A. A. Pyalling

Pressure-produced ionization of nonideal plasma in a megabar range of dynamic pressures

The low-frequency electrical conductivity of strongly nonideal hydrogen, helium, and xenon plasmas was measured in the megabar range of pressures. The plasmas in question were generated by the method of multiple shock compression in planar and cylindrical geometries, whereby it was possible to reduce effects of irreversible heating and to implement a quasi-isentropic regime. As a result, plasma states at pressures in the megabar range were realized, where the electron concentration could be as high as ne≈2×023 cm−3, which may correspond to either a degenerate or a Boltzmann plasma characterized by a strong Coulomb ΓD=1–10) and a strong interatomic Γa=rana1/3∼1) interaction. A sharp increase (by three to five orders of magnitude) in the electrical conductivity of a strongly nonideal plasma due to pressure-produced ionization was recorded, and theoretical models were invoked to describe this increase. Experimental data available in this region and theoretical models proposed by various authors are analyzed. The possibility of a first-order “phase transition” in a strongly nonideal plasma is indicated.

THERMODYNAMIC PROPERTIES AND ELECTRICAL CONDUCTIVITY OF HYDROGEN UNDER MULTIPLE SHOCK COMPRESSION TO 150 GPA

Abstract The results of experiments on simultaneous registration of optical emission intensity and electrical resistivity of hydrogen layer at a multiple shock compression to pressure 106, 123 and 150 GPa are presented. The experimentally determined thermodynamic parameters of hydrogen at the first steps of compression are consistent with results of a semi-empirical equation of state for molecular hydrogen. Hydrogen electrical conductivity was traced from 0.1 to 1000 1 /( Ω cm ) under various regimes of compression and subsequent expansion.

Electrical conductivity of nonideal hydrogen plasma at megabar dynamic pressures

The electrical conductivity of a nonideal hydrogen plasma is measured under shock-wave compression to pressures ∼1.5 Mbar. It is found that the conductivity increases sharply (by five orders of magnitude) at a density ρ∼0.3−0.4 g/cm3, reaching close to liquid-metal values ∼103 S/cm. The data obtained can be described by a nonideal-plasma model taking into account the increase in the number of conduction electrons as a result of “ionization by pressure.”

Experimental determination of the conditions for the transition of Jupiter’s atmosphere to the conducting state

The intensity of optical radiation and resistance of a hydrogen-helium layer with He mass fraction Y=mHe/(mHe+mH)≅0.24, which corresponds to the composition of the outer layers of Jupiter’s atmosphere [2], were simultaneously measured under multiple shock compression up to 164 GPa in plane geometry. The initial pressure and temperature of the mixture were equal to 8 MPa and 77.4 K, respectively, and the velocity of steel strikers was equal to 6.2 km/s. These conditions allowed the generation of the final compressed curve close to the adiabatic states of Jupiter’s atmosphere according to the models proposed in [2, 3]. The conditions for the appearance of the conducting phase in the compression process and the achieved level of electrical conductivity were determined. The experimental data were compared with the one-dimensional fluid-dynamic simulation of the compression process using the equation of state for the mixture in a model similar to the one proposed in [3, 8]. The experimental data were also compared with the behavior of pure components having the same initial density as in the mixture and compressed to the same final pressure.

Near-Critical-Point Thermodynamics from Shock Experiments with Porous Ni Samples

Results of experiments on the expansion of shock-compressed nickel samples into helium are presented. An isentrope with an initial pressure of 170 GPa was studied. The radiance temperature of the nickel sample and the velocity of the shock wave, generated in helium, were measured by a fast multichannel optical pyrometer; other parameters, such as the particle velocity, the pressure on the He-Ni interface, and the temperature of He were calculated using He Hugoniot (chemical plasma model). To increase the shock entropy up to a near-critical value and to intensify the process of heat-mass transfer, porous samples were used. Final states with pressures below 1.5 GPa, determined by the initial He pressure, were generated. The isobaric overheat of nickel by hot shocked helium provided an information about the nickel liquid spinodal. The change in slope of an isentrope in the pressure-particle velocity plane allowed an estimate of the point of its entrance in the two-phase region. Estimates of the critical temperature and pressure were made from peculiarities of P-T path, using various models of the nickel liquid spinodal to represent experimental data.

