Abstract:
The present invention relates to a method and system for automatically focusing an electron beam. Such an invention is based on a Faraday Cup diagnostic system, often a Modified Faraday Cup (MFC) system that enables tomographic reconstruction of the beam so as to measure beam parameters. Such a reconstruction method and system is automated using a servo-feedback loop to determine, for example, power distributions of the beam so as to provide appropriate adjustments to system controls to enable desired beam focus conditions.

Description:
[0001]     The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to beam focusing, and more particularly to an automatic method for determining a desired beam focus condition for electron beams in an electron beam welder.  
         [0004]     2. State of Technology  
         [0005]     A number of factors affect the “sharp focus” condition of an electron beam: beam current; beam voltage; filament current; focus coil current; travel speed; distance from the electron gun to the workpiece; chamber vacuum level; etc. Of these parameters, the determination of the focus coil current setting which corresponds to the “sharp focus” condition of the beam is the most difficult to define and reproduce on a consistent basis.  
         [0006]     Typically, focusing of an electron beam in an electron beam welding machine is a manual operation performed by an operator directing the beam onto a consumable tungsten block, under the conditions to be used during welding. The operator observes the brightness of a spot heated by the beam and adjusts the focus coil current until the spot is at its brightest, which is then assumed to be the “sharp focus” setting. Such a procedure is highly speculative because it depends on the operator&#39;s judgment to interpret visible light emission from the locally heated target block. Thus subjectivity can result in variations in the “sharp focus” setting between different operators. Accuracy in the choice of the “sharp focus” setting may also be degraded by slow changes in the brightness of the spot with changes in the focus current coil setting, or by the electron beam damaging the tungsten target. In addition, such a rudimentary focusing method only allows the operator to adjust a beam close to sharp focus and does not allow the beam to be reliably defocused by a known amount in order to produce a beam of greater width and lower power density. Moreover, visual-manual determination is generally satisfactory at lower current levels, but becomes difficult to apply at higher levels, e.g., currents above 10 mA.  
         [0007]     Background information for systems and methods of profiling power distributions within an electron beam can be found in WO 01/51183, Japanese Patents No. 11,154,489 and No. 9,166,698, in addition to U.S. Pat. No. 5,198,676, No. 6,300,755, No. 5,468,966, No. 5,554,926, No. 5,382,895 and No. 5,583,427. Further background information on such diagnostic methods and devices is described by J. W. Elmer et al. in, “Tomographic Imaging of Non-Circular and Irregular Electron Beam Power Density Distributions,” Welding Journal 72 (ii), p. 493-s, 1993; A. T. Teruya et al.; “Fast Method for measuring power-density distribution of non-circular and irregular electron beams,” Science and Technology of Welding and Joining, 3(2):51 Elmer, J. W. and Teruya A. T.; “An Enhanced Faraday Cup for Rapid Determination of Power Density Distribution in Electron Beams,” Welding Journal 80(12), pp. 288-s to 295-s, Elmer, J. W. and Teruya A. T.  
         [0008]     Background information on an automatic focusing method of an electron beam is described in U.S. Pat. No. 5,483,036, including the following: “An automated procedure changes the focus coil current until the focal point location is just below a workpiece surface. A parabolic equation is fitted to the calculated beam sizes from which optimal focus coil current and optimal beam diameter are determined.” 
         [0009]     Accordingly, there is a need for an improved automatic and quantitative method and system for focusing electron beams. The present invention is directed to such a need.  
       SUMMARY OF THE INVENTION  
       [0010]     Accordingly, the present invention provides an automatic method to provide a desired focus for a beam that often includes: setting an arbitrary sharp focus coil current, providing a feedback loop so as to provide automatically, a predetermined plurality of focus coil current increments positively above and negatively below the arbitrary sharp focus coil current, tomographically reconstructing a plurality of beams resulting from a plurality of received Faraday cup measurements, wherein each of the beams correlate to a respective focus coil current; calculating Peak Power Densities and a corresponding locus of beam diameters resulting from respective tomographically reconstructed beams; and determining a desired focus coil current based on respective calculated beam diameters so as to provide a desired beam focus condition for a predetermined application.  
