Implementation of X-Ray Phase-Contrast Imaging (XPCI) with Principal Component Analysis (PCA) to Retrieve Areal Density of Laser-Shocked Materials in Extreme Conditions with a Single XFEL Pulse*

The characterization of the dynamic behavior of materials in extreme conditions requires an ultra-fast x-ray probe, such as x-ray free electron lasers (XFELs). Nevertheless, the stochastic nature of the x-ray pulses makes quantitative characterization of the physical and chemical behavior challenging. At the Matter in Extreme Conditions (MEC) Instrument at the Linac Coherent Light Source (LCLS), we perform in-situ 2D x-ray phase contrast imaging (XPCI), combining single femtosecond x-ray pulses from LCLS with laser shock compression to study heterogeneities that arise in low-density polymers with voids at the micron scale. Insight into the behavior of the void can be gained by performing phase retrieval on the images and measuring the areal density of the polymer during laser-shock ablation. To address the shot-to-shot fluctuations of the x-ray pulse, we implement principal component analysis on a sequence of recorded white-field images. The correct flat-field images can then be simulated to normalize the dynamic images against heterogeneities in the x-ray wavefront not induced by the sample. For the first time, we uniquely characterize the areal density of a dynamically compressed material in a single shot, which is crucial for understanding important attributes of matter.