We developed and experimentally investigated a Nb/Si multilayer mirror for high power EUV light sources. The multilayer deposited by magnetron sputtering system has a reflectivity of 64% at 13.5 nm. As a first demonstration, this value is not inferior to that of conventional Mo/Si multilayer mirror. For investigating highpower irradiation tolerance, EUV damage threshold was measured by using SACLA BL1. The input beam specifications are as below; power density of 60 mJ/cm2, pulse duration of 1100 fs, wavelength of 13.5 nm, 10 shots, and incident angle of 45 deg. The observed beam profiles clearly show that the damage threshold of Nb/Si multilayer mirror is higher than that of Mo/Si multilayer mirror. In detail measurements, damage threshold of Nb/Si multilayer mirror was estimated to as twice as that of Mo/Si multilayer mirror. The results will be extremely useful to drivie new standard multilayer mirrors developments for industrial uses.International Conference on Synchrotron Radiation Instrumentation (SRI 2018
