High Resolution Simulation of Beam Dynamics in Electron Linacs for Free Electron Lasers

2009 
CBP 815 High Resolution Simulation of Beam Dynamics in Electron Linacs for Free Electron Lasers J. Qiang, R. D. Ryne, M. Venturini, A. A. Zholents Lawrence Berkeley National Laboratory, Berkeley, CA 94720 I. V. Pogorelov Tech-X Corporation, Boulder, CO 80303 In this paper we report on large scale multi-physics simulation of beam dy- namics in electron linacs for next generation free electron lasers (FELs). We describe key features of a parallel macroparticle simulation code including three- dimensional (3D) space-charge effects, short-range structure wake fields, longitu- dinal coherent synchrotron radiation (CSR) wake fields, and treatment of radio- frequency (RF) accelerating cavities using maps obtained from axial field profiles. A macroparticle up-sampling scheme is described that reduces the shot noise from an initial distribution with a smaller number of macroparticles while maintaining the global properties of the original distribution. We present a study of the mi- crobunching instability which is a critical issue for future FELs due to its impact on beam quality at the end of the linac. Using parameters of a planned FEL linac at Lawrence Berkeley National Laboratory (LBNL), we show that a large number of macroparticles (beyond 100 million) is needed to control numerical shot noise that drives the microbunching instability. We also explore the effect of the longitudinal grid on simulation results. We show that acceptable results are obtained with around 2048 longitudinal grid points, and we discuss this in view of the spectral growth rate predicted from linear theory. As an application, we present results from simulations using one billion macroparticles of the FEL linac under design at LBNL. We show that the final uncorrelated energy spread of the beam depends not only on the initial uncorrelated energy spread but also depends strongly on the shape of the initial current profile. By using a parabolic initial current profile, 5 keV initial uncorrelated energy spread at 40 MeV in- jection energy, and improved linac design, those simulations demonstrate that a reasonable beam quality can be achieved at the end of the linac, with the final distribution having about 100 keV energy spread, 2.4 GeV energy, and 1.2 kA
    • Correction
    • Source
    • Cite
    • Save
    • Machine Reading By IdeaReader
    0
    References
    0
    Citations
    NaN
    KQI
    []