Beam focus modifications by cropping partially coherent X-ray beams

TL;DR

This paper simulates how cropping partially coherent X-ray beams affects their focus, revealing that aperture size influences focus position and size, which is crucial for beamline optimization.

Contribution

It introduces a simulation framework for analyzing focus modifications of partially coherent X-ray beams due to cropping and pairing focusing elements.

Findings

Focus position shifts with slit cropping.

Focus size enlarges when beam is cropped.

Pairing focusing elements maintains fixed focus position.

Abstract

We simulate the focusing of a partially-coherent X-ray beam emitted by an undulator in a fourth-generation storage ring performing coherent mode decomposition and wave optics propagation. The focus position is shifted, and its size is enlarged when an entrance slit crops the beam. This is the usual case when a slit is used to select the coherent fraction. The pairing of two focusing elements (mirrors, lenses or transfocators) to ensure a fixed focal position is also analyzed. Our results show that the image of a partially coherent source, such an undulator in a low-emittance storage ring, is a non-trivial function of the aperture used to control the coherence fraction.