Publications

A principal goal of the shock physics program at Sandia is to establish a capability to make accurate equation of state (EOS) measurements on the Z pulsed radiation source. The Z accelerator is a source of intense x-ray radiation, which can be used to drive ablative shocks for EOS studies. With this source, ablative multi shocks can be produced to study materials over the range of interest to both weapons and ICF physics programs. In developing the capability to diagnose these types of studies on Z, techniques commonly used in conventional impact generated experimental were implemented. The primary diagnostic presently being used for this work is velocity interferometry, VISAR, which not only provides Hugoniot particle velocity measurements, but also measurements of non-shock EOS measurements, such as isentropic compression. In addition to VISAR capability, methods for measuring shock velocity have also been developed for shock studies on Z. When used in conjunction with the Rankine- Hugoniot jump conditions, material response at high temperatures and pressures can be inferred. Radiation in the Z accelerator is produced when approximately 18 MA are passed through a cylindrical wire array typically 20 to 50 mm in diameter and 10 to 20 mm in height. 200-300 wires with initial diameters on the order of 8 to 20 micron form, upon application of the current, a plasma shell, which is magnetically imploded until it collapses and stagnates on axis, forming a dense plasma emitter in the shape of a column, referred to as a" z pinch". The initial wire array and subsequent plasma pinch are confined within a metallic can, referred to as a primary hohlraum, which serves as both a current return path and a reflective surface to contain the radiation. Attached to openings in the primary hohlraum wall are smaller tubes referred to as secondaries. Multiple secondaries can be fielded on most experiments, which are the typical location for mounting EOS samples. In this configuration, the secondary S1 contains two separate VISAR probes for making velocity measurements at different material thicknesses. By correlating the resulting velocity profiles in time, a measurement of shock velocity can be determined. In addition, the velocity profiles provide the Hugoniot particle velocity after the records were impedance-matched. Secondaries S2 and S3 provide measurements of shock velocity using laser light reflected from steps. As the shock arrives at each of these surfaces, the surface reflectivity significantly decreases, which causes a sharp drop in return light. The shock velocity can be inferred from shock arrival at different steps The z-pinch technique is particularly useful for producing high amplitude shock waves for EOS applications. An alternative approach for using Z is to produce shockless loading directly with the magnetic pressure in the accelerator.

In this paper, we describe a new technique for using a pulsed power source (Z pinch) to produce planar shock waves for high-pressure equation of state (EOS) studies. Initial EOS experiments conducted with techniques indicate that these sources are effective for shock wave studies in samples with diameters of a few millimeters and thicknesses of a fraction of one millimeter, and thus provide the possibility for achieving accuracy in shock and particle velocity measurements of a few percent. We have used the Z pinch source to produce the first in-situ time-resolve particle velocity profiles obtained with pulsed radiation sources in the Mbar regime. Particle velocity profiles obtained with a VISAR interferometer are compared with I-D numerical simulations performed with a radiation-hydrodynamics code, ALEGRA. Good agreement with experimental results was achieved in the simulations and suggests that Z pinch source should be a valuable tool for high-pressure EOS studies in thermodynamic regimes important to hypervelocity impact.

The recent development of Z pinch drivers for producing intense radiation envkomn~ enables study of physical and mechanical properties of condensed materials in regimes previously inaccessible in the Mm-am-y. With Z pinch radiation sources, it is possible fo subject mm-sized sampies to pianar compressions of a fe w Mbar. Tie-resolved velocity interferometry was used to perform the first shock loading and unloading profiles in Al and Be for ablatively driven shock$s to 3 Mbar and the first iseritropic loading of iron specimens to 300 War. A principai goai of our shock physics program is to establish a capability to make accurats eqwion of state measurements on the Z pulsed radiation source. The Z accelerator is a source of intense radntion, which can be used to drive ablative shocks for E(X$ studies. With this source, ablative muki-Mbar shocks can be produced to study materials over the range of interest to both weapons and ICF physics programs. In developing the capability to diagnose these types of studies on Z, techniques commonly used in conventional impact generated experiments were implemented. The primary diagnostic presently being used for this work is ve"!ocity interferoinetry, VL%4R, [2] which not only provides Hugoniot particle velocity measurements, but also measurements of non-shock EOS measummenu,, such as isentropic compression. In addition to VKSAR capability, methods for measuring shock velocity have also been developed for shock studies on Z. When used in conjunction with the Rankine-Hugoniot jump conditions, material response at high temperatures and pressures can be inferred. The next section discusses the basic approach for conducting EOS experiments on Z for both shock loading and istmtropic compression on the Z accelerator.

