Abstract:
The present invention relates to a probe for determining the orientation of electron beams being profiled. To accurately time the location of an electron beam, the probe is designed to accept electrons from only a narrowly defined area. The signal produced from the probe is then used as a timing or triggering fiducial for an operably coupled data acquisition system. Such an arrangement eliminates changes in slit geometry, an additional signal feedthrough in the wall of a welding chamber and a second timing or triggering channel on a data acquisition system. As a result, the present invention improves the accuracy of the resulting data by minimizing the adverse effects of current slit triggering methods so as to accurately reconstruct electron or ion beams.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Patent Application No. 60/582,754 filed Jun. 24, 2004 and titled “A Trigger Probe for Determining The Orientation of the Power Distribution of an Electron Beam” and is herein incorporated by reference in its entirety. 

   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 
   1. Field of the Invention 
   The present invention relates to beam profiling electron and/or ion beams, and more particularly to a trigger diagnostic and method for determining the orientation of profiled electron beams. 
   2. State of Technology 
   Many of the diagnostic methods for measuring the power density distribution in electron beams are variations of the Faraday cup. A version of the Faraday cup diagnostic method can include an electrically conductive trap, which contains and measures a beam current. In order to measure the power density distribution of the beam, modifications to the Faraday cup are required so that only a selected portion of the beam enters the cup at any one time. One type of Faraday cup isolates a portion of the beam by placing a single slit or knife-edge above the Faraday cup while the beam is swept over this slit. This technique measures the beam intensity along the sweep direction and provides a one-dimensional profile of the beam. By maximizing the amplitude of the profile measured through the slit while adjusting the focus, the minimum beam width, which corresponds to the sharpest focus along this direction, can be determined. This technique provides a one-dimensional view of the beam along the sweep direction and is useful for inspecting beams with radial symmetry; however, if the beam is non-circular or has an irregular power distribution, more sophisticated techniques are required to map the power density distribution in the beam. 
   Pinhole devices have also been used to measure the power distribution of irregularly shaped electron beams. Pinhole measurements are made using a small aperture (&lt;10% of the beam diameter) placed over a Faraday cup. The electron beam sweeps over the pinhole several times at regularly spaced intervals to provide enough information to map the power density distribution in the beam. The drawbacks of this technique are that variations in the side-to-side position of the beam on successive sweeps can lead to errors in the measured power density distribution and that this device has a relatively low signal-to-noise ratio since the pinhole collects only a small percentage of the beam current. 
   Computed tomography (CT) coupled with a modified Faraday cup (MFC) technique was developed at Lawrence Livermore National Laboratory as an improvement to the above methods for measuring the power density distribution of electron beams used for welding. The Lawrence Livermore National Laboratory device includes a Faraday cup assembly within an electrically insulating ceramic cup, a tungsten disk containing 17 thin radially positioned slits (0.1 mm wide each), and a cylindrical copper heat sink that holds the tungsten disk above the Faraday cup. During operation, the electron beam deflection coils are used to sweep the beam in a circle of known diameter and at a constant frequency over the tungsten slit disk. The majority of the beam current is intercepted by the tungsten disk and is conducted by the copper heat sink to ground. However, when the beam passes over a slit, a portion of the beam current passes through the slit and into the Faraday cup where it can be measured as a voltage drop across a known resistor. 
   A current versus time profile is collected using a fast sampling analog to digital converter as the beam passes over each slit, wherein each slit provides a profile of the beam at an angle perpendicular to that slit. Such profiles are then compiled and tomographically reconstructed in order to determine the size, shape, and power density distribution of the beam. When defocused, the electron beams are asymmetric in both shape and power density distribution. The beam orientation is currently determined by the positioning of an oversized radial slit on the tungsten disk. Since this one slit is wider than the others, its profile is larger than the others and can be easily identified by the reconstruction software. However, such an oversized slit can adversely affect the reconstruction of the beam, especially in cases in which the width of this slit is no longer small relative to the width of the beam. 
   Background information for systems and methods of profiling power distributions within an electron beam can be found in U.S. Pat. Nos. 6,300,755, 5,468,966, 5,554,926, 5,382,895 and 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. 
