  Using the ONEDFEL code we perform Free Electron Laser simulations in the
astrophysically important guide-field dominated regime. For wigglers' (Alfven
waves) wavelengths of tens of meters and beam Lorentz factor $\sim 10^3$, the
resulting coherently emitted waves are in the centimeter range. Our simulations
show a growth of the wave intensity over fourteen orders of magnitude, over the
astrophysically relevant scale of $\sim$ few kilometers. The signal grows from
noise (unseeded). The resulting spectrum shows fine spectral sub-structures,
reminiscent of the ones observed in Fast Radio Bursts (FRBs).
