Abstract
In this paper we report on large scale multi-physics simulation of beam dynamics 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, longitudinal coherent synchrotron radiation (CSR) wake fields, and treatment of radiofrequency (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 microbunching 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 anmore » 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 injection 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 peak current.« less
Highlights
Generation of high-power coherent radiation in x-ray free electron lasers (FELs) requires high peak current, low emittance, and low energy spread multi-GeV electron beams
Collective effects driven by space charge, linac structure wakefields, and coherent synchrotron radiation (CSR) tend to cause an unacceptable degradation of the beam emittance and energy spread limiting the maximum achievable peak current
Previous studies have shown [1,2,3,4,5] that the accumulated effect of the longitudinal space-charge force along the linac driven by small density fluctuations can cause large energy modulations along the bunch
Summary
Generation of high-power coherent radiation in x-ray free electron lasers (FELs) requires high peak current, low emittance, and low energy spread multi-GeV electron beams. After passing through a bunch compressor, the energy modulation is further amplified in the presence of CSR and is converted into larger density modulations This phenomenon, known as ‘‘microbunching instability,’’ results in the appearance of large current fluctuations and significant growth of the uncorrelated and correlated energy spread, where the latter is seen as electron beam fragmentation in the longitudinal phase space. Macroparticle codes provide well established and successful methods for studying beam dynamics but they tend to overestimate the effect of instabilities that are sensitive to small fluctuations of the beam density, like those induced byqsffihffiffioffiffiffitffiffinffiffiffioffiffiiffiffise. These will be artificially magnified by a factor N=Nmp in a simulation employing Nmp macroparticles to represent a beam of N electrons. A macroparticle up-sampling scheme that reduces the macroparticle shot noise from an initial distribution with a smaller number of macroparticles while maintaining the global properties of the original distribution is described in the Appendix
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