Rayleigh–Taylor and Richtmyer-Meshkov instability induced flow, turbulence, and mixing. II

The direct-drive, laser-based approach to inertial confinement fusion (ICF) is reviewed from its inception following the demonstration of the first laser to its implementation on the present generation of high-power lasers. The review focuses on the evolution of scientific understanding gained from target-physics experiments in many areas, identifying problems that were demonstrated and the solutions implemented. The review starts with the basic understanding of laser–plasma interactions that was obtained before the declassification of laser-induced compression in the early 1970s and continues with the compression experiments using infrared lasers in the late 1970s that produced thermonuclear neutrons. The problem of suprathermal electrons and the target preheat that they caused, associated with the infrared laser wavelength, led to lasers being built after 1980 to operate at shorter wavelengths, especially 0.35 μm—the third harmonic of the Nd:glass laser—and 0.248 μm (the KrF gas laser). The main physics areas relevant to direct drive are reviewed. The primary absorption mechanism at short wavelengths is classical inverse bremsstrahlung. Nonuniformities imprinted on the target by laser irradiation have been addressed by the development of a number of beam-smoothing techniques and imprint-mitigation strategies. The effects of hydrodynamic instabilities are mitigated by a combination of imprint reduction and target designs that minimize the instability growth rates. Several coronal plasma physics processes are reviewed. The two-plasmon–decay instability, stimulated Brillouin scattering (together with cross-beam energy transfer), and (possibly) stimulated Raman scattering are identified as potential concerns, placing constraints on the laser intensities used in target designs, while other processes (self-focusing and filamentation, the parametric decay instability, and magnetic fields), once considered important, are now of lesser concern for mainline direct-drive target concepts. Filamentation is largely suppressed by beam smoothing. Thermal transport modeling, important to the interpretation of experiments and to target design, has been found to be nonlocal in nature. Advances in shock timing and equation-of-state measurements relevant to direct-drive ICF are reported. Room-temperature implosions have provided an increased understanding of the importance of stability and uniformity. The evolution of cryogenic implosion capabilities, leading to an extensive series carried out on the 60-beam OMEGA laser [Boehly et al., Opt. Commun. 133, 495 (1997)], is reviewed together with major advances in cryogenic target formation. A polar-drive concept has been developed that will enable direct-drive–ignition experiments to be performed on the National Ignition Facility [Haynam et al., Appl. Opt. 46(16), 3276 (2007)]. The advantages offered by the alternative approaches of fast ignition and shock ignition and the issues associated with these concepts are described. The lessons learned from target-physics and implosion experiments are taken into account in ignition and high-gain target designs for laser wavelengths of 1/3 μm and 1/4 μm. Substantial advances in direct-drive inertial fusion reactor concepts are reviewed. Overall, the progress in scientific understanding over the past five decades has been enormous, to the point that inertial fusion energy using direct drive shows significant promise as a future environmentally attractive energy source.

Direct-drive inertial confinement fusion: A review

The Z accelerator [R. B. Spielman, W. A. Stygar, J. F. Seamen et al., Proceedings of the 11th International Pulsed Power Conference, Baltimore, MD, 1997, edited by G. Cooperstein and I. Vitkovitsky (IEEE, Piscataway, NJ, 1997), Vol. 1, p. 709] at Sandia National Laboratories delivers ∼20MA load currents to create high magnetic fields (>1000T) and high pressures (megabar to gigabar). In a z-pinch configuration, the magnetic pressure (the Lorentz force) supersonically implodes a plasma created from a cylindrical wire array, which at stagnation typically generates a plasma with energy densities of about 10MJ∕cm3 and temperatures >1keV at 0.1% of solid density. These plasmas produce x-ray energies approaching 2MJ at powers >200TW for inertial confinement fusion (ICF) and high energy density physics (HEDP) experiments. In an alternative configuration, the large magnetic pressure directly drives isentropic compression experiments to pressures >3Mbar and accelerates flyer plates to >30km∕s for equation of state ...

Pulsed-Power-Driven High Energy Density Physics and Inertial Confinement Fusion Research

Pulsed-power-driven high energy density physics and inertial confinement fusion research

Pressure- and temperature-induced phase transitions have been studied for more than a century but very little is known about the non-equilibrium processes by which the atoms rearrange. Shock compression generates a nearly instantaneous propagating high-pressure/temperature condition while in situ X-ray diffraction (XRD) probes the time-dependent atomic arrangement. Here we present in situ pump-probe XRD measurements on shock-compressed fused silica, revealing an amorphous to crystalline high-pressure stishovite phase transition. Using the size broadening of the diffraction peaks, the growth of nanocrystalline stishovite grains is resolved on the nanosecond timescale just after shock compression. At applied pressures above 18 GPa the nuclueation of stishovite appears to be kinetically limited to 1.4±0.4 ns. The functional form of this grain growth suggests homogeneous nucleation and attachment as the growth mechanism. These are the first observations of crystalline grain growth in the shock front between low- and high-pressure states via XRD.

