US3185882A

US3185882A - Electron discharge device including cathode-focus electrode assemblies therefor

Info

Publication number: US3185882A
Application number: US82869A
Authority: US
Inventors: Vernon W Dryden; Bertram G James
Original assignee: Eitel Mccullough Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1961-01-16
Filing date: 1961-01-16
Publication date: 1965-05-25
Anticipated expiration: 1982-05-25

Description

y 25, 1955 v. w. DRYDEN ETAL 3, 82

ELECTRON DISCHARGE DEVICE INCLUDING CATHODE-FOCUS ELECTRODE ASSEMBLIES THEREFOR 2 Sheets- Sheet 1 Filed Jan. 16, 1961 mvsnrons VERNON m DRYDEN asnmm 6. JAMES WiEw ATTORNEY v. w. DRYDEN ETAL 3,185,882

DEVICE INCLUDING CATHODE-FOCU May 25, 1965 ELECTRON DISCHARGE ELECTRODE ASSEMBLIES THEREFOR 2 Sheets-Sheet 2 Filed Jan. 16, 1961 INVENTORS VERNON W, DRYDEN BERT RAM 6. JAMES BY M-fl 9 ATTORNEY United States Patent ELECTRUN DlSCHARGE DEVICE INCLUEENG CATHODE-FOQUS ELECTRODE ASSEMBLIES TIEREFQR Vernon W. Dryden, Palo Alto, and Bertram G. .lames, San Mateo, Calif., assignors to Eitel-McCnllough, Inn, San Carlos, Catlin, a corporation of California Filed Jan. 16, 1961, Ser. No. 82,869 6 Claims. ('Jl. 313-83) The invention relates to electron beam guns, and particularly to an electron beam gun suitable for electron beam metallurgical applications such as welding, machining, vaporizing and refining.

The advent of space travel and rocketry has spurred development of exotic alloys and pure metals to meet the exacting requirements imposed on materials used in these applications. Some of these materials are not readily susceptible to conventional fabrication processes, and it has been necessary and continues to be necessary and desirable to develop the art of electron beam metallurgy to cope with these new products to render them usable in applications where they have not heretofore been utilized, such as in the manufacture of electron tubes.

A paramount advantage realized from electron beam metallurgy is that extremes in heat may be generated on extremely small areas to effect whatever process is required, e.g., welding, drilling, vaporizing or refining. Such localized heating is the result of a concentrated high current density electron beam being caused to impinge on the particular area which it is desired to heat. By restricting the application of heat to such localized areas, it will be obvious that adjacent structure will be protected therefrom, thus rendering it feasible to closely associate in structures, materials having widely differing thermal characteristics.

The cleanliness with which metallurgical and fabrication processes can be effected with an electron beam is another advantage of paramount importance. Autogenous unions of materials such as tungsten, tantalum, rhenium, molybdenum, vanadium, platinum, nickel, silicon, and ruthenium may be effected without the introduction of a bonding material, and the localized application of heat reduces to a minimum deleterious liberation of impurities from the material being welded.

Conventional electron guns capable of generating a beam useful in electron beam metallurgy are usually of the type disclosed in United States Patents, 2,771,568, 2,793,281, and 2,793,282 in which a filamentary type cathode of thin tungsten wire constitutes the electron source. Such electron gun structures are extremely sensitive to vibration and impact shock damage, and by their very nature are short-lived, usually less than about ten hours of operation.

It is therefore one of the important objects of the invention to provide a rugged electron gun useful in electron beam metallurgical applications, and which can be produced by mass production techniques to render them substantially insensitive to vibration and impact shock.

