US3497743A - Annular multiple beam contoured cathode device - Google Patents

Description

ii'bfiiim ma 219-12u OR 394979743 SR M 6 3 1970 A. P. WALCH ET AL 3,497,743

ANNULAR MULTIPLE BEAM GONTOURED CATHODE DEVICE Filed June 16, 1967 3 Sheets-Sheet 1 FIG.

Q INVENTORS ALLAN P. WALCH ALBERT w. ANGELBECK BY @Qmfi ATTORNEY Feb. 24, 1910 ARWALCH ETAL I 3,497,743

ANNULAR MULTIPLE BEAM CONTOUREDCATHODE DEVICE Filed June 16, 1967 3 Sheets-Sheet 2 FIG. 4

,1970 A. P.'WALCH ETAL 3,497,743

7 ANNULAR MULTIPLE BEAM CONTOURED CATHODE DEVICE Filed June 16, 1967 s Sheets-Sheet 5 United States Patent 3,497,743 ANNULAR MULTIPLE BEAM CONTOURED CATHODE DEVICE Allan P. Walch, Manchester, and Albert W. Angelbeck,

East Hartford, Conn., assignors to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed June 16, 1967, Ser. No. 646,730 Int. Cl. H01j 29/46, 31/00 US. Cl. 31369 8 Claims ABSTRACT OF THE DISCLOSURE An apparatus and method for welding the seam of workpieces in any orientation without loss of molten material is accomplished with multiple beam cathodes operating in the glow discharge electrOn beam mode. Annular contoured and hollow cathode devices producing multiple beams requiring mechanical or electronic control for relative movement are shown.

BACKGROUND OF THE INVENTION Conventional means for producing electron beams involve the liberation of electrons from the surface of a heated cathode by thermionic emission. Recently, elec-,

these modes produces a well-defined electron beam which i may predictably and advantageously be used to mork materials. Other modes, such as the arc mode, also are a good source of electrons, but in general they exhibit erratic behavior and do not produce a high energy collimated electron beam.

These narrow apertured hollow cathodes have hollow chambers fabricated from a wire mesh or solid metal with a single aperture in one end. When the cathode is subjected to a high negative potential with respect to an anode and with the proper cathode geometry and pressure level in the hollow chamber, a Well-defined pencil beam of high current density, high energy electrons emanates from the aperture. An example of the versatility of the configurations possible with the apertured hollow cathode may be found in the copending application Ser. No. 417,399, filed Dec. 10, 1964, now US. Patent No. 3,381,157, entitled Annular Hollow Cathode Discharge Apparatus, by Fernand J. Ferriera, and assigned to the same assignee. The hollow cathode has a cathode fall region adjacent its external surface. The electrons for the beam are obtained, however, from a plasma generated by an intense discharge within the chamber enclosed by the cathode.

The range of gas densities and voltages in which a narrow apertured hollow cathode operates with stability with little tendency to enter the arc mode is generally small. For instance a narrow apertured cathode operating in argon at 24 microns of mercury pressure must operate at 1200-volt potential and will enter the arc mode if the potential is raised to 2000 volts or the pressure is increased 3,497,743 Patented Feb. 24, 1970 "ice to 40 microns. At about 7 microns of pressure, a potential of 10,000 volts can be used but an increase in the pressure to 10 microns may result in an arc mode. Hence, with each particular gas density necessary for an electron beam mode, there is a maximum voltage at which a relatively small increase in gas density will shift the operation into the arc mode.

Another glow discharge cathode has been developed such as described in copending application by Jack W. Davis, entitled Contoured Cathode, Ser. No. 508,314, filed Nov. 17, 1965, now abandoned but forming a portion of a continuation-in-part issued as US Patent No. 3,430,091. In this contoured cathode, a highly focused electron beam is produced under high pressures and high voltages. This cathode can operate in a high pressure region up to 1000 microns and produces the electron beam with a different mechanism from that relied upon in the hollow cathode. The electrons in the contoured cathode are essentially produced from secondary emission processes such as from ions striking the surface and is therefore essentially a surface emitter. Focusing is accomplished by contouring the surface of the cathode.

In the glow discharge mode, practically all of the potential drop from cathode to anode occurs across a small cathode fall region adjacent the cathode. Since equipotential lines near the cathode follow the contoured surface, strong electrostatic focusing of the electrons emitted from the cathode surface may be accomplished. By contouring the surface of the cathode along a constant radius, the majority of the electrons will be focused at the center of curvature. For practical operations, the electrons are focused at a center of curvature which lies outside the cathode fall region.

