Publications

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A dynamic hohlraum is created when an annular z-pinch plasma implodes onto a cylindrical 0.014 g/cc 6-mm-diameter CH{sub 2} foam. The impact launches a radiating shock that propagates toward the axis at {approx}350 {micro}m/ns. The radiation trapped by the tungsten z-pinch plasma forms a {approx}200 eV hohlraum that provides X-rays for indirect drive inertial confinement fusion capsule implosion experiments. We are developing the ability to diagnose the hohlraum interior using emission and absorption spectroscopy of Si atoms added as a tracer to the central portion of the foam. Time- and space-resolved Si spectra are recorded with an elliptical crystal spectrometer viewing the cylindrical hohlraum end-on. A rectangular aperture at the end of the hohlraum restricts the field of view so that the 1D spectrometer resolution corresponds approximately to the hohlraum radial direction. This enables distinguishing between spectra from the unshocked radiation-heated foam and from the shocked foam. Typical spectral lines observed include the Si Ly{alpha} with its He-like satellites and the He-like resonance sequence including He{alpha}, He{beta}, and He{gamma}, along with some of their associated Li-like satellites. Work is in progress to infer the hohlraum conditions using collisional-radiative modeling that accounts for the radiation environment and includes both opacity effects and detailed Stark broadening calculations. These 6-mm-scale radiation-heated plasmas might eventually also prove suitable for testing Stark broadening line profile calculations or for opacity measurements.

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Z-pinch plasmas are susceptible to the magnetic Rayleigh-Taylor (MRT) instability. The Z-pinch dynamic hohlraum (ZPDH), as implemented on the Z machine at Sandia National Laboratories, is composed of an annular tungsten plasma that implodes onto a coaxial foam convertor. The collision between tungsten Z pinch and convertor launches a strong shock in the foam. Shock heating generates radiation that is trapped by the tungsten Z pinch. The radiation can be used to implode a fuel-filled, inertial confinement fusion capsule. Hence, it is important to understand the influence that the MRT instability has on shock generation. This paper presents results of an investigation to determine the affect that the MRT instability has on characteristics of the radiating shock in a ZPDH. Experiments on Z were conducted in which a 1.5 cm tall, nested array (two arrays with initial diameters of 2.0 and 4.0 cm), tungsten wire plasma implodes onto a 5 mg/cc, CH{sub 2} foam convertor to create a {approx}135 eV dynamic hohlraum. X-ray pinhole cameras viewing along the ZPDH axis recorded time and space resolved images of emission produced by the radiating shock. These measurements showed that the shock remained circular to within +/-30-60 {micro}m as it propagated towards the axis, and that it was highly uniform along its height. The measured emission intensities are compared with synthetic x-ray images obtained by postprocessing two-dimensional, radiation magnetohydrodynamic simulations in which the amplitude of MRT perturbations is varied. These simulations accurately reproduce the measured shock trajectory and spatial profiles of the dynamic hohlraum interior emission as a function of time, even for large MRT amplitudes. Furthermore, the radiating shock remains relatively uniform in the axial direction regardless of the MRT amplitude because nonuniformities are tamped by the interaction of the tungsten Z-pinch plasma with the foam. These results suggest that inertial confinement fusion implosions driven by a ZPDH should be relatively free from random radiation symmetry variations produced by Z-pinch instabilities.

Progress in understanding the physics of dynamic-hohlraums is reviewed for a system capable of generating 13 TW of axial radiation for high temperature (>200 eV) radiation-flow experiments and ICF capsule implosions.

