Wasserstein metric analysis of X-ray Doppler-shifted line shapes from laser-ablated Ti plasma*

The creation and study of high energy density (HED) plasmas informs the understanding of astrophysical events, inertial fusion, and medical imaging. This project compares and sorts spectral line profiles from a solid-density plasma. X-ray emission was previously collected by high-resolution spectrometers from HED plasma formed by laser-ablation of Al foil containing embedded Ti tracer layers. Measurements included the intercombination (IC) line of the Ti He⍺ complex, ideal for examining a depth-dependent blue shift. Here, the IC line shape from hundreds of shots over several layer depths is background-subtracted and statistically compared via the Wasserstein metric and Guassian fitting. Periodic variation is observed in the line shape of shots with the Ti tracer layer on the target surface, indicating a variation in target orientation–likely a tilt or curvature–with respect to the laser focus. Monitoring the Wasserstein metric between shots of the same layer depth during the experiment can inform target alignment. With proper alignment, the IC line shape is used to infer ion velocity distributions of the different tamper layer depths.