David Franklin Wenger

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 ...

Development and characterization of a Z-pinch-driven hohlraum high-yield inertial confinement fusion target concept

Initial experiments to study the Z-pinch-driven hohlraum high-yield inertial confinement fusion (ICF) concept of Hammer, Tabak, and Porter [Hammer et al., Phys. Plasmas 6, 2129 (1999)] are described. The relationship between measured pinch power, hohlraum temperature, and secondary hohlraum coupling (“hohlraum energetics”) is well understood from zero-dimensional semianalytic, and two-dimensional view factor and radiation magnetohydrodynamics models. These experiments have shown the highest x-ray powers coupled to any Z-pinch-driven secondary hohlraum (26±5 TW), indicating the concept could scale to fusion yields of >200 MJ. A novel, single-sided power feed, double-pinch driven secondary that meets the pinch simultaneity requirements for polar radiation symmetry has also been developed. This source will permit investigation of the pinch power balance and hohlraum geometry requirements for ICF relevant secondary radiation symmetry, leading to a capsule implosion capability on the Z accelerator [Spielman et...

Monochromatic x-ray imaging experiments on the Sandia National Laboratories Z facility (invited)

The Z facility is a 20 MA, 100 ns rise time, pulsed power driver for z-pinch plasma radiation sources. The Z facility can make >200 TW, 1–2 MJ, near-blackbody radiation sources through the compression of cylindrical wire arrays. These sources are being used as drivers to study inertial-confinement fusion capsule implosions, complex radiation–hydrodynamic jet experiments, and wire-array z-pinch physics tests. To backlight plasmas in this environment we have built diagnostics based on spherically bent crystals that provide high spatial resolution (9–10 μm), a narrow spectral bandpass (<0.5 eV), and a large field of view (4 mm×20 mm). These diagnostics use the 2 TW, multi-kJ Z-Beamlet laser to produce x-ray emission sources at 1.865 or 6.151 keV for backlighting.

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.

Evaluation of bent-crystal x-ray backlighting and microscopy techniques for the Sandia Z machine

X-ray backlighting and microscopy systems for the 1-10-keV range based on spherically or toroidally bent crystals are discussed. These systems are ideal for use on the Sandia Z machine, a megajoule-class x-ray facility. Near-normal-incidence crystal microscopy systems have been shown to be more efficient than pinhole cameras with the same spatial resolution and magnification [Appl. Opt. 37, 1784 (1998)]. We show that high-resolution (< or = 10 microm) x-ray backlighting systems using bent crystals can be more efficient than analogous point-projection imaging systems. Examples of bent-crystal-backlighting results that demonstrate 10-microm resolution over a 20-mm field of view are presented.

Monochromatic x-ray backlighting of wire-array z-pinch plasmas using spherically bent quartz crystals

X-ray backlighting systems are being developed to diagnose z-pinch, inertial confinement fusion capsule, and complex hydrodynamics experiments on the 20 MA Sandia Z machine. The x-ray backlighter source is a laser-produced plasma created using the Z-Beamlet laser, a 2 TW, 2 kJ Nd:glass laser recently constructed at Sandia. As an alternative to point-projection radiography, we are investigating a different geometry [S. A. Pikuz et al., Rev. Sci. Instrum. 68, 740 (1997)] that uses spherically bent crystal mirrors to simultaneously obtain high spatial resolution and a narrow spectral bandwidth. Backlighting systems using the Si Heα line (1.865 keV) and the Mn Heα line (6.15 keV) are discussed. These systems are capable of spatial resolutions in the 5–10 μm range, a field of view as large as 5 mm by 20 mm, and a spectral bandwidth comparable to the width of the emission line used for backlighting.

Measurements and simulations of the ablation stage of wire arrays with different initial wire sizes

Comparisons of 20mm diameter, 300-wire tungsten arrays with different initial wire sizes were made on the 20MA Sandia Z facility. Radiographic measurements of each wire array, taken at the same point in the current during the wire ablation stage, show systematic differences. A detailed comparison of the radiography and self-emission data with simulations and analytic models suggests that a variation in the mass ablation rate with wire size may be responsible.

Symmetric inertial confinement fusion capsule implosions in a high-yield-scale double-Z-pinch-driven hohlraum on Z

Detailed radiation-hydrodynamics calculations indicate that the dual-63-MA Z-pinch high-yield (HY) 220-eV inertial confinement fusion concept [Phys. Plasmas 6, 2129 (1999)] may release 400 MJ of fusion yield, if pulse shaping, capsule preheat, and x-radiation drive uniformity can be acceptably controlled. Radiation symmetry is under detailed investigation in an advanced, 70-eV HY-scale scoping hohlraum [Phys. Rev. Lett. 88, 215004 (2002)] driven by the single 20-MA power feed of Sandia National Laboratories’ Z accelerator. The time-averaged polar radiation asymmetry, 〈ΔI〉/I, is inferred from direct distortion measurements of an imploding capsule’s limb-darkened (“backlit”) shell, via 6.7 keV point projection x-ray imaging. Thus far, 〈ΔI〉/I has been measured at the 3.0±1.4 (%) level, on the best shots, in hohlraums (cylindrical) with length/radius ratios L/R of 1.61 and 1.69, either side of a L/R=1.66 predicted optimum for a zeroed P2 Legendre mode. Simulations suggest that when scaled to 220 eV with zeroe...

Compact, rugged in-chamber transmission spectrometers (7-28 keV) for the Sandia Z facility.

We describe a pair of time-integrated transmission spectrometers that are designed to survey 7-28 keV (1.9 to 0.43 Å) x-ray photons produced by experiments on the Sandia Z pulsed power facility. Each spectrometer uses a quartz 10-11 crystal in a Cauchois geometry with a slit to provide spatial resolution along one dimension. The spectrometers are located in the harsh environment of the Z vacuum chamber, which necessitates that their design be compact and rugged. Example data from calibration tests and Z experiments are shown that illustrate the utility of the instruments.

Measurement of the efficiency of gold transmission gratings in the 100 to 5000 eV photon energy range

Three x-ray spectrometers, each with a transmission grating dispersion element, are routinely used at the Z soft x-ray facility to measure the spectrum and temporal history of the absolute soft x-ray power emitted from z-pinch and hohlraum radiation sources. Our goal is to make these measurements within an accuracy of ±10%. We periodically characterize the efficiency of the gratings used in the spectrometers by using an electron-impact soft x-ray source, a monochromator, grazing-incidence mirrors, thin filters, and an x-ray charge-coupled device (CCD) detector. We measure the transmission efficiency of the gratings at many photon energies for several grating orders. For each grating, we calculate efficiency as a function of photon energy using published optical constants of gold and multiple-slit Fraunhofer diffraction theory and fit the calculation to the measurements using the physical parameters of the grating as variables. This article describes the measurement apparatus and calibration techniques, di...