US3428776A

US3428776A - Method and apparatus for extracting a charged particle beam into a higher pressure atmosphere

Info

Publication number: US3428776A
Application number: US523583A
Authority: US
Inventors: Lynn H Stauffer
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1966-01-28
Filing date: 1966-01-28
Publication date: 1969-02-18
Anticipated expiration: 1986-02-18
Also published as: DE1615267A1; GB1138905A

Description

} mm?" mm? Feb. 18, 1969 H. STAUFFER 3,428,776

METHOD AND APPARATUS FOR EXTRACTING A CHARGED PARTICLE BEAM INTQ A HIGHER PRESSURE ATMOSPHERES t hee Filed Jan. 28, 1966 [)7 vent or: Lynn H Sauf'f'er,

3,428,776 PARTICLE BEAM E Sheet 3 of TING A CHAR E URE ATMOSP 8. 1969 L. H. STAUFFER METHOD AND APP ATUS FOR EXTRAC IN; A HIGHER PRESS Filed Jan. 28, 1966 Inventor: Lynn H SZJaz/f'f'er, by Q W QWMA United States Patent 3,428,776 METHOD AND APPARATUS FOR EXTRACTING A CHARGED PARTICLE BEAM INTO A HIGHER PRESSURE ATMOSPHERE Lynn H. Stautrer, Pattersonville, N.Y., assignor to General Electric Company, a corporation of New York Filed Jan. 28, 1966, Ser. No. 523,583 US. Cl. 219-121 12 Claims Int. Cl. B23k 15/00, 9/16 ABSTRACT OF THE DISCLOSURE A charged particle beam such as an electron beam issuing from a low pressure gaseous environment into a high pressure gaseous environment is transmitted to a workpiece therein without substantial loss in beam power and with minimum beam scattering by a process of vaporizing a highly condensible and nondecomposable metallic vapor supplied to the beam exit aperture at a controlled rate. The material to be vaporized is supplied to the aperture in the solid or liquid state. The vaporized material generates a first jet of the vapor directed outwardly from the aperture to displace the external high pressure gaseous environment in the region of the aperture. The vaporized material also generates a second jet directed inwardly into the aperture and this jet condenses on an inner wall thereof to function as a getter of undesired gases and also to constrict the aperture and completely seal it when the intensity of the beam is reduced below a critical value.

My invention relates to the extraction of charged particle beams from low pressure gaseous environments into relatively high pressure gaseous environments without substantial loss in beam power, and in particular, to a method and apparatus for accomplishing this beam extraction by generating jets of metal vapor at a beam exit aperture separating the two environments to thereby displace the high pressure environment tending to flow toward the low pressure environment.

Electron beam welding and other irradiation processes employing charged particle beams (electron or ion) are advantageously accomplished in relatively high pressure, selected gaseous environments whereas the beam source or generator must operate in a substantial vacuum, or at least in a relatively low pressure gaseous environment. A.

particular problem when extracting a charged particle beam from the low pressure gaseous environment within the generator to the relatively high pressure environment external thereof is a substantial loss in beam power due to beam heating of thin windows which conventionally separate the two environments at the beam exit aperture of the generator. The thin windows, which may comprise a material such as a one mil thickness stainless steel foil, further present a cooling problem due to the natural tendency of the window to become overheated by the beam. The thin windows are generally associated with high energy, low intensity beams. For high intensity beams, having energies from to 200 kilovolts, a series of small difierentially pumped apertures are conventionally used. The latter arrangement is expensive, space consuming and inconvenient for many applications. Multiple apertures further create serious beam alignment problems and require more than one beam focussing lens.

Therefore, one of the principal objects of my invention is to provide a new method and apparatus for extracting a charged particle beam from a relatively low pressure gaseous environment into a relatively high pressure gaseous environment without substantial loss in beam power.

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A second problem inherent in the extraction of a charged particle beam into a higher pressure gaseous environment is the introduction of objectionable scattering of the beam. The use of either conventional approach, thin window or multiple apertures, does not remedy this problem.

Therefore, another object of my invention is to provide a new method and apparatus for extracting the beam with reduced beam scattering.

A still further object of my invention is to extract the beam through a single aperture in which are generated jets of metal vapor.

