S. R. Goldman

Shock structuring due to fabrication joints in targets

The use of copper-doped beryllium ablators on National Ignition Facility [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] targets, in place of plastic, can require the bonding together of hemispheres with a joint of differing composition. Indirect drive experiments have been conducted on the Nova laser [J. L. Emmet, W. F. Krupke, and J. B. Trenholme, Sov. J. Quantum Electron. 13, 1 (1983)], and the resulting shock structuring compared with code simulations. It is concluded that one of the available codes, the RAGE code [R. M. Baltrusaitis et al., Phys. Fluids 8, 2471 (1996)] provides useful insight into the effect of joints. This code is then employed to obtain a physical picture of the shock front nonuniformity in terms of a secondary rarefaction and an oblique shock interacting with the main shock that propagates in the absence of the joint. A simple analysis reinforces this picture.

Computational study of laser imprint mitigation in foam-buffered inertial confinement fusion targets

Recent experiments have shown that low density foam layers can significantly mitigate the perturbing effects of beam nonuniformities affecting the acceleration of thin shells. This problem is studied parametrically with two-dimensional LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2, 51 (1975)]. Foam-buffered targets are employed, consisting typically of 250 A of gold, and 50 μm of 50 mg/cm3 C10H8O4 foam attached to a 10 μm foil. In simulation these were characteristically exposed to 1.2 ns, flat-topped green light pulses at 1.4×1014 W/cm2 intensity, bearing 30 μm lateral perturbations of up to 60% variation in intensity. Without the buffer layers the foils were severely disrupted by 1 ns. With buffering only minimal distortion was manifest at 3 ns. The smoothing is shown to derive principally from the high thermal conductivity of the heated foam. The simulation results imply that (1) the foam thickness should exceed the disturbance wavelength; (2) intensities exceeding ...

Tuning indirect-drive implosions using cone power balance

We demonstrate indirect-drive implosion symmetry tuning in a vacuum hohlraum 6.6 mm in length and 3.56 mm in diameter with a CH capsule 6.38 μm in thickness and 1414 μm in diameter, scaled roughly 0.7 × from a National ignition facility (NIF) [E. Moses and C. R. Wuest, Fusion Sci. Technol. 47, 314 (2005)] The hohlraums have radiation drives of 117 ± 4 eV relevant to conditions for the first ∼1 ns of ignition experiments. By varying the relative ratio of the energy between inner and outer beam cones illuminating the hohlraum at OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. the shape of the x-ray self-emission, and hence the shape of the emitting object, can be tuned from prolate to oblate. The second-order Legendre coefficient, used to characterize the shape, changes from a negative to a positive value at the time of peak x-ray emission during the implosion through the variation of the cone power balance. With the appropriate selection of the cone power balance, the implosion can be tuned to p...

Modeling ion interpenetration, stagnation, and thermalization in colliding plasmas

Ion interpenetration, stagnation, and energization processes are studied in colliding laser‐produced plasma configurations relevant to Trident [R. G. Watt, Rev. Sci. Instrum. 64, 1770 (1993)] experiments using four different numerical methods: one‐dimensional Monte Carlo and Lagrangian multifluid codes, and one‐ and two‐dimensional hybrid (particle ions, fluid electrons) and single‐fluid Lagrangian codes. Results from the four methodologies are compared for plasmas generated with gold and deuterated polyethylene (CD2) targets. Overall, the various codes give similar results concerning the initial expansion of the plasmas and their collisional interaction, the degree of stagnation, stagnation time, and amount of ion thermalization for gold targets, while multispecies techniques indicate a much softer stagnation for CD2 plasmas than the single‐fluid model. Variations in the results of the calculations due to somewhat different initializations and parameters, as well as to different physics in the codes, are...

Sensitivity of ignition scale backlit thin-shell implosions to hohlraum symmetry in the foot of the drive pulse

A necessary condition for igniting indirectly driven inertial confinement fusion spherical capsules on the National Ignition Facility (NIF) is controlling drive flux asymmetry to the 1% level time-integrated over the pulse and with <10%∕ns swings during the pulse [J. D. Lindl, P. Amendt, R. L. Berger et al., Phys. Plasmas 11, 339 (2003)]. While drive symmetry during the first 2ns of the pulse can be inferred by using the re-emission pattern from a surrogate high Z sphere and symmetry during the last 5ns inferred from the shape of fully imploded capsules, the midportion (≈2–10ns) has been shown to be amenable to detection by the in-flight shape of x-ray backlit thin-shell capsules. In this paper, we present sensitivity studies conducted on the University of Rochester’s OMEGA laser [J. Soures, R. L. McCrory, C. P. Verdon et al., Phys. Plasmas 3, 2108 (1996)] of the thin-shell symmetry measurement technique at near NIF-scale for two candidate capsule ablator materials: Ge-doped CH and Cu-doped Be. These expe...

