A focused beam in nanometer scale is important for soft X - ray scanning microscopy and microscopic spectroscopy. A plan has been proposed to realize diffraction - limited focusing of X - ray

during the construction of synchrotron radiation beamline. In this design

light source from undulator radiation

with high coherent flux

is employed. A zone plate

a kind of diffraction optical element

is used to focus the soft X - ray beam into diffraction - limited spot. A concept of redundant illumination is introduced to ensure the spatial coherent illumination of the imaging element. In order to improve the output

special consideration is posed to phase space matching in horizontal and vertical directions. The designed focus spot size is 50nm

flux output is 109photons/s·0. 1% BW.