Briefly stated, my invention provides a method for extracting a charged particle beam from a relatively low pressure gaseous environment internal of a charged particle beam generator into a relatively high pressure gaseous environment through a beam exit aperture by the process of vaporizing a selected material supplied thereto at a controlled rate. The material to be vaporized may be a metal, and is supplied to the aperture in the solid state, in the form of a thin tape or wire, or in the liquid state. The charged particle beam upon being transmitted through the aperture impinges upon the material supplied thereto and vaporizes a small amount thereof at a controlled rate. The vaporized material generates a first jet of the vapor directed outwardly from the aperture and a second jet directed inwardly thereto. The outwardly directed jet is of magnitude sufficient to displace the external high pressure gaseous environment in the region of the aperture normally tending to flow toward the low pressure gaseous environment. This region of displaced high pressure environment is also the region between the aperture and workpiece being processed by the beam, and the beam is therefore transmitted from a relatively low pressure gaseous environment to a workpiece in a relatively high pressure gaseous environment without substantial loss in beam power and with minimum scattering of the beam. The inwardly directed jet of vapor material condenses on an inner wall of the aperture and, if comprised of suitable material, functions as a getter of undesired gases which may be present therein.

The apparatus for practicing my beam extraction method includes a novel beam exit assembly for a charged particle beam generator. The beam exit assembly which includes the beam exit aperture, is preferably provided with an inner chamber for storing the condensed material from the inwardly directed jet of vapor material. The means for supplying the (highly condensible and nondecomposable) material to the aperture is determined by the form of the material. Thus, in the case of a thin metal tape, the tape is controllably unwound from a first spool, and wound on a second spool oriented to feed the tape in contacting relationship with the exit aperture such that during the time no beam is being generated, the exit aperture is sealed from the external high pressure environment and during generation of the beam the moving tape tends to maintain the aperture substantially sealed.

The features of my invention which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, wherein like parts in each of the several figures are identified by the same reference character, and wherein:

FIGURE 1 is an elevation view, partly in section, illustrating a first embodiment of my charged particle beam extraction apparatus employing a movable tape;

FIGURE 2 is a second embodiment of the apparatus employing a movable tape;

FIGURE 3 illustrates the apparatus provided with a wire feed; and

FIGURE 4 illustrates the apparatus supplied with material in the liquid state.

Referring particularly to the apparatus illustrated in FIGURE 1, there is shown a first embodiment of the apparatus constructed in accordance with my invention and my novel method for extracting a charged particle beam into controlled atmospheres. A constriction through which the beam passes from a low pressure to high pressure region may be a beam exit aperture 5 of a charged particle beam generator shown only in part and identified as a whole by numeral 6, and comprises an elongated channel or nozzle having an inner side wall (or walls) 7. The nozzle side walls may be parallel (when viewed in section), but in general are slightly inclined, as shown in FIGURES l and 2 to conform to the cross section of the generally converging beam passing therethrough. The cross section of the exit aperture along the beam axis is, in general, dependent upon the cross section of the charged particle beam 8 which is generated internal of generator 6, and for many applications is circular in shape. Aperture 5 is preferably made as small in cross section as possible to permit passage of the beam therethrough while leaving little room for influx of gaseous medium from the high pressure region. The charged particle beam may be an electron or ion beam, as determined by the particular beam generating structure internal of generator 6, such particular structure not comprising a part of my invention. The region 9 internal of the charged particle beam generator is a relatively low pressure gaseous environment, and in many of the applications may be substantially a vacuum. The region 10 external of the charged particle beam generator is, in general, a relatively high pressure gaseous environment which in many applications comprises the ambient atmosphere. The charged particle beam 8 upon transmission through the beam exit aperture 5 is employed for processing materials in the ambient atmosphere or in controlled gaseous environments. Thus, a workpiece 11 may be maintained in stationary or moving relationship with respect to aperture 5 for appropriately processing by the beam 8 impinging thereupon.

