Leveraging high temporal and spectral resolution to study solid-density plasma ablation

X-ray emission spectroscopy is a key diagnostic for laser-produced plasmas, especially at high densities where most optical- and particle-based measurement techniques cannot probe. We investigate plasma phenomena near solid densities by pairing an ultrafast streak camera (temporal resolution Δt < 1 ps) with focusing quartz crystal spectrometers (spectral resolving power E/ΔE ~ 10⁴) to capture the evolution of emitted x-ray line shapes. Since line shapes encode plasma properties such as densities, temperatures, and flows, the measurements access new information about high-density plasma physics. One application of this capability is the study of solid material ablation by relativistic laser pulses (intensity > 10²¹ W/cm²). Streaked x-ray spectra emitted from discrete layers of highly ionized tracer material contain unshifted x-ray line transitions at early times, representative of hot, non-flowing plasma. Later in time, the same line shapes develop Doppler-shifted components, indicating time-evolving ion flows as plasma ablates into vacuum. This data can quantify a nonzero and depth-dependent time delay between laser-induced plasma heating and the initiation of expansion.