The high-pressure response of cryogenic liquid deuterium (LD{sub 2}) has been studied to pressures of {approx}400GPa and densities of {approx}1.5g/cm{sup 3}. Using intense magnetic pressure produced by the Sandia National Laboratories Z accelerator, macroscopic aluminum or titanium flyer plates, several mm in lateral dimensions and a few hundred microns in thickness, have been launched to velocities in excess of 22 km/s, producing constant pressure drive times of approximately 30 ns in plate impact, shock wave experiments. This flyer plate technique was used to perform shock wave experiments on LD{sub 2} to examine its high-pressure equation of state. Using an impedance matching method, Hugoniot measurements of LD{sub 2} were obtained in the pressure range of {approx}22-100GPa. Results of these experiments indicate a peak compression ratio of approximately 4.3 on the Hugoniot. In contrast, previously reported Hugoniot states inferred from laser-driven experiments indicate a peak compression ratio of approximately 5.5-6 in this same pressure range. The stiff Hugoniot response observed in the present impedance matching experiments was confirmed in simultaneous, independent measurements of the relative transit times of shock waves reverberating within the sample cell, between the front aluminum drive plate and the rear sapphire window. The relative timing was found to be sensitive to the density compression along the principal Hugoniot. Finally, mechanical reshock measurements of LD{sub 2} using sapphire, aluminum, and {alpha}-quartz anvils were made. These results also indicate a stiff response, in agreement with the Hugoniot and reverberating wave measurements. Using simple model-independent arguments based on wave propagation, the principal Hugoniot, reverberating wave, and sapphire anvil reshock measurements are shown to be internally self-consistent, making a strong case for a Hugoniot response with a maximum compression ratio of {approx}4.3-4.5. The trends observed in the present data are in very good agreement with several ab initio models and a recent chemical picture model for LD{sub 2}, but in disagreement with previously reported laser-driven shock results. Due to this disagreement, significant emphasis is placed on the discussion of uncertainties, and the potential systematic errors associated with each measurement.

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Inertial confinement fusion capsule implosions absorbing up to 35 kJ of x-rays from a {approx}220 eV dynamic hohlraum on the Z accelerator at Sandia National Laboratories have produced thermonuclear D-D neutron yields of (2.6 {+-} 1.3) x 10{sup 10}. Argon spectra confirm a hot fuel with Te {approx} 1 keV and n{sub e} {approx} (1-2) x 10{sup 23} cm{sup -3}. Higher performance implosions will require radiation symmetry control improvements. Capsule implosions in a {approx}70 eV double-Z-pinch-driven secondary hohlraum have been radiographed by 6.7 keV x-rays produced by the Z-beamlet laser (ZBL), demonstrating a drive symmetry of about 3% and control of P{sub 2} radiation asymmetries to {+-}2%. Hemispherical capsule implosions have also been radiographed in Z in preparation for future experiments in fast ignition physics. Z-pinch-driven inertial fusion energy concepts are being developed. The refurbished Z machine (ZR) will begin providing scaling information on capsule and Z-pinch in 2006. The addition of a short pulse capability to ZBL will enable research into fast ignition physics in the combination of ZR and ZBL-petawatt. ZR could provide a test bed to study NIF-relevant double-shell ignition concepts using dynamic hohlraums and advanced symmetry control techniques in the double-pinch hohlraum backlit by ZBL.

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We present results from crystal spectroscopic analysis of silicon aero-gel foams heated by dynamic hohlraums on Z. The dynamic hohlraum on Z creates a radiation source with a 230-eV average temperature over a 2.4-mm diameter. In these experiments silicon aero-gel foams with 10-mg/cm{sup 3} densities and 1.7-mm lengths were placed on both ends of the dynamic hohlraum. Several crystal spectrometers were placed both above and below the z-pinch to diagnose the temperature of the silicon aero-gel foam using the K-shell lines of silicon. The crystal spectrometers were (1) temporally integrated and spatially resolved, (2) temporally resolved and spatially integrated, and (3) both temporally and spatially resolved. The results indicate that the dynamic hohlraum heats the silicon aero-gel to approximately 150-eV at peak power. As the dynamic hohlraum source cools after peak power the silicon aero-gel continues to heat and jets axially at an average velocity of approximately 50-cm/{micro}s. The spectroscopy has also shown that the reason for the up/down asymmetry in radiated power on Z is that tungsten enters the line-of-sight on the bottom of the machine much more than on the top.

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Using intense magnetic pressure, a method was developed to launch flyer plates to velocities in excess of 20 km s{sup -1}. This technique was used to perform plate-impact, shock wave experiments on cryogenic liquid deuterium (LD{sub 2}) to examine its high-pressure equation of state (EOS). Using an impedance matching method, Hugoniot measurements were obtained in the pressure range of 22--100 GPa. The results of these experiments disagree with the previously reported Hugoniot measurements of LD2 in the pressure range above {approx}40 GPa, but are in good agreement with first principles, ab initio models for hydrogen and its isotopes.