         [0011]     A further aspect of the present invention provides a method for providing a desired beam focus condition for an electron beam welder to include: setting a predetermined focus coil current; sweeping a beam across a disk having a plurality of slits, the disk being arranged in a Faraday cup system, positioning a probe to detect secondary and backscattered electrons from a predetermined position on the disk; sensing a signal produced by the probe; calculating the proper orientation of the beam based on the signal so as to produce a set of beam profile data; and processing the beam profile data so as to tomographically reconstruct the power distribution in the beam; calculating a beam diameter resulting from the tomographically reconstructed beam; providing a predetermined incremental focus coil current; iterating the steps described above until a desired locus of Peak Power Densities and beam diameters are computed; and setting a desired focus coil current based on calculated beam diameters to provide a desired beam focus condition for a given application.  
         [0012]     Another aspect of the present invention provides a system having a feedback loop means coupled to a predetermined Faraday cup so as to automatically determine a locus of tomographically produced Peak Power Densities and respective beam diameters as a function of respective focus coil current settings so as to provide a desired best focus condition.  
         [0013]     The present invention provides an improved Faraday cup based feedback system and method that enables automatic quantitative optimization of beam focus conditions in an electron beam welder. Such a system and method is cost-effective and can be used rapidly and automatically to set a desired focus coil current so as to provide a best focus condition for a given application. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The accompanying drawings, which are incorporated into and form a part of the disclosure, illustrate an embodiment of the invention and, together with the description, serve to explain the principles of the invention.  
         [0015]      FIG. 1  shows an example automatic focusing beam welding system for determining a best focus condition for a beam weld.  
         [0016]      FIG. 2 ( a ) shows a resultant data plot of Peak Power Density (W/mm 2 ) versus focus coil current values of a defocus run.  
         [0017]      FIG. 2 ( b ) shows the same data of  FIG. 2 ( a ) plotted as a function of the relative machine focus setting. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     Referring now to the following detailed information, and to incorporated materials; a detailed description of the invention, including specific embodiments, is presented.  
         [0019]     Unless otherwise indicated, numbers expressing quantities of ingredients, constituents, reaction conditions and so forth used in the specification and claims are to be understood as being modified by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the subject matter presented herein. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the subject matter presented herein are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.  
         [0000]     General Description  
         [0020]     Electron beam welds are made by first determining the “sharp focus” condition, which is used as a reference point, and then setting the welding focus above, below, or directly on this sharp focus to produce the desired weld properties. To reiterate, during focusing of, for example, an electron beam welder, the strength of the magnetic lens in the focus coils is typically manually adjusted by an operator, thus arbitrarily raising or lowering the sharp focus position in the weld chamber. The operator directs the beam onto a high melting point target material, such as, but not limited to tungsten, and adjusts the focus coil current setting while observing the intensity of the light emitted from the target so as to determine the sharp focus setting. When the emitted light reaches a maximum intensity, the beam is considered to be at sharp focus. However, producing a sharply focused beam at a given focus setting on even a single machine is not guaranteed. For example, electron beams have a degree of symmetry at a subjective sharp focus setting but such beams become substantially elliptical around a defined range on either side of the “sharp focus”. Different operators may miss the best focus condition by interpreting the brightest emission from the target material differently, resulting in different definitions of “sharp focus” and the use of beams with different properties in the welding of potentially high-value components. These difficulties are only compounded when the parameters selected for one machine are transferred to other machines. For example, the beam produced at a given focus setting on one machine may not match that produced on another, due to differences in the focusing lens and the construction of the upper column. As a result, the current density of each beam can differ, resulting in welds of differing dimensions.  
         [0021]     The proposed concept of the present invention is directed to addressing manual adjustment subjectivity so as to produce a “sharp focus” condition in an electron beam welder. Accordingly, the present invention is based on a configured feedback system and corresponding method that includes a Faraday Cup diagnostic, more often a Modified Faraday Cup diagnostic, for automatically providing a best focus condition (i.e., a sharp focus at a predetermined position with respect to a sample of interest or a defocus position below or above the sharp focus) for circular and irregularly (e.g., elliptical) shaped electron beams.  