The efficiency of these glow discharge cathodes may be substantially improved by the use of shields surrounding them. The shields may be made of a conductor material or an insulator. The shield suppresses the emission of electrons from those surfaces on the cathode where the electrons would not contribute to the main beam. The shield generally is selectively spaced from the cathode so that glow discharge is suppressed within this space.

In the above-mentioned copending applications, an annular cathode for both the hollow and contoured versions are described. The utility of these devices is quite varied and includes the welding of materials. In welding with an annular cathode, a uniform disc-like beam is formed which produces a single molten zone all around the entire circumference of the joint. In any diameter thin-wall materials, such as less than inch pipe or tubing, such welds generally are quite excellent. The welding plane, however, in such a case is horizontal for in that position the surface tension forces act essentially vertically across the weld and tend to contain the molten material. However, as soon as the weld plane is inclined to the horizontal, the molten metal, except for that in the surface film, is free to fiow circumferentially around the weld joint to the lowest point under the action of gravity. When the pressure of the melt at this lowpoint exceeds that which can be contained by surface tension forces, the surface film ruptures and molten material spills out. For the welding of large pipe or rod sizes such a problem can be tremendous and the solution to such a problem is to divide the molten material into small individual Zones in a manner which maintains a high ratio of surface force relative to the mass of molten material.

3 SUMMARY OF INVENTION It is therefore an object of this invention to provide an annular cathode operating in the glow discharge electron beam mode with a plurality of electron beams for pro ducing welds in any desired position.

This object is accomplished by this invention wherein an annular cathode is provided which operates in the glow discharge mode and produces a plurality of beams around its periphery and utilizes these beams in a controlled manner to obtain the full circumferential working of the workpiece with the annular structure.

DESCRIPTION OF THE PREFERRED EMBODIMENT Several embodiments for accomplishing this object are shown in the drawings wherein:

FIGURES 1 and 1A illustrate an annular cathode of the contoured type providing multiple beams from a multiple number of contoured surfaces.

FIGURE 2 shows a similar cathode as that in FIG- URE 1 but with the emitting surfaces arranged to produce the multiple beams away from the concentric axis of the annular contoured cathode.

FIGURE 3 shows a contoured cathode arrangement with independent segments for producing a plurality of electron beams which can be adjusted to accommodate workpieces of varying diameter.

FIGURE 4 illustrates the control of the multiple electron beams of FIGURE 3 to preclude damage to cathode or workpiece due to arc formation during high power applications, and to obtain electronic beam rotation by energizing the segments according to a particular schedule.

:In FIGURE 1 an annular cathode 10, generally made of stainless steel, has an internal surface 12 and an external surface 18. The internal surface is provided with a plurality of alike contoured sections 11. There are a total of eleven sections distributed around the circumference of the internal surface 12. Each section 11 is curved and contoured both in the circumferential and in the axial directions. The circumferential curvature indicated at 14 and the axial curvature indicated at 15 are such as to provide a rectangular section 11 cut from the surface of a sphere. The total cross-sectional dimensions of each section 11 as, for instance, obtained by measuring the cord interconnecting the bottom and top circumferential curves 14 and the cord across the section 11 between the side axial curves 15, are such that they are much larger than the cathode fall distance. The concave surface of each section need not necessarily be commensurate with the rectangular section cut from the surface of a sphere but may be any other desired shape which focuses or directs the beam towards a particular point. In FIGURES 1 and 1A each of the sections 11 generate a beam which is essentially focused at a point 17 on the circumference of the circle 16 which has a diameter usually commensurate with the outside diameter of the workpiece 2. In this instance, the radius of the circle 16 is approximately two inches, the outside diameter of the stainless steel annular cathode structure is 8% inches, the height from one circumferential curve 14 to the opposite circumferential curve 14 is two inches. The radius of the circumference around which the sections 11 are distributed is approximately four inches. The purpose of each section 11 is to focus the beam to a small spot and therefore the curvature of such section is sharper than the curvature of the annular circumference 14 of the cathode 10. In order to obtain the desired annular weld seam on a workpiece, it will be necessary to move or rotate the cathode relative to the workpiece. Such rotation may be accomplished by conventional mechanical means andeither the workpiece or the cathode may be rotated.

