B. R. Poole, Y.-J. Chen (LLNL)
Advanced x-ray radiography machines require that multiple electron beam pulses be delivered to x-ray converter targets over several lines of sight. This can be accomplished with a single accelerator by using a fast kicker to deliver the electron beam pulses to several beamlines. This type of radiography machines requires transport lines with several large achromatic bends in the individual transport lines. To maintain a small spot size and a large dose for a x-ray pulse created at the converter target at each transport line requires that emittance growth to be kept minimum on each beamline. Emittance growth can arise from nonlinear forces associated with the external focusing elements, nonlinear image forces, and nonlinear space-charge fields associated with the curvature of the beam and the transport line. We have used a multi-slice, particle-in-cell code to study emittance growth in a bend. The code uses the beam slicems local coordinates. Typically, the radius of curvature, R for such a beam and the transport line is much larger than the pipe radius, b. The space charge fields can be approximated as that in a straight line with correction terms to first order in b/R. To include the effects of the bend geometry on the space charge fields, these correction terms are implemented in the code. The relative importance of emittance growth due to nonlinear image forces associated with envelope oscillations of a non-round beam in the bend and due to nonlinear space charge fields associated with the bend geometry will be quantified. Simulation results for the baseline design orbit as well as off-energy transport will be presented.
*This work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.
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