Picosecond Time-Resolved X-Ray Absorption Spectroscopy of Ultrafast Aluminum Plasmas

We have used point-projection $K$-shell absorption spectroscopy to infer the ionization and recombination dynamics of transient aluminum plasmas. Two femtosecond beams of the 100 TW laser at the LULI facility were used to produce an aluminum plasma on a thin aluminum foil (83 or 50 nm), and a picosecond x-ray backlighter source. The short-pulse backlighter probed the aluminum plasma at different times by adjusting the delay between the two femtosecond driving beams. Absorption x-ray spectra at early times are characteristic of a dense and rather homogeneous plasma. Collisional-radiative atomic physics coupled with hydrodynamic simulations reproduce fairly well the measured average ionization as a function of time.

Figure 1

Experimental setup. (a) Layout of the different laser beams. (b) Top and side views of the conically bent crystal spectrograph; $L=90\mathrm{m}\mathrm{m}$, $d=3\mathrm{m}\mathrm{m}$.

Figure 2

Space-resolved absorption lines obtained at three different times after laser peak for two different thicknesses of Al, (a) 83 nm, (b) 50 nm irradiated at a laser intensity of $3\times {10}^{15}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. The shadow of the $300\mathrm{\text{−}}\mu \mathrm{m}$ diameter gold pinhole (red ellipse), the cold Al $K$ edge at 7.949 Å, and the $1s\mathrm{\text{−}}3p$ ${\mathrm{A}\mathrm{l}}^{3+}$ line at 7.883 Å can be seen, together with the spatial shape of the pump laser pulse.

Figure 3

Experimental spectrum (dashed line) of the $1s\mathrm{\text{−}}3p$ line of ${\mathrm{A}\mathrm{l}}^{3+}$ obtained during the laser pulse, compared to the prediction of the TOTAL code at $N_e=5\times {10}^{22}{\mathrm{c}\mathrm{m}}^{-3}$ (solid line), $N_e=3\times {10}^{22}{\mathrm{c}\mathrm{m}}^{-3}$ (thin dashed line), and $T_e=50\mathrm{e}\mathrm{V}$.

Figure 4

Transmission, 7 ps after the heating pulse, of a 83-nm Al foil heated at a laser intensity of $3\times {10}^{15}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ (dashed line). The best HULLAC/GA fit is shown for comparison (solid line).

Figure 5

Time evolution of the measured average ionization of (a) 83 nm and (b) 50 nm Al foils compared to the numerical simulations of the areal-mass-weighted average ionization under LTE (crosses) and non-LTE (open squares) conditions.