a first ion drawing electrode concentrically mounted within said passage between said upper and lower housing for drawing ions from said high pressure chamber, a second ion drawing electrode concentrically mounted within said passage beneath said high pressure chamber for drawing ions from said high pressure chamber, a plurality of magnetic lenses mounted concentrically around the outside of said cylinder and a melting chamber disposed beneath said cylinder, said cathode and anode being adapted, in cooperation with said electrodes, chambers and magnifying lenses to produce an electron beam which is focused upon an element in said melting chamber.

References Cited UNITED STATES PATENTS JOSEPH V. TRUHE, Primary Examiner.

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