G. W. Collins

Line-imaging velocimeter for shock diagnostics at the OMEGA laser facility

A line-imaging velocity interferometer has been implemented at the OMEGA laser facility of the Laboratory for Laser Energetics, University of Rochester. This instrument is the primary diagnostic for a variety of experiments involving laser-driven shock-wave propagation, including high-pressure equation of state experiments, materials characterization experiments, shock characterization for Rayleigh–Taylor experiments, and shock timing experiments for inertial confinement fusion research. Using a laser probe beam to illuminate a target, the instrument measures shock breakout times at temporal resolutions as low as 20 ps, and spatial resolution ∼4 μm. For velocity measurements the detection limit is <0.1 km/s, and velocities of interfaces, free surfaces, and shock fronts traveling through transparent media can be measured with accuracies ∼1% over the range from 4 km/s to greater than 50 km/s. Quantitative measurements of the optical reflectance of ionizing shock fronts can also be obtained simultaneously wi...

The experimental plan for cryogenic layered target implosions on the National Ignition Facility—The inertial confinement approach to fusion

Ignition requires precisely controlled, high convergence implosions to assemble a dense shell of deuterium-tritium (DT) fuel with ρR>∼1 g/cm2 surrounding a 10 keV hot spot with ρR ∼ 0.3 g/cm2. A working definition of ignition has been a yield of ∼1 MJ. At this yield the α-particle energy deposited in the fuel would have been ∼200 kJ, which is already ∼10 × more than the kinetic energy of a typical implosion. The National Ignition Campaign includes low yield implosions with dudded fuel layers to study and optimize the hydrodynamic assembly of the fuel in a diagnostics rich environment. The fuel is a mixture of tritium-hydrogen-deuterium (THD) with a density equivalent to DT. The fraction of D can be adjusted to control the neutron yield. Yields of ∼1014−15 14 MeV (primary) neutrons are adequate to diagnose the hot spot as well as the dense fuel properties via down scattering of the primary neutrons. X-ray imaging diagnostics can function in this low yield environment providing additional information about ...

Capsule implosion optimization during the indirect-drive National Ignition Campaign

Capsule performance optimization campaigns will be conducted at the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, 228 (2004)] to substantially increase the probability of ignition. The campaigns will experimentally correct for residual uncertainties in the implosion and hohlraum physics used in our radiation-hydrodynamic computational models using a variety of ignition capsule surrogates before proceeding to cryogenic-layered implosions and ignition experiments. The quantitative goals and technique options and down selections for the tuning campaigns are first explained. The computationally derived sensitivities to key laser and target parameters are compared to simple analytic models to gain further insight into the physics of the tuning techniques. The results of the validation of the tuning techniques at the OMEGA facility [J. M. Soures et al., Phys. Plasmas 3, 2108 (1996)] under scaled hohlraum and capsule conditions relevant to the ignition design are shown ...

Phase Transformations and Metallization of Magnesium Oxide at High Pressure and Temperature

Planetary Interiors Under Pressure The interiors of Earth and other rocky planets generally consist of a few common minerals. Depending largely on the size of the planet, the distribution and relative abundance of these minerals varies; for example, MgO is abundant in the mantles of Earth and large Earth-like planets, but is present in Jupiters core. The properties of MgO also vary with planetary size as a function of temperature and pressure. McWilliams et al. (p. 1330, published online 22 November) performed laser-shock experiments at pressures over three times higher than Earths inner core. MgO underwent two phase transformations, first to a solid with a modified crystal structure, and then to a conductive liquid. In terrestrial planets greater than eight Earth masses, MgO in the mantle could generate a magnetic field–generating dynamo such as those that typically found in planetary cores. Mantle minerals conductive at the high pressures and temperatures of planetary interiors could induce a magnetic field. Magnesium oxide (MgO) is representative of the rocky materials comprising the mantles of terrestrial planets, such that its properties at high temperatures and pressures reflect the nature of planetary interiors. Shock-compression experiments on MgO to pressures of 1.4 terapascals (TPa) reveal a sequence of two phase transformations: from B1 (sodium chloride) to B2 (cesium chloride) crystal structures above 0.36 TPa, and from electrically insulating solid to metallic liquid above 0.60 TPa. The transitions exhibit large latent heats that are likely to affect the structure and evolution of super-Earths. Together with data on other oxide liquids, we conclude that magmas deep inside terrestrial planets can be electrically conductive, enabling magnetic field–producing dynamo action within oxide-rich regions and blurring the distinction between planetary mantles and cores.

