What is Diffraction-limited storage ring?

Diffraction-limited storage rings , or ultra-low emittance storage rings, are synchrotron light sources where the emittance of the electron-beam in the storage ring is smaller or comparable to the emittance of the x-ray photon beam they produce at the end of their insertion devices.These facilities operate in the soft to hard x-ray range with extremely high brilliance

Together with X-ray free-electron lasers, they constitute the fourth generation of light sources, characterized by a relatively high coherent flux and enable extended physical and chemical characterizations at the nano-scale.