Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA
JLab, Newport News, Virginia, USA
The coherent synchrotron radiation (CSR) of a high brightness electron beam traversing a series of dipoles, such as recirculation or transport arcs, may lead to the microbunching instability. We extend and develop a semi-analytical approach of the CSR-induced microbunching instability for a general lattice, based on the previous formulation with 1-D CSR model [Phys. Rev. ST Accel. Beams 5, 064401 (2002)] and apply it to investigate the physical processes of microbunching amplification for two example transport arc lattices. We find that the microbunching instability in transport arcs has a distinguishing feature of multistage amplification (e.g, up to 6th stage for our example arcs in contrast to two stage amplification for a 3-dipole chicane). By further extending the concept of stage gain as proposed by Huang and Kim [Phys. Rev. ST Accel. Beams 5, 074401 (2002)], we developed a method to quantitatively characterize the microbunching amplification in terms of iterative or staged orders that allows the comparison of optics impacts on microbunching gain for different lattices. The parametric dependencies and Landau damping for our example lattices are also studied. Excellent agreement of the gain functions and spectra from Vlasov analysis with results from ELEGANT is achieved which helps to validate our analyses.
JLab, Newport News, Virginia, USA
Understanding CSR effects for the generation and transport of high brightness electron beams is crucial for designs of modern FELs. Most studies of CSR effects focus on the impacts of the longitudinal CSR wakefield. In this study, we investigate the impact of the initial retarded potential energy of particles, due to bunch collective interaction, on the transverse dynamics of particles on a curved orbit. It is shown that as part of the remnants of the CSR cancellation effect when both the longitudinal and transverse CSR fores are taken into account, this initial potential energy at the entrance of a bending system acts as a pseudo kinetic energy, or pseudo energy in short, because its effect on particle optics through dispersion and momentum compaction is indistinguishable from effect of the usual kinetic energy offset from the design energy. Our estimation indicates that the resulting effect of pseudo energy spread can be measurable only when the peak current of the bunch is high enough such that the slice pseudo energy spread is appreciable compared to the slice kinetic energy spread. The implication of this study on simulations and experiments of CSR effects will be discussed.