We present developments on a hard X-ray wavefront sensing instrument for characterizing and monitoring the beam of the European X-ray Free Electron Lasers (EuXFEL). The pulsed nature of the intense X-ray beam delivered by this new class of facility gives rise to strong challenges for the optics and their diagnostic. In the frame of the EUCALL project Work Package 7, we are developing a sensor able to observe the beam in the X-ray energy range [8-40] keV without altering it. The sensor is based on the speckle tracking principle and employs two semi-transparent optics optimized such that their X-ray absorption is reduced. Furthermore, this instrument requires a scattering object with small random features placed in the beam and two cameras to record images of the beam at two different propagation distances. The analysis of the speckle pattern and its distortion from one image to the other allows absolute or differential wavefront recovery from pulse to pulse. Herein, we introduce the stakes and challenges of wavefront sensing at an XFEL source and explain the strategies adopted to fulfil the high requirements set by such a source.
The aim of this research is to calculate the refractive index of transparent atmospheric aerosols, which have biological
origin, using a digital holographic microscopy technique (DHM). The samples are collected on filters, using miniature
impactors for particles with dimensions smaller than 10μm (on even one axis), from a height of over 20 meters, in
Magurele, a rural location near the urban and industrial agglomeration of the capital city, Bucharest. Due to their organic
or inorganic origin, each atmospheric aerosol particle has different size, shape and optical properties which have a
determinant role in LIDAR measurements. We record on a CCD camera hundreds of holograms which contain the
diffraction pattern from every aerosol particle superposed with the reference wave. Digitally, we scan the entire volume
of one particle with nanometric resolution (using an algorithm based on the Fresnel approximation). The calibration was
done using an object with known dimensions fabricated by e-beam lithography and some complementary measurements
were done in confocal microscopy. Our analysis separates four main classes of atmospheric aerosols particles (wires,
columns, spherical fragments, and irregular). The predominant class in the investigated period is the first one, which has
biological origin and the refractive index was calculated starting from the phase shift introduced by them in the optical
path and models for their cylindrical shape. The influence of spatial filtering in the reconstructed object images was
investigated.
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