         [0022]     Existing Faraday cup embodiments to provide feedback information for automatically focusing a welding chamber can be found in U.S. Pat. No. 5,468,966, by Elmer et al., entitled “System For Tomographic Determination Of The Power Distribution in Electron Beams”; U.S. Pat. No. 5,583,427, by Teruya et al., entitled “Tomographic Determination Of The Power Distribution In Electron Beams”; U.S. Pat. No. 5,554,926, by Elmer et al., entitled “Modified Faraday Cup”; U.S. Pat. No. 6,300,755, by Elmer et al., entitled “Enhanced Modified Faraday Cup For Determination Of Power Density Distribution Of Electron Beams” and pending U.S. application Ser. No. 11/158481, entitled “A trigger Probe for Determining the Orientation of an Electron Beam”, by Elmer et al.; all of which are herein incorporated by reference in its entirety.  
         [0000]     Specific Description  
         [0023]      FIG. 1  illustrates an example embodiment of an automatic focusing system and is generally designated by the reference numeral  100 . The system of  FIG. 1  is substantially the same as that of above-incorporated by reference U.S. Pat. No. 6,300,755 and pending U.S. application Ser. No. 11/158481, entitled “A trigger Probe for Determining the Orientation of an Electron Beam”, by Elmer et al., and includes interconnected components or sub-systems, such as, for example, an electron beam gun assembly generally indicated at  50 , a Faraday cup assembly, such as, a modified Faraday cup (MFC) assembly indicated by reference numeral  51 , and a control and data acquisition system  52 . System  52  functions to control elements of the gun assembly  50 , such as a deflection coil  58  and a focusing coil  57 , in addition to controlling the MFC assembly  51  and acquiring and storing the acquired tomographic profile data.  
         [0024]     Gun assembly  50 , as shown in  FIG. 1 , which may be used in a welding machine, often includes a filament  53 , a cathode  54 , an anode  55 , an alignment coil  56 , a magnetic lens controlled by focus coil  57 , and a defection coil  58 . Filament  53  may be of any desired configuration, such as a ribbon type or a hairpin type as known in the art. The various components of electron beam gun assembly  50  and details of filament  53  are known in the art. Deflection coil  58  is coupled and controlled by system  52  so as to deflect an electron beam  59  produced by gun assembly  50  in a pattern, such as, for example, an elliptical or circular pattern as indicated by arrow  60  in or order to be swept across slits  12  arranged in, for example, a Faraday cup, often an enhanced MFC  20  disposed within Faraday Cup assembly  51 .  
         [0025]     MFC  20 , as shown in  FIG. 1 , can be mounted on a movable assembly  61 , via a support member  62  and an actuator  63  connected via line  64  to a tilt controller  69  of control and data acquisition system  52 . Movable assembly  61 , that includes x, y, and z translation stages as denoted by the double arrows x, y, and z, provides the capability of movement of enhanced MFC  20  as desired so as to accurately align slits  12  of slit disk  10  with electron beam  59  as it moves in a pattern (e.g., a circular pattern) around disk  10 .  
         [0026]     The electrical contact  36 , as shown in  FIG. 1 , of MFC  20  is connected via an electrical cable or lead  66  to a current viewing or sensing resistor  67  and to a common ground as indicated at  68 , and to a computer  65  of system  52 . As another arrangement, signal wire  36  can be replaced by a coaxial-type electrical cable and connector as detailed in above-incorporated by reference U.S. Pat. No. 6,300,755. The voltage across resistor  67  (e.g., a 100 ohm resistor) is measured and stored in computer  65  for each slit signal as beam  59  passes thereacross. Housing  21  of MFC  20 , having a lower plate section  28  that includes a radially extending passageway or groove  30 , is electrically connected to the common ground  48  via a cable or lead  70  connected to electrical contact  70 ′. Positioned within housing  21  is a liner or insulator  32  composed, for example, of Macor ceramic, alumina, and boron nitride; and an annular bottom cap or plate  33  (also composed of Macor ceramic, alumina, boron nitride, or other insulator material) having a central opening  34  which aligns with groove  30 . Also located within Faraday cup  20  is a second disk  37  having a ring  39 , constructed of graphite, copper, or tantalum and is often fixidly secured therein by bolts, screws, etc.  
         [0027]     Computer  65 , as shown as part of control and data acquisition system  52  of  FIG. 1 , is configured with digital to analog communication capabilities (e.g. via a digital to analog card) and coupled to a Beam focus power supply  81  via a predetermined cable (not shown) known to one of ordinary skill in the art (e.g., an RS 232 or USB having data transfer capability). Such a configuration enables computer  65  to supply a desired focus coil voltage setting and thus a desired focus coil current  82  to focus coil  57  via a software directed or manual command. Computer  65  is also coupled to tilt controller  69  via a cable or lead  71  and to deflection coils  58  of electron gun  50  via leads or cables  72  and  73 . To accurately position the MFC  20  with respect to the sweep of the electron beam  59  across the slits  12  of disk  10 , the computer  65  through tilt controller  69  enables an actuator  63  to move the movable assembly  61  in any desired direction.  
