X-ray laser seeding for short pulses and narrow bandwidth

The performance of an X-ray free electron laser can be substantially enhanced if a coherent seed source is used to provide sufficient initial power that it dominates the spontaneous emission in the early part of the undulator. The FEL output then becomes an amplified reproduction of the input seed having, under certain conditions, the same pulse length and bandwidth. For studies of molecular and atomic time dynamics the pulse length may be very short, reaching below 1 fs with a transform limited bandwidth of a few tenths of a percent. For spectroscopy and diffraction experiments the beam may alternatively be made monochromatic at the level of one part in 10⁵ or better, with a pulse length of tens of femtoseconds. Simulation studies of seeded FEL performance at 0.3 nm using the code GINGER are presented.