A charged particle beam upon transmission through a beam exit aperture generally suffers beam scattering and considerable loss in beam power due to the higher density of the gaseous environment external of the generator. as hereinbefore described. The method and apparatus hereinafter described substantially reduces this beam scattering and loss in beam power. The essence of my invention is to supply a material characterized as being a highly condensible and nondecomposable vapor at elevated temperatures to the beam exit aperture 5 at a controlled rate. The nondecomposable characteristic is restricted to those compounds, alloys and mixtures whose elements are not readily condensible and/or reactive with the constituents of the electron beam source and workpiece. The material should also be capable of being conveniently supplied to the aperture, having low vapor pressure, high heat conductivity and high gettering ability, which characteristics are inherent in many elemental metals such as aluminum and titanium. Finally, the material should have wetting ability to provide thermal contact with the aperture walls. The material is supplied into the aperture or along the end thereof in a manner such that the charged particle beam 8 impinges thereupon. The beam upon impinging on the material causes heating thereof sufiicient to produce vaporization (and the attendant expansion) of the material. The expanding vapor material generates a twin jet action wherein a first jet (indicated by arrows 12) is directed outwardly away from aperture 5, and by conservation of momentum a second jet (indicated by arrows 13) is directed inwardly of the aperture. The outwardly directed jet 12 displaces the relatively high pressure gas in external environment 10 adjacent aperture 5 which tends to pass through the aperture to the lower pressure environment 9 internal of the generator. Jet 12 thus provides a path of low air or other gaseous density for the emitting beam thereby reducing the heating of such external atmosphere which is one of the primary causes of loss in beam power as well as causing scattering of the beam.

The vaporization process has the feature of cooling the aperture as an added benefit. An additional desirable characteristic of the material to be vaporized is the capability ofwetting (adhering to) the Walls of aperture 5. This wetting feature is desirable in that it provides protection of the aperture walls from melting and erosion by the charged particle beam 8 passing thereby. Another advantage of the deposition of this material on the aperture walls upon condensation thereof is the utilization of the lower intensity fringe of the charged particle beam to melt the material and maintain vaporization in the aperture. The deposition feature also removes the low intensity fringe of the charged particle beam which results in a slight reduction of the total beam power but an increase of the average power density in the beam transmitted through the aperture, this feature being especially desirable for deep narrow beam applications such as in electron beam welding. The self constriction of the beam exit aperture, which is produced by the deposition of the condensed material on the walls of the aperture automatically constricts the effective cross section of the aperture to the intense core of the charged particle beam thereby further minimizing any influx of atmospheric gas into the generator. Finally, the self constriction of the beam exit aperture functions to completely close the aperture when the beam intensity is reduced below a critical value, such that in the condition wherein no beam is being gene-rated, the relatively low pressure gaseous environment internal of the generator is completely sealed from the external environment.

The material which is supplied to the aperture 5 and subsequently vaporized to form the outwardly and inwardly directed

jets

12 and 13, respectively, may be in the solid or liquid state. In FIGURE 1, the material is in the solid state in the form of a thin metal strip or tape 14 made of a suitable material such as aluminum or stainless steel by wayof example and not by way of limitation on the materials which may be employed. Thus, any material which is characterized as being a highly condensible and nondecomposable (into volatile components) vapor at elevated temperatures is suitable, the material further preferably characterized as being a wetting agent and functioning as a getter for any undesirable gas that may be present in the aperture or generator. The metal tape may be up to several millimeters in thickness and passes across the bottom end of aperture 5 at a speed of a few inches per minute. The thickness and speed of travel of the tape 14 is determined by factors such as the diameter or cross sectional area of the beam exit aperture at the feed point, the difference in gaseous pressures external and internal of the beam generator, the atomic mass of the vapor material and the temperature generated during the vaporization process. The feed rate in grams per second for an exit aperture having a circular cross section, it being assumed that a perfect vacuum is maintained internal of the generator, may be determined from the following equation which provides that the outwardly directed jet reaction balances the force of atmospheric pressure 21r7 P M: 3KT 1/2 wherein r=radius of the exit aperture at the material feed point in centimeters P=atmospheric pressure in dynes per centimeter K=the Boltzmann constant, 1.3805 X10 ergs per degree Kelvin T=absolute temperature in degrees Kelvin m=atomic mass of the vapor in grams For aluminum, m'=4.5 10 T is approximately 2,000 and P= Thus, for an aperture having a radius r of 0.05 cm., the feedrate M of aluminum tape (or wire is 0.120 gram/sec.

