What is it about?
We evaluate the damage threshold of an Au coated flat mirror, which is one of the reflective optics installed on FEL-1 beamline of Dalian Coherent Light Source (DCLS), upon far UV free electron laser (FEL) irradiation. The surface of the coating is characterized by profilometer and optical microscope. We present also theoretical approach of the phenomenon by applying conventional single-pulse damage threshold calculation, one-dimensional thermal diffusion model as well as finite element analysis with ANSYS to the case.
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Why is it important?
In this paper, we presented the result from an experiment performed at Dalian Coherent Light source, a FEL facility operating in UV wavelength range. We took an Au-coated mirror and bombarded its surface with UV FEL (121.6nm) in normal incidence geometry. As the repetition rate was merely 20Hz, such experiment was considered for single shot. Trend of the damage crater evolving with different pulse energy and time of irradiation (or number of pulses) was presented, together with different theroretical apporaches that shown mismatch. Such mismatch was later discussed and explained by the experimental conditions. The result can be good reference for the community and is worthy of some attention.
Perspectives
Perspectively, grazing incidence geometry can be considered to reduce the absorption of the FEL on the Au coating and increase the irradiated surface. Mathematically this leads to lower energy density for the exposure. The SSDT in this case can be converted and compared to the normal incidence case. Furthermore, a photodiode can be placed after the sample mirror to monitor the reflectivity of the mirror throughout the entire FEL exposure process. In theory, by synchronizing the read of such photodiode with the data from an intensity monitor installed upstream of the beamline, one may get accurate reflectivity of the mirror for each pulse.
Meiyi Wu
Institute of Advanced Science Facilities, Shenzhen
Read the Original
This page is a summary of: Study on the UV FEL single-shot damage threshold of an Au thin film, Journal of Synchrotron Radiation, July 2024, International Union of Crystallography,
DOI: 10.1107/s1600577524005332.
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