Phase imaging of irradiated foils at the OMEGA EP facility using phase-stepping X-ray Talbot-Lau deflectometry

Diagnosing the evolution of laser-generated High Energy Density (HED) systems is fundamental to developing a correct understanding of the behavior of matter under extreme conditions. Interferometry methods are a very powerful tool for diagnosing these systems, as they can provide valuable information about the plasma electron and ion density in a simple manner. However, current diagnostic methods mostly rely on visible radiation and thus, HED plasma probing is difficult since these plasmas are mostly opaque to visible wavelengths. Considering this, Talbot-Lau grating interferometry is a promising approach to diagnosing HED systems as it extends interferometry methods to the X-ray regime. In recent years, with the aim of imaging dense plasmas, there have been several efforts to adapt Talbot-Lau interferometry to high power laser facilities such as PALS, the Multi-TeraWatt (MTW) facility and OMEGA EP, as well as proof-of-concepts experiments at lower energy high-repetition rate lasers. A schematic drawing of a Talbot-Lau interferometer and its different components is shown in Figure 1.

Talbot-Lau interferometry has been widely used in the field of medical sciences since it permits imaging of softer tissue than traditional X-ray radiography, commonly using the method of phase-stepping. This consists of taking several sequential images for different lateral positions of one of the gratings in the interferometer. The set of images can be used to reconstruct high-resolution phase and transmission images. However, this method is not often used in HED experiments owing to the limited amount of data that can usually be obtained at high power laser facilities, which impedes taking several images at the same conditions for different positions of the grating. Furthermore, the laser beams are often close enough to the interferometer to cause grating ablation. Therefore, instead of phase-stepping, a deflectometry configuration is used, where one of the gratings is rotated at a small angle with respect to another. This generates a Moiré pattern on the image that introduces an additional periodicity, larger than that of the individual gratings.

Figure 1. Schematic drawing of the experimental setup. The figure shows the different elements of the interferometer together with the backlighter target, the plasma target and the corresponding laser beams. In this figure, G0 corresponds to the source grating, G1 is the beamsplitter and G2 is the analyzer grating described in the text. The dot-dashed line across all elements corresponds to the optical axis of the interferometer.

Figure 2. (a) Region of interest of the interferometer data. (b), (c) Transmission and phase-shift data integrated lineouts. The shaded regions in all images correspond to the original position of the target. In (c) the brown dotted line corresponds to the phase shift obtained without applying phase-stepping techniques. The vertical dashed lines across all figures correspond to the shocked plasma.

An international team including collaborators from Europe and USA, combined these two techniques (phase-stepping and Moiré deflectometry) in a recent experiment at the OMEGA EP laser facility, to image the ablation front of a laser-generated plasma. While Moiré deflectometry was used to image the ablated plasma, reference images were taken by applying phase-stepping to the same Moiré configuration. The research results are published on High Power Laser Science and Engineering, Vol.11, Issue 4, (G. Pérez-Callejo, V. Bouffetier, L. Ceurvorst, T. Goudal, S. R. Klein, D. Svyatskiy, M. Holec, P. Perez-Martin, K. Falk, A. Casner, T. E. Weber, G. Kagan, M. P. Valdivia. Phase imaging of irradiated foils at the OMEGA EP facility using phase-stepping X-ray Talbot–Lau deflectometry[J]. High Power Laser Science and Engineering, 2023, 11(4): 04000e49).

They present the first X-ray interferometry image of an ablating HED plasma obtained at a high-power laser facility, probed by a Talbot-Lau X-ray interferometer. From this image, they obtained, X-ray transmission and phase-shift information of a laser-produced compression wave through a solid, as shown in Figure 2. The data obtained with this technique can provide meaningful insights into the physics of the ablation zone in laser-generated plasmas (a requirement to benchmark current theoretical models), as it allows direct probing into the dense ablated/compressed regions.