Fatigue damage of X-ray optics under large number of laser pulses (Conference Presentation)

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With the LCLS-II high-repetition-rate FEL, the number of pulses on the optics over ten years reaches 20 trillion. The thermal fatigue damage and lifetime of the optics, beam transport components under such large number of FEL pulses are important issues that should be addressed. For the optics, the definition of the damage should be the significant (for instance, 50%) reduction of reflectivity, which is premonitory of damage, and much more stringent than the ablation threshold. Silicon is widely used for X-ray optics both as mirror substrate and crystal monochromator. As the power absorption length of the silicon in visible optical laser wavelength (400 – 800 nm) is comparable as the one in soft X-ray FEL (300 – 1600 eV), we use optical laser of 515 nm wavelength, 200 fs pulse length and up to 0.928 MHz repetition rate for fatigue damage test on silicon. This allows remediating the limited availability or unavailability of high rep-rate FEL beam time. By monitoring the reflectivity of the sample (polished silicon wafer), we can measure the damage versus the number of pulses. Our first results show the damage threshold decreases significantly when the number of pulses increases. We have pushed the number of pulses over 100 billion, which corresponds to several days of MHz operation. The damage here can be explained as thermal mechanical fatigue damage. The outcome of this project will be a fatigue damage model predicting the lifetime of the key components, such as X-ray optics, used for high-repetition-rate X-ray FELs and also sample material under large number of laser pulses.