Implosion dynamics measurements at the National Ignition Facility

Measurements have been made of the in-flight dynamics of imploding capsules indirectly driven by laser energies of 1–1.7 MJ at the National Ignition Facility [Miller et al., Nucl. Fusion 44, 228 (2004)]. These experiments were part of the National Ignition Campaign [Landen et al., Phys. Plasmas 18, 051002 (2011)] to iteratively optimize the inputs required to achieve thermonuclear ignition in the laboratory. Using gated or streaked hard x-ray radiography, a suite of ablator performance parameters, including the time-resolved radius, velocity, mass, and thickness, have been determined throughout the acceleration history of surrogate gas-filled implosions. These measurements have been used to establish a dynamically consistent model of the ablative drive history and shell compressibility throughout the implosion trajectory. First results showed that the peak velocity of the original 1.3-MJ Ge-doped polymer (CH) point design using Au hohlraums reached only 75% of the required ignition velocity. Several capsu...

Ramp compression of diamond to five terapascals

The recent discovery of more than a thousand planets outside our Solar System, together with the significant push to achieve inertially confined fusion in the laboratory, has prompted a renewed interest in how dense matter behaves at millions to billions of atmospheres of pressure. The theoretical description of such electron-degenerate matter has matured since the early quantum statistical model of Thomas and Fermi, and now suggests that new complexities can emerge at pressures where core electrons (not only valence electrons) influence the structure and bonding of matter. Recent developments in shock-free dynamic (ramp) compression now allow laboratory access to this dense matter regime. Here we describe ramp-compression measurements for diamond, achieving 3.7-fold compression at a peak pressure of 5 terapascals (equivalent to 50 million atmospheres). These equation-of-state data can now be compared to first-principles density functional calculations and theories long used to describe matter present in the interiors of giant planets, in stars, and in inertial-confinement fusion experiments. Our data also provide new constraints on mass–radius relationships for carbon-rich planets.

Dense Matter Characterization by X-ray Thomson Scattering

Abstract We discuss the extension of the powerful technique of Thomson scattering to the X-ray regime for providing an independent measure of plasma parameters for dense plasmas. By spectrally resolving the scattering, the coherent (Rayleigh) unshifted scattering component can be separated from the incoherent Thomson component, which is both Compton and Doppler shifted. The free electron density and temperature can then be inferred from the spectral shape of the high-frequency Thomson scattering component. In addition, as the plasma temperature is decreased, the electron velocity distribution as measured by incoherent Thomson scattering will make a transition from the traditional Gaussian Boltzmann distribution to a density-dependent parabolic Fermi distribution. We also present a discussion for a proof-of-principle experiment appropriate for a high-energy laser facility.

Accurate measurement of laser-driven shock trajectories with velocity interferometry

We describe a velocity interferometer used to measure the velocity and trajectory of laser driven shocks in liquid deuterium accurately and continuously. This demonstration of velocity interferometry to measure shock velocity and shock trajectory in condensed matter shows strong potential for future studies of laser-driven shocks in transparent media. Accuracy of this technique can be better than 1% in velocity and ±0.2 μm in position during a 10 ns interval.

High-precision measurements of the equation of state of hydrocarbons at 1-10 Mbar using laser-driven shock waves

The equation of state (EOS) of polystyrene and polypropylene were measured using laser-driven shock waves with pressures from 1 to 10 Mbar. Precision data resulting from the use of α-quartz as an impedance-matching (IM) standard tightly constrains the EOS of these hydrocarbons, even with the inclusion of systematic errors inherent to IM. The temperature at these high pressures was measured, which, combined with kinematic measurements, provide a complete shock EOS. Both hydrocarbons were observed to reach similar compressions and temperatures as a function of pressure. The materials were observed to transition from transparent insulators to reflecting conductors at pressures of 1 to 2 Mbar.

Convergent ablator performance measurements

The velocity and remaining ablator mass of an imploding capsule are critical metrics for assessing the progress toward ignition of an inertially confined fusion experiment. These and other convergent ablator performance parameters have been measured using a single streaked x-ray radiograph. Traditional Abel inversion of such a radiograph is ill-posed since backlighter intensity profiles and x-ray attenuation by the ablated plasma are unknown. To address this we have developed a regularization technique which allows the ablator density profile ρ(r) and effective backlighter profile I0(y) at each time step to be uniquely determined subject to the constraints that ρ(r) is localized in radius space and I0(y) is delocalized in object space. Moments of ρ(r) then provide the time-resolved areal density, mass, and average radius (and thus velocity) of the remaining ablator material. These results are combined in the spherical rocket model to determine the ablation pressure and mass ablation rate during the implos...