Damage threshold of amorphous carbon mirror for 177eV FEL radiation

We present results of damage studies performed at the Free-Electron LASer in Hamburg (FLASH) on amorphous carbon (a-C). The experiment was performed in the total external reflection geometry representing the working configuration of X-ray mirrors. The 177eV photon laser beam was focused on a 40nm thin layer of a-C coated on a silicon substrate. Single-shot damages were produced at two different grazing angles (4.3° and 7.7°) below the critical angle (θc=8°) calculated for this photon energy. The corresponding damage fluence thresholds were determined to be 43.7 and 74mJ/cm2. No evidence of delamination or mechanical damages (cracks) under grazing angles on the 40nm thin sample was observed. The results are compared with the results of the previous experiment done on thicker layers (900nm) of a-C with the same experimental conditions [1].