The tape material 14 may be fed to aperture 5 in any convenient manner such as by unwinding from a first reel 15 and winding on a driven second reel 16 which preferably is controllably driven by a suitable apparatus such as a controlled speed electric motor drive 17. One or more takeup reels (not shown) may also be employed in association with reels 15, 16 and tape 14.

Referring now to FIGURE 2, there is shown a second embodiment of my apparatus employing a material in the form of tape 14 passing along the end of beam exit aperture 5 and preferably in direct contact therewith. An electromagnetic or electrostatic focussing lens 20 is preferably employed within the charged particle beam generator 6 for converging the beam 8 to its smallest cross section in the region of tape 14 to thereby utilize a minimum size aperture 5. The very small cross section of the beam at this point has the further advantage of impinging upon only a very narrow portion of tape 14 such that the tape may become narrowly slitted by the beam passing therethrough, and in many cases the slit becomes resealed as that portion of the tape passes away from the beam and the molten state thereof solidifies. A guide plate 21 may be conveniently attached to a bottom portion of the charged particle beam generator 6 and functions to maintain the tape 14 positioned between the bottom of aperture 5 and guide plate 21.

The major distinction between the apparatus illustrated in FIGURES 1 and 2 is that in the FIGURE 1 embodiment, the beam exit assembly merely comprises beam exit aperture 5 defined by side walls 7 whereas in FIG- URE 2, the assembly further includes a chamber 22 which is totally enclosed along the side walls thereof. Chamber 22 is defined at the bottom end by a beam exit aperture plate having beam exit aperture 5 therethrough, and at the top end by a second aperture plate 23 having an aperture 24 therethrough wherein aperture 24 is of larger size in cross section than aperture 5. Apertures 5 and 24 are in alignment with beam 8.

Aperture plates

23 and 25 are each preferably cooled due to the high temperatures sustained near the aperture walls thereof in the presence of a generated charged particle beam 8. Cooling ducts 26 may be provided in the aperture plates and supplied with a cooling fluid medium such as water as one example, although it is to be understood that other cooling means may also be employed. The surface of the tape 14 which contacts the bottom'of aperture plate 25 may be lubricated or otherwise treated to improve the sealing action of the moving tape with the bottom surface of plate 25 to further isolate the internal low pressure environment from the external high pressure environment. The use of chamber 22 as illustrated in FIGURE 2 permits relatively long term operation of the beam extracting apparatus as compared to the aperture assembly in FIGURE 1 in that chamber 22 forms a condensation chamber for storage of the condensed material from the inwardly directed jet 13 of the vaporized material from tape 14. The volume of condensation chamber 22 determines the length of time that such beam exit assembly may be utilized before such volumeis completely filled in by the condensing material. The beam exit assembly of FIGURE 2 forms a nozzle-like member which may conveniently be attached to the lower portion of the charged particle beam generator 6 in a suitable manner such as by being screwed on along threaded section 27. Thus, upon chamber 22 becoming substantially filled in by the condensed material, the beam is interrupted and the nozzle assembly replaced by a second nozzle assembly having an empty chamber 22. The condensed material within chamber 22 may be removed by any of a number of known methods such as dissolving such material with a suitable reagent or melting out the material. It is also foreseeable that chamber 22 may be provided with means for removing the material from the inwardly directed jet of vapor without the necessity of detatching the nozzle member from the charged particle beam generator and even without interrupting the generation of the beam.

Referring now to FIGURE 3, there is shown a second means for introducing a condensible material in close proximity to the beam exit aperture 5. In particular, the material is in the solid state in the form of a metal wire 30 which is fed at a controlled rate by a wire feed mechanism 31 through a small diameter hole 32 which passes through one of the side walls 7 defining beam exit aperture 5. To accomplish this wire feed, the bottom aperture plate 25 of FIGURE 2 is preferably replaced by a more massive member 33 having a funnel-shaped channel 34 in the lower portion thereof for providing further acceleration of the outwardly directed jet of vapor 12 to thereby increase the displacing effect of the external atmosphere by such jet. This increase of acceleration of jet 12 may also be obtained in the FIGURE 2 embodiment by attaching a funnel-shaped member 28 to guide plate 21 as illustrated therein.

