Multi FEL Lines with Compact Undulator Layout

Today the high repetition rate X-ray Free Electron Lasers based on superconducting radiofrequency technologies have come to their age. Such kind of facilities are able to serve many FEL photon beamlines simultaneously with each of which have large flexibilities in selecting wavelength, intensity, polarization, coherence and other properties through independent tuning of the undulator magnets. In reality the space needed to accommodate many undulator lines could be a limiting factor of user capacity, especially for the high rep rate XFELs that tend to utilize the underground tunnel to host long superconducting accelerator machines. In this paper we present a concept of compact undulator layout for more FEL lines in the precious tunnel spaces or similar environment. Shanghai Coherent Light Facility(SCLF) is a high repe-tition rate X-ray Free Electron Lasers installed in under-ground tunnels with an overall length of more than 3 km. The concept described in this paper could be applied to the SCLF or similar FEL facilities. The design and R&D progress will be presented.