Design Considerations for X-Ray FEL Optics

John Arthur LCLS, SLAC 

Abstract
New facilities based on free-electron laser (FEL) x-ray sources are becoming operational in several countries. These facilities generate sub-picosecond pulses of extremely intense, monochromatic, coherent x-rays. FEL x-ray optics must perform the functions of transmission, absorption, and reflection, while preserving the extreme brightness of the x-ray pulses. This places constraints on the optics design:

• Extreme peak intensity - FEL pulses are very short but very intense. Most absorber materials will suffer local damage due to the energy delivered by a single pulse. Apertures and shutters that must intercept the beam can mitigate the peak intensity by using materials made of very light elements (Be, B, C) to diffuse the pulse energy.
• Coherence preservation – FEL pulses are spatially coherent. Windows, lenses, and solid attenuators, made of light elements (see above), must also be very homogeneous with polished boundaries. Care must taken in using beryllium; most commercially-available Be includes microscopic inhomogeneities which distort a highly-coherent x-ray beam. Reflection optics must be precisely figured to avoid distortions of the reflected wave. A typical grazing-incidence mirror must have figure errors over a 1-100mm spatial frequency range of less than 2nm rms. This level of surface precision can only be met by incorporating interferometric metrology as part of the mirror polishing process.
• Mechanical stability – FEL beamlines are often very long (> 100m), and FEL x-rays have extremely low divergence. All beamline components must be stable, and reflective optics must maintain angular stability at the nanoradian level. Temperature control at the level of 0.01 degree is typically required.