KAOS latest results with FEL-2

KAOS, the Kirkpatrick-Baez Active Optics System, serves as a refocusing section for three beamlines at FERMI (DiProI, LDM and MagneDYN), and is going to be used also at FLASH2 facility in Hamburg. The system is capable of providing variable-size focal spots into the endstations assuring the fine correction of the wavefront incoming on the sample. Recently, it was upgraded mechanically in order to guarantee opto-mechanically limited focusing to the experiments.

A wafefront sensor is used as diagnostic for the characterization of the focused spot and for the optimization of the parameters of the KAOS as well. It is mounted out of focus behind the DiProI chamber, allowing 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 1.8 μm x 3.0 μm at 4.14 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.

For further details check the following publication:

Wavefront sensor based diagnostic of FERMI FEL photon beam
L. Raimondi, N. Mahne, M. Manfredda, C. Svetina, D. Cocco, F. Capotondi, E. Pedersoli, M. Kiskinova, M. Zangrando

Proc. SPIE 9965, Adaptive X-Ray Optics IV, 99650F (2016)
Ultima modifica il Venerdì, 21 Aprile 2017 16:18