Damages to B4C/W multilayer mirrors by intense soft x‐ray bursts

B4C/W multilayer mirrors designed for an x‐ray beamline at the MEGAJOULE facility are susceptible to x‐ray damage. The OCTAL laser was implemented to generate a gold plasma with a continuous x‐ray spectrum (0.1 to 1.5 keV) used to irradiate the mirror. The fluence on the sample is 0.05 J/cm2 per shot. The pulse duration is about 1 ns. The temporally integrated spectrum reflected by the mirror is analyzed by a bent crystal spectrometer and recorded on photographic film. The spectral analysis suggests that an increase of the mean period of the multilayer happens during the second shot. This expansion is estimated to be 5% with respect to the unexposed mirror. After the second shot, destruction of the multilayer coating in the central part of the sample and irreversible damages in the other part characterized by a decrease of Bragg reflectivity have been observed.