Speaker
Description
In 1990s, the emergence of the 3rd generation synchrotron user facilities equipped with undulator sources, stimulated pioneering works on interaction of coherent X-ray beams with matter (Brauer et al. 1995), resulted in the development of X-ray photon correlation spectroscopy (XPCS) technique, capable to probe nanoscale fluctuations up to kHz frequencies using dynamics of speckle pattern given by scattering of coherent X-ray beam from the sample. More recently (and highly aided by the development of modern high-throughput data analysis algorithms), coherent X-ray diffraction imaging (CXDI), also known as lensless imaging, revolutionized the field of X-ray microscopy, finally bringing users the possibility of non-destructive 2D and 3D imaging of complex structures with unprecedented ~10 nm resolution. However, strict requirements to transverse coherence of X-ray probe put serious limits on further development of CXDI-based techniques at the 3rd generation facilities, where coherent flux after spatial filtering becomes unacceptably small at energies above ~20 keV. At the same time, the latter was used as one of the strongest arguments of the user community for push towards construction of MBA-based 4th generation of storage rings with ultra-low-emittance.
Since coherent flux can be simply expressed as $F_{coh}={\lambda^2\over4}B$, where $\lambda$ is X-ray wavelength, and B – source brightness, the demand for higher coherent flux is often translated as demand for higher brightness – a recognizable motto for both accelerator and user communities. From the same relationship one can also easily see that even more brightness is needed to obtain the same coherent flux at shorter wavelengths / higher energies. Another expression $F_{coh}={\lambda^2\over 16\pi^2}F/\varepsilon_{tot}$, where F is total flux, and $\varepsilon_{tot}$ – 4D phase volume of X-ray beam, illustrates that it is the phase volume of the undulator X-ray source that should be minimized in order to deliver a maximum number of coherent photons to end-user (given total flux has already reached its limit imposed by machine current and undulator technology). Whereas at the 3rd generation facilities the phase volume of undulator source was dominated by electron beam emittance with negligible influence of other factors, the estimation and minimization of undulator source phase volume at 4th generation facilities requires more elaborate approach taking into account electron beam energy spread, undulator phase error, and matching of phase-space ellipses between electron and X-ray beams.
In our contribution we will present estimations of coherent flux at different energies available for future users of ultra-low-emittance SKIF storage ring (to be commissioned in 2023) and discuss ways of its optimization, as well as future scientific program of user experiments with diffraction-limited X-ray beams.
Brauer, S., Stephenson, G.B., Sutton, M., Brüning, R., Dufresne, E., Mochrie, S.G.J., Grübel, G., Als-Nielsen, J., and Abernathy, D.L. (1995) X-Ray Intensity Fluctuation Spectroscopy Observations of Critical Dynamics in F${\mathrm{e}}_{3}$Al. Physical Review Letters, 74, 2010–2013.
