Abstract
The experimental characterization of the spatial and temporal coherence properties of the free-electron laser in Hamburg (FLASH) at a wavelength of 8.0 nm is presented. Double pinhole diffraction patterns of single femtosecond pulses focused to a size of about 10×10 μm(2) were measured. A transverse coherence length of 6.2 ± 0.9 μm in the horizontal and 8.7 ± 1.0 μm in the vertical direction was determined from the most coherent pulses. Using a split and delay unit the coherence time of the pulses produced in the same operation conditions of FLASH was measured to be 1.75 ± 0.01 fs. From our experiment we estimated the degeneracy parameter of the FLASH beam to be on the order of 10(10) to 10(11), which exceeds the values of this parameter at any other source in the same energy range by many orders of magnitude.
Highlights
Free-electron lasers (FELs) based on the self-amplified spontaneous emission (SASE) principle produce extremely brilliant, highly coherent radiation in the extreme ultraviolet (XUV) [1] and hard x-ray [2] range
From our experiment we estimated the degeneracy parameter of the free-electron laser in Hamburg (FLASH) beam to be on the order of 1010 to 1011, which exceeds the values of this parameter at any other source in the same energy range by many orders of magnitude
In this paper we present measurements of the transverse coherence properties of FLASH and investigate how they fluctuate from shot to shot
Summary
Free-electron lasers (FELs) based on the self-amplified spontaneous emission (SASE) principle produce extremely brilliant, highly coherent radiation in the extreme ultraviolet (XUV) [1] and hard x-ray [2] range. Utilizing the high photon flux, the femtosecond pulse duration and the high degree of coherence, techniques like coherent x-ray diffraction imaging (CXDI) [3, 4, 5, 6] x-ray holography [7] and, recently, nano-crystallography [8] promise important new insights in biology [9, 10], condensed matter physics [11] and atomic physics [12] Some of these methods can be implemented only if the radiation is sufficiently coherent, both spatially and temporally. Temporal and transverse coherence effects play a role in the accessible field of non-linear excitations of atoms and molecules [15]
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