to be done in this area. Face recognition is a problem that scarcely clamoured for attention before the computer age but, having surfaced, has involved a wide range of techniques and has attracted the attention of some fine minds (David Mumford was a Fields Medallist in 1974). This singular application of mathematical modelling to a messy applied problem of obvious utility and importance but with no unique solution is a pretty one to share with students: perhaps, returning to the source of our opening quotation, we may invert Duncan's earlier observation, 'There is an art to find the mind's construction in the face!'.

Linear and nonlinear waves, by G. B. Whitham. Pp.636. £50. 1999. ISBN 0 471 35942 4 (Wiley).

Physics of shock waves and high-temperature hydrodynamic phenomena - Ya.B. Zeldovich and Yu.P. Raizer (translated from the Russian and then edited by Wallace D. Hayes and Ronald F. Probstein); Dover Publications, New York, 2002, 944 pp., $34.

Ignition is needed to make fusion energy a viable alternative energy source, but has yet to be achieved. A key step on the way to ignition is to have the energy generated through fusion reactions in an inertially confined fusion plasma exceed the amount of energy deposited into the deuterium-tritium fusion fuel and hotspot during the implosion process, resulting in a fuel gain greater than unity. Here we report the achievement of fusion fuel gains exceeding unity on the US National Ignition Facility using a 'high-foot' implosion method, which is a manipulation of the laser pulse shape in a way that reduces instability in the implosion. These experiments show an order-of-magnitude improvement in yield performance over past deuterium-tritium implosion experiments. We also see a significant contribution to the yield from α-particle self-heating and evidence for the 'bootstrapping' required to accelerate the deuterium-tritium fusion burn to eventually 'run away' and ignite.

https://materias.df.uba.ar/fc1a2014c1/files/2014/05/Hurricane-2014-Nature-Fuel-gain-exceeding-unity-in-an-inertially-confined-fusion-implosion.pdf

Fuel gain exceeding unity in an inertially confined fusion implosion

With the advent of high-energy-density (HED) experimental facilities, such as high-energy lasers and fast Z-pinch, pulsed-power facilities, millimeter-scale quantities of matter can be placed in extreme states of density, temperature, and/or velocity. This has enabled the emergence of a new class of experimental science, HED laboratory astrophysics, wherein the properties of matter and the processes that occur under extreme astrophysical conditions can be examined in the laboratory. Areas particularly suitable to this class of experimental astrophysics include the study of opacities relevant to stellar interiors, equations of state relevant to planetary interiors, strong shock-driven nonlinear hydrodynamics and radiative dynamics relevant to supernova explosions and subsequent evolution, protostellar jets and high Mach number flows, radiatively driven molecular clouds and nonlinear photoevaporation front dynamics, and photoionized plasmas relevant to accretion disks around compact objects such as black holes and neutron stars.

/pdf/experimental-astrophysics-with-high-power-lasers-and-z-2ws0p4rgie.pdf

Experimental astrophysics with high power lasers and Z pinches

Accurate x-ray scattering techniques to measure the physical properties of dense plasmas have been developed for applications in high energy density physics. This class of experiments produces short-lived hot dense states of matter with electron densities in the range of solid density and higher where powerful penetrating x-ray sources have become available for probing. Experiments have employed laser-based x-ray sources that provide sufficient photon numbers in narrow bandwidth spectral lines, allowing spectrally resolved x-ray scattering measurements from these plasmas. The backscattering spectrum accesses the noncollective Compton scattering regime which provides accurate diagnostic information on the temperature, density, and ionization state. The forward scattering spectrum has been shown to measure the collective plasmon oscillations. Besides extracting the standard plasma parameters, density and temperature, forward scattering yields new observables such as a direct measure of collisions and quantum effects. Dense matter theory relates scattering spectra with the dielectric function and structure factors that determine the physical properties of matter. Applications to radiation-heated and shock-compressed matter have demonstrated accurate measurements of compression and heating with up to picosecond temporal resolution. The ongoing development of suitable x-ray sources and facilities will enable experiments in a wide range of research areas including inertial confinement fusion,more » radiation hydrodynamics, material science, or laboratory astrophysics.« less

X-ray Thomson scattering in high energy density plasmas

Summary form only given. Magnetically driven, Quasi Spherical Direct Drive (QSDD) fusion capsules potentially offer much higher efficiency implosions for inertial fusion energy than x-ray driven implosions with the comparable pulsed power generator. However, these solid liners must initiate uniformly to produce a quality implosion. Previous experiments by Stinnett, et al, showed that 20 to 200 nm thick aluminum liners on thick insulating substrates require a local action integral of ~6×108 (A/cm2)2-s or ~8×1016 A/cm2 rising in 7 ns to initiate uniformly. The underlying physical processes are complicated by the presence of ~3 monolayers of adsorbed gas and 7.5 nm of AlO2O3. Since QSDD liners are typically 100 micron thick beryllium, we have used a rad-hydro MHD code to analyze the initiation of both thin aluminum liners, to understand the underlying physics, and thick beryllium liners, to predict the drive requirements for uniform initiation of QSDD implosions. Both simulations had 3 monolayers of adsorbed hydrogen on the metal. The simulations provided the local electric field, density, and temperature profile of the metal and gas. The results were analyzed with J. C. Martin's multi-channel switching relationships. We infer that the metal must reach approximately 1 eV temperature before the gas, which is heated by thermal conduction, can expand to provide an alternative path for current channel formation through the gas. The results also suggest that an ignition-class QSDD capsule will need ~40 ns implosion times with a 40 MA drive pulse. A 2 micron polyimide coating may significantly improve the initiation of solid liners, as it has improved the initiation of wire arrays in work by D. B. Sinars, et. al., and by G. S. Sarkisov, et. al.

