N. C. Holmes

The nature of the interior of uranus based on studies of planetary ices at high dynamic pressure.

Data from the Voyager II spacecraft showed that Uranus has a large magnetic field with geometry similar to an offset tilted dipole. To interpret the origin of the magnetic field, measurements were made of electrical conductivity and equation-of-state data of the planetary ices ammonia, methane, and synthetic Uranus at shock pressures and temperatures up to 75 gigapascals and 5000 K. These pressures and temperatures correspond to conditions at the depths at which the surface magnetic field is generated. Above 40 gigapascals the conductivities of synthetic Uranus, water, and ammonia plateau at about 20(ohm-cm)-1, providing an upper limit for the electrical conductivity used in kinematic or dynamo calculations. The nature of materials at the extreme conditions in the interior is discussed.

Raman spectrum and structure of silica aerogel

Raman spectra have been obtained from silica aerogel, a porous low‐density material having grain sizes near 70 A. The Raman spectra are qualitatively similar to those from fused silica, thus indicating that the aerogel is amorphous. However, a greatly intensified peak (relative to ordinary fused silica) occurs near 478 cm−1 and is assigned to small rings, e.g., six (3‐SiO) or eight (4‐SiO) membered. Such rings may be more important in the aerogel than in fused silica, but the observed absence of the 600 cm−1 ring peak indicates that only one ring type, e.g., eight membered, is predominant. Other marked frequency and intensity changes in the Si–O–Si bending and Si–O stretching region, compared to fused silica, were also observed.

Silica at ultrahigh temperature and expanded volume

Shock compression experiments were performed on samples of 0.13 g/cm3 density silica aerogel. The aerogel is a transparent, homogeneous, open cell, SiO2, glass structure with a pore size of 10 nm. A shock velocity of 11.4 km/s was measured at 14.6 GPa (146 kbar) and over sevenfold compression of the specimen, which is threefold expanded relative to the density of crystalline α‐quartz. A shock temperature of nearly 1 eV was measured at 6.7 GPa. These experiments access a new regime for accurate laboratory measurements on high‐temperature, expanded‐volume states of glass.

Shock wave pressure enhancement using short wavelength (0.35 μm) laser irradiation

Shock velocities in planar aluminum targets irradiated at 0.35‐ and 1.06‐μm laser wavelengths have been measured. Absorbed intensities of ∼1.0×1014 W/cm2, produced by 700‐ps full width at half‐maximum Gaussian pulses, generated shock pressures of 1.0±0.2 and 0.6±0.2 TPa, respectively, demonstrating an enhancement of shock pressure at decreased laser wavelength.

Chemiluminescence of shock-pyrolyzed benzene

Abstract C 2 and other small molecules produced during shock-induced (24–63 GPa) decomposition of liquid benzene (initially at 1 bar at 295 K) have been detected by time-resolved (50–120 ns) spectroscopy. These molecular products have been identified by characteristic electronic bands observed together with a gray-body background in visible and near-ultraviolet shock-induced chemiluminescence spectra collected during the passage of the shock front through the focus of spectrographs equipped with gated, intensified diode-array detectors. The spectroscopic techniques are described, and the dependence of the spectra on shock parameters will be discussed in terms of known decomposition mechanisms of highly excited benzene molecules.

Ultrahigh pressure laser‐driven shock wave experiments

We review recent laser‐driven shock wave experiments, with a view toward assessing the prospects of making accurate physical properties measurements at ultrahigh pressures. Recent experimental results on the scaling of shock pressure with laser intensity and wavelength are presented, and preliminary impedance matching experiments are discussed.

Impedance‐match experiments using high intensity lasers

We present the results of a series of impedance‐match experiments using copper‐aluminum targets irradiated using the Janus Laser Facility. The results are compared to extrapolations of data obtained at lower pressures using impact techniques. The sources of errors are described and evaluated. We discuss the potential of lasers for high accuracy equation‐of‐state investigations.

Fluids at high dynamic pressures and temperatures

Abstract Electrical conductivity data for shocked liquid nitrogen, Hugoniot data for liquid air, shock temperatures for liquid ammonia, and double-shock equation-of-state data for Al are discussed.