Measurement of Preheat Due to Nonlocal Electron Transport in Warm Dense Matter

This Letter presents a novel approach to study electron transport in warm dense matter. It also includes the first x-ray Thomson scattering (XRTS) measurement from low-density CH foams compressed by a strong laser-driven shock at the OMEGA laser facility. The XRTS measurement is combined with velocity interferometry (VISAR) and optical pyrometry (SOP) providing a robust measurement of thermodynamic conditions in the shock. Evidence of significant preheat contributing to elevated temperatures reaching 17.5–35 eV in shocked CH foam is measured by XRTS. These measurements are complemented by abnormally high shock velocities observed by VISAR and early emission seen by SOP. These results are compared to radiation hydrodynamics simulations that include first-principles treatment of nonlocal electron transport in warm dense matter with excellent agreement. Additional simulations confirm that the x-ray contribution to this preheat is negligible.

Figure 1

A schematic of the targets. The driven target sits inside the larger Au cone and consists of a plastic ablator, a Au radiation shield, an Al pusher, and ${\mathrm{C}}_8{\mathrm{H}}_8$ foam. The XRTS target also includes a second smaller cone containing a Ni backlighter and Ta slit used to collimate the x rays. Laser beams illuminate the ablator from inside of each cone. The VISAR target is identical, but has no backlighter and includes four steps on the back side of the foam. Additional Ta shielding was used to restrict the view of VISAR and XRTS.

Figure 2

X-ray scattering data from shocked ${\mathrm{C}}_8{\mathrm{H}}_8$ polystyrene foam. The best fit conditions were $T_e=26\pm 3\mathrm{eV}$, $n_e=8.43\times {10}^{22}{\mathrm{cm}}^{-3}$ and $Z\sim 2.4$. The error bars were estimated from a ${\chi }^2$ fit (inserted image).

Figure 3

Hydrodynamic simulation of a laser driven shock with the pete code. The position of the propagating shock can be seen, where the shock velocities corresponding to the break-out times at the three steps on the rear side of the target are shown. The preheat due to nonlocal electrons can be observed and the corresponding thermodynamic conditions are in excellent agreement with the elevated temperature measured by XRTS.

Figure 4

The shock propagating in the foam layer can be seen in the hydrodynamic simulation profiles of density $\rho$, electron temperature $T_e$, and pressure $p$ corresponding to the time of the XRTS measurement in Fig. 3. The preheat zone is highlighted in the inset further accompanied by the profile of electron energy flux density $q_H$. Since the Knudsen number Kn within the highlighted zone reaches a high value 0.1 (diffusive limit 0.001), the nonlocal electron transport becomes significant [$q_H\sim u(E+p)$], thus forming the preheat.