부산대학교 도서관

We describe a camera to record coherent scattering patterns with a soft-X-ray free-electron laser. The camera consists of a laterally-graded multilayer mirror which reflects the diffraction pattern onto a CCD detector. The mirror acts as a bandpass filter both for wavelength and angle, which isolates the desired scattering pattern from non-sample scattering or incoherent emission from the sample. The mirror also solves the particular problem of the extreme intensity of the FEL pulses, which are focused to greater than 10{sup 14} W/cm{sup 2}. The strong undiffracted pulse passes through a hole in the mirror and propagates on to a beam dump at a distance behind the instrument rather than interacting with a beamstop placed near the CCD. The camera concept is extendable for the full range of the fundamental wavelength of the FLASH FEL (i.e. between 6 nm and 60 nm) and into the water window. We have fabricated and tested various multilayer mirrors for wavelengths of 32 nm, 16 nm, 13.5 nm, and 4.5 nm. At the shorter wavelengths mirror roughness must be minimized to reduce scattering from the mirror. We have recorded over 30,000 diffraction patterns at the FLASH free-electron laser with no observable mirror damage or degradation of performance.