K-B bendable system optimization at FERMI@Elettra FEL: impact of different spatial wavelengths on the spot size

摘要

Kirkpatrick-Baez (K-B) active X-ray optics system is a good choice for focusing the free electron laser (FEL) pulses delivered by FERMI, the first seeded EUV-SXR FEL user facility operated at Elettra Sincrotrone Trieste. The present work reports the results obtained tuning this optical system used at the Diffraction and Projection Imaging (DiProI) beamline in order to improve and optimize its performance in terms of quality and size of the focal spot onto the sample controlling the fluence as well. To characterize the performance 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 allowed for optimizing the bending acting on the mirrors curvature and the angle positions (pitch and roll) of the K-B system. The experimental results are compared with the predictions from simulations obtained using the WISE code, starting from mirror actual surface metrology characterization. Filtering the Fourier transform of the mirror surface profiles we have analyzed the impact of spatial wavelengths on the focal spot degradation. From the wavefront measurements we have inferred a focal spot of 10 μm × 10 μm confirmed by the PMMA ablation imprints. The results from simulations with the WISE code are in agreement with the measurements and for different energies of the incident beam we established the threshold when the focal spot degradation is not affected by the spatial wavelengths of the K-B mirror surfaces.