Abstract
Optical damage of multi-layer mirrors used in an ultraviolet free electron laser (UV-FEL) resonator at linac-based FEL has been observed. The peak power of the UV-FEL at the linac-based FEL is about 100 times as large as that of the ring-based FELs. The mirror degradation mechanism in the linac-based FEL therefore is entirely different from that of the ring-based FEL which has been reported. Several 0.2 mm diameter craters have been found on both surfaces of upstream and downstream Ta 2O 5/SiO 2 multi-layer mirrors of the resonator after 50 h UV-FEL oscillation at 310–370 nm. At each crater, several layers near the surface are crushed by ablation induced by the FEL. Results from both the X-ray micro-analysis (XMA) on the craters and luminescence analysis during ablation induced with the pico-second pulsed Nd:YLF laser, which is the same pulse format as the FEL, explain that the pico-second pulses of the laser affect the damages.
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