Abstract
An analysis of the coherence properties of the fourth-generation high-energy storage rings with emittance values of 10 pm rad is performed. It is presently expected that a storage ring with these low emittance values will reach diffraction limit at hard X-rays. Simulations of coherence properties were performed with the XRT software and an analytical approach for different photon energies from 500 eV to 50 keV. It was demonstrated that a minimum photon emittance (diffraction limit) reached at such storage rings is λ/2π. Using mode decomposition it is shown that, for the parameters of the storage ring considered in this work, the diffraction limit will be reached for soft X-ray energies of 500 eV. About ten modes will contribute to the radiation field at 12 keV photon energy and even more modes give a contribution at higher photon energies. Energy spread effects of the electron beam in a low-emittance storage ring were analysed in detail. Simulations were performed at different relative energy spread values from zero to 2 × 10-3. A decrease of the degree of coherence with an increase of the relative energy spread value was observed. This analysis shows that, to reach the diffraction limit for high photon energies, electron beam emittance should go down to 1 pm rad and below.
Highlights
It was realized that, due to a new conceptual approach, i.e. the multi-bend achromat synchrotron storage ring design, the brightness of next-generation X-ray storage rings may be increased by two to three orders of magnitude (Eriksson et al, 2014; Hettel, 2014)
X-ray radiation of the electron beam was simulated for the synchrotron storage ring with the parameters presented in Tables 1 and 2
We have provided a detailed analysis of the coherence properties of a high-energy synchrotron storage ring with ultra-low emittance values near 10 pm rad and a wide range of photon energies from 500 eV to 50 keV
Summary
It was realized that, due to a new conceptual approach, i.e. the multi-bend achromat synchrotron storage ring design, the brightness of next-generation X-ray storage rings may be increased by two to three orders of magnitude (Eriksson et al, 2014; Hettel, 2014). The high degree of coherence will allow the focusing of the synchrotron beams efficiently to the nanometre range without spatial filtering of flux (Singer & Vartanyants, 2014) It will allow an effective application of coherence-based techniques such as coherent diffraction imaging (CDI) potentially reaching sub-nanometre resolution (Schroer & Falkenberg, 2014). In a recent paper by Geloni et al (2018), energy spread effects on brightness and coherence were carefully analysed for the low-emittance storage rings using an analytical approach. It is a big demand to develope adequate and effective methods that may correctly describe properties of radiation from the ultimate storage rings close to the diffraction limit Such a description should be based on the application of the first- and higher-order correlation functions (Mandel & Wolf, 1995; Vartanyants & Singer, 2016).
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