The Sandia Z machine is a source of intense radiation which can be used to drive ablative shocks for equation of state studies. In developing the capability to diagnose these types of studies on Z, techniques commonly used in conventional impact generated experiments were leveraged. The primary diagnostic transferred was velocity interferome~, VLSAR, [1] which not only provides Hugoniot particle velocity measurements, but also indications of shock stability and wave attenuation. In addition to a VISAR capability on the Z machine, methods for measuring shock velocity have been developed. When these measured parameters are used in conjunction with the Rankine-Hugoniot jump conditions, [2] material response at high temperatures and pressures can be inferred. With sample sizes used on Z being much smaller than those fielded in typical impact experiments, temporal resolution and methods of interfacing the diagnostics with the targets had to be improved. In this paper, a "standard" equation of state experiment, associated diagnostics, and some recent results in aluminum and beryllium will be discussed.

Recent developments in pulsed power technology demonstrate use of intense radiation sources (Z pinches) for driving planar shock waves in samples with spatial dimensions larger than possible with other radiation sources. Initial indications are that the use of Z pinch sources can be used to produce planar shock waves in samples with diameters of a few millimeters and thicknesses approaching one half millimeter. These dimensions allow increased accuracy of both shock velocity and particle velocity measurements. The Z pinch radiation source uses imploding metal plasma induced by self-magnetic fields applied to wire arrays to produce high temperature x-ray environments in vacuum hohlraum enclosures. Previous experiments have demonstrated that planar shock waves can be produced with this approach. A photograph of a wire array located inside the vacuum hohlraum is shown here. Typically, a few hundred individual wires are used to produce the Z pinch source. For the shock wave experiments being designed, arrays of 120 to 240 tungsten wires with a diameter of 40 mm and with individual diameters of about 10 {micro}m are used. Preliminary experiments have been performed on the Z pulsed radiation source to demonstrate the ability to obtain VISAR measurements in the Z accelerator environment. Analysis of these results indicate that another effect, not initially anticipated, is an apparent change in refractive index that occurs in the various optical components used in the system. This effect results in an apparent shift in the frequency of reflected laser light, and causes an error in the measured particle velocity. Experiments are in progress to understand and minimize this effect.

Shockwave techniques have been used for decades to study the dynamic states of matter in temperature and pressure regimes inaccessible by other methods. These techniques have been employed in a wide variety of scientific, military, and commercial applications. A principal scientific objective has been to determine high-pressure equations of state (EOS) to ultra-high pressures; pressures of tens of Mbar have been reported for several materials. Most recently, these methods have been used for studies of thermophysical properties under shock compression, including phase transition kinetics, and mechanical properties, such as the high-pressure yield strength. In this paper, a brief discussion of recent developments in high velocity launchers is given. Advances in techniques for subjecting materials to a wide range of loading conditions is presented, including selected illustrations of shockwave measurements to Mbar pressures. 54 refs.

Hypervelocity launchers are used to study the high-pressure equation of state of materials in regimes inaccessible by other methods. Two-stage light gas guns have been extremely useful for these applications, but have a practical velocity limit of about 8--9 km/s for impact studies. In this paper, we describe a three-stage launcher consisting of a two-stage light gas gun combined with a third-stage railgun, which overcomes previous velocity limitations pertaining to two-stage guns. This launcher is being developed for operation to 15 km/s and has achieved projectile velocities of 7.2 km/s to date. 10 refs., 9 figs., 1 tab.