N. E. Lanier

Three-Dimensional Hydrodynamic Experiments on the National Ignition Facility

The production of supersonic jets of material via the interaction of a strong shock wave with a spatially localized density perturbation is a common feature of inertial confinement fusion and astrophysics. The behavior of two-dimensional (2D) supersonic jets has previously been investigated in detail [J. M. Foster et. al, Phys. Plasmas 9, 2251 (2002)]. In three-dimensions (3D), however, there are new aspects to the behavior of supersonic jets in compressible media. In this paper, the commissioning activities on the National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] to enable hydrodynamic experiments will be presented as well as the results from the first series of hydrodynamic experiments. In these experiments, two of the first four beams of NIF are used to drive a 40 Mbar shock wave into millimeter scale aluminum targets backed by 100 mg/cc carbon aerogel foam. The remaining beams are delayed in time and are used to provide a point-projection x-ray backlighter source for diagnosing the three-dimensional structure of the jet evolution resulting from a variety of 2D and 3D features. Comparisons between data and simulations using several codes will be presented.

Characterization and cross calibration of Agfa D4, D7, and D8 and Kodak SR45 x-ray films against direct exposure film at 4.0–5.5keV

Kodak direct exposure film (DEF) [B. L. Henke et al., J. Opt. Soc. Am. B 3, 1540 (1986)] has been the standard for moderate energy (1–10keV) x-ray diagnostic applications among the high-energy-density and inertial confinement fusion research communities. However, market forces have prompted Kodak to discontinue production of DEF, leaving these specialized communities searching for a replacement. We have conducted cross-calibration experiments and film characterizations on five possible substitutes for Kodak DEF. The film types studied were Kodak’s Biomax MR (BMR) and SR45 along with Agfa’s D8, D7, and D4sc. None of the films tested matched the speed of DEF. BMR and D8 were closest but D8 exhibited lower noise, with superior resolution and dynamic range. Agfa D7, Agfa D4sc, and Kodak SR45 were significantly less sensitive than BMR and D8, however, the improvements they yielded in resolution and dynamic range warrant their use if experimental constraints allow.

A monochromatic x-ray imaging system for characterizing low-density foams

In high energy density laser experiments, targets often require small, low-density, foam components. However, their limited size can preclude single component characterization, forcing one to rely solely on less accurate bulk measurements. We have developed a monochromatic imaging system to characterize both the density and uniformity of single component low-mass foams. This x-ray assembly is capable of determining line-averaged density variations near the 1% level, and provides statistically identical results to those obtained at the Brookhavens NSLS. This system has the added benefit of providing two-dimensional density data, allowing an assessment of density uniformity.

Effect of convergence on growth of the Richtmyer-Meshkov instability

Strongly shocked cylindrically convergent implosions were conducted on the OMEGA laser. The directly driven targets consist of a low-density foam core and an embedded aluminum shell covered by an epoxy ablator. The outer surface of the aluminum shell has imposed single-mode perturbations with wave numbers k = 0.25, 0.7, 1.05, and 2.5 (rad/μm) and initial amplitudes η 0 /λ = 0.04, 0.11, 0.33, and 0.4. In our convergent geometry, perturbation growth without evidence of saturation, for η/λ as large as 4.5 is observed for k k > 1 growth rate scaling with wavenumber breaks down and transition to turbulence is suggested.

Observation and simulation of plasma mix after reshock in a convergent geometry

Experiments to study the effect of a second, counterpropagating shock on the growth of hydrodynamic instabilities in a convergent, compressible system have been performed on the Omega Laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] at the University of Rochester. Direct laser illumination of a cylindrical target launches a strong shock across hydrodynamically unstable interfaces formed between an epoxy ablator material on the outside, a buried aluminum marker layer and low-density CH foam on the inside. The Richtmyer–Meshkov instability mixes the marker into the two adjacent materials. Of particular interest is what happens when the mixing region is reshocked by using a second, coaxial central cylinder to reflect the incident shock back into the mixing region. These experiments have been extensively modeled, in two dimensions, using the hydrocodes NYM [P. D. Roberts et al., J. Phys. D 13, 1957 (1980)], PETRA [D. L. Youngs, Physica D 12, 32 (1984)], and RAGE [R. M. Baltrusaitis et al., Phys. Flui...