Electron guns for electron beam metallurgical applica tions must generate concentrated high current density beams of small cross-section to possess utility in this field. The current density of an electron beam is controlled in large measure by the electron emission of the cathode which is determined by the equation Where A is a constant determined by the cathode material, T is temperature, and qb is the true work function of the material from which the cathode is formed. The true work function of course is determined by the cathode Patented May 25, 1965 material and the state of the cathode surface, both of which may vary with temperature. In filamentary hairpin or coil type cathodes as used, for example, in electron guns such as those disclosed in the patents previously referred to, the area or surface from which electrons are emitted is extremely limited, and electron emission is therefore also limited. For a cathode of given area, the electron emission may be increased by either increasing the temperature or decreasing the work function by changing the cathode material or changing the state of the cathode surface. To achieve any appreciable increase in electron emission over average operating emission, filamentary cathodes of the hairpin or coil type must be 0perated near the evaporation temperature of the cathode material. At this temperature some evaporation of cathode material occurs, rendering an already thin filament even thinner, thus reducing the effective area of the cathode, and consequently reducing the emission of electrons. For these reasons filamentary type cathodes are short lived when used in electron beam metallurgical applications. Another reason for their short life is that in electron beam metallurgical applications, evaporation or melting of the workpiece results in increased ion bombardment of the fragile filament, with destructive results. It is accordingly another object of the invention to provide an electron gun capable of generating a high current density electron beam supported by high electron emission from a rugged cathode structure of thick cross-section and having a large emitting area contributing to a long life expectancy.

Another requirement of electron beam guns useful for electron beam metallurgical applications is that they be capable of projecting a high current density beam of extremely small cross-section. In a filamentary type cathodes this requirement is met to a large extent by restricting the area of emission to a so-called point source. In a cathode having a large area for high electron emission, a small beam cross-section is dilficult to achieve. It is therefore another object of the invention to provide a gun structure incorporating focusing means adapted to provide a small cross-section beam having the requisite high current density.

Still another object of the invention is the provision of an electron gun of the bombarded cathode type utilizing a pure tantalum cathode button as the emitting source of electrons. Such a cathode is especially resistant to the deleterious effects of ion bombardment, and is capable of being raised to a high temperature to restore it to its original condition if it is inadvertently poisoned" by ion bombardment.

A still further object is the provision of an electron gun in which the beam generating and controlling electrodes are cooperatively arranged to provide stability against relative movement due to thermal expansion and contraction and mechanical vibration.

in most conventional electron guns the least reliableelement is the filament or coil used to heat the cathode, either by direct radiation or by electron bombardment. This element, or the associated cathode, usually determines the life expectancy of the electron gun. It is therefore another object of the invention to provide an electron gun structure in which the cathode is practicably indestructible, and in which a defective heater therefore may be easily removed and replaced without disturbing the relationship between cathode and focus electrode.

The invention possesses other objects and features of value, some of which, with the foregoing, will become apparent from the following description and the drawings. It is to be understood, however, that the invention is not limited to the specific embodiment illustrated and described, as various forms of the invention may be adopted Within the scope of the appended claims.

Briefly. described, the electron beam gun' embodying the invention comprises a hollow enclosure adapted to be detachably secured to and have its interior communicate with the interior of an associated vacuum chamber within which a metallurgical process may be carried out. The hollow gun enclosure includes a centrally apertured base plate on which the main cathode and focus electrode are rigidly supported in a manner to compensate for thermal expansion and contraction of the parts. The cathode heater structure, including an auxiliary cathode and focus electrode therefor, extends through the base plate into operative position adjacent the main cathode to effect bombardment of the main cathode. To removably support the heater structure on the base plate; a complex of integrally and hermetically united dielectric rings and metallic electrode support shells are heliarc welded to the base plate in association with the central aperture therein. concentrically arranged about the electron gun electrodes and hermetically united at one end to the base plate is a tubular dielectric shell or housing portion, the end thereof remote from the base plate terminating adjacent the main focus electrode and serving to support an apertured accelerating electrode operatively arranged adjacent the cathode. A tubular metallic housing extension mounted at one end on the accelerating electrode concentric with the dielectric housing portion is provided with a flange utilized to secure the gun assembly to the evacuated chamber into which the beam of electrons will be projected.

Referring to the drawings:

areasea 21, the focus electrode converges inwardly'toward the central axis of the electron gun in a conically tapered portion 24 apertured at its apex end 26 to form a truncated conical shell'section closely associated to a main cathode button 27.

