Theoretical and numerical analyses of a slit-masked chicane for modulated bunch generation

摘要

Density modulations on electron beams can improve machine performance of beamdriven accelerators and FELs with resonance beam-wave coupling. The beam modulation is studied with a masked chicane by the analytic model and simulations with the beam parameters of the Fermilab Accelerator Science and Technology (FAST) facility. With the chicane design parameters (bending angle of 18°, bending radius of 0.95m and R_(56) ~ -0.19 m) and a nominal beam of 3 ps bunch length, the analytic model showed that a slit-mask with slit period 900 μm and aperture width 300 μm induces a modulation of bunch-to-bunch spacing ~ 100μm to the bunch with 2.4% correlated energy spread. With the designed slit mask and a 3 ps bunch, particle-incell (PIC) simulations, including nonlinear energy distributions, space charge force, and coherent synchrotron radiation (CSR) effect, also result in beam modulation with bunch-to-bunch distance around 100 μm and a corresponding modulation frequency of 3 THz. The beam modulation has been extensively examined with three different beam conditions, 2.25 ps (0.25 nC), 3.25 ps (1 nC), and 4.75 ps (3.2 nC), by tracking code Elegant. The simulation analysis indicates that the sliced beam by the slit-mask with 3 ~ 6% correlated energy spread has modulation lengths about 187 mm (0.25 nC), 270 μm (1 nC) and 325 μm (3.2 nC). The theoretical and numerical data proved the capability of the designed masked chicane in producing modulated bunch train with micro-bunch length around 100 fs.