Ablating Ion Velocity Distributions in Short-Pulse-Heated Solids via X-Ray Doppler Shifts

Solids ablate under laser irradiation, but experiments have not previously characterized the initiation of this process at ultrarelativistic laser intensities. We present first measurements of bulk ion velocity distributions as ablation begins, captured as a function of depth via Doppler-shifted x-ray line emission from two viewing angles. Bayesian analysis indicates that bulk ions are either nearly stationary or flowing outward at the plasma sound speed. The measurements quantitatively constrain the laser-plasma ablation mechanism, suggesting that a steplike electrostatic potential structure drives solid disassembly.

Figure 1

(a) Cartoon of the experimental setup, including tracer layer targets and three spectrometer views. (b) Front-viewed x-ray spectra versus photon energy $E_{\gamma }$ (solid lines and shaded regions are ten-shot means and standard deviations) at $z_{\mathrm{Ti}}=250\mathrm{nm}$ with the Ti $\mathrm{He}\alpha$ complex and $\mathrm{Ly}\alpha$ resonance lines. Model predictions from the scram code [22] (vertically offset, upper left) show satellite intensity ratios at three labeled densities; similar calculations at different $T_e$ generate the upper right inset, where the shaded band reflects observations. “Ratio” is the line intensity ratio between the hatched regions, notably avoiding opacity effects from the $\mathrm{He}\alpha$ resonance line. (c) IC line shapes, tamped and untamped as labeled, shown from front and rear views.

Figure 2

IC line shapes from six noted depths $z_{\mathrm{Ti}}$ viewed from the front (left) and side (right). Solid lines and shaded regions are ten-shot intensity means and standard deviations. The untamped front-view line shape is expanded for clarity.

Figure 3

(a) Normal-direction velocity distributions for $z_{\mathrm{Ti}}=250\mathrm{nm}$ fit from the one-Gaussian (1G) and two-Gaussian (2G) models. Shaded regions encompass 95% confidence intervals. (b) Best-fit line shapes corresponding to distributions in (a) overlaying the measurement. (c) Velocities of faster (shaded circle) and slower (plus) peaks versus $z_{\mathrm{Ti}}$ with 98% confidence intervals, overlaid with the inferred range of $c_s$. (d) Fraction of ions in the faster peak plotted as in (c).