' Cathode button 27 is preferably formed from tantalum to provide a concave emissive surface 28, and is supported in operative relation to the focus electrode by radially extending circumferentially spaced tantalum spokes 29, each having one end welded to the back surface 31 of the cathode button and its other end welded adjacent the inner periphery of the adjacentannular concave shell 32. Shell 32 is integral with cylindrically extending cathode support shell 33, arranged concentrically within shell 21,

and has its end remote from shell 32 Welded to the inner eripheral surface of shell 21 at a point spaced from the union of shell 21 and shell 16. The end of cathode support shell 33. is flared outwardly to effect contact with FIGURE 1 is a vertical half-sectional view illustrating ,7

the internal structure of the electron gun.

FIGURE 2 is a fragmentary horizontal sectional View taken in the plane indicated by the line 22 in FIG- URE 1.

FIGURE 3 is a fragmentary horizontal sectional View taken in the plane indicated by the line 3--3 in F1"- URE 1.

All figures are drawn to a scale approximately twice actual size.

In more specific detail, the electron gun embodying the invention comprises an annular metallic base plate 2 of high' electrical and thermal conductivity serving to rigidly support the other elements of the gun structure. I Adjacent its outer peripheral edge the base plate is grooved to receive a concentrically arranged dielectric backing ring member 3 axially aligned with tubular dielectric shell 4 forming part of the electron gun envelope. Metallic sealing flanges 6 and '7 integrally interposed between the ring, shell and plate hermetically unite this end of the shell to the base plate. At the other end of the shell 4, a similar sealing structure integrally unites the accelerating anode 8 to the shell 4. The anode is centrally apertured as shown at 9 to permit the passage of an electron beam as indicated by dash lines 16. Atubular housing extension 12 brazed to the accelerating anode on the opposite side thereof from shell 4 is provided with a radially extending flange 13 adapted to secure the electron gun housing structure to a chamber Within which a metallurgical process is adapted to be carried out.

7 Rigidly supported on the base plate within the shell 4 is a main cathode and focus electrode assembly designated generally by the numeral 14, and comprising a truncated conical shell 16 having a radial baseflange 17, and a cylindrical apex flange 18. Apertures 19 formed in the conical shell lighten the assembly, While detachably securing the radially extending flange 17 to the plate 2 by means of screws 20 provides a continuous electrically conductive electrode 23. From its point of attachment to the shell shell 21 and to provide a space 34 between

shells

21 and 33 over a greater portion of the length of shell 33.

From the foregoing it Will be apparent that main cathode button 27 lies suspended on spokes 29 in close proximity to aperture 26 in the apex end of focus electrode 23. By virtue of this construction, when shell 21 expands and contracts due to temperature fluctuations,

shell 33 will also expand in about the same proportions and in the same direction, thus maintaining the close relationship between the focus electrode and cathode button 27. Shells 2i, 33, and focus electrode 23 are preferably formed from tantalum. For maximum efiiciency, the

. surface of the shell 32 closely adjacent the cathode, and

the inner surface of the conical section 24 of the focus electrode are rhodium plated for maximum reflectivity.

To effect heating of the main cathode button 27, it is preferred that a heater structure be provided which may be removed from the assembly without disturbing the physical relationships existing between the main cathode and its associated focus electrode. In the present embodiment of the invention it has been found that an electron bombarded cathode type construction provides the desired efficiency. To provide a source of such electrons, an auxiliary tungsten cathode 41 is provided. Auxiliary cathode 41 is preferably formed of tungsten wire supported in flPPropriately spaced relation to surface 31 of the main cathode by leads 42 and 43. Leads 42 and 43 merge with heavier support leads or

posts

44 and 46, respectively, the lead 46 at its other end being welded to a central support block 47. The other end of lead is welded to the upper end portion of a cylindrical shell 48, the lower end of which is flared and welded to a conical shell 49 having a radially extending flange 51 integrally and hermetically brazed between the adjacent metaliz ed edges of a pair of axially arranged

ceramic cylinders

52 and 53. An outwardly extending lug 54 on flange 51 provides a means for connecting the heater lead 42 to a source of power. 1