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Laboratory measurements provide benchmark data for wavelength-dependent plasma opacities to assist inertial confinement fusion, astrophysics, and atomic physics research. There are several potential benefits to using z-pinch radiation for opacity measurements, including relatively large cm-scale lateral sample sizes and relatively-long 3-5 ns experiment durations. These features enhance sample uniformity. The spectrally resolved transmission through a CH-tamped NaBr foil was measured. The z-pinch produced the X-rays for both the heating source and backlight source. The (50+4) eV foil electron temperature and (3±1) × 1021 cm-3 foil electron density were determined by analysis of the Na absorption features. LTE and NLTE opacity model calculations of the n=2 to 3, 4 transitions in bromine ionized into the M-shell are in reasonably good agreement with the data.

Using intense magnetic pressure, a method was developed to launch flyer plates to velocities in excess of 20 km/s. This technique was used to perform plate-impact, shock wave experiments on cryogenic liquid deuterium (LD{sub 2}) to examine its high-pressure equation of state (EOS). Using an impedance matching method, Hugoniot measurements were obtained in the pressure range of 30-70 GPa. The results of these experiments disagree with previously reported Hugoniot measurements of LD{sub 2} in the pressure range above {approx}40 GPa, but are in good agreement with first principles, ab-initio models for hydrogen and its isotopes.

A new method for measuring the wavelength dependence of the transit time, material dispersion, and attenuation of an optical fiber is described. The authors inject light from a 4-ns risetime pulsed broad-band flashlamp into various length fibers and record the transmitted signals with a time-resolved spectrograph. Segments of data spanning an approximately 3,000 {angstrom} range are recorded from a single flashlamp pulse. Comparison of data acquired with short and long fibers enables the determination of the transit time and the material dispersion as functions of wavelength dependence for the entire recorded spectrum simultaneously. The wavelength dependent attenuation is also determined from the signal intensities. The method is demonstrated with experiments using a step index 200-{micro}m-diameter SiO{sub 2} fiber. The results agree with the transit time determined from the bulk glass refractive index to within {+-} 0.035% for the visible (4,000--7,200 {angstrom}) spectrum and 0.12% for the ultraviolet (2,650--4,000 {angstrom}) spectrum, and with the attenuation specified by the fiber manufacturer to within {+-} 10%.

The maximum power achieved in a wide variety of high-power devices, including electron and ion diodes, z pinches, and microwave generators, is presently limited by anode-cathode gap breakdown. A frequently-discussed hypothesis for this effect is ionization of fast neutral atoms injected throughout the anode-cathode gap during the power pulse. The authors describe a newly-developed diagnostic tool that provides the first direct test of this hypothesis. Time-resolved vacuum-ultraviolet absorption spectroscopy is used to directly probe fast neutral atoms with 1 mm spatial resolution in the 10 mm anode-cathode gap of the SABRE 5 MV, 1 TW applied-B ion diode. Absorption spectra collected during Ar RF glow discharges and with CO{sub 2} gas fills confirm the reliability of the diagnostic technique. Throughout the 50--100 ns ion diode pulses no measurable neutral absorption is seen, setting upper limits of 0.12--1.5 x 10{sup 14} cm{sup {minus}3} for ground state fast neutral atom densities of H, C, N, O, F. The absence of molecular absorption bands also sets upper limits of 0.16--1.2 x 10{sup 15} cm{sup {minus}3} for common simple molecules. These limits are low enough to rule out ionization throughout the gap as a breakdown mechanism. This technique can now be applied to quantify the role of neutral atoms in other high-power devices.

The Z Accelerator is a fast pulse power facility capable of performing high-pressure studies of the dynamic response of materials under loading conditions unachievable with other methods. A variety of advanced laser diagnostics have been implemented on the facility for shock physics experiments. These include multipoint laser velocity interferometry,line and full field velocity interferometry, time-resolved optical and uv spectroscopy, and both active and passive shock breakout.