FIGURE 4 illustrates a fourth embodiment of my invention wherein the material supplied to the beam exit aperture 5 is in the liquid state as distinguished from the solid state in the FIGURE 1, 2 and 3 embodiments. One or more ducts or small diameter holes 44 passing through the side walls 7 of beam exit aperture 5 form passages for the liquid material flowing from an external reservoir 41 to the beam exit aperture. An external heat supply (not shown) may be employed to maintain the material 42 in the molten state within the reservoir, or, the (reduced amount of) heat dissipated from interaction of the beam passing through the aperture may meet this requirement. An inert gas may be admitted to the reservoir 41 at controlled pressure to regulate the flow of the liquid material through the ducts 40. The gas is illustrated as being supplied from a gas source 43 and being pressure controlled by means of valve 44. Alternatively, the liquid material may be supplied to the aperture by feeding the material in a solid state through a small resistively heated tube to obtain the liquid state in place of the reservoir 41 gas pressure control combination. In the liquid material supply system such as illustrated in FIGURE 4 wherein the reservoir 41 is in direct contact with the beam exit assembly, the vaporized material of the inwardly directed vapor jet may remain in the liquid state upon condensation within chamber 22. In such case, a means for draining the material into aperture 5 or back into the reservoir for recycling purposes may be employed.

From the foregoing description, it can be appreciated that my invention attains the objectives set forth in that it makes available a new method and apparatus for extracting a charged particle beam from a low pressure gaseous environment to a relatively high pressure gaseous environment without substantial loss in the beam power and with reduced beam scattering. The problem of maintaining a positive separation between the two different pressure gaseous environments and also minimizing power losses and scattering of the charged particle beam upon extraction through a constriction separating the two environments is accomplished by supplying a material characterized as forming a highly condensible and nondecomposable (as hereinbefore clarified) vapor at elevated temperatures to the constriction such that this material when impinged upon by the beam forms an out wardly directed jet of the vapor material in quantity sufficient to displace any gas in the immediate higher pressure environment which would normally tend to flow through the constriction. This highly condensible material, which is preferably a single or combination of metals, may be supplied in the solid state in the form of a tape or wire, or in the liquid state.

Having described four embodiments of an apparatus for practicing my new method of charged particle beam extraction into relatively high pressure gaseous environ ments, it is believed obvious that modification and varia tion of my invention is possible in the light of the above teachings. Thus, the beam exit assembly may take other forms than that illustrated and the material supplied to the aperture in the solid state may take forms other than a thin tape or wire. For example, the material in solid wire form may be supplied to capillary channels cut into the exterior surface of the nozzle member and in communication with the aperture such that surface tension and wetting forces supply the material, melted by the hot nozzle, to the aperture similar to soldering iron action. Finally, the funnel-shaped member 28 in FIGURE 2 for increasing the acceleration of the outwardly directed jet of vapor material may also be employed in FIGURES 1 and 4, as desired. It is, therefore, to be understood that changes may be made in the particular embodiments as described which are within the full intended scope of the invention as defined by the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a method for extracting a charged particle beam from a relatively low pressure gaseous environment into a relatively high pressure gaseous environment through a constriction which separates the environments, the step of:

supplying a material characterized as being a highly condensible and nondecomposable vapor at elevated temperatures to a constriction at a controlled rate whereby a charged particle beam being extracted from a relatively low pressure gaseous environment on a first side of the constriction into a relatively high pressure gaseous environment on a second side of the constriction is transmitted thereto without substantial beam power loss due to the high pressure environment in the region of the constriction being displaced by a first jet of the material in the vapor state being directed into the high pressure environment.

2. In the method set forth in claim 1 wherein the material is supplied to the constriction in the solid state, the further steps of:

generating a charged particle beam within the low pressure gaseous environment, and

focussing the beam to obtain a beam cross section sufficiently small in the region of the constriction at which the solid material is supplied whereby the beam upon passage within the constriction impinges on the material supplied thereto and vaporizes a small amount thereof at a controlled rate, the vaporized material also generating a second jet of the material in the vapor state being directed into the low pressure environment and functioning as a getter.

