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Wavefront sensor based diagnostic of FERMI FEL photon beam (Conference Presentation)
Author(s): Lorenzo Raimondi; Nicola Mahne; Michele Manfredda; Cristian Svetina; Daniele Cocco; Flavio Capotondi; Emanuele Pedersoli; Maya Kiskinova; Marco Zangrando
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Paper Abstract

FERMI is the first seeded EUV-SXR free electron laser (FEL) user facility, and it is operated at Elettra Sincrotrone Trieste. Two of the four already operating beamlines, namely LDM (Low Density Matter) 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. A wafefront sensor is used as diagnostic for the characterization of the focused spot and for the optimization of the parameters of these active optical systems as well. The aim of this work is, first, to describe in detail the optimization procedure using the wavefront sensor through the minimization of the Zernike coefficients, and second, report on the final results obtained on the K-B optical system at the DiProI endstation. The wavefront sensor, mounted out of focus behind the DiProI chamber, allows to compute the intensity distribution of the FEL beam, typically a mix between several modes resulting in a ”noisy hyper-Gaussian” intensity profile, and the wavefront residual from ideal propagation shape and after tilt correction. Combining these two measures we can obtain the electric field of the wave out of focus. Propagating back the electric field we reconstruct the focal spot in far field approximation. In this way the sensor works as a diagnostic reconstructing the focal spot. On the other hand, after modelling the electric field with a Zernike polynomial it is easy and fast to optimize the mirror bending and the optical system angles by minimizing the aberrations, quantified in terms of Zernike coefficients. Since each coefficient corresponds to a single parameter, they can be minimized one at the time. 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 for DiProI of 5.5 μm x 6.2 μm at 32 nm wavelength, confirmed by the PMMA ablation imprints. The experimental results were 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 were found to be in agreement with the experimental measurements.

Paper Details

Date Published: 2 November 2016
PDF: 1 pages
Proc. SPIE 9965, Adaptive X-Ray Optics IV, 99650F (2 November 2016); doi: 10.1117/12.2239212
Show Author Affiliations
Lorenzo Raimondi, Elettra-Sincrotrone Trieste S.C.p.A. (Italy)
Nicola Mahne, Elettra-Sincrotrone Trieste S.C.p.A. (Italy)
Michele Manfredda, Elettra-Sincrotrone Trieste S.C.p.A. (Italy)
Cristian Svetina, Elettra-Sincrotrone Trieste S.C.p.A. (Italy)
Univ. of Trieste (Italy)
Daniele Cocco, SLAC National Accelerator Lab. (United States)
Flavio Capotondi, Elettra-Sincrotrone Trieste S.C.p.A. (Italy)
Emanuele Pedersoli, Elettra-Sincrotrone Trieste S.C.p.A. (Italy)
Maya Kiskinova, Elettra-Sincrotrone Trieste S.C.p.A. (Italy)
Marco Zangrando, Elettra-Sincrotrone Trieste S.C.p.A. (Italy)
CNR – Consiglio Nazionale delle Ricerche Istituto Officina dei Materiali - TASC (Italy)


Published in SPIE Proceedings Vol. 9965:
Adaptive X-Ray Optics IV
Stephen L. O'Dell; Ali M. Khounsary, Editor(s)

Video Presentation

Wavefront sensor based diagnostic of FERMI FEL photon beam (Conference Presentation)



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