Abstract
The brightness and transverse coherence of undulator radiation in synchrotron light sources is investigated using the Wigner formulation in a full 4D treatment. New results are presented on the impact of energy spread on the brightness and coherence as a function of the emittance. In contrast to other work on this topic, it is found that as emittance decreases, the effect of energy spread increases, but as the diffraction limit is approached, the behavior of brightness and coherence diverge. The accuracy of the commonly used Gaussian approximation for brightness and coherence is also explored in detail, resulting in a new conclusion that the commonly used projected source sizes and divergences are not the appropriate quantities to use in the Gaussian approximation to obtain the most accurate results. New expressions are derived for the rms source size and divergence as a function of the energy spread for use in the Gaussian approximation.
Highlights
A revolution is currently under way in the field of synchrotron light sources with the advent of lower emittance storage rings based on multibend achromat (MBA) lattices [1], which promise synchrotron radiation with a much higher brightness and degree of transverse coherence than previously obtainable
Even at the emittance which is widely considered to be needed to reach the diffraction limit, ε 1⁄4 λ=4π, which corresponds to 10 pm for a photon energy of 10 keV, the overall degree of spectral coherence is only 17%, and to reach 50% would require an emittance of 2.5 pm in each plane, which is beyond currently conceived machines
Energy spread further impacts the brightness and coherence, and in the regime of emittance that is currently of interest, the relative effect increases as the electron beam emittance is reduced
Summary
A revolution is currently under way in the field of synchrotron light sources with the advent of lower emittance storage rings based on multibend achromat (MBA) lattices [1], which promise synchrotron radiation with a much higher brightness and degree of transverse coherence than previously obtainable. The first of these fourthgeneration storage ring light sources, MAX-IV, recently came into operation in Sweden [2], and this will soon be followed by Sirius in Brazil and the European Synchrotron Radiation Facility upgrade in France. We will shed further light on the recently highlighted “paradox” that energy spread by itself does not influence the peak brightness but does degrade the transverse coherence off axis [4]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
