Hugoniot Data for Helium in the Ionization Regime

Hugoniot data were obtained for fluid He in the 100 GPa pressure range by shock compression of samples statically precompressed in diamond-anvil cells. The initial (precompressed) He density (${\rho }_1$) for each experiment was tuned to a value between ${\rho }_{0L}<{\rho }_1<3.3{\rho }_{0L}$, where ${\rho }_{0L}$ is the zero-pressure density of the cryogenic liquid (${\rho }_{0L}=0.123\mathrm{g}/{\mathrm{cm}}^3$). The maximum observed shock-compression ratios range from $\rho /{\rho }_1=6$ for ${\rho }_1={\rho }_{0L}$ to $\rho /{\rho }_1=4$ for ${\rho }_1\ge 3{\rho }_{0L}$ (i.e., $\rho /{\rho }_{0L}\ge 12$). Data show an increase in compressibility at the onset of ionization, similar to theoretical predictions.

Figure 1

(a) Sketch of a precompressed target. (b) Example VISAR record obtained from precompressed He (${\rho }_1=0.122\mathrm{g}/\mathrm{cc}$, shocked to 117 GPa). The shock front in both quartz and in He are reflecting. Before 5.9 ns, fringes are produced by reflection of the probe beam from the stationary Al coating between the thin diamond and quartz plate. At $\sim 5.9\mathrm{ns}$, the shock enters the quartz and the shock front in the quartz is reflecting. At 6.9 ns, the shock front enters the He and again the shock is so strong, He is reflecting. The shock then passes through the He and enters the back anvil at 10.6 ns. (c) Shock velocity (left scale) and shock trajectory (right scale) in both the quartz and He. The dotted lines represent the quartz and He thicknesses (right scale) measured before the shock experiment. The calculated thickness obtained from shock trajectory agrees to within 2% with the quartz and He thicknesses determined before the shot.

Figure 2

He shock-wave velocity $\mathrm{U}_s{}^{\mathrm{He}}$ vs particle velocity $\mathrm{U}_p{}^{\mathrm{He}}$ determined from the experimentally measured values of $\mathrm{U}_s{}^{\mathrm{He}}$ and $\mathrm{U}_s{}^Q$ (inset). Open and closed symbols indicate whether the shocked state is reflecting or opaque, respectively, with the color showing the initial density, ${\rho }_1$, relative to the liquid density, ${\rho }_{0L}$. Previous gas-gun data [7] (${\rho }_1={\rho }_{0L}=0.1233\mathrm{g}/\mathrm{cc}$) are plotted as triangles, and the dashed curve shows the results of ACTEX calculations [3].

Figure 3

(a) Hugoniot data for an initial density ${\rho }_1=0.123\mathrm{g}/\mathrm{cc}$, i.e., of cryogenic He and of He precompressed at 0.12 GPa at room temperature. The symbols are the same as in Fig. 2. Also shown are the gas-gun data from Nellis et al. [7] with ${\rho }_1=0.123\mathrm{g}/\mathrm{cc}$ (▴) and model calculations from SCVH [8] (solid line), ACTEX [3] (dotted line), and DFT/PIMC [2] (short dashed line). To compare these new data to the expected Hugoniot without electronic excitations, the lower pressure gas-gun data are extrapolated assuming a linear ${\mathrm{U}}_s$ vs ${\mathrm{U}}_p$ (long dashed line). Inset shows the estimation of the 1-sigma error bars from a Monte Carlo calculation for the highest pressure datum.

Figure 4

He Hugoniots for different ${\rho }_1$ from ${\rho }_1={\rho }_{0L}$ to ${\rho }_1=3.3{\rho }_{0L}$. ○ are the present data. The full line is the SCVH model [8] and the dashed line the ab initio calculations [2]. The color and symbol notation for the data is the same as in Fig. 2.