  A simple method to reduce the numerical cost in free electron laser (FEL)
simulations is presented, which is based on retrieving a spatially-coherent
component of microbunching to suppress artifact effects that can potentially
overestimate the FEL gain; this significantly reduces the number of
macroparticles to reach the numerical convergence and enables the direct
computation of amplified radiation without solving the wave equation. Examples
of FEL simulations performed to demonstrate the proposed method show that the
computation time to get a reliable result is reduced by 1-2 orders of magnitude
depending on the simulation condition.
