Abstract
Author(s): Li, B; Qiang, J | Abstract: The microbunching instability seeded by small initial density modulation and driven by collective effects can cause significant electron beam quality degradation in next generation x-ray free electron lasers. A method exploiting longitudinal mixing derived from the natural transverse spread of the electron beam through a dispersive bending magnet was proposed to suppress this instability several years ago [Phys. Rev. Lett. 111, 054801 (2013)PRLTAO0031-900710.1103/PhysRevLett.111.054801]. Instead of using bending magnets to introduce the transverse-to-longitudinal coupling, which will lead to an inconvenient deflection of the downstream beam line, in this paper, we propose a scheme to mitigate the microbunching instability by inserting a quadrupole magnet inside a bunch compressor of the accelerator. This results in transverse-to-longitudinal phase space mixing and large slice energy spread that can efficiently mitigate the growth of the microbunching instability through the major accelerator section. Finally, at the exit of the accelerator, a dogleg section is used to restore the emittance and slice energy spread before entering the undulator radiation section. Multiparticle simulations show that the transverse space charge, structure wakefield, and the coherent synchrotron radiation effects will have a relatively small impact on this scheme.
Highlights
X-ray free electron lasers (FEL) provide an important tool for scientific discovery in physics, chemistry, biology, and material science. The performance of those x-ray FEL facilities critically depends on the electron beam quality out of linear accelerators, i.e., linacs
In the following calculation of microbunching instability gain, we included only longitudinal space-charge (LSC) effect to account for collective effects of electron beam through the accelerator since it is the dominant factor during the process of microbunching amplification [4]
Up to 100 million macroparticles with 512 × 64 × 512 grid points were used in the simulation to test the convergence. These results suggest that the effects of transverse spacecharge and structure wakefield, and the effects of coherent synchrotron radiation (CSR) through the above linear accelerator system could be relatively small
Summary
X-ray free electron lasers (FEL) provide an important tool for scientific discovery in physics, chemistry, biology, and material science. The induced energy modulation will cause even larger density modulation inside the beam after passing through a magnetic bunch compressor to increase the electron beam peak current. This leads to the amplification of the initial density modulation, i.e., the microbunching instability. A conventional method to control the instability is to use a laser heater to increase the electron beam uncorrelated energy spread before bunch compression to damp the instability [4,5,15]. We propose a scheme to mitigate the microbunching instability by inserting a quadrupole magnet inside a four dipole bunch compressor chicane to allow dispersion leakage and transverse-to-longitudinal coupling.
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