D.E. Bliss

Pulsed-power-driven high energy density physics and inertial confinement fusion research

The Z accelerator [R. B. Spielman, W. A. Stygar, J. F. Seamen et al., Proceedings of the 11th International Pulsed Power Conference, Baltimore, MD, 1997, edited by G. Cooperstein and I. Vitkovitsky (IEEE, Piscataway, NJ, 1997), Vol. 1, p. 709] at Sandia National Laboratories delivers ∼20MA load currents to create high magnetic fields (>1000T) and high pressures (megabar to gigabar). In a z-pinch configuration, the magnetic pressure (the Lorentz force) supersonically implodes a plasma created from a cylindrical wire array, which at stagnation typically generates a plasma with energy densities of about 10MJ∕cm3 and temperatures >1keV at 0.1% of solid density. These plasmas produce x-ray energies approaching 2MJ at powers >200TW for inertial confinement fusion (ICF) and high energy density physics (HEDP) experiments. In an alternative configuration, the large magnetic pressure directly drives isentropic compression experiments to pressures >3Mbar and accelerates flyer plates to >30km∕s for equation of state ...

Direct observation of an abrupt insulator-to-metal transition in dense liquid deuterium

Driving liquid deuterium into metal Quick and powerful compression can force materials to change their properties dramatically. Knudson et al. compressed liquid deuterium to extreme temperatures and pressures using high-energy magnetic pulses at the Sandia Z-machine (see the Perspective by Ackland). Deuterium began to reflect like a mirror during compression, as the electrical conductivity sharply increased. The observed conditions for metallization of deuterium and hydrogen help us to build theoretical models for the universes most abundant element. This a our understanding of the internal layering of gas giant planets such as Jupiter and Saturn. Science, this issue p. 1455; see also p. 1429 Magnetic compression drives an insulator-to-metal transition in dense liquid deuterium. [Also see Perspective by Ackland] Eighty years ago, it was proposed that solid hydrogen would become metallic at sufficiently high density. Despite numerous investigations, this transition has not yet been experimentally observed. More recently, there has been much interest in the analog of this predicted metallic transition in the dense liquid, due to its relevance to planetary science. Here, we show direct observation of an abrupt insulator-to-metal transition in dense liquid deuterium. Experimental determination of the location of this transition provides a much-needed benchmark for theory and may constrain the region of hydrogen-helium immiscibility and the boundary-layer pressure in standard models of the internal structure of gas-giant planets.Eighty years ago, it was proposed that solid hydrogen would become metallic at sufficiently high density. Despite numerous investigations, this transition has not yet been experimentally observed. More recently, there has been much interest in the analog of this predicted metallic transition in the dense liquid, due to its relevance to planetary science. Here, we show direct observation of an abrupt insulator-to-metal transition in dense liquid deuterium. Experimental determination of the location of this transition provides a much-needed benchmark for theory and may constrain the region of hydrogen-helium immiscibility and the boundary-layer pressure in standard models of the internal structure of gas-giant planets.

Magnetically driven isentropic compression to multimegabar pressures using shaped current pulses on the Z accelerator

A technique has previously been developed on the Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] to generate ramped compression waves in condensed matter for equation-of-state studies [C. A. Hall, J. R. Asay, M. D. Knudson, W. A. Stygar, R. B. Spielman, T. D. Pointon, D. B. Reisman, A. Toor, and R. C. Cauble, Rev. Sci. Instrum. 72, 3587 (2001)] by using the Lorentz force to push on solid electrodes rather than to drive a Z pinch. This technique has now been extended to multimegabar pressures by shaping the current pulse on Z to significantly increase the sample thickness through which the compression wave can propagate without forming a shock. Shockless, free-surface velocity measurements from multiple sample thicknesses on a single experiment can be analyzed using a backward integration technique [D. B. Hayes, C. A. Hall, J. R. Asay, and M. D. Knudson, J. Appl. Phys. 94, 2331 (2003)] to extract an isentropic loading curve. At very high pressures, the accuracy of this method is dominat...

Magnetically accelerated, ultrahigh velocity flyer plates for shock wave experiments

The intense magnetic field produced by the 20 MA Z accelerator is used as an impulsive pressure source to accelerate metal flyer plates to high velocity for the purpose of performing plate impact, shock wave experiments. This capability has been significantly enhanced by the recently developed pulse shaping capability of Z, which enables tailoring the rise time to peak current for a specific material and drive pressure to avoid shock formation within the flyer plate during acceleration. Consequently, full advantage can be taken of the available current to achieve the maximum possible magnetic drive pressure. In this way, peak magnetic drive pressures up to 490 GPa have been produced, which shocklessly accelerated 850μm aluminum (6061-T6) flyer plates to peak velocities of 34km∕s. We discuss magnetohydrodynamic (MHD) simulations that are used to optimize the magnetic pressure for a given flyer load and to determine the shape of the current rise time that precludes shock formation within the flyer during ac...

