Abstract
The Linac Coherent Light Source (LCLS) is an X-ray source of unmatched brilliance, that is advancing many scientific fields at a rapid pace. The highest peak intensities that are routinely produced at LCLS take place at the Coherent X-ray Imaging (CXI) instrument, which can produce spotsize at the order of 100 nm, and such spotsizes and intensities are crucial for experiments ranging from coherent diffractive imaging, non-linear x-ray optics and high field physics, and single molecule imaging. Nevertheless, a full characterisation of this beam has up to now not been performed. In this paper we for the first time characterise this nanofocused beam in both phase and intensity using a Ronchi Shearing Interferometric technique. The method is fast, in-situ, uses a straightforward optimization algoritm, and is insensitive to spatial jitter.
Highlights
We find an RMS error of less than 1 of a wavelength for each Ronchigram, ensuring that the inversion algorithm works accurately
We note that the40use of three Ronchigrams over-constrains the optimization problem
In order to demonstrate the value of the information gained from the Ronchigram wavefront retrieval, a simulation of the aberrations due to misalignment of the KB mirror pair was performed
Summary
The start of operations of Free electron laser both in the Extreme UV1,2 and in the hard X-ray regime[3,4,5] has created sources of unmatched brilliance, that are advancing many scientific fields at a rapid pace (see[6,7] and references therein). A standard technique currently used involves evaluating the size of damage craters created by the focused X-ray beam in a target[9]. While this method has the advantage that it measure the whole intensity profile (i.e. both the coherent and incoherent part), it requires a time consuming post mortem analysis of many such imprints, is not an in-situ method, and has limited spatial resolution. The area where we do see fringes only yields information on the phase differences in one direction: we do not have any information on phase changes in the beam parallel to the fringes This problem can be overcome by using enough Ronchigrams to ensure that every area where there is appreciable beam intensity is sheared along at least two angles. A full description of the inversion algorithm can be found in the methods section of this paper
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