An anode can be provided at a convenient location anywhere within the evacuated chamber which contains 4 the cathode assembly and the workpiece. The chamber is evacuated to a pressure commensurate with obtaining the electron beam mode from the cathode.

This cathode is operate within a shield and support assembly similar to the one described in copending application Ser. No. 508,201, entitled, Mounting for a Glow Discharge Cathode, filed Nov. 17, 1965, by Allan P. Walch, and assigned to the same assignee.

In FIGURE 2 a cathode of the type shown in FIGURE 1 is illustrated but in this instance the sections 11 are placed on the external surface of the annular cathode 10. Each surface of a section still produces a focused beam and this is accomplished by making the cross-sectional dimensions of each section again larger than the cathode fall distance. In addition, the circumferential curvature of each section 11 is now opposite to that of the circumference of the annular structure. With a device of the type shown in FIGURE 2, one may weld the seam of a tube from the inside thereof without regard for orientation. The excessive heat radiated back toward the cathode 10 from the workpiece 35 requires cooling. The coolant is supplied at orifice 26 and removed at 27 after passage through coils 28.

FIGURE 3 illustrates another embodiment wherein again a multiple number of contoured cathode sections are used, but each section is electrically and mechanically independent and isolated. The cathode 30 comprises a plurality of contoured sections 31 placed around the internal periphery 32 as shown in FIGURE 2. Each section comprises a contoured cathode 31, a shield 33, and a support tube 34 which isheld in position by a locking device 35. The high voltage lead 36 passes through the center of the support tube 34 and attaches to the cathode 31.

Each cathode section has an approximately spherically contoured emission surface, but may be either circular, rectangular or any other arbitrary shape, in the circum: ferential plane. Circular shapes will minimize end effects while rectangular shapes will permit close packing of the emitting surfaces. The adjustable support tube 34 permits the individual elements to be moved radially to accommodate different diameter workpieces at the optimum focal distance. Also, for applications such as heat treating, surface hardening, etc., the beams can be defocused to obtain a larger zone of heating.

The electrical connection to the cathode 31 may be as described in the copending application to Allan P. Walch, entitled, Mounting for a Glow Discharge Cathode, Ser. No. 508,201, filed Nov. 17, 1965, and assigned to the same assignee. The connection is to be made so as to avoid any sharp corners and eliminate the possibilities of arcing.

An important advantage of a segmented cathode of the type shown in FIGURE 3 is that it can be used to significantly reduce the severity of arcing problems. When large diameter welding operations are required of heavy workpieces, the total current requirements of the cathode may be extremely large. These high current operations tend to induce sporadic arcing causing damage to the workpiece, the shield or the cathode. Suppressing these arcs and also protecting the power supply present great difficulties. A segmented cathode of the type shown in FIG- URE 3 lends itself to prevent such arcing problems as is illustrated by the following description of its electrical interconnections in relation to FIGURE 4.

In FIGURE 4, three leads 37, 38 and 39 are shown entering the cathode 30 to specific apertures provided therefor. These leads selectively interconnect one or more of the cathode segments 31 and are provided with special current limiting devices 41, 42, 43 and switches 44, 45, 46 in series between the leads and the power supply 40. Each current limiter comprises an emission limited diode in which the current is set by the electron emission from the cathode. This emission in turn is determined by the temperature of the cathode which in turn is controlled by the variable transforms 55. It is well known that, for

a given combination of gas, pressure, and type of electrode, a critical value of current exists below which an arc is not self-sustaining. Each current limiting device 41, 42, 43 is set to limit the current to a value less than this critical value, which is typically of the order of 3 amps. If the normal operating current of the cathode structure is less than this value, a single current limiting device is sufficient and can be connected to all segments 31 or to a single element cathode as in FIG. 1. However, if the total normal operating current is, for example, 9 amps, three current limiting devices would be used with each one connected to one-third of the elements 31, as shown in FIG. 4. Any number of cathode segments can be connected to one current limiting device provided that the sum of normal operating currents to these segments does not exceed the critical current level for a self-sustaining arc.

Further arc protection may be provided with the switches 44, 45, and 46. A voltage sensitive control 52, 53, and 54 is coupled respectively to each lead 37, 38 and 39 to sense the occurrence of an excessive drop of the voltage on each lead. For instance, the drop in voltage below a predetermined value *will produce an output signal from each voltage sensitive device and this is used to actuate the high voltage switches 44, 45 or 46 and disconnect the cathode from the power supply for a short time to extinguish the arc.

