# In-situ single-shot diffractive fluence mapping for X-ray free-electron laser pulses

## TL;DR

This paper introduces a novel in-situ method for simultaneously measuring diffraction signals and the actual spatial fluence distribution of FEL pulses, enabling better control and understanding of non-linear light-matter interactions.

## Contribution

It presents the first in-situ, single-shot measurement technique that captures both diffraction data and the beam's fluence profile during FEL experiments.

## Key findings

- Enables real-time monitoring of beam fluence during experiments
- Improves understanding of shot-to-shot fluctuations in FEL experiments
- Facilitates quantitative analysis of non-linear light-matter interactions

## Abstract

Free-electron lasers (FEL) in the extreme ultraviolet (XUV) and X-ray regime opened up the possibility for experiments at high power densities, in particular allowing for fluence-dependent absorption and scattering experiments to reveal non-linear light-matter interactions at ever shorter wavelengths. Findings of such non-linear effects in the XUV and X-ray regime are met with tremendous interest, but prove difficult to understand and model due to the inherent shot-to-shot fluctuations in photon intensity and the often structured, non-Gaussian spatial intensity profile of a focused FEL beam. Presently, the focused beam spot is characterized and optimized separately from the actual experiment. Here, we present the first simultaneous measurement of diffraction signals from solid samples in tandem with the corresponding single-shot spatial fluence distribution on the actual sample. This new in-situ characterization scheme enables fast and direct monitoring and thus control of the sample illumination which ultimately is necessary for a quantitative understanding of non-linear light-matter interaction in X-ray and XUV FEL experiments.