3. In the method set forth in claim 1 wherein the material is supplied to the construction in the liquid state, the further step of:

focussing the beam to obtain a beam cross section sufficiently small in the region of the constriction at which the liquid material is supplied whereby the beam upon passage within the constriction impinges on the material supplied thereto and vaporizes a small amount thereof at a controlled rate, the vaporized material also generating a second jet of the material in the vapor state being directed into the low pressure environment and condensing on an inner wall of the constriction and functioning to constrict the aperture to further minimize influx of the high pressure environment into the low pressure environment.

4. A method for extracting a charged particle beam from a relatively low pressure gaseous environment into a relatively high pressure gaseous environment through a beam exit aperture of a charged particle beam generator comprising the steps of:

supplying a material at a controlled rate to the beam exit aperture of a charged particle beam generator, the material characterized as being nondecomposable and forming a highly condensible vapor at elevated temperatures,

generating a charged particle beam within a relatively low pressure gaseous environment internal of the generator wherein the beam is directed through the exit aperture to impinge upon the material supplied thereto, and

vaporizing the material at the exit aperture at a controlled rate whereby the charged particle beam is transmitted into a relatively high pressure gaseous environment external of the generator without substantial beam power loss due to the external gaseous environment in the region of the aperture being displaced by an outwardly directed jet of the material in the vapor state.

5. The method set for in claim 4 and further comprising the step of focussing the beam to obtain a beam cross section sufficiently small in the region of the aperture at which the material is supplied whereby the beam upon impinging on the material vaporizes a small amount thereof at the controlled rate.

6. The method set forth in claim 4 wherein the material is supplied to the aperture in the liquid state.

7. The method set forth in claim 4 wherein the material is supplied to the aperture in the solid state, and

the step of vaporizing the material also provides an inwardly directed jet of the material in the vapor state which condenses on an inner wall of the aperture to constrict and completely seal the aperture when the intensity of the charged particle beam is reduced below a critical value.

8. Apparatus for extracting a charged particle beam from a relatively low pressure gaseous environment into a relatively high pressure gaseous environment comprising:

means for generating a charged particle beam, said means comprising:

a beam exit assembly including a beam exit aperture for separating a region of low pressure gaseous environment internal of the beam generating means from a region of relatively high pressure gaseous environment external thereof, and

means for supplying a material characterized by being highly condensible and nondecomposable at elevated temperatures to said beam exit aperture at a controlled rate whereby the beam in passing outwardly through said aperture impinges on the material causing it to vaporize and generate an outwardly directed jet of the vapor which displaces the external gaseous environment in the region of the aperture to provide transmission of the beam from the relatively low pressure gaseous environment into the relatively high pressure gaseous environment without substantial loss in beam power.

9. The apparatus set forth in claim 8 wherein said beam exit assembly further comprises:

a chamber for storing the material from an inwardly directed jet of the vapor which is also generated when the material is vaporized, the stored material being condensed on a wall of said chamber, and

said material supplying means comprising means for feeding a metal tape in contacting relationship with an outer surface of said exit aperture, said tape being stationary during the time no beam is generated to seal the internal low pressure environment from the external high pressure environment, said tape moving during beam generation at a controlled rate sufl'lcient to supply a controlled amount of metal vapor for generating the outwardly and inwardly directed jets of vapor.

10. The apparatus set forth in claim 9 wherein said beam exit assembly further comprises:

a funnel-shaped channel in communication with said chamber, said funnel shape providing increased acceleration of the outwardly directed jet of vapor for increasing external gaseous environmental displacement eifect thereof.

11. The appartus set forth in claim 8 wherein said beam exit assembly further comprises:

said material supplying means comprising:

means for feeding a metal wire at a controlled rate to said exit aperture.

12. The apparatus set forth in claim 8 wherein said beam exit assembly further comprises:

a chamber for storing the material from an inwardly directed jet of the vapor which is also generated References Cited UNITED STATES PATENTS Ruhle 219-121 Trump 219121 Radtke 219-121 Peracchio 219121 Bowers et al 219-121 Niedzielski et al 219--121 Eklund 219-121 Harris 219121 Sciaky 219121 Greene 219121 RICHARD M. WOOD, Primary Examiner.

W. DEXTER BROOKS, Assistant Examiner.