Session J6: Focus Session: Realizing the Promise of ICE II

Chair: Jon Boettger, Los Alamos National Laboratory
Room: Hyatt Regency Chesapeake A/B

Tuesday, August 2, 2005
11:00AM - 11:30AM

J6.00001: Quantitative Equation-of-State Results from Isentropic Compression Experiments to Multimegabar Pressures
Invited Speaker: Jean-Paul Davis

Isentropic ramp-wave loading of condensed matter has long been hailed as a possible experimental technique to obtain accurate equation-of-state (EOS) data in the solid phase at relatively low temperatures and multimegabar pressures. In this range of pressure, isothermal diamond-anvil techniques have limited accuracy due to reliance on theoretical EOS of calibration standards, thus accurate isentropic compression data would help immensely in constraining EOS models. An isentropic compression technique developed using the Z Machine at Sandia as a magnetic drive has been extended to the multimegabar regime by recent advances in current-pulse shaping. Diagnostics typically consist of time-resolved velocity interferometry to monitor the back surfaces of samples having different thickness but subjected to the same magnetic loading. Extraction of a stress-density curve from such data requires that the experiment has been designed to avoid coupling (during the time of interest) between the back surface and the joule-heated region where the stress wave is generated. Uncertainty in the result at multimegabar pressure is dominated by uncertainty in the transit time, or difference in arrival times for a particular velocity, between samples of different thickness. After a brief discussion of experiment design issues, a detailed analysis of data on aluminum to 240 GPa will be presented, followed by some results from recent experiments on other materials. \break * Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000    [Preview Abstract]

Tuesday, August 2, 2005
11:30AM - 11:45AM

J6.00002: Isotherms Reduced from Isentropes and Hugoniots up to several 100 GPa
Akobuije D. Chijioke , W.J. Nellis , Isaac F. Silvera

To obtain pressure standards for use in diamond anvil cells at static pressures as high as 300 GPa, we have reduced Hugoniot curves of Al, Cu, Ta and W to 300-K isotherms. These shock-wave reduced isotherms (SWRIs) are used to obtain pressures of metal markers whose densities are determined by x-ray diffraction, which in turn can be used to calibrate the shift with pressure of the ruby fluorescence line. Hugoniot data were reduced to isotherms using calculated thermal pressures and measurements of strengths along these Hugoniots. At still higher shock pressures, calculated thermal pressures are sufficiently large to introduce significant systematic uncertainties. For this reason, quasi-isentropes measured under dynamic compression are needed as reference curves for calibration of static pressures well above 300 GPa. Thus, measured quasi-isentropes, material strengths, and some Hugoniot data are needed for metals, including Al, Cu, Mo, Ta, W, Pt, and Au, up to dynamic pressures significantly higher than 300 GPa.    [Preview Abstract]

Tuesday, August 2, 2005
11:45AM - 12:00PM

J6.00003: Off-Hugoniot Compression of Tantalum to Megabar Pressures
Jeffrey Nguyen , Daniel Orlikowski , Frederick Streitz , Roger Minich , Neil Holmes

We recently carried out off-Hugoniot experiments on tantalum at the LLNL two-stage light gas gun. In these experiments, tantalum samples are subjected to a combination of shock, release and quasi-isentropic compression. High density and a lack of phase transitions at low pressures make tantalum an interesting material for this kind of experiments. In an approach similar other quasi-isentropic compression experiments, we used a VISAR to record the particle velocities at two different sample thicknesses in each of these experiments. These data and any associated analysis will be presented. This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under contract W-7405-Eng-48.    [Preview Abstract]

Tuesday, August 2, 2005
12:00PM - 12:15PM

J6.00004: \textit{Ab initio} calculations of principal and release isentropes for aluminum
Michael P. Desjarlais , Marcus D. Knudson

We present a direct first-principles approach to calculating isentropes using quantum molecular dynamics (QMD) simulations with density functional theory. The results are compared to several popular EOS models for aluminum as well as principal isentrope and release data from several sources. The agreement with data for aluminum is very good, as is agreement with aluminum data on the principal Hugoniot up to 1200 GPa. We find that our QMD isentropes and Hugoniot for aluminum represent a better overall match to available data than any of the existing aluminum equations of state. We have employed these methods to perform \textit{ab initio} impedance matching calculations for aluminum flyer plates impacting a deuterium sample.    [Preview Abstract]

Tuesday, August 2, 2005
12:15PM - 12:30PM

J6.00005: Results from Isentropic Compression Experiments (ICE)
D.G. Tasker , J.H. Goforth , H. Oona , P.A. Rigg , D. Dennis-Koller , J. King , D. Torres , D. Herrera , F. Sena , F. Abeyta , L. Tabaka

We have developed high explosive pulsed power (HEPP) methods to obtain accurate isentropic EOS data with the ICE technique.\footnote{D.G. Tasker, et al., in Proc. APS Shock Compression of Condensed Matter, 2003, p.1239.} In our ICE experiment, fast rising current pulses (with risetimes from 400 to 600~ns) at current densities exceeding many~MA/cm, create continuous magnetic compression of materials to Mbar pressures. The response of materials to this isentropic loading provides the required isentropic EOS. The LANL ICE system comprises a flat-plate explosively-driven magnetic flux compression generator (FCG), a small explosively formed fuse (EFF) opening switch, and explosively-driven closing switches. By precise timing of the various components in this system we are able to optimize the current profile for various applications. The prototype system produces isentropic compression profiles in the vicinity of 0 to 3 Mbar but our larger systems may achieve 20 Mbar or more. We will discuss the factors affecting the accuracy of the results. To demonstrate the viability and accuracy of the technique we will present recent EOS data from experiments at pressures of the order of 1 Mbar.    [Preview Abstract]