Matching a variable-included-angle grating monochromator to the properties of a soft X-ray FEL source to achieve a controlled temporal stretch

The use of a grating monochromator on a soft X-ray FEL source raises concerns about the induced pulse stretch because a diffraction grating stretches the pulse due to the optical path difference along the illuminated length. This paper describes a method for controlling the pulse stretch by matching the grating angles to the properties of the source, so that the illuminated footprint is defined. The source properties are deduced by wavefront propagation of simulations of the FEL output. The pulse stretch can be kept to about 10 fs RMS from 250 to 850 eV. The monochromator uses a single grating in the classical mount and gives high efficiency combined with flexibility to permit other operation modes such as high spectral resolving power. The gratings must be of very low line density to work at 250 eV with such low pulse stretch.