   Accordingly, there is a need for an improved method and system of identifying the orientation of ion /electron beams being profiled. The present invention is directed to such a need. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention provides a method of profiling a beam that includes: sweeping a beam across a disk having a plurality of slits with the disk being arranged in a modified 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 beam profile; and processing data resulting from the beam profile so as to reconstruct the power distribution in the beam. 
   Another aspect of the present invention provides a beam profiling system that includes a probe that can detect secondary and backscattered electrons and an electronic circuit arranged to combine signals resulting from the probe and from the modified Faraday cup arrangement, wherein the power distribution from an energy beam can be accurately reconstructed. 
   The present invention provides an improved Faraday cup system that eliminates changes in slit geometry, an additional signal feedthrough in the wall of a welding chamber and a second timing or triggering channel on a data acquisition system. Accordingly, the present invention improves the accuracy of the resulting data by minimizing the adverse effects of current slit triggering methods so as to accurately reconstruct electron or ion beams. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. 
       FIG. 1(   a ) shows a prior art refractory metal disk having radially extending slits, with one of the slits being twice as wide as the other slits. 
       FIG. 1(   b ) illustrates a cross section of the prior art refractory metal disk. 
       FIG. 2  shows a partial cross-section of an example enhanced modified Faraday cup of the present invention. 
       FIG. 3(   a ) shows a cross-sectional perspective of the modified flange clamp of the present invention. 
       FIG. 3(   b ) shows a top-down perspective of a slit disk utilized with the modified flange clamp of the present invention. 
       FIG. 4  shows an example system for determining the power density distribution in high power electron beams. 
       FIG. 5(   a ) shows example signals obtained by the data acquisition system of the present invention and illustrates the conversion of a signal from the trigger probe to a positive value. 
       FIG. 5(   b ) shows a flow chart path of both signals from the vacuum chamber to the data acquisition system and the general location of the electronic circuit used to convert the trigger probe signal. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the following detailed information, and to incorporated materials; a detailed description of the invention, including specific embodiments, is presented. 
   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. 
   General Description 
   The proposed concept is based on a configured external probe and circuit arranged in a system for tomographic determination of the power distribution of non-circular and irregularly (e.g., elliptical) shaped electron beams. Specifically, such a probe and circuit can be configured in existing Faraday cup embodiments to provide an improved method of orienting a measured beam profile with respect to a welding chamber. Similar systems and methods for determining the power distribution in electron beams 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”; and 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”; all of which are herein incorporated by reference in its entirety. 
     FIG. 1(   a ) shows a refractory metal disk, as disclosed in incorporated by reference U.S. Pat. No. 6,300,755, and is generally designated by the reference numeral  10 . Such a disk  10  is often constructed from tungsten, but may be constructed of tantalum, tungsten-rhenium, or other refractory metals and is configured with a center hole  11  and an odd number of radially extending slits spaced apart from center hole  11 , with a predetermined slit  12 ′ having an enlarged width that enables orientation of a beam profile with respect to the coordinates of a welding chamber. Disk  10  can be arranged to have a diameter of about 1.5 inches and a thickness of about 0.125 inches ±0.005 inches. Center hole  11  can be configured to have a diameter of about 0.040 inches ±0.002 inch and further configured to be about ±0.002 from the true center. Slits  12  in disk  10  can be equally spaced apart, such as, for example, at about 21.18°±0.02°, as indicated by double arrow  13 , each having a width of about 0.004 inches ±0.002 inches on an upper surface  14  of disk  10  and a length of about 0.500 inches ±0.005 inches, as indicated by double arrow  15 , wherein such slits  12  can terminate a distance of about 0.250 inches from true center hole  11 . 
     FIG. 1(   b ) is a cross-section of a section of disk  10  and widened slit  12 ′ looking radially outwardly in the direction of arrows  2 - 2  of  FIG. 1(   a ), wherein the slit  12 ′ has tapered surfaces  16  and  17  tapering outwardly and downwardly from upper surface  14  to a lower surface  18  of disk  10 . The slits  12  and slit  12 ′ are tapered, beveled, etc. to remove material from the disk  10  behind the slits in surface  14  of the disk to improve sensitivity and prevent beam reflections and/or secondary electrons, while providing adequate heat dissipation generated by the electron beam crossing slits  12  and  12 ′. The tapered surfaces  16  and  17  are exemplified as being at a 10° angle and can be increased or decreased by about 2-4 degrees. 
