Probing the Evolution of Over-Dense Plasma using New X-ray Schlieren Imaging at the Matter in Extreme Conditions Instrument of LCLS

High power laser systems used in laser-matter interactions routinely generate optically opaque plasmas, increasing the challenge of probing the internal structure of these plasmas. Recently, the Matter in Extreme Conditions (MEC) [1] instrument at the Linac Coherent Light Source (LCLS) developed a new direct X-ray imaging diagnostic [2], capable of directly probing these over-dense plasmas. Here we present experimental results expanding this direct X-ray imaging to an X-ray Schlieren imaging system. Thick Silicon wafer targets were pumped with the MEC short pulse laser (1J, 40fs, >10¹⁹ W/cm²) and probed via X-rays from LCLS (9.5keV, 50fs, <1mJ) in this new X-ray Schlieren imaging configuration. The probe was aligned to be perpendicular to the target face and provide a side-on view of the evolution of hot electrons streaming through the target. A pump-probe temporal delay scan was performed, ranging from <1ps to >2ns. These results are compared against a second pump-probe delay scan using the traditional, direct X-ray imaging configuration. The Schlieren imaging system clearly shows an increase in contrast, typical of optical Schlieren systems, and maintains the same image resolution measured in the traditional imaging system. Ongoing investigations into the plasma evolution in the target will be presented.

[1] B. Nagler, et al. J. Synchrotron Rad. 22, 3 520-525 (2015)

[2] E. Galtier, et al. Sci Rep 15, 7588 (2025).