Gilbert W. Collins

Powder diffraction from solids in the terapascal regime

A method of obtaining powder diffraction data on dynamically compressed solids has been implemented at the Jupiter and OMEGA laser facilities. Thin powdered samples are sandwiched between diamond plates and ramp compressed in the solid phase using a gradual increase in the drive-laser intensity. The pressure history in the sample is determined by back-propagation of the measured diamond free-surface velocity. A pulse of x rays is produced at the time of peak pressure by laser illumination of a thin Cu or Fe foil and collimated at the sample plane by a pinhole cut in a Ta substrate. The diffracted signal is recorded on x-ray sensitive material, with a typical d-spacing uncertainty of ~0.01 Å. This diagnostic has been used up to 0.9 TPa (9 Mbar) to verify the solidity, measure the density, constrain the crystal structure, and evaluate the strain-induced texturing of a variety of compressed samples spanning atomic numbers from 6 (carbon) to 82 (lead). Further refinement of the technique will soon enable diffraction measurements in solid samples at pressures exceeding 1 TPa.

Investigation of Carbon

We report on the first investigations of the use of carbon fibers in an X-pinch load using a 250-kA linear transformer driver (LTD). Multiframe laser shadowgraphy is used to examine the evolution of the pinch and shows that carbon loads demonstrate wire expansion and cross-point pinch and gap formation as observed in X-pinches constructed from high Z materials. Radiographs taken using the carbon X-pinch as the source demonstrate both that sufficient flux is emitted to provide a good contrast image at source-to-image distances of > 10 cm and that the cross point produces a relatively small hot spot. Radiographs of a series of fine wires (5-30 mum) using X-rays > 500 eV demonstrated that 25-mum wires can be resolved in this energy range. Time-resolved X-ray emission measurements showed that, while emission in the hnu > 500 eV range shows long (> 100 ns) timescales, emission in the hnu > 1 keV range shows a multiple-peaked structure with durations as short as 20 ns.

X

X-pinches driven by an 80-kA current are investigated by using high-resolution dark-field laser Schlieren imaging. Results demonstrate the evolution of the low-density coronal plasma and the formation of an axial jet. Quantitative measurements of plasma expansion rates and the jet velocity are carried out for several materials.

-Pinches as a Source for Point-Projection Radiography

Bright spots in the hot spot intensity profile of gated x-ray images of ignition-scale implosions at the National Ignition Facility [G. H. Miller et al., Opt. Eng. 443, (2004)] are observed. X-ray images of cryogenically layered deuterium-tritium (DT) and tritium-hydrogen-deuterium (THD) ice capsules, and gas filled plastic shell capsules (Symcap) were recorded along the hohlraum symmetry axis. Heterogeneous mixing of ablator material and fuel into the hot spot (i.e., hot-spot mix) by hydrodynamic instabilities causes the bright spots. Hot-spot mix increases the radiative cooling of the hot spot. Fourier analysis of the x-ray images is used to quantify the evolution of bright spots in both x- and k-space. Bright spot images were azimuthally binned to characterize bright spot location relative to known isolated defects on the capsule surface. A strong correlation is observed between bright spot location and the fill tube for both Symcap and cryogenically layered DT and THD ice targets, indicating the fill ...

High-Resolution Laser Schlieren Imaging of Coronal Plasma Evolution in 80-kA

We propose a method for thermal conductivity measurements of high energy density matter based on differential heating. A temperature gradient is created either by surface heating of one material or at an interface between two materials by different energy deposition. The subsequent heat conduction across the temperature gradient is observed by various time-resolved probing techniques. Conceptual designs of such measurements using laser heating, proton heating, and x-ray heating are presented. The sensitivity of the measurements to thermal conductivity is confirmed by simulations.

X

We present the preliminary results from the first ablation studies of wire-array z-pinches using a short-pulse linear transformer driver (LTD) providing a current of 200 kA in 150 ns. Laser imaging is used to recover the ablation-flare wavelength for comparison with previous studies and to provide quantitative information on the axial-density variation of this structure. The results are used to determine the general form of the variation of the ablation velocity in the axial direction, and fits to data can infer the range of values required to describe observed modulation. In addition, conical arrays demonstrate the formation and propagation of moderate atomic-number plasma jets comparable with those observed in mega-ampere experiments.

-Pinches

Studies of the late time diode gap formation in two- and four-wire tungsten x-pinches using an 80kA, 50ns current pulse are presented. Quantitative measurements of the coronal plasma density are recovered using interferometry simultaneously with laser shadowgraphy. Axial expansion of the gap occurs at ∼106cm∕s for both two- and four-wire systems and is likely to be driven by an axial J×B force resulting from radial current flow in the plasma minidiode “electrodes.” Radial density profiles suggest repinching of the low density plasma occurs after the main pinch resulting in secondary x-ray emission peak >10ns after the first, which is recorded with a pair of pin diodes.

Experimental investigation of bright spots in broadband, gated x-ray images of ignition-scale implosions on the National Ignition Facility

A compact linear transformer driver, capable of producing 250 kA in 150 ns, was used to study instability formation on the surface of thin liners. In the experiments, two different materials, Cu and Ni, were used to study the effect of the liners resistivity on formation and evolution of the instabilities. The dimensions of the liners used were 7 mm height, 1 mm radius, and 3 μm thickness. Laser probing and time resolved extreme ultraviolet (EUV) imaging were implemented to diagnose instability formation and growth. Time-integrated EUV spectroscopy was used to study plasma temperature and density. A constant expansion rate for the liners was observed, with similar values for both materials. Noticeable differences were found between the Cu and Ni instability growth rates. The most significant perturbation in Cu rapidly grows and saturates reaching a limiting wavelength of the order of the liner radius, while the most significant wavelength in Ni increases slowly before saturating, also at a wavelength clo...

Differential heating: A versatile method for thermal conductivity measurements in high-energy-density matter

We report on experiments examining the dynamics of low current (25 kA), long rise-time (400 ns) X-pinches. The experiments used two- and four-wire 5 μm tungsten X-pinches. We discuss coronal plasma dynamics, X-ray emission, cross-point evolution, and axial column dynamics. We show that the pinch dynamics are akin to those in X-pinches driven by 0.1–1 MA and at rise-rates from 0.25 to 10 kA/ns. We observe soft X-rays (∼1–2 keV range) emitted from the cross point in both two-wire and four-wire X-pinches at about peak current. These results show that compact current generators consisting of 2–4 small capacitors in a simple capacitive discharge circuit can produce good quality pinches for applications in Thomson X-ray scattering and backlighting among others.

Ablation Studies of Low-Number Wire Arrays at 200 kA Using A Linear Transformer Driver

We report on results from experiments using laser-cut X shaped foils (or foil X-pinches) driven by the 250 kA, 150 ns rise-time GenASIS Linear Transformer Driver. Optical probing of the laser-cut foil X-pinches shows considerable dynamic and qualitative similarity with the more traditional wire X-pinches. Results demonstrate excellent quality X-ray emission flux, timing, and source size. The foil X-pinches offer advantages such as improved, reproducible cross-point symmetry, and the opportunity for rep-rated operation, which is not possible with conventional X-pinches using wires.