Support block 47 is also supported on ceramic ring 53 by a conical shell 56 having a radially extending flange 57 at its base end integrally brazed to the opposite edge of ceramic cylinder 53. A backing ring 58 brazed'on the opposite side'of flange 57 equalizes the stresses imposed on the ceramic ring 53 due to thermal expansion and contraction. The apex end59 of shell 56 is integrally brazed meansfor connecting the auxiliary cathode'41 to a source of energizing power. 'From the foregoing it will be seen that currentfrom an appropriate source passes through the heavy conductor 61, support body- 47, support lead 45, cathode lead. 43, auxiliary cathode 41, and then through output lead 42 from the cathode, support lead 44, and through output shell'sdS and 49 to an appropriate lead connectc'd to lug 54. 'Tox eifectuate the; passage of current in' thismanner the upper end portion-of shell 48 is provided in the vicinity of support rod 46 with a' notch 62, providing electrical insulation between rod 46 and shell 43. On the opposite side of block 47 however, the block is provided with a notch 63 functioning to electrically isolate the lower end of support rod 44 from the block.

To focus electrons from auxiliary cathode 41 onto the back surface 31 of main cathode 27, an auxiliary cathode focus electrode is provided. Such electrode comprises a concentrically arranged shell 67, supported at its lower end by a radially extending flange 6S, detachably connected to a supporting bracket 69 welded or brazed about the upper end of support block 47 in conductive contact therewith and with the upper end of shell 48. The opposite end of cylindrical focus electrode shell 67 is spun inwardly in a portion 66 having a generally truncated conical cross section apertured as at 71 to provide a passage for electrons.

From the foregoing, it will be apparent that main cathode 27 and its associated focus electrode 23 are electrically connected together and operate at the same potential. It will likewise be noted that auxiliary cathode 41 with its appropriately associated focus electrode 66 are also electrically connected together and operate at the same electrical potential.

For increased efficiency in a device of this type, it is important that heat radiated by the cathodes be redirected in a. manner to provide maximum heat reflectivity in a direction to impinge upon the cathodes. The auxiliary cathode is therefore provided with a heat shield comprising a transversely extending dish-shaped member 72 supported by a cylindrical flange 73 welded to the upper end of shell 67. The transversely extending shield member 72 is apertured appropriately as at 74 to provide for the passage of leads 42 and 43. As with

shells

24 and 32, the upper surface of member 72 is rhodium plated so that heat from the auxiliary cathode will be reflected toward the under side of main cathode 31. Because shield 72, forming a wall under auxiliary cathode 41, will become quite hot and radiate heat in a direction opposite to the main cathode 31, auxiliary heat shield means in the form of transversely extending dish-shaped plates 76 are provided supported immediately below shield member 72. Plates 7:: are supported on central support rod 77, the lower end of which is brazed in support block 47. It will thus be seen that heat radiated from member 72 will strike the rhodium plated upper shield plate 76 and be reflected back toward member 72, thus effectively containing most of the heat in the immediate vicinity of the auxiliary and main cathodes.

Because a heater structure, even as rugged as the one described, is usually the weakest element of the combination, means are provided enabling removal of the heater structure from the assembly. Such means comprise a sealing flange 81 brazed to the upper end of ceramic cylinder 52 and having a cylindrical extension 82 adapted to be heliarc welded to the cylindrically extending flange 83 brazed on plate 2. A backing member 84 is interposed between flange 81 and plate 2 supports the construction and provides for a sliding contact between the heater support structure and the plate 2.

In operation, mounting flange 13 is adapted to be detachably connected to a complementary flange on the vacuum chamber (not shown) to which the electron gun will be connected. Appropriate lead means (not shown) connected to plate 2 and to accelerating anode 8 charge these elements to any desired potential, and cooperate with the heater structure previously described to generate a beam of electrons as indicated at 10. Such an electron gun construction may be mounted either horizontally or vertically or at some intermediate angle without danger of the electrode elements within the gun sagging or being otherwise displaced.