   While having a configured widened slit  12 ′ is a beneficial embodiment, as shown in as shown in  FIGS. 1(   a )- 1 ( b ), such an arrangement can adversely affect the reconstruction of the beam, especially in cases in which the width of this 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. 
   The present invention generally involves an improvement of previously disclosed systems and is substantially the same as that of above-incorporated by reference U.S. Pat. No. 6,300,755, except for a configured external probe to provide a fiducial locator (i.e., a timing or triggering signal) mounted in a modified flange clamp and a sensing circuit 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, as shown in  FIGS. 1(   a )- 1 ( b ), by taking advantage of the secondary and backscattered electrons generated by the interaction between the beam and an integrated tungsten disk. Such a probe rests above the tungsten 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. 
   Specific Description 
   Turning back to the drawings,  FIG. 2  shows a partial cross-section of an example enhanced MFC of the present invention, capable of being utilized in accordance with the present invention, and is generally designated by the reference numeral  20 . MFC  20  includes an outer shield or heat sink body  21 , made of high electrically conductive metal or alloy, such as copper, silver or steel, with an opening extending therethrough that includes sections  22  and  22 ′, with opening section  22  being of a smaller diameter than opening section  22 ′, and with a cutaway or counter-sink  23  about the outer end of opening section  22 . Disk  10 , arranged with, for example, an odd number of slits of equal width, can be manufactured from a beneficial slit refractory metal, such as, for example, tungsten and can be positioned in counter-sink  23  and secured in heat sink body  21  by a modified flange clamp or plate  24  made of copper, silver, or steel that is removably attached via bolts, screws, etc., arranged in a plurality of through holes  25  in plate  24  so as to be received by threaded opening  25 ′ in heat sink body  21 . 
   An electrical ground, such as a wire  26 , made of, for example, tantalum, copper, or silver, is brazed or otherwise secured to the slit disk  10  and attached to the heat sink body  21  via any means known to those skilled in the art, such as, but not limited to, a screw  27 . Outer shield or heat sink body  21  includes a lower plate section  28 , also capable of being constructed of copper, silver, or steel and secured to heat sink body  21  by bolts or screws extending through openings, e.g.,  29  and  29 ′. Plate section  28  includes a radially extending passageway or groove  30  and a cutaway or counter-sink  31 . Positioned within opening section  22 ′ of heat sink body  21  is a liner or insulator  32  that can be arranged from, for example, Macor ceramic, alumina, and boron nitride; and an annular bottom cap or plate  33  that can also be arranged from Macor ceramic, alumina, boron nitride, or any other insulator material capable of meeting the specifications of the present invention. Liner  32  may be secured by screws extending through opening  21 ′ in heat sink body  21 . The annular bottom cap or plate  33  includes a central opening  34  which aligns with groove  30  in plate section  28  and is provided with a protruding pin  33 ′ that fits into cutaway  31 . A Faraday cup  35 , constructed of copper, silver, or steel, is located within liner  32  and abuts annular cap  33 , and a signal wire  36 , such as a shielded copper wire, is secured to the bottom of Faraday cup  35 , as indicated at  36 ′, and extends outwardly via opening  34  in cap  33  and groove  30  in plate section  28 . 
   Positioning in the upper end of Faraday cup  35  is a second slit disk  37 , constructed of copper, silver, tantalum, or tungsten having a center hole  37 ′ and slits  37 ″ which align with center hole  11  and slits  12 - 12 ′ of disk  10 . The second disk  37  can be secured to Faraday cup  35  by bolts, screws, etc. extending in, for example, openings  38  and  38 ′ in disk  37  and the Faraday cup  35 . Liner or insulator  32  includes an inwardly extending flange  32 ′ which extends over the periphery of the second disk  37 . Located within Faraday cup  35  and below the second disk  37  is a ring  39 , constructed of graphite, copper, or tantalum, which is secured therein by bolts, screws, etc., which extend through openings  40  in Faraday cup  35 , only one shown. Located in Faraday cup  35  below ring  39  is a beam trap  41 , constructed of copper, silver, or steel, and secured in the Faraday cup  35  by bolts, screws, etc., extending through openings, e.g.,  42 , in Faraday cup  35 . Faraday cup  35  is aligned with plate  33  by a cutaway/pin arrangement, indicated at  31 ′/ 33 ″. The second slit disk  37  is aligned with Faraday cup  35  and the first slit disk  10  is aligned with heat sink body  21  via cutaway/pin arrangements  31 ′/ 33 ″, as shown in  FIG. 2 . It is to be appreciated that a beneficial embodiment of the present invention is shown at the cutaway portion of modified flange  24  that includes a thru hole  82  for an inserted trigger or timing probe (not shown) held in place by, for example, a set screw (not shown) threaded into thru hole  81 . 
