Shock Compressing Diamond to a Conducting Fluid

Laser generated shock reflectance data show that diamond undergoes a continuous transition from optically absorbing to reflecting between Hugoniot pressures $600<P_H<1000\mathrm{G}\mathrm{P}\mathrm{a}$. The data are consistent with diamond having a thermal population of carriers at $P_H\sim 600\mathrm{G}\mathrm{P}\mathrm{a}$, undergoing band overlap metallization at $P_H\sim 1000\mathrm{G}\mathrm{P}\mathrm{a}$ and melting at $800<P_H<1000\mathrm{G}\mathrm{P}\mathrm{a}$. The results agree well with an equation of state model that predicts that elemental carbon remains solid throughout the interior of Neptune.

Figure 1

Temperature versus pressure phase diagram for diamond together with the melt (yellow line), Hugoniot (red line), constant densities (black lines), and entropy (color shading) as calculated by VTR [12]. Other Hugoniot predictions include Kerley and Chhabildas (KC) [8] (green line), Molodets [13] (purple line), Fried [11] (blue line), and Sesame [17] (orange line). The pink dashed line is a calculated isentrope of Neptune [18]. The inset VISAR (velocity interferometer system for any reflector) images are from diamond shock compressed to (a) 550–600 GPa and (b) 1100 GPa and are discussed in the text. The graphite phase of carbon lies to lower temperatures and pressures than those shown in the figure.

Figure 2

Typical target and VISAR image for decaying shock used to measure shock reflectance versus shock velocity over the transition region between reflective and nonreflective states. Time zero on this image is set to the beginning of the drive pulse. (i) Before the start of laser-drive pulse, unshifted fringes are seen. (ii) After the start of drive, fringes are still unshifted as shock has not reached rear surface. (iii) The shock has broken out into diamond. Shifted fringes are reflected from the shock front in diamond. Unshifted fringes are still seen from the thick Al step. (iv) The shock has broken out from the thick Al step.

Figure 3

Measured shock reflectance versus shock velocity. The shock pressure axis is calculated from the shock velocity using the VTR EOS [12], which is limited to pressures below 2000 GPa. The solid lines are fits to the Sommerfeld model described in the text using $\gamma =4$, $\tau ={\tau }_{\min }$, and $m=m_e$. The solid lines shown use the following EOS and values for $A$: red line—VTR [12], $A=5.8\mathrm{e}\mathrm{V}$; green line—KC [8] $A=6.3\mathrm{e}\mathrm{V}$; orange line—Sesame [17], $A=6.0\mathrm{e}\mathrm{V}$. The blue dotted line is the liquid volume fraction using the VTR equation of state (EOS).