  The development of the microbunching instability is studied for the linac of
the proposed Shanghai Soft X-ray Free-Electron-Laser facility (SXFEL) by
analytic formulae as well as numerical simulations with the aid of two
different codes. The process is investigated in detail and the growth rates
(gains) of the instability under various conditions are compared. The results
indicate that the limitations from numerical computations in the present
simulation model must be taken into account. Moreover, the appearance of
higher-order mode excitations in the simulations suggests that further
improvement of the current theory is necessary. A mechanism of introducing shot
noise into beam profile as the beam passes through a chicane is proposed. The
key issues that drive the instability are analyzed.