     FIG. 3(   a ) schematically illustrates a different cross-sectional perspective of modified flange clamp  24 , as shown in  FIG. 2 , and as discussed above. Modified flange clamp  24  holds slit disk  10  to a fixed position and is also adapted to hold an external trigger or timing probe  44  of the present invention. In particular, modified flange clamp  24  is arranged with an outside diameter of about 3.0 inches so as to include a thin flange  84  region having a thickness of about 0.250 inches ±0.010 inches and a thicker flange region  85  of about 0.588 inches ±0.010 inches configured around a center hole region  86  having a diameter of about 1.250 inches ±0.005 inches. 
   Thicker flange region  85  can include a threaded hole  81  for receiving a predetermined fastener, such as a set screw (not shown) so as to affix external probe  44  disposed within thru hole  82 . Thru hole  82 , has been beneficially designed at a predetermined angle (θ), such as, for example, an angle of about 45 degrees as measured from a center-line (C/L) of modified flange clamp  24  and is often further configured with a smaller diameter portion  83  that provides a stop and viewing aperture for probe  44 . Such an arrangement provides a predetermined field of view for receiving backscattered/secondary electrons from disk  10 , as stated above. Finally, thicker flange region  85  also includes a passageway or groove  90  for receiving wire  26 , as shown in  FIG. 2 , to provide an electrical ground. 
   It is to be appreciated that the use of external probe  44 , which often is a piece of a semi-rigid copper-jacketed coaxial cable, eliminates the need for a wide slit, as shown in  FIGS. 1(   a )-( b ). Accordingly, by positioning probe  44  to be aimed at a point located between two predetermined slits located on disk  10  via modified flange clamp  24 , probe  44  can beneficially detect secondary and backscattered electrons generated by the interaction between an ion or electron beam and disk  10 . 
     FIG. 3(   b ) schematically shows a perspective of disk  10  looking top down in the direction of side A, as shown in  FIG. 3(   a ). Such a perspective shows a predetermined point  5  (shown encircled for illustration purposes only) between a pair of predetermined slits (e.g.,  12 ″ and  12 ″′), as viewed by external probe  44  for receiving backscattered/secondary electrons so as to eliminate the need for a wide slit embodiment as illustrated in  FIGS. 1(   a )-( b ). As the beam passes the probe locations, i.e., predetermined point  5 , the secondary and back-scattered electrons, which originate from slit disk  10 , are captured by the inner conductor of a coaxial cable that is utilized as probe  44 . In order to limit the amount of electrons detected by probe  44 , it is configured to accept electrons from only a narrowly defined area by first indenting the edge of the probe slightly below the surface of the modified flange clamp  24  via thru hole  82  and adding a small diameter portion  83 , as stated above. Such features limit the locations from which electrons are scattered to those only directly in front of the probe. By placing probe  44  in such an arrangement, it does not interfere with the oscillation of the beam or the capture of the beam by the individual slits  12 . 
   Disk  10 , capable of being constructed from tantalum, tungsten-rhenium, or other refractory metals, is similar to the embodiment shown in  FIGS. 1(   a )-( b ), except there is no longer the requirement for a wide slit to provide proper beam orientation. In the present invention, slits  12 , often an odd number of radially extending slits  12 , are spaced apart from center hole  11  and can be angularly spaced apart from other, for example, at about 21.18°±0.02°, as indicated by double arrow  13 . Moreover, each slit  12 , which terminates at distance of about 0.250 inches from true center hole  11 , can be configured with a width of about 0.004 inches ±0.002 inches on an upper surface  14  of disk  10  and a length of about 0.500 inches ±0.005 inches, as indicated by double arrow  15 . The sensing of scattered electrons by probe  44 , produced as the beam passes between predetermined slits, e.g. slits  12 ″ and  12 ″′, of slit disk  10  can thus be used as a timing or triggering fiducial to orient the beam profile data, thus enabling the reconstruction software utilized in the present invention to calculate the correct orientation of the beam using beam profiles obtained by slits  12  having a uniform width. 
