Temperature measurements of shocked silica aerogel foam

We present recent results of equation-of-state (EOS) measurements of shocked silica (${\mathrm{SiO}}_2$) aerogel foam at the OMEGA laser facility. Silica aerogel is an important low-density pressure standard used in many high energy density experiments, including the novel technique of shock and release. Due to its many applications, it has been a heavily studied material and has a well-known Hugoniot curve. This work then complements the velocity and pressure measurements with additional temperature data providing the full EOS information within the warm dense matter regime for the temperature interval of 1–15 eV and shock velocities between 10 and 40 km/s corresponding to shock pressures of 0.3–2 Mbar. The experimental results were compared with hydrodynamic simulations and EOS models. We found that the measured temperature was systematically lower than suggested by theoretical calculations. Simulations provide a possible explanation that the emission measured by optical pyrometry comes from a radiative precursor rather than from the shock front, which could have important implications for such measurements.

Figure 1

Schematic of the target setup with the laser drive coming from the top of the image incident onto a plastic ablator inside a $25\text{−}\mu \mathrm{m}$-thick gold cone. The target consists of a $25\text{−}\mu \mathrm{m}$-thick CH ablator, $125\text{−}\mu \mathrm{m}$-thick $\alpha$-quartz, and ${\mathrm{SiO}}_2$ aerogel foam with a $40\mu \mathrm{m}$ step on the back side (total foam thickness was 100–140 $\mu \mathrm{m}$).

Figure 2

Comparison of shock and particle velocity measurements by Knudson et al. [5, 6], Miller et al. [39, 40], and Vildanov [41], with SESAME table 7387 [42]. Our measurements (NIF-5) of the shock velocity and calculated values of particle velocities are also included in the plot.

Figure 3

Quartz reflectivity measurement as a function of shock velocity from Ref. [21].

Figure 4

Measured temperature of shocked $\alpha$-quartz as a function of shock velocity from Ref. [21].

Figure 5

Reflectivity of silica aerogel from experiment, determined from VISAR and SOP data.

Figure 6

Comparison of measured temperatures as a function of shock velocity in silica aerogel with Saha calculations [53, 54], our QMD simulations, and SESAME equation of state 7387 [42].

Figure 7

${\mathrm{Log}}_{10}$ temperature (solid line) and ${\log }_{10}$ mean free path (${\lambda }_{\mathrm{mfp}}$) as a function of position for the shock 1.1 ns after the start of the laser drive. The transition region is quite narrow, but it persists even when our simulation resolution drops below 0.15 $\mu \mathrm{m}$. The fact that this is only a few zones thick implies that numerical diffusion (despite the steepener in the rage code) could be the dominant effect. However, we do expect some radiative preheat, and a portion of this preheat is likely due to this radiative heating. The position of the photosphere is marked by a vertical dot-dashed line.

Figure 8

Reflectivity of silica aerogel for various conditions along the principal Hugoniot calculated using QMD simulations [24]. The optical pyrometry measurement is carried out at a narrow wavelength band around ${\lambda }_0$ = 684 nm.