Gas-filled hohlraum experiments at the National Ignition Facility

Experiments done at the National Ignition Facility laser [J. A. Paisner, E. M. Campbell, and W. Hogan, Fusion Technol. 26, 755 (1994)] using gas-filled hohlraums demonstrate a key ignition design feature, i.e., using plasma pressure from a gas fill to tamp the hohlraum-wall expansion for the duration of the laser pulse. Moreover, our understanding of hohlraum energetics and the ability to predict the hohlraum soft-x-ray drive has been validated in ignition-relevant conditions. Finally, the laser reflectivity from stimulated Raman scattering in the fill plasma, a key threat to hohlraum performance, is shown to be suppressed by choosing a design with a sufficiently high ratio of electron temperature to density.

Production of enhanced pressure regions due to inhomogeneities in inertial confinement fusion targets

It is shown that regions of enhanced pressure have been produced in targets with indirect radiation drive in planar and cylindrically convergent geometry through the interaction between the flows caused by target inhomogeneities and the main target drive. Design calculations for National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] targets with beryllium ablators formed by bonded hemi-shells [D. C. Wilson et al., Bull. Am. Phys. Soc. 43, 1667 (1998)] indicate that related behavior produces a seed perturbation in the ablator which can in some cases lead to the suppression of ignition. From simulation and analysis of the NIF problem in the planar geometry analog, a scaling for the perturbation, which should be useful for validation of the behavior with lower energy drive and smaller-scale geometries, is derived.

Radiation hydrodynamic effects in two beryllium plates with an idealized aluminum joint

A beryllium capsule formed from two hemispherical shells with a thin bond is one possible ignition target for the National Ignition Facility [J. A. Paisner et al., Laser Focus World 30, 75 (1994)] Nonuniformities in density, opacity, and interface position at the joint between these hemishells will initiate two-dimensional (2-D) perturbations of the shock wave and material behind the shock as the shock passes through the shell perpendicular to the joint width. Rarefaction of material flow behind the shock front can cause the interface between the shell and joint material to oscillate in position. The amplitude of these oscillations may be comparable to the joint width. The evolution of these perturbations is studied by numerically simulating shock passage through flat beryllium plates containing aluminum joints. Using the MIMOSA-ND code [D. Sofronov et al., Vopr. At. Nauki Tekh., Ser: Mat. modelirovanie fizicheskih processov 2, 3 (1990)] two different cases are calculated—a wide (10 μm) and a narrow (1 μm...

Foam-buffered laser-matter interactions

Recent experiments indicate that low-density foam buffer layers can significantly mitigate the perturbing effects of beam non-uniformities in direct drive laser-matter interactions. Results of a computational study with a 2D ALE code are reported here. Typical targets consisted of 50 {micro}m of 50 mg/cm{sup 3} C{sub 10}H{sub 8}O{sub 4} foam attached to a 10 {micro}m foil and covered with 250 {angstrom} of gold. These targets were exposed to {approximately} 1.2 ns, flat topped, green light pulses at {approximately} 1.4 {times} 10{sup 14} W/cm{sup 2} intensity, bearing 30 {micro}m lateral perturbations. Without the buffer layers the foils were severely disrupted after 1 ns of laser illumination. Buffering could provide stability for more than 2 ns of full shell acceleration. This study shows that the high thermal conductivity of the foam results in flattened shocks in the foam plasma, communicating a smoothed laser drive to the accelerated shells. Preheat from the gold hastens conversion of solid foam to the smoothing heated plasma.

INERTIAL CONFINEMENT FUSION RESEARCH AT LOS ALAMOS NATIONAL LABORATORY

Inertial confinement fusion research at Los Alamos National Laboratory is focused on high‐leverage areas of thermonuclear ignition to which LANL can apply its historic strengths and that are complementary to high‐energy‐density‐physics topics. Using the Trident and Omega laser facilities, experiments are pursued in laser‐plasma instabilities, symmetry, Be technologies, neutron and fusion‐product diagnostics, and defect hydrodynamics.