L. E. Ruggles

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 ...

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.

X-ray imaging techniques on Z using the Z-Beamlet laser

The Z-Beamlet laser backlighter system at Sandia National Laboratories, which will be operational in 2001, will create a point or area source of high (or moderate) energy x rays behind a Z-accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105 (1998)] -driven target. In the former case with >2 kJ in up to four pulses of 80% of the 2ω energy in a ∼50-μm-diam focal spot, the resulting >4×1016 W/cm2 irradiances will generate ⩾8.950, 8.999 keV (zinc He-α, etc.) x rays. This high-energy source, as either a single point or four separate spots, will be used directly for four-frame point-projection x-ray imaging, and will attain spatial resolutions and signal-to-noise levels significantly better than presently possible on Z using existing methods. In combination with a ∼1 cm field of view, the technique will be well suited to the large, relatively opaque objects characteristic of Z experiments. This addition is anticipated to have a major impact upon the basic phy...

Measurements of 4–10 keV x-ray production with the Z-Beamlet laser

In order to characterize the current backlighting capability of Sandia’s Z-Beamlet laser (ZBL) over a range of high photon energies, we measured the x-ray conversion efficiency of the focused 527 nm ZBL beam into 4–10 keV x rays from He-like emission of the elements Sc through Ge (excluding Ga). The measurements approximated ZBL’s nominal backlighting geometry and laser performance at Sandia’s Z soft x-ray facility by irradiating planar foil targets several microns thick rotated 30° to the laser beam axis with a 600 ps pulse at 1 TW. The focal spot diameter was about 150 μm. This study includes measurements of the K-shell x-ray spectrum, x-ray power, and x-ray spot size with an array of filtered high-bandwidth silicon diodes, a convex LiF crystal spectrometer, step wedge filtered x-ray film, and a filtered x-ray pinhole camera. We found agreement with previous work for comparable laser parameters and recorded decreasing conversion efficiency versus atomic number and He-like photon energy.

Zero-dimensional energetics scaling models for z-pinch-driven hohlraums

Wire array z pinches on the Z accelerator provide the most intense laboratory source of soft X rays in the world. The unique combination of a near-Planckian radiation source with high X-ray production efficiency (10 to 15% wall plug), large X-ray powers and energies (>100 TW, ≥0.8 MJ in 6 ns to 7 ns), large characteristic hohlraum volumes (0.5 to >10cm 3 ), long pulse lengths (5 to 20 ns), and low capital cost ( 200 MJ yield) ICF capsules with adequate radiation symmetry and margin. The z-pinch-driven hohlraum approach of Hammer et al. (1999) may provide a conservative and robust solution to the requirements for high yield, and is currently being studied on the Z accelerator. This paper describes a multiple-region, 0-D hohlraum energetics model for z-pinch-driven hohlraums in four configurations. We observe consistency between the model and the measured X-ray powers and hohlraum wall temperatures to within ±20% in X-ray flux, for the four configurations. The scaling of pinch energy and radiation-driven anode-cathode gap closure with drive current are also examined.

High-brightness, high-spatial-resolution, 6.151keV x-ray imaging of inertial confinement fusion capsule implosion and complex hydrodynamics experiments on Sandia’s Z accelerator (invited)

When used for the production of an x-ray imaging backlighter source on Sandia National Laboratories’ 20MA, 100ns rise-time Z accelerator [M. K. Matzen et al., Phys. Plasmas 12, 055503 (2005)], the terawatt-class, multikilojoule, 526.57nm Z-Beamlet laser (ZBL) [P. K. Rambo et al., Appl. Opt. 44, 2421 (2005)], in conjunction with the 6.151keV, Mn–Heα curved-crystal imager [D. B. Sinars et al., Rev. Sci. Instrum. 75, 3672 (2004)], is capable of providing a high quality x radiograph per Z shot for various high-energy-density physics experiments. Enhancements to this imaging system during 2005 have led to the capture of inertial confinement fusion capsule implosion and complex hydrodynamics images of significantly higher quality. The three main improvements, all leading effectively to enhanced image plane brightness, were bringing the source inside the Rowland circle to approximately double the collection solid angle, replacing direct exposure film with Fuji BAS-TR2025 image plate (read with a Fuji BAS-5000 sc...

Measurement of radiation symmetry in Z-pinch-driven hohlraums

The Z-pinch-driven hohlraum (ZPDH) [J. H. Hammer et al., Phys. Plasmas 6, 2129 (1999)] is a promising approach to high yield inertial confinement fusion currently being characterized in experiments on the Sandia Z accelerator [M. E. Cuneo et al., Phys. Plasmas 8, 2257 (2001)]. Simulations show that capsule radiation symmetry, a critical issue in ZPDH design, is governed primarily by hohlraum geometry, dual-pinch power balance, and pinch timing. In initial symmetry studies on Z without the benefit of a laser backlighter, highly-asymmetric pole-hot and equator-hot single Z-pinch hohlraum geometries were diagnosed using solid low density foam burnthrough spheres. These experiments demonstrated effective geometric control and prediction of polar flux symmetry at the level where details of the Z-pinch implosion and other higher order effects are not critical. Radiation flux symmetry achieved in Z double-pinch hohlraum configurations exceeds the measurement sensitivity of this self-backlit foam ball symmetry di...

Soft x-ray measurements of z-pinch-driven vacuum hohlraums

This article reports the experimental characterization of a z-pinch driven-vacuum hohlraum. The authors have measured soft x-ray fluxes of 5 x 10{sup 12} W/cm{sup 2} radiating from the walls of hohlraums which are 2.4--2.5 cm in diameter by 1 cm tall. The x-ray source used to drive these hohlraums was a z-pinch consisting of a 300 wire tungsten array driven by a 2 MA, 100 ns current pulse. In this hohlraum geometry, the z-pinch x-ray source can produce energies in excess of 800 kJ and powers in excess of 100 TW to drive these hohlraums. The x-rays released in these hohlraums represent greater than a factor of 25 in energy and more than a factor of three in x-ray power over previous laboratory-driven hohlraums.

Enhancement of x-ray yield from the Z-Beamlet laser for monochromatic backlighting by using a prepulse

The Z-Beamlet Laser (ZBL) is capable of providing on-target energies of up to 1.5kJ at 527nm in up to four separate 0.3–1.5ns pulses during a 20ns window. ZBL is routinely used as a source of x rays for backlighting experiments on the Sandia Z facility, a 20MA, 100ns rise-time, pulsed-power driver for z-pinch plasma radiation sources. Most backlighting experiments use monochromatic crystal imaging diagnostics at 1865 or 6151eV. We present calibration data demonstrating that the use of a 0.3–0.6ns, ∼200J pulse, followed 2ns later by a 1.0ns, ∼1kJ pulse results in more than twice the x-ray yield at 6151eV (a He-like Mn emission line) compared to a single 1.0ns, ∼1kJ pulse. The first pulse creates a plasma (and few x rays) that expands and approaches the critical density for the laser when the second pulse arrives, creating a more efficient coupling of laser light to the plasma. A similar improvement was also noted for He-like Ni emission lines, suggesting that this simple technique scales to higher photon e...

Measurement and simulation of apertures on Z hohlraums

We have performed aperture measurements and simulations for vacuum hohlraums heated by wire array implosions. A low-Z plastic coating is often applied to the aperture to create a high ablation pressure which retards the expansion of the gold hohlraum wall. However this interface is unstable and may be subject to the development of highly nonlinear perturbations (“jets”) as a result of shocks converging near the edge of the aperture. These experiments have been simulated using Lagrangian and Eulerian radiation hydrodynamics codes.