     FIG. 4  schematically illustrates an example embodiment of the enhanced modified Faraday cup  20  illustrated in  FIG. 2  and incorporating trigger or timing probe  44  arranged in modified flange  24 , as shown in  FIG. 3(   a ), in a system for taking electron beam profile data. The system of  FIG. 4  involves three (3) interconnected components or sub-systems: an electron beam gun assembly generally indicated at  50 , a modified Faraday cup (MFC) assembly generally indicated at  51 , and a control and data acquisition system  52 . System  52  functions to control elements of the gun assembly  50  and the MFC assembly  51  as well as storing the acquired data. The system of  FIG. 4 , as stated above, is substantially the same as that of above-incorporated by reference U.S. Pat. No. 6,300,755, except for probe  44 , an electronic circuit  100  capable of converting probe signals to positive values and operably coupled to probe  44 , modified flange clamp  24 , and slit  10  having slits  12  of about the same width. 
   The electronic signal produced by trigger probe  44  is transmitted through the same circuit, e.g., circuit  100 , as the beam profiles obtained by the slits. Because an electron beam is being collected, the voltages obtained by the data acquisition system have a negative value when passed across the current viewing resistor. In order for both the triggering signal and the beam profile signals to be transmitted on the same circuit without interference, the signal from the trigger probe is converted into a positive voltage through the addition of electronic circuit  100 , which can be placed inside the electron beam vacuum chamber as shown in  FIG. 4 . However, while placing circuit  100  within the vaccum chamber is a beneficial arrangement, such a circuit can also be placed external to the vaccum chamber without departing from the scope and spirit of the present invention. In addition to the probe, a second beneficial component to the invention includes an electronic circuit operably connected to signals produced by the Faraday cup and to probe  44 . 
     FIG. 5(   a ) schematically illustrates example slit signals  112  and trigger probe signals  114  converted into a positive value. Because a positive voltage cannot be part of the beam profile data, the reconstruction software of the present invention ignores it while calculating the beam power distribution. The software thus utilizes the timing of trigger probe signal  114  to match the beam profiles to the orientation of the slit disk and to determine the proper beam orientation. 
   The addition of electronic converter circuit  100 , as shown in  FIG. 4 , in the electron beam vacuum chamber or adapted external to the vacuum beam chamber, eliminates the need for a second feed-through, thus simplifying machine setup. In order for such a circuit  100  to function, it must be able to accept the signals from both the electron trigger probe and the slits, while at the same time converting the signal from the trigger probe and leaving the signals from the slits unchanged. 
     FIG. 5(   b ) shows a flow chart of the path of the signals from both the trigger probe  114  and the slits  112 . Each signal, once obtained by the trigger probe or the slits, is transmitted to electronic circuit  100 , which converts the trigger probe signal into a positive value. The two input signals can then be joined within electronic circuitry  100  in order for the signal to be transmitted through the single feed through  102  of the vacuum chamber. As the signal passes through current viewing resistor  67  (mounted internal or external to the vacuum chamber), the current is converted to a voltage, with the slits displaying negative voltage signals  112  and the trigger probe displaying a positive voltage signal  114 . 
   Turning back to  FIG. 4 , electron beam gun assembly  50 , such as may be used in a welding machine, basically includes a filament  53 , a cathode  54 , an anode  55 , an alignment coil  56 , a magnetic lens  5 , 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. The deflection coil  58  is connected so as to be controlled by system  52  to deflect an electron beam produced by gun assembly  50  and indicated at  59  in a circular pattern as indicated by arrow  60 . Beam  59  is thus moved via deflection coil  18  to sweep across each of the slits  12  and  12 ′ in slit disk  10  in the enhanced MFC  20  of the MFC assembly  51  as the beam  59  is deflected in a circular pattern as indicated by arrow  60 . 