The switches 44, 45 and 46 may be so controlled to effectively rotate the electron beams from the peripherally located cathodes. This is accomplished with a variable frequency oscillator-timer 48 driving a two-stage counter comprising flip flops 49 and 50. The 1, 2, and 3 counts are decoded sequentially by decoder 51 and applied respectively to switches 44, 45 and 46. The fourth count along lead 55 is used to reset the counter and inhibit further pulses from the timer 48. The frequency adjust of the timer 48 allows for different electron beam pulses lengths needed for welding different materials and thickness as may be encountered. The cathode contours and focal distances may be adjusted to provide overlapping adjacent segments. Individual cathodes may be so-connected to a switching circuit rather than selected groups depending upon the power requirements and economic factors considered for the high voltage switches.

The contoured cathode sections 31 may be replaced with hollow cathodes of the type described before and as specifically shown in the copending application Ser. No. 417,339.

It is to be understood that the invention is not limited to the specific embodiments herein illustrated and described but may be used in other ways without departure from its spirit as defined by the following claims.

Having thus described our invention, what we claim is:

1. An annular circumferential contoured cathode structure operating at a high negative potential with respect to an anode in a gaseous chamber evacuated to a predetermined pressure range of the gas comprising:

means for establishing a glow discharge for the production of an electron beam from the cathode,

said cathode structure having an axis and an internal annular electron emitting surface for directing an electron beam toward and concentric with said axis, said emitting surface being further shaped to include a plurality of contoured sections, each section having a concave surface with a curvature both in the circumferential and the axial directions of the annular cathode, the circumferential curvature of each section being of like direction but sharper than the annular curvature of the cathode, and

the concave surface of each section having cross-sectional dimensions greater than the cathode fall for producing an individually focused beam of electrons from each section.

2. A device as recited in claim 1 wherein the surface of each section resembles a rectangular section cut from the surface of a sphere.

3. An annular circumferential contoured cathode structure operating at a high negative potential with respect to an anode in a gaseous chamber evacuated to a predetermined pressure range of the gas comprising:

means for establishing a glow discharge for the production of an electron beam from the cathode,

said cathode structure having an axis and an external electron emitting surface for directing an annular electron beam away from and substantially concentric with said axis,

said emitting surface being further shaped to include a plurality of contoured sections,

each section having a concave surface with a curvature both in the circumferential and the axial directions of the annular cathode, the circumferential curvature of each section being of opposite direction to the annular curvature of the cathode, and the cross-sectional dimensions of each concave section being greater than the cathode fall for producing an individual focussed beam of electrons from each sec- 5 tion.

4. A device including an annular cathode structure which functions at a high negative potential with respect to an anode for operating on a workpiece positioned substantially concentrically with the cathode structure in a gaseous chamber evacuated to a predetermined pressure range of the gas suitable for the production of a glow discharge comprising:

power supply means coupled to the anode and the cathode structure for establishing a glow discharge for the production of an electron beam therefrom,

said annular cathode structure having an axis and an electron emitting surface for directing an electron beam concentric with said axis,

said emitting surface including a plurality of electrically isolated electron emitting sections, each section having a concave surface with a curvature in both the axial and circumferential directions of the annular cathode structure,

current limiting means interposed between said power supply means and each of said electron emitting sections for reducing the current to a section to a value below that necessary for sustaining an are from a section whenever such are tends to be produced.

5. A device as recited in claim 4 and further including:

means for selectively coupling said emitting sections to said power supply means for alternate working of selected annular portions of the workpiece. 6. A device as-recited in claim 5 wherein said selective coupling means comprises:

a plurality of switches interposed between selected cathode emitting sections and the power supply means,

means producing a plurality of switching signals of substantially alike duration for activation of said switches to electrically connect said selected cathode emitting sections to the power supply means for said durations, and

means for varying the time duration of said switching signals.

7. A device as recited in claim 6 and further comprising:

means for sensing the voltage applied to a cathode emitting section and producing an output signal When 3,497,743 7 8 a supporting member spaced opposite from the emitting 3,067,350 12/ 1962 Stebler et a1 313-93 surface of the section, and 2,977,496 3/1961 Doolittle 313-69 means for radially movably mounting said emitting sections to said supporting member. JOHN W. HUCKERT, Primary EXaminer References Cited 5 B. ESTRIN, Asslstant Examlner UNITED STATES PATENTS 2/1961 Coeterier 3137O US. Cl. X.R.

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