         [0028]     It is to be appreciated that Faraday cup  20  includes a flange clamp  24  so as to secure disk  10  to the housing assembly  21 . In such an arrangement, disk  10  is often constructed from tungsten, but may be constructed of tantalum, tungsten-rhenium, or other refractory metals and is configured having an odd number of radially extending slits spaced apart. As one embodiment, a predetermined slit, is arranged to have a greater width to enable orientation of a beam profile with respect to the coordinates of a welding chamber. A detailed explanation of such an example embodiment can be found in incorporated by reference U.S. Pat. No. 6,300,755.  
         [0029]     While having a configured widened slit as detailed in U.S. Pat. No. 6,300,755 is a beneficial embodiment; such an arrangement can in some circumstances adversely affect the reconstruction of the beam, especially in cases in which the width of the widened slit is no longer small relative to the width of the beam. Therefore, the beam width of the reconstructed beam may be slightly elongated in cases of tightly focused beams as the width of the beam approaches that of an enlarged slit.  
         [0030]     Accordingly, flange clamp  24  as another beneficial embodiment, can be modified and configured with an external electron probe to provide a fiducial locator (i.e., a timing or triggering signal) and a sensing circuit can be added to the data acquisition hardware of the present invention to detect such a fiducial locator so as to properly orient an ion or electron beam. The use of such an external electron probe eliminates the need for an enlarged slit by taking advantage of secondary and backscattered electrons generated by the interaction between the beam  59  and integrated disk  10 . Such a probe rests above the slit disk and is aimed at a point located between two of the slits so that the reconstructed beam profile can be determined with the proper orientation. A detailed description of such an arrangement can be found in pending U.S. application Ser. No. 11/158481, entitled “A trigger Probe for Determining the Orientation of an Electron Beam”, by Elmer et al.; also herein incorporated by reference in its entirety.  
         [0031]     In the method of the present invention, the strength of the magnetic lens provided by focus coil  57  is often first manually adjusted by the operator, thus arbitrarily raising or lowering the sharp focus position in the weld chamber. In order to determine an arbitrary machine “sharp focus” current setting, the operator, in manually making such an adjustment, directs the beam onto a high melting point target material, such as tungsten, and adjusts the focus coil  57  current setting while observing the intensity of the light emitted from the target. When the emitted light reaches a maximum intensity, the beam is considered to be substantially close to the desired machine sharp focus and computer  65 , is utilized to generate the signals actuating the electron beam sweep, to acquire beam profile data from, for example, an MFC  20 . Specifically, electron gun  50  is turned on and computer  65  is arranged to activate deflection coil  58  of electron gun  50  to move beam  59  in a predetermined pattern, e.g., circular, so as to cross each slit  12  of disk  10 , and thereafter computer  65  receives the output data from a predetermined MFC  20  via lead  66  and resistor  67 . The beam is then tomographically reconstructed by computer  65  from the received beam profile data and a characteristic beam spot size based on the full width half maximum (FWHM) and/or the full width at the 1/e 2  point of the beam (FWe 2 ) and/or an average of a full beam diameter, can be determined based on the computed peak power density. From such an initial procedure, the resultant tomographically reconstructed beam enables the system  100  of the present invention to calculate a Peak power Density and a corresponding calculated spot size to provide a starting point for quantitatively determining a quantitative “sharp focus” current setting in addition to a locus of defocus conditions.  
