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 (<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.