Conventional electron beam metallurgical chambers usually provide a movable worktable within the chamber on which the material to be processed is movably supported. In these chambers the electron beam is usually stationary, and the work is moved in relation thereto. Other conventional devices provide for electrostatic control and direction of the beam while the work remains stationary or is movable and thus derive a somewhat broader range of movement. Such arrangements, however, are complex and cumbersome because of the electrostatic lenses and power supplies that are required to effect control of the beam. Because of its ruggedness, the gun structure here disclosed is especially adapted to be movably mounted on a vacuum chamber.

We claim:

1. In an electron discharge device, a cathode-focus electrode assembly comprising a hollow focus electrode shell, a tubular support shell surrounding at least a portion of the focus electrode shell and integral therewith, a cathode button within the tubular support shell spaced from one end of the focus electrode shell, and means including a plurality of radially extending spokes interposed between the underside of the cathode button and the tubular support shell to support the button in operative relation to the focus electrode.

2. In an electron discharge device having a base, a pair of electron beam forming electrode assemblies arranged in tandem on the base, each electrode assembly including a mechanically and electrically interconnected cathode and focus electrode cooperating to form an electron beam having a predetermined minimum cross-sectional area at a predetermined distance from the cathode, the predetermined minimum cross-sectional area of the beam projected by one of said electrode assemblies being substantially equal to the emissive area of the cathode of the other electrode assembly and the distance between the cathodes of the two assemblies being substantially equal to the predetermined distance of the minimum cross-sectional area of one of the beams from the associated cathode.

3. The combination according to claim 2, in which said base is apertured, one electrode assembly is mounted on one side of the base, and the other electrode assembly is mounted on the opposite side of the base and extends through the aperture therein into operative tandem relation with said one electrode assembly.

4. The combination according to claim 2, in which the focus electrode of each electrode assembly comprises a hollow shell including a truncated conical portion.

5. The combination according to claim 4, in which the apex ends of said truncated conical portions are axially spaced, and the cathode of one of said electrode assemblies is supported between the apex ends of said focus electrodes and operatively associated with one of them.

6. The combination according to claim 4, in which the apex end of the truncated conical portion of each focus electrode is apertured, a heat shield extends transversely across one of said focus electrodes adjacent the base thereof, and a cathode is operatively disposed between said transverse shield and the truncated conical portion of the focus electrode.

References Cited by the Examiner UNITED STATES PATENTS 2,888,591 5/59 Schmidt et a]. 313-305 2,912,616 11/59 Marchese et al 313-821 DAVID I. GALVIN, Primary Examiner.

RALPH G. NIL ON, xaminer.

Claims

2. IN AN ELECTRON DISCHARGE DEVICE HAVING A BASE, A PAIR OF ELECTRON BEAM FORMING ELECTRODE ASSEMBLIES ARRANGED IN TANDEM ON THE BASE, EACH ELECTRODE ASSEMBLY INCLUDING A MECHANICALLY AND ELECTRICALLY INTERCONNECTED CATHODE AND FOCUS ELECTRODE COOPERATING TO FORM AN ELECTRON BEAM HAVING A PREDETERMINED MINIMUM CROSS-SECTIONAL AREA AT A PREDETERMINED DISTANCE FROM THE CATHODE, THE PREDETERMINED MINIMUM CROSS-SECTIONAL AREA OF THE BEAM PROJECTED BY ONE OF SAID CATHODE ASSEMBLIES BEING SUBSTANTIALLY EQUAL TO THE EMISSIVE AREA OF THE CATHODE OF THE OTHER ELECTRODE ASSEMBLY AND THE DISTANCE BETWEEN THE CATHODES OF THE TWO ASSEMBLIES BEING SUBSTANTIALLY EQUAL TO THE PREDETERMINED DISTANCE OF THE MINIMUM CROSS-SECTIONAL AREA OF ONE OF THE BEAMS FROM THE ASSOCATED CATHODE.