Diffraction-limited storage rings (DLSR), 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 (100eVâÂÂ100keV) with extremely high brilliance (in the order of 10<sup>21</sup>âÂÂ10<sup>22</sup> photons/s/mm<sup>2</sup>/mrad<sup>2</sup>/0.1%BW)
Together with X-ray free-electron lasers, they constitute the fourth generation of light sources, characterized by a relatively high coherent flux (in the order of 10<sup>14</sup>âÂÂ10<sup>15</sup>photons/s/0.1%BW for DLSR) and enable extended physical and chemical characterizations at the nano-scale.