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Ultrafast visualization of crystallization and grain growth in shock-compressed SiO2.

The current capsule target design for the first ignition experiments at the NIF Facility beginning in 2009 will be a copper-doped beryllium capsule, roughly 2 mm in diameter with 160-µm walls. The capsule will have a 75-µm layer of solid deuterium-tritium on the inside surface, and the capsule will be powered by X-rays generated from a gold/uranium cocktail hohlraum. The design specifications are extremely rigorous, particularly with respect to interfaces, which must be very smooth to inhibit Rayleigh-Taylor instability growth. This paper outlines the current design, and focuses on the challenges and advances in capsule fabrication and characterization; hohlraum fabrication, and deuterium-tritium layering and characterization.

National Ignition Facility target design and fabrication

All planned National Ignition Facility (NIF) capsule targets except machined beryllium require a glow discharge polymer (GDP) mandrel upon which the ablator is applied. This mandrel, {approx}2 mm in diameter, must at least meet if not exceed the symmetry and surface finish requirements of the final capsule. Such mandrels are currently produced by the three-step depolymerizable mandrel technique. The quality of the final mandrel depends upon precise optimization and execution of each of the three steps. We had shown previously that fabrication of a mandrel which met the symmetry and surface finish requirements was feasible using this technique. In this paper we will discuss recent progress towards converting this process into a high yield, production scale process.

/pdf/progress-in-2-mm-glow-discharge-polymer-mandrel-development-4khlth0bmc.pdf

Progress in 2 mm Glow Discharge Polymer Mandrel Development for NIF

OAK-B135 New high gain designs for direct drive ignition on NIF require foam shells. Scaled down versions of these designs are needed for near term experiments on the OMEGA laser facility at the Laboratory Laser Energetics (LLE). These shells need to be about 1 mm in diameter and 50-100 {micro}m wall thickness and densities of 100-250 mg/cc. In addition, a full density permeation seal needs to be deposited for retention of the fill gas at room temperature or the ice at cryogenic temperatures. They have fabricated such shells using Resorcinol-formaldehyde (R/F) as the selected foam material due to its transparency in the optical region. Extensive characterization of the wall uniformity of these shells has been performed. The foam shells have {approx} 5%-6% non-concentricities on the average. A full density permeation seal has been deposited on the R/F shells using two different techniques. In the first technique R/F shells are coated directly with plasma polymer to thicknesses of 3-4 {micro}m. In the second technique, R/F shells are coated with polyvinylphenol, using a chemical interfacial polymerization technique. Data on surface finish and gas retention for R/F shells coated by both methods are provided.

https://web.gat.com/pubs-ext/TFSM03/A24451.pdf

Fabrication and properties of overcoated resorcinol-formaldehyde shells for omega experiments

Previously we have developed a production process for both standard density (100 mg/cc) and high-density (180-200 mg/cc) resorcinol formaldehyde (RF) foam shells with a triple orifice droplet generator. These foam shells are needed for direct drive inertial confinement laser fusion experiments on the OMEGA laser facility at the University of Rochester. Although this process has been developed into production mode, the yield of high density RF (HDRF) and standard density (SDRF) shells with acceptable wall uniformity has been poor. This yield depends on the type of RF shell that is being fabricated. For HDRF this yield is ~5% while for the SDRF shells the yield is ~30%. We have made improvements in the yield of these shells that meet the wall uniformity specification by modifying the composition of the outer oil solution (O2) in the microencapsulation emulsion. This improvement was achieved by a small addition (0.60 wt.%) of a styrene-butadiene-styrene (SBS) block copolymer into the outer oil (O2) s...

https://web.gat.com/pubs-ext/TFSM06/A25652.pdf

Improving the wall uniformity of resorcinol formaldehyde foam shells by modifying emulsion components

Recently, we developed a new method for making spherical poly(α-methylstyrene) (PαMS) mandrels. The process utilizes a small amount (<0.1wt%) of high-molecular-weight poly(acrylic acid) (PAA) in th...

M. Takagi

Papers

National Ignition Facility target design and fabrication

Progress in 2 mm Glow Discharge Polymer Mandrel Development for NIF

Fabrication and properties of overcoated resorcinol-formaldehyde shells for omega experiments

Improving the wall uniformity of resorcinol formaldehyde foam shells by modifying emulsion components

Development of High Quality Poly(α-Methylstyrene) Mandrels for NIF