J. W. Shaner

Issues and future directions in subsecond thermophysics research

The key issues and anticipated future directions in subsecond thermophysics research are presented and discussed. The main emphasis is placed on experimental techniques for measurements of selected thermophysical properties utilizing rapid volume heating (resistive self-heating) and rapid surface heating (laser pulse-heating) methods. The time regime covered is from 1 to 10−12s. Specific research topics and key research areas are identified and discussed.

Sound speed measurements in liquid lead at high temperature and pressure

Recently sound speed measurements in high temperature liquid lead have been made over a wide density range. Such measurements may be combined with other thermophysical properties measured with the isobaric expansion experiment (IEX) to yield several additional thermodynamic quantities. Results of calculations based on the sound speed measurements are presented, and their impact on liquid metal phenomenology is discussed.

Sound speed in liquid lead at high temperatures

A dynamic technique, the isobaric expansion experiment (IEX), is used to reach high-temperature and pressure states in liquid lead. A unique technique is described for making sound-speed measurements once a final equilibrium end state is obtained. Data over an extended density range are presented. The sound speed in liquid lead over this range appears to vary linearly with density and has no dependence on temperature within our experimental precision (±7 %).

Coherent anti-stokes Raman scattering in benzene and nitromethane shock-compressed to 10 GPa

The frequency shifts of the ring-stretching mode of shock-compressed liquid benzene and the CN stretching mode of nitromethane have been measured using coherent anti-Stokes Raman scattering. Shock pressures up to 11 GPa were achieved using a two-stage light gas gun. The frequency shifted Raman signal was generated using single pulse Nd:YAG and broadband-type lasers. 16 refs., 3 figs.

Raman spectroscopies in shock-compressed materials

Spontaneous Raman spectroscopy, stimulated Raman scattering and coherent anti-Stokes Raman scattering have been used to measure temperatures and changes in molecular vibrational frequencies for detonating and shocked materials. Inverse Raman and Raman induced Kerr effect spectroscopies have been suggested as diagnostic probes for determining and phenomenology of shock-induced chemical reactions. The practicality, advantages, and disadvantages of using Raman scattering techniques as diagnostic probes of microscopic phenomenology through and immediately behind the shock front of shock-compressed molecular systems are discussed.

Detection of free surface motion using a masked aperture technique

A new noncontacting technique for measuring the arrival of a stress wave at the free surface of a very hot fluid metal has been developed. By use of an optical lever arm and a coded aperture, the new technique avoids the extreme sensitivity of optical interferometers, while allowing accurate measurement of rapid surface motions.

Acoustic Velocity Measurements on Fluid Metals from Two-Fold Compressions to Two-Fold Expansions

Fluid metals around normal density can be thought of as low temperature non-ideal plasmas. They are plasmas in that the Coulomb interactions among the constituent particles are important in determining thermodynamic and transport properties. They are low temperature in that even up to 1 eV the temperature is less than or comparable to the average interionic electrostatic energy. The resulting high Γ′s (Γ = Z2e2/ kT) cause the pair distribution function to look more like that of a normal liquid than that of an ideal gas. For the present studies Γ ~ 10-100 Z2, where Z is the effective charge of the ions. The real metallic systems have several important differences from idealizations such as the one component plasma model. For example, since the mean distance between electrons, rs, varies between 2 and 3 in atomic units with temperatures between 0.1 and 1 eV, the electron gas is degenerate, but polarizable. Therefore, unless the electron screening is well known, the effective Coulomb interaction between ions cannot be specified. Furthermore, according to the model of Ashcroft and Lekner,1 the packing fraction along the liquidus remains roughly constant at 0.46. As a result the excluded volume of the ions probably cannot be ignored anywhere over the density range from two-fold compressed to four-fold expanded. These complexities make realistic modeling of dense fluid metals very difficult in practice.