         [0032]     Computer  65 , configured with data acquisition hardware, Digital to Analog (D/A) communication capabilities, and further configured with an algorithm generated out of available software, such as, but not limited to, Fortran, Basic, Visual Basic, LabView, Visual C++, C++, or any programmable language capable of operating within the scope and spirit of the invention herein, then, as one example desired embodiment, initiates auto focusing of system  100 . As part of the designed logic, an incremental focus coil power supply voltage (PSV) derived about the arbitrary machine “sharp focus” current stored setting (e.g., often of about a 1 mA increment from the arbitrary machine “sharp focus” current setting and up to about 10 mA increments from the arbitrary machine “sharp focus” current setting), is computed. A converted digital to analog signal is then directed by computer  65  along communication line  83 , such as an RS 232 or USB communication interface cable, to set beam focus power supply  81  to the desired voltage setting. Beam focus power supply  81  then is directed via communication line  83  to output focus coil current  82  based on the desired voltage setting to focus coil  57  so as to provide the desired focus coil increment. Next, computer  65  again generates signals actuating the electron beam sweep along lines  72  and  73 , as shown in  FIG. 1 , to acquire beam profile data from the configured MFC  20 . The beam is once again tomographically reconstructed by computer  65  from the received beam profile data and a characteristic beam spot size (e.g., FWHM) correlated to the respective focus coil current increment is then determined based on the computed peak power density. Computer  65 , then increments the focus coil current and system  100  can perform the operations as disclosed above to again provide a Peak Power Density and a corresponding characteristic beam spot size for a given focus coil current increment. Such current increments and resultant spot sizes based on respective computed peak power densities are then performed for a predetermined number of times, e.g., thirty increments of about 1 mA above and below the arbitrary “best focus” setting, to provide a locus of data points so as to determine the maximum Peak Power Density and thus a non-arbitrary “sharp focus” condition. Such a procedure can often require minutes to hours, but more often requires 1-2 minutes depending on the number of desired tomographic profiles that is chosen in the automatic focusing program. Once, the non-arbitrary “sharp focus” condition is determined, a desired automatic or manual setting of the welding focus position, i.e., above, below, or directly on sharp focus, is chosen to produce desired weld properties of given materials and conditions.  
         [0033]      FIG. 2 ( a ) shows a resultant data  102  plot of computed Peak Power Density (W/mm 2 ) versus focus coil current values of a defocus run in addition to an operator determined sharp focus (denoted with an arrow and pointing to about 0.45 A) that can be determined by the automatic focusing of example system  100 , as shown in  FIG. 1 . It is to be appreciated that such Peak Power Density (W/mm 2 ) values, as shown in  FIG. 2 ( a ), are capable of being determined from the system of  FIG. 1  by computer analyzing corresponding tomographically constructed beams so as to determine, if needed, a (FWHM) and/or (FWe 2 ) of such resultant beams (e.g., Gaussian, elliptical, super Gaussian, semi-Gaussian, etc.) for each focus coil current setting.  
         [0034]      FIG. 2 ( b ) shows the same data  104  plotted as a function of the relative machine focus setting, wherein the zero value for the relative machine focus setting corresponds to the focus setting at which a maximum peak power density is measured. Since the focus coil current settings vary between welders, the method of assigning a relative machine focus setting enables users of the present invention to define a relative setting to describe the same focus condition on different welders. The machine sharp focus setting (i.e.,  103 , as shown in  FIG. 2 ( b )), and a corresponding stored beam spot size based on the (FWHM) and/or (FWe 2 ) as defined by the resultant obtained data from the automatic focusing example system  100  of  FIG. 1 , is used as the reference value and is set to a value of zero. Focus coil current settings above this value are given a positive value, while those below are given a negative value, as shown in the following relationship of equation (1): 
 Relative Machine Focus Setting=Focus Coil Current−Sharp Focus Coil Current,   (1)  
 wherein the focus coil current (mA) is the value for each focus setting and the sharp focus coil current (mA) represents the focus coil current at the machine sharp focus setting. Accordingly, a particular machine focus setting and thus a corresponding computed beam spot size of  FIG. 2 ( b ) represents the amount of defocus above (positive) or below (negative) the sharp focus setting  103 . Specifically, the focus coil current values plotted in  FIG. 2 ( a ) have been converted to the relative machine focus settings, and the results are shown in  FIG. 2 ( b ) for the same peak power density values. 
 
         [0035]     The present invention thus provides an enhanced method and system that automatically generates a locus of tomographically analyzed data for determining a best focus condition (e.g., a sharp or defocused condition) for electron beam welds.  
         [0036]     Accordingly, the present invention can be utilized with high-power, high-intensity multiple kilowatt (20 kv plus) electron beams, or with low-power (1 kv) beams in addition to analysis of ion beams. Moreover, the present invention can be utilized for the automatic analysis of any type of energy producing beams such as the generation of x-rays or use in electron beam lithography.  
         [0037]     While the invention may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the following appended claims.