Investigation of tin thermodynamics in near critical point region

Near critical point states of tin were generated by the expansion of shocked metal into helium to pressures of several kbars and less. Thermodynamic states on release isentropes originated from shock pressures of 137, 180, and 220 GPa were investigated. Gas dynamic parameters and temperature have been measured by the use of the method of optical pyrometry. The heating of tin on the boundary with shocked gas was described due to the difference between tin and gas temperatures. The problem of the interface heat transfer was solved to obtain the temperature of expanded tin. The point of intersection of experimental and theoretical curves for maximum overheat temperatures was used to estimate parameters of the critical point, which occurred at Pcrit=0.25 GPa, Tcrit=7850 K (dT=300K, dP=0.02 GPa). The result is in a good agreement with available evaluations of the critical point. Experimental data obtained have been used for construction of multi-phase wide-range equation of state for tin.

Time-resolved optical spectroscopy of lead at near critical-point states

Experiments were performed to investigate the behavior of lead near its critical point. Emission spectra of shocked lead samples during unloading into helium at different initial pressures were measured by an optical multichannel analyzer (OMA), as well as by a fast optical pyrometer. To describe the obtained experimental data, a model of a thin mixture layer was suggested, in which helium emits due to the presence of external electrons from lead.

Temperature measurements and hydrogen transformation under dynamic compression up to 150 GPA.

Lithium fluoride single crystal window was used for optical light emission registration during quasi-isentropic compression of hydrogen to the pressures 100-150 GPa. Initially gaseous hydrogen samples at 78 K temperature and different pressures in the range 3-30 MPa were investigated. Recorded brightness temperature profiles in near infrared range of wavelength were analyzed to evaluate optical and transport properties of the investigated hydrogen sample and window. Two EOS models of hydrogen, with and without metallic region were used for 1-D simulation of its properties under dynamic compression and estimation of hydrogen temperature within compressed layer. The obtained data demonstrate abrupt change of final temperatures after heating higher then 3500K.

CONDUCTIVITY OF MULTIPLE SHOCK COMPRESSED HYDROGEN ALONG 135 AND 180 GPA ISOBARS

The results of temperature and conductivity measurements of hydrogen, multiple shock compressed to the pressures 135 and 180 GPa are presented. Explosively driven steel plate with velocity up to 8 km/s was used for shock wave generation. Hydrogen with various initial pressures and temperatures was multiple shock compressed between steel bottom and sapphire window. Brightness temperature of hydrogen was measured by fast optical pyrometer. Electrical resistance of shocked hydrogen was measured simultaneously with optical pyrometer records. The conductivity of hydrogen decreased from 424 1/Om/cm at 2700 K down to 20 1/Om/cm at 6000 K along 135 GPa isobar. The conductivity of hydrogen decreased from 800 1/Om/cm at 5000 K down to 100 1/Om/cm at 6700 K along 180 GPa isobar. Experimental results are compared with various theoretical predictions.

INVESTIGATION OF NEAR CRITICAL POINT STATES OF LITHIUM, SODIUM AND ALUMINIUM BY PULSE HEATING DURING LAUNCHING

The results of experimental investigation of near—critical point states of liquid‐vapour phase transition of of lithium, sodium and aluminium are presented. The metal foil samples were launched by explosively driven steel plate in Helium atmosphere; Li and Na—by direct impact and Al—by impact through the layer of helium. The heating of the Li and Na foils were performed by heat exchange with shocked He layer from the free side of sample; Al—by heating by multiple‐shocked He from the back side of the foil. The temperature of sample surface was measured by fast multi‐channel optical pyrometer. For Li and Na experiments the pressure was obtained from measured shock velocity in helium using base length technique; the 1‐D simulation of the process of launching was performed to obtain pressure for Al experiments. The obtained experimental information allowed to evaluate liquid spinodal line, and the position of critical points on pressure—temperature plane for investigated metals.