Analysis of Ti K-shell emission produced from solid targets using nanosecond pulses on the TRIDENT laser facility

Measurements of Ti He-like x-ray emission (4.75 keV) from solid targets irradiated with nanosecond pulses on the TRIDENT laser facility are examined. Relative x-ray emission and conversion efficiency was measured as a function of laser irradiance conditions using a crystal spectrometer and step-filtered charge coupled device and x-ray film. Data on the x-ray emission with and without random phase plates is presented. Difficulties and caveats in the diagnostic techniques are also presented. Analysis of step-filtered data suggests a high-energy x-ray tail.

Characterization of surface roughness and initial conditions for cylindrical hydrodynamic and mix experiments

Abstract Hydrodynamic experiments in cylindrical geometry are used to study both mix (compressible, in convergent geometry) and mode coupling (impact of short wavelengths on long). For both types of experiments, knowledge of the initial conditions (the surface roughness spectrum, amplitude versus wavelength, as well as all target metrology) is very important. This paper is a discussion of the techniques and efforts to document and understand our initial conditions and their uncertainties and how well we can control them.

A technique for measuring the propagation of a supersonic radiation front in foam via spatially resolved spectral imaging of a tracer layer

We present a technique for measuring the propagation of a supersonic radiation front in low-density foam, where the lack of motion of the objects in its wake makes it difficult to determine its location. We illuminate a thin tracer foil embedded in the foam with a broadband x-ray source, and measure its changing absorption of these x rays as it ionizes. We record both spatial and spectral information of the heated tracer, and thus obtain its ionization state as a function of distance along the front propagation direction. We extrapolate this information to determine the state of the foam and the location of the radiation front. We present the experimental configuration used to test this technique at the Omega laser facility along with experimental results.

Highly resolved measurements of defect evolution under heated-and-shocked conditions

One of the principal advantages of a double-shell capsule design is the potential for ignition without requiring cryogenic implosions. These designs compress deuterium fuel by transferring kinetic energy from a laser-ablated outer shell to an inner shell by means of a nearly elastic symmetric collision. However, prior to this collision the inner shell experiences varying levels of preheat such that any nonuniformities can evolve significantly. It is the condition of these perturbations at the time the collision-induced shock compresses the inner shell that ultimately dictates capsule performance. With this in mind, a series of experiments have been performed on the OMEGA laser facility [R. T. Boehly et al., Opt. Comm. 133, 495 (1997)] that produce highly resolved measurements of defect evolution under heated-and-shocked conditions. Tin L-shell radiation is used to heat a layered package of epoxy and foam. The epoxy can be engineered with a variety of surface perturbations or defects. As the system evolves...

Validation of the radiation hydrocode RAGE against defect-driven mix experiments in a compressible, convergent, and miscible plasma system

Accurate predictive hydrodynamics codes increase the efficiency with which ignition will be achieved at the National Ignition Facility (NIF) [J. W. Hogan et al., J. Nucl. Fus. 41, 567 (2001)]. By validating these codes against well-diagnosed experiments, additional confidence in their predictive capability is attained. This work presents comparisons between the predictive simulations of the Los Alamos hydrocode RAGE [R. M. Baltrusaitus et al., Phys. Fluids 8, 2471 (1996)] and data obtained from cylindrical defect-driven mix experiments conducted on the OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser. The results show that RAGE accurately captures much of the bulk hydrodynamics of the experiments. However, persistent discrepancies with respect to the small-scale fluid flows remain.