Mordecai D. Rosen

Design and modeling of ignition targets for the National Ignition Facility

Several targets are described that in simulations give yields of 1–30 MJ when indirectly driven by 0.9–2 MJ of 0.35 μm laser light. The article describes the targets, the modeling that was used to design them, and the modeling done to set specifications for the laser system in the proposed National Ignition Facility. Capsules with beryllium or polystyrene ablators are enclosed in gold hohlraums. All the designs utilize a cryogenic fuel layer; it is very difficult to achieve ignition at this scale with a noncryogenic capsule. It is necessary to use multiple bands of illumination in the hohlraum to achieve sufficiently uniform x‐ray irradiation, and to use a low‐Z gas fill in the hohlraum to reduce filling of the hohlraum with gold plasma. Critical issues are hohlraum design and optimization, Rayleigh–Taylor instability modeling, and laser–plasma interactions.

Hydrodynamics of exploding foil x‐ray lasers

An accurate simple model for the hydrodynamics of laser heated exploding foils is presented. Particular emphasis is given to applications in the design of soft x‐ray lasers. The model predicts the conditions in the foil plasma (e.g., temperature, density, and scale length), given the experimental parameters (e.g., optical laser intensity, laser pulse duration, target thickness, and target composition). The simple model is based on an isothermal, homogeneous expansion similarity solution of the ideal hydrodynamic equations. Both analytical and numerical solutions of the similarity equations are studied. The numerical solutions agree closely with computational hydrodynamic simulations at times of interest—after the laser burns through the foil. Analytic solutions for constant intensity laser irradiation provide useful power‐law scaling relations between the input laser and target parameters and the plasma variables. The simple model is a powerful design tool that reproduces the essential results of more expensive and time‐consuming simulations over a large and important range of parameter space.

The interaction of 1.06 μm laser radiation with high Z disk targets

Gold disks have been irradiated with 1.06 μm laser light at intensities between 7 × 1013 and 3 × 1015 W/cm2, and pulse lengths between 200 and 1000 psec. Due to the high Z and long pulse, inverse bremsstrahlung becomes an important absorption mechanism and competes strongly with resonance absorption and stimulated scattering. In addition to measured absorptions, data on the temporal, spatial, angular, and spectral characteristics of the x‐ray emission are presented. Temporally and spectrally resolved back‐reflected light, and polarization‐dependent sidescattered light are detected, providing estimates for the amount of stimulated scattering and of the coronal electron temperature. Inhibited electron thermal conduction and nonlocal thermodynamic equilibrium ionization physics play key roles in bringing numerical simulations of these experiments into agreement with all of the above‐mentioned data.

Laser irradiation of disk targets at 0.53 μm wavelength

Results and analyses are presented for laser irradiation of Be‐, CH‐, Ti‐, and Au‐disk targets with 0.53 μm light in 3–200 J, 600–700 psec pulses, at nominal incident intensities from 3×1013 to 5×1015 W/cm2. The measured absorptions are higher than observed in similar 1.06 μm irradiations, and are largely consistent with modeling which shows the importance of inverse‐bremsstrahlung and Brillouin scattering. Observed red‐shifted back‐reflected light shows that Brillouin scattering occurs at low to moderate levels. Backscattering fractions up to 30% were observed in the f/2 focusing lens. The measured fluxes of multi‐keV x rays indicate hot‐electron fractions of 1% or less, with temperatures of 6 to 20 keV which are consistent with resonance absorption or perhaps 2ωpe. Measurements show 30%–50% efficient conversion of absorbed light into sub‐keV x rays, with time‐, angular‐, and spatial‐emission distributions which are generally consistent with non‐local‐thermodynamic‐equilibrium modeling using inhibited th...

Wavelength choice for soft x-ray laser holography of biological samples

The choice of an optimal wavelength for soft x-ray holography is discussed, based on a description of scattering by biological structures within an aqueous environment. We conclude that wavelengths slightly longer than the 43.7-A carbon K-edge provide a good trade off between minimizing the necessary source power and the dose absorbed by the sample and maximizing the penetrability of the x-rays through wet samples. This differs from the previous notion that wavelengths within the water window (between 23.2 A and 43.7 A) would be the best for holography. The problem of motion resulting from the absorption of x rays during a short exposure is described. The possibility of using ultrashort exposures in order to capture the image before motion can compromise the resolution is explored. The impact of these calculations on the question of the feasibility of using an x-ray laser for holography of biological structures is discussed.

X-ray laser research at the Lawrence Livermore National Laboratory Nova laser facility

We describe our optical-laser-pumped x-ray laser program. Our long-term goal is to develop and utilize a fully coherent, gigowatt-power-level sub-44-A laser. To this end we have been studying the characteristics of the exploding-foil amplifier coupled with various inversion schemes: Ne-like and Ni-like collisional excitation as well as H-like three-body recombination. Most of our experimental results to date are for the Ne-like schemes; we have observed ~15 laser transitions in Se, Y, and Mo having wavelengths from 26.3 to 10.6 nm. Output power to at least 1 MW has been observed for the Se J = 2 to 1 lines at 20.6 and 20.9 A along with geometrical divergence patterns for the beam. We have also observed time-dependent beam refraction from these amplifiers and have been able to demonstrate double-pass amplification by using a multilayer mirror operated at normal incidence. Future plans for improving beam coherence and producing lasing at wavelengths shorter than 44 A are discussed.

