Figure 1

(a) Variation of observed temperature versus Hugoniot pressure for two Hugoniots centered at ${\rho }_{0L}=0.123\mathrm{g}/{\mathrm{cm}}^3$ and $3.3{\rho }_{0L}$, where ${\rho }_{0L}$ is the cryogenic liquid density. Also shown are the temperatures predicted by several models for the ${\rho }_{0L}$ initial density: PIMC 25, ACTEX 24, SCVH 23, and Ross and Young [24, 27]. (b) Solid diamonds show the observed reflectivity as a function of temperature and final density indicated by the color scale. Curves show the reflectivity obtained from a fit to the data using the semiconductor Drude model for three final state densities: 0.8, 1.1, and $1.4\mathrm{g}/{\mathrm{cm}}^3$. Gray triangles are calculated reflectivities by Kowalski et al. [5] near $1\mathrm{g}/{\mathrm{cm}}^3$ with a $+3\mathrm{eV}$ gap correction.Reuse & Permissions

Figure 2

(a) Energy gap as a function of density (solid curve) as determined from the best fit to the reflectivity data along with the 68% joint-parameter confidence band for the fit (dashed curves). The shaded region spans the density range of our measurements. The dotted curves show the gap function obtained when the parameter $B$ is set to $+20$ and $-20$, respectively. Solid (red) circles with error bars show the gap calculated by Kowalski et al. [5] at $T=3\mathrm{eV}$; solid (blue) triangles show the gap calculated by Stixrude and Jeanloz [7] at $T=1.7\mathrm{eV}$ (inverted) and $T=4.3\mathrm{eV}$ (upright). Annotations indicate the ionization energy of the isolated atom and the excitonic gap of solid He at 0.2 GPa [3]. (b) Solid diamond symbols show the DC conductivity inferred from the reflectivity; dashed curves show the conductivity extracted from the semiconductor Drude fit to the reflectivity data at 17, 25, 35, 45, and 60 kK. Colors indicate temperature (inset scale). Other symbols show DC conductivities as follows: solid squares calculated by Kietzmann et al. [18]; solid inverted triangles calculated by Kowalski et al. [5]; open circles show the data set collected by Ternovoi et al. [16] (temperatures not reported). The dot-dash line connects a pair of calculations at 17.4 kK by Kowalski et al. to show the density dependence.Reuse & Permissions

Figure 3

Phase diagram of He showing the data set (solid diamond symbols) obtained from the precompressed samples along with contours (dash-dot curves) of 4 levels of conductivity: 1, 2, 5, and $10\times {10}^4(\Omega -\mathrm{m}{)}^{-1}$. Colors of the symbols and dash-dot curves indicate the DC conductivity (inset scale). Ionization boundaries (black line) of two chemical models are from Foerster et al. [33] (dotted line) and Winisdoerffer and Chabrier. [34] (dashed line). The melt curve (blue) is an extrapolation of a Ketchin fit to experimental data [37] (solid line) and extrapolated to higher densities along a path similar to Ref. 33 (dotted line). The temperature-density profile of the WD star is from 2.Reuse & Permissions