Status of the K-B bendable optics at FERMI@Elettra FEL

Abstract

FERMI is the first seeded EUV-SXR free electron laser (FEL) user facility operated at Elettra Sincrotrone Trieste. Two of the three already operating beamlines, namely LDM (Low DensityMatter) and DiProI (Diffraction and Projection Imaging), use a Kirkpatrick-Baez (K-B) active X-ray optics system for focusing the FEL pulses onto the target under investigation. The present work reports on the final results obtained from the optimization of the K-B optical system at the DiProI endstation. The aim of the optimization is to improve the system performances in terms of quality and size of the focal spot onto the sample, controlling the fluence as well. To characterize the performances and develop reliable and reproducible focusing procedures we performed a campaign of measurements with several diagnostic systems, including a wavefront sensor mounted after the DiProI chamber. Online wavefront measurements have made possible the optimization of the bending acting on the mirror curvature and of the (pitch and roll) angle positions of the K-B system. From the wavefront measurements we have inferred a focal spot of 8 μm x 9.5 μm, confirmed by the PMMA ablation imprints. The experimental results are compared with the predictions from simulations obtained using the WISE code, starting from the characterization of the actual mirror surface metrology. The results from simulations are in agreement with the experimental measurements. Filtering the Fourier transform of the mirror surface profiles, using the WISE code we have analyzed the impact of different spatial wavelengths on the focal spot degradation. For different energies of the incident beam we established the threshold where the focal spot degradation is no longer affected by the spatial wavelengths of the K-B mirror surfaces. In the very last period we were starting to observe a degradation of the focal spot. After a metrology analysis we concluded that the problem was due to a failure of the substrate material. We temporally solved the problem checking the mounting, but we have planned an improvement of the material for the future.