Compact single and nested tungsten-wire-array dynamics at 14–19MA and applications to inertial confinement fusiona)

Wire-array z pinches show promise as a high-power, efficient, reproducible, and low-cost x-ray source for high-yield indirect-drive inertial confinement fusion. Recently, rapid progress has been made in our understanding of the implosion dynamics of compact (20-mm-diam), high-current (11–19MA), single and nested wire arrays. As at lower currents (1–3MA), a single wire array (and both the outer and inner array of a nested system), show a variety of effects that arise from the initially discrete nature of the wires: a long wire ablation phase for 50%-80% of the current pulse width, an axial modulation of the ablation rate prior to array motion, a larger ablation rate for larger diameter wires, trailing mass, and trailing current. Compact nested wire arrays operate in current-transfer or transparent mode because the inner wires remain discrete during the outer array implosion, even for interwire gaps in the outer and inner arrays as small as 0.21mm. These array physics insights have led to nested arrays that...

Measurements of the mass distribution and instability growth for wire-array Z-pinch implosions driven by 14–20 MA

The mass distribution and axial instability growth of wire-array Z-pinch implosions driven by 14–20 MA has been studied using high-resolution, monochromatic x-ray backlighting diagnostics. A delayed implosion is consistently observed in which persistent, dense wire cores continuously ablate plasma until they dissipate and the main implosion begins. In arrays with small interwire gaps, azimuthally correlated axial instabilities appear during the wire ablation stage and subsequently seed the early growth of magneto-Rayleigh–Taylor instabilities. The instabilities create a distributed implosion front with trailing mass that may limit the peak radiation power.

Progress in symmetric ICF capsule implosions and wire-array z-pinch source physics for double-pinch-driven hohlraums

Over the last several years, rapid progress has been made evaluating the double-z-pinch indirect-drive, inertial confinement fusion (ICF) high-yield target concept (Hammer et al 1999 Phys. Plasmas 6 2129). We have demonstrated efficient coupling of radiation from two wire-array-driven primary hohlraums to a secondary hohlraum that is large enough to drive a high yield ICF capsule. The secondary hohlraum is irradiated from two sides by z-pinches to produce low odd-mode radiation asymmetry. This double-pinch source is driven from a single electrical power feed (Cuneo et al 2002 Phys. Rev. Lett. 88 215004) on the 20 MA Z accelerator. The double z-pinch has imploded ICF capsules with even-mode radiation symmetry of 3.1 ± 1.4% and to high capsule radial convergence ratios of 14–21 (Bennett et al 2002 Phys. Rev. Lett. 89 245002; Bennett et al 2003 Phys. Plasmas 10 3717; Vesey et al 2003 Phys. Plasmas 10 1854). Advances in wire-array physics at 20 MA are improving our understanding of z-pinch power scaling with increasing drive current. Techniques for shaping the z-pinch radiation pulse necessary for low adiabat capsule compression have also been demonstrated.

Multi-dimensional high energy density physics modeling and simulation of wire array Z-pinch physics

The two- and three-dimensional (2D and 3D) versions of ALEGRA-HEDP [A. C. Robinson and C. J. Garasi, “Three-dimensional Z-pinch wire array modeling,” Computer Physics Communications, submitted] have been utilized to simulate discrete wire effects including precursor formation in 2D (r-θ plane) and nonuniform axial ablation (3D). Comparisons made between 2D and 3D simulations indicate that 2D simulations overestimate the mass ablation rate by a factor of 10–100 with respect to the 3D case, causing pre-mature motion of the array with respect to experimental data. Additionally, the 2D case advects a factor of 5 more current to axis than the 3D case. The integrity of the simulations is assessed by comparing the results to laser imaging of wire ablation and array trajectory information inferred from visible and x-ray imaging. Comparisons to previously proposed ablation models are also presented. These simulations represent the first high-fidelity three-dimensional calculations of wire-array pinch geometries.

The ZR refurbishment project

ZR is a refurbished (R) version of Z aiming to improve its overall performance, reliability, precision, pulse shape tailoring and reproducibility. Z, the largest pulsed power machine at Sandia, began in December 1985 as the Particle Beam Fusion Accelerator II (PBFA II). PBFAII was modified in 1996 to a z-pinch driver by incorporating a high-current (20-MA, 2.5-MV) configuration in the inner ∼ 4.5 meter section. Following its remarkable success as z-pinch driver, PBFA II was renamed Z in 1997. Currently Z fires 170 to 180 shots a year with a peak load current of the order of 18–20 MA. The maximum z-pinch output achieved to date is 1.6-MJ, 170-TW radiated energy and power from a single 4-cm diameter, 2-cm tall array, and 215 eV temperature from a dynamic hohlraum. ZR in turn will, operating in double shift, enable 400 shots per year, deliver a peak current of 26 MA into a standard 4cm × 2cm Z-pinch load, and should provide a total radiated x-ray energy and power of 3 MJ and 350 TW, respectively, achieve a maximum hohlraum temperature of 260 eV, and include a pulse-shaping flexibility extending from 100ns to 300ns for equation of state and isentropic compression studies. To achieve this performance ZR will incorporate substantial modifications and upgrades to Marx generator, intermediate store capacitors, gas and water switches, water transmission lines and the laser triggering system. Test beds are already in place, and the new pulsed power components are undergoing extensive evaluation. The Z refurbishment (ZR) will be operational by 2006 and will cost approximately

Progress in z-pinch driven dynamic-hohlraums for high-temperature radiation-flow and ICF experiments at Sandia National Laboratories

60M.