Initiation of Quasi Spherical Direct Drive capsules for inertial fusion

Summary form only given. The presence of Magneto-Rayleigh-Taylor (MRT) instabilities limit the achievable controlled compression of fusion fuel in pulsed power inertial confinement fusion concepts such as MagLIF [1]. It is therefore essential to not only mitigate the growth of MRT instabilities to the extent possible, but also validate simulation tools used to evaluate and assess MRT instability growth in these types of targets. Previously, we have shown single mode MRT experiments in solid Al liners that have proven extremely fruitful in benchmarking instability growth in several simulation codes [2]. It is also important to test our understanding and validate simulation predictions of multimode MRT growth. Computational studies of multimode perturbations have been done in the past with wire array z-pinch implosions [3]. However, heretofore no experiment has been performed to quantitatively study either linear or nonlinear multimode MRT instability growth in a controlled manner. In this paper, we discuss new simulations and experiments designed specifically to investigate multimode MRT instability growth. These experiments utilize x-ray radiography to measure the growth of two superimposed sinusoidal perturbations (400 gm and 550 gm in wavelength with peak to valley amplitudes of 20 gm) imposed on solid Al liners 292 gm thick with an initial outer radii of 3.16 mm. These experiments are also designed to evaluate MRT growth in the quasi-linear and nonlinear regimes. As such, we will also discuss effects such as the generation of higher order mode harmonics, mode saturation, and mode coupling.

Multi-mode Magneto-Rayleigh-Taylor instability growth experiments in pulsed power liners

Peak x-ray powers as high as 280 {+-} 40 TW have been generated from the implosion of tungsten wire arrays on the Z Accelerator at Sandia National Laboratories. The high x-ray powers radiated by these z-pinches provide an attractive new driver option for high yield inertial confinement fusion (ICF). The high x-ray powers appear to be a result of using a large number of wires in the array which decreases the perturbation seed to the magnetic Rayleigh-Taylor (MRT) instability and diminishes other 3-D effects. Simulations to confirm this hypothesis require a 3-D MHD code capability, and associated databases, to follow the evolution of the wires from cold solid through melt, vaporization, ionization, and finally to dense imploded plasma. Strong coupling plays a role in this process, the importance of which depends on the wire material and the current time history of the pulsed power driver. Strong coupling regimes are involved in the plasmas in the convolute and transmission line of the powerflow system. Strong coupling can also play a role in the physics of the z-pinch-driven high yield ICF target. Finally, strong coupling can occur in certain z-pinch-driven application experiments.

/pdf/the-role-of-strong-coupling-in-z-pinch-driven-approaches-to-4pf96owk98.pdf

The Role of Strong Coupling in Z-Pinch-Driven Approaches to High Yield Inertial Confinement Fusion

Summary form only given. In Z-pinch experiments on the Z accelerator, we are evaluating the feasibility of using a radiation-driven vacuum secondary hohlraum for indirect-drive ICF. To develop a system with the potential to scale to ignition and high yield, we must demonstrate a high degree of radiation flux symmetry at the capsule position. In initial experiments with single-sided illumination of the secondary, we have used a 5.0-mm-diameter low density foam ball imaged by a gated X-ray framing pinhole camera looking through a 6.5-mm tamped aperture to diagnose the large time-dependent asymmetry in polar radiation flux. For peak hohlraum radiation temperatures in the range of 70 100 eV, foams of about 50 mg/cc will support a transonic radiation front propagating with a sharp total transmission edge (combined foam self-emission and transmitted hohlraum wall backlight). The transmission edge can be tracked for comparison with radiation-hydrodynamics and 3D radiosity simulations.

Polar flux symmetry in Z-pinch-driven hohlraums using a self-backlit foam ball diagnostic

In Z-pinch implosions, K-shell radiation often comes from isolated bright spots, whose density and/or temperature significantly differ from the background plasma parameters [1] - [3] . The prominence of bright spots in many radiating materials raises questions about the applicability of the analytical [4] and numerical predictions of K-shell yields that do not accurately describe small-scale radiating structures. Our study was motivated by the spectroscopic analysis done at Weizmann [5] , indicating high densities and small sizes of K-shell emitting plasmas in wire-array loads on Z. We analyzed the data from argon gas-puff and stainless-steel wire-array shots on Z, which produced high yields in K-shell lines and the hard-photon continuum. We compared the ion densities inferred from the observed K-shell spectra using the CRE model with the average densities of the radiating plasmas. The Ar plasma was determined to be energy-rich, its average parameters being consistent with K-shell yields and spectra. On the contrary, the stainless-steel plasma is energy-poor. Its spectra indicate that the emission comes from bright spots whose density is much greater than the background plasma.

Roger Alan Vesey

Papers

Initiation of Quasi Spherical Direct Drive capsules for inertial fusion

Multi-mode Magneto-Rayleigh-Taylor instability growth experiments in pulsed power liners

The Role of Strong Coupling in Z-Pinch-Driven Approaches to High Yield Inertial Confinement Fusion

Polar flux symmetry in Z-pinch-driven hohlraums using a self-backlit foam ball diagnostic

Bright-Spot Contributions to Hardphoton Continuum K-Shell Yield from Argon and Stainless-Steel Load Implosions on Z