Compression of gaseous hydrogen into warm dense states up to 95 GPa using multishock compression technique

The thermodynamic properties of warm dense hydrogen, such as the equation of state (EOS) and sound velocity, affect our understanding of the evolution and interior structures of gas giant planets. However, for this low-$Z$ gas, obtaining the EOS and sound velocity experimentally under relevant planetary conditions is challenging because the extremely warm states are difficult to reach, characterize, and interrogate. Here, the multishock compression technique is used to generate the thermodynamic states of warm dense hydrogen. This provides a dynamic loading from shock adiabatic to quasi-isentropic compressions, covering wide pressure and density ranges of 0.02–95 GPa and $0.01–0.64 \mathrm{g}/{\mathrm{cm}}^3$, which enables us to determine the EOS and infer the high-pressure sound velocities of warm dense hydrogen relevant to planetary interiors. The data obtained in this way are comprehensively compared with several important theoretical models for astrophysics applications. The present experiments provide desirable principal- and off-Hugoniot states for revisiting the thermodynamic space of existing experimental data. These observations and thermodynamic states may be important in guiding future theoretical developments and constructing interior structure models for gas giant planets.

Figure 1

Representative signals from the MCOP and DLHV instruments for shot no. H18704. The light radiation histories of ${\mathrm{H}}_2$ recorded by the 504-nm-wavelength channel of MCOP-I and 672-nm-wavelength channel of MCOP-II are represented as green and blue lines, respectively. The intensity is indicated by the voltage signal. For clarity, the MCOP-I signal amplitude has been shifted downward by 0.55 V. The red line is the velocity history at the ${\mathrm{H}}_2$-window interface recorded by DLHV-II. Note that the values directly observed by DLHV-II are apparent velocities and have been corrected using the refractive index of lithium fluoride (LiF) [51, 52, 53]. $t_0, t_1, t_2, \cdots$ are the shock arrival times at the baseplate-${\mathrm{H}}_2$ and ${\mathrm{H}}_2$-window interfaces. Inset: Schematic diagram of a target designed for the multishock compression (dimensions are not to scale).

Figure 2

(a) Pressure as a function of the density of ${\mathrm{H}}_2$ for shot no. H18704 under multishock compressions. Theoretical multishock Hugoniots are shown for the SESAME model [26] (dash-dotted lines), the SCVH model [27] (dotted lines), the SFVT model [40] (dash-double-dotted lines), the REOS.3 model [33] (solid lines), and the CMS model [35] (dashed lines). Our experimental data are represented by half-filled diamonds. (b) Single- and double-shock temperature vs density for shot no. H18704. The lines and symbols are as in (a).

Figure 3

Comparisons of the present data with available experimental data for hydrogen in pressure vs density plane. Our data are represented by half-filled diamonds. The numbers in circles represent the $i\mathrm{th}$-shock state. Data from previous shock compression experiments on hydrogen using a gas gun (black hexagons [5] and gray downward triangles [6]), convergent explosives (gray upward triangles [55] and cyan stars [22]), and laser shocks (blue squares [17] and purple circles [19, 56]) are also shown. The theoretical principal Hugoniot curve (solid line) and isentrope (dashed line) calculated by the REOS.3 model [33] start from the initial conditions of the laser experiment [17] and Jovian condition, respectively.

Figure 4

Sound velocities of hydrogen (deuterium) vs pressure. Half-filled diamonds and circles correspond to the sound velocities of shots no. H18622 and no. H18704, respectively. The previous experimental data are from Holmes et al. (open circles) [61], Zha et al. (open triangles) [46], and Fratanduono et al. (open squares) [62]. The pure hydrogen isentropes for the Jovian conditions include the calculated result by the REOS.3 model (solid line) and extrapolated experimental results (dash-dotted line) by Duffy et al. [63]. A Jupiter adiabat (dashed line) based on the hydrogen-helium-water mixtures EOSs [64] is also shown. The temperature of all data points and curves is indicated by the color bar.