   Enhanced MFC  20 , such as the embodiment illustrated in detail in  FIG. 2 , 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  65  of control and data acquisition system  52 . Movable assembly  61 , that includes x, y, and z translation stages as indicated by the double arrows x, y, and z, provides the capability of movement of enhanced MFC  20  as desired to accurately align slits  12  of slit disk  10  with the electron beam  59  as it moves in a circular pattern around the disk  1 , as discussed in greater detail hereinafter. The electrical contact  36 , as shown in  FIG. 2 , 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  69  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  is measured and stored in computer  69  for each probe signal  114  and each slit signal  112 , as shown in  FIGS. 5(   a ) and  5 ( b ), as beam  59  passes thereacross. Housing  21  of MFC  20  is electrically connected to the common ground  48  via a cable or lead  70  connected to electrical contact  70 ′. By way of example, the resistor  67  may be 100 ohms. 
   The control and data acquisition system  52  includes computer  69  and tilt controller  65 , with computer  69  being connected to tilt controller  65  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  69  through tilt controller  65  actuates actuator  63  to move the movable assembly  61  in any desired direction. To initiate acquisition of beam profile data via MFC  20 , electron gun  50  is turned on and computer  69  activates deflection coils  58  of electron gun  50  to move the beam  59  in a circular pattern so as to cross each slit  12  of disk  10 , and thereafter computer  69  receives the output data from MFC  20  via lead  66  and resistor  47 . Thus, a single computer is used to generate the signals actuating the electron beam sweep, to acquire the data from the MFC, and to do the reconstruction of the beam profile data to produce a tomographic profile of the power distribution in the electron beam. 
   Because each of the angular profiles is acquired using a different slit  12  in slit disk  10 , it is important that the slit disk be accurately centered on and made perpendicular to an undetected beam. In order to facilitate this, small hole  11  has been drilled in the center of the slit disk, which allows the modified Faraday cup of the present invention to be used as a traditional Faraday cup and measure the total beam current. Disk  10  may then be centered on beam  59  by moving the MFC  20  around, via movable assembly  61 , until a signal read across sensing resistor  67  indicates that the center hole  11  is aligned with beam  59 . If slit disk  10  is tilted and slits  12  are not perpendicular to the beam, then the beam path through some of the slits  12  will be narrowed or cut off completely since slit disk  10  has a thickness that is much larger than the width of the slits. Tilt of the MFC  20  is checked by sweeping beam  59  in its circular pattern, indicated by arrow  60 , and adjusting the tilt via tilt controller  65 , or manually adjusted, and assembly  61  until a clear signal comes through each slit. 
   The problem of sensitivity to tilt may be minimized by removing material behind the top surface forming the slits so that only material at the top surface  14  of the slit disk  10  forms the slits  12  while remembering that it is important to provide as much disk material possible to adequately dissipate heat generated by the electron beam. Although the walls of the slits can be parallel so as not to have a tapered configuration, tapering the walls of the slits so that the walls are not perpendicular to the front or top surface  14  of slit disk  10 , as shown in  FIG. 1(   b ), can also be utilized to resolve tilt sensitivity. With respect to the second slit disk, the slits must be large enough to let all electrons pass through. As another arrangement, such wall surfaces can also be leveled or otherwise configured such that the bottom of the slits is wider than the width of the slits at the top surface  14  of disk  10 . 
   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. Without the use of diagnostic tools, the sharp focus condition is somewhat subjective and is generally determined by the electron beam operator. By use of computer tomography (CT), such as described in above-referenced U.S. Pat. No. 5,583,427 and by using trigger or timing probe  44  and electronic circuit  100 , as shown in  FIGS. 3(   a ) and  4  respectively, a power density distribution can be measured as a function of focus coil setting for electron beams at about 80 kV and 1 about 40 kV generated by, for example, a 150 kV, 50 mA Hamilton-Standard electron beam welder, fitted with a ribbon filament and an R-40 electron gun. 
   The present invention provides an enhanced method and system for use in a system for tomographically determining the power distribution in electron beams. 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 analysis of any type of energy producing beams such as the generation of x-rays or use in electron beam lithography. 
   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.