A model is presented which demonstrates that the attosecond pulse structure of a High Harmonic Generation (HHG) seed may be retained through to saturation in an FEL amplifier. At wavelengths of ~12nm a train of attosecond pulses of widths ~300 attose
conds with peak powers in excess of 1 GW are predicted from full 3D simulation. Methods for improving these results are discussed.