The physics issues that determine inertial confinement fusion target gain and driver requirements: A tutorial

This paper presents a simplified, tutorial approach to determining the gains of inertial confinement fusion (ICF) targets, via a basic, zero-dimensional (“0-D”), energy “bookkeeping” of input (parametrized by ICF drivers’ coupling efficiencies to the target, and subsequent hydrodynamic efficiencies of implosion) versus output (thermonuclear burn efficiency and target fuel mass). Physics issues/constraints such as hydrodynamic instabilities, symmetry and implosion velocity requirements will be discussed for both the direct drive (driver impinging directly on the target) and indirect drive (x-ray implosion within a driver heated hohlraum) approaches to ICF. Supplementing the 0-D model with simple models for hohlraum wall energy loss (to predict coupling efficiencies) and a simple one-dimensional (1-D) model of the implosion as a spherical rocket (to predict hydrodynamic implosion efficiencies) allows gains to be predicted that compare well with the results of complex two-dimensional (2-D) radiation hydrodyn...

Observation of soft x‐ray amplification in neonlike molybdenum

Thin molybdenum coated foils have been irradiated in line focus geometry with from 3 to 8×1014 W cm−2 of 0.53‐μm light at the Nova laser. The resulting exploding foil plasma has demonstrated x‐ray laser gain at four wavelengths (106.4, 131.0, 132.7, and 139.4 A), identified as 3s‐3p transitions in neonlike Mo. The J=0–1, a 3s–3p transition at 141.6 A has been identified, but does not show evidence of significant gain in disagreement with the theory.

X-ray conversion efficiency of high-Z hohlraum wall materials for indirect drive ignition

The conversion efficiency of 351nm laser light to soft x rays (0.1–5keV) was measured for Au, U, and high Z mixture “cocktails” used as hohlraum wall materials in indirect drive fusion experiments. For the spherical targets in a direct drive geometry, flattop laser pulses and laser smoothing with phase plates are employed to achieve constant and uniform laser intensities of 1014 and 1015W∕cm2 over the target surface that are relevant for the future ignition experiments at the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion 44, 228 (2004)]. The absolute time and spectrally resolved radiation flux is measured with a multichannel soft x-ray power diagnostic. The conversion efficiency is then calculated by dividing the measured x-ray power by the incident laser power from which the measured laser backscattering losses are subtracted. After ∼0.5ns, the time resolved x-ray conversion efficiency reaches a slowly increasing plateau of 95% at 1014W∕cm2 laser intensity and of 80...

The science applications of the high‐energy density plasmas created on the Nova laser

Since the late 1970s it has been realized that the laser‐heated hohlraums envisioned for indirect drive Inertial Confinement Fusion (ICF) could also serve as ‘‘physics factories’’ by providing a high‐energy density environment for the study of a wide variety of physics with important applications. In this review we will describe some of these studies, accomplished in the early 1990s using the Nova laser [J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 (1989)] at the Lawrence Livermore National Laboratory. They include measuring the opacity of Fe, thus confirming that the OPAL low Z opacity code [C. A. Iglesias and F. J. Rogers, Astrophys. J. 443, 460 (1995)] is quantitatively more accurate than ‘‘standard’’ models, with important astrophysical implications such as modeling the Cepheid variables [F. J. Rogers and C. A. Iglesias, Science 263, 50 (1994)]; measuring the Rosseland mean opacity of Au, confirming the correctness of the ‘‘Super Transition Array’’ (STA) high‐Z code [Bar Shalom et al., Phys. Rev. A 40, 3183 (1989)] with important implications for ignition targets designed for the National Ignition Facility (NIF); sophisticated Rayleigh–Taylor and other hydrodynamic turbulence experiments and analysis that serve as a test bed for understanding astrophysical observations such as supernova explosions; using laboratory x‐ray lasers for probing high‐density ICF plasmas as well as biology; and creating near Gbar pressures [Cauble et al. Phys. Rev. Lett. 70, 2102 (1993)]. Expanded opportunities for such research on the NIF will also be described.Since the late 1970s it has been realized that the laser‐heated hohlraums envisioned for indirect drive Inertial Confinement Fusion (ICF) could also serve as ‘‘physics factories’’ by providing a high‐energy density environment for the study of a wide variety of physics with important applications. In this review we will describe some of these studies, accomplished in the early 1990s using the Nova laser [J. T. Hunt and D. R. Speck, Opt. Eng. 28, 461 (1989)] at the Lawrence Livermore National Laboratory. They include measuring the opacity of Fe, thus confirming that the OPAL low Z opacity code [C. A. Iglesias and F. J. Rogers, Astrophys. J. 443, 460 (1995)] is quantitatively more accurate than ‘‘standard’’ models, with important astrophysical implications such as modeling the Cepheid variables [F. J. Rogers and C. A. Iglesias, Science 263, 50 (1994)]; measuring the Rosseland mean opacity of Au, confirming the correctness of the ‘‘Super Transition Array’’ (STA) high‐Z code [Bar Shalom et al., Phys. Rev. A 4...