  Density modulations on electron beams can improve machine performance of
beam-driven 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{\deg}, bending radius of 0.95 m and R56 ~ - 0.19 m) and a nominal beam of
3 ps bunch length, the analytic model showed that a slit-mask with slit period
900 {\mu}m and aperture width 300 {\mu}m induces a modulation of bunch-to-bunch
spacing ~100 {\mu}m to the bunch with 2.4% correlated energy spread. With the
designed slit mask and a 3 ps bunch, particle-in-cell (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 {\mu}m 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 {\mu}m (0.25 nC), 270 {\mu}m (1 nC) and 325 {\mu}m
(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.
