C. J. Keane

The National Ignition Facility: Ushering in a new age for high energy density science

The National Ignition Facility (NIF) [E. I. Moses, J. Phys.: Conf. Ser.112, 012003 (2008); https://lasers.llnl.gov/], completed in March 2009, is the highest energy laser ever constructed. The high temperatures and densities achievable at NIF will enable a number of experiments in inertial confinement fusion and stockpile stewardship, as well as access to new regimes in a variety of experiments relevant to x-ray astronomy, laser-plasma interactions, hydrodynamic instabilities, nuclear astrophysics, and planetary science. The experiments will impact research on black holes and other accreting objects, the understanding of stellar evolution and explosions, nuclear reactions in dense plasmas relevant to stellar nucleosynthesis, properties of warm dense matter in planetary interiors, molecular cloud dynamics and star formation, and fusion energy generation.

Three-dimensional simulations of Nova high growth factor capsule implosion experiments

Capsule implosion experiments carried out on the Nova laser [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] are simulated with the three‐dimensional HYDRA radiation hydrodynamics code [NTIS Document No. DE‐96004569 (M. M. Marinak et al. in UCRL‐LR‐105821‐95‐3)]. Simulations of ordered, near single mode perturbations indicate that structures which evolve into round spikes can penetrate farthest into the hot spot. Bubble‐shaped perturbations can burn through the capsule shell fastest, in which case they cause even more damage. A simulation of a capsule with a multimode perturbation of moderate amplitude shows spike amplitudes evolving in good agreement with a saturation model during the deceleration phase. The presence of sizable low mode asymmetry, caused either by drive asymmetry or perturbations in the capsule shell, can dramatically affect the manner in which spikes approach the center of the hot spot. Three‐dimensional coupling between the low mode shell perturbations intrinsic to Nova caps...

Short wavelength x-ray laser research at the Lawrence Livermore National Laboratory*

Laboratory x‐ray lasers are currently being studied by researchers worldwide. This paper reviews some of the recent work carried out at Lawrence Livermore National Laboratory. Laser action has been demonstrated at wavelengths as short as 35.6 A while saturation of the small signal gain has been observed with longer wavelength schemes. Some of the most successful schemes to date have been collisionally pumped x‐ray lasers that use the thermal electron distribution within a laser‐produced plasma to excite electrons from closed shells in neon‐ and nickel‐like ions to metastable levels in the next shell. Attempts to quantify and improve the longitudinal and transverse coherence of collisionally pumped x‐ray lasers are motivated by the desire to produce sources for specific applications. Toward this goal there is a large effort underway to enhance the power output of the Ni‐like Ta x‐ray laser at 44.83 A as a source for x‐ray imaging of live cells. Improving the efficiency of x‐ray lasers in order to produce s...

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.

Soft X-ray laser source development and applications experiments at Lawrence Livermore National Laboratory

Recent progress in experimental laboratory soft X-ray laser research at Lawrence Livermore National Laboratory (LLNL) is reviewed. Research at LLNL in this area has concentrated on further characterising and understanding neon-like X-ray laser plasmas, investigating soft X-ray amplification at shorter wavelengths, and demonstrating examples of X-ray laser applications. For the standard 200 AA neon-like selenium collisional excitation laser, the output source size as well as the beam time history, divergence, energy and spatial profile have been measured. Gain has been demonstrated at wavelengths as short as 50.3 AA in nickel-like ytterbium. Several recombination X-ray laser schemes have also been investigated. X-ray laser holography, cavity operation of an X-ray laser, and the capability to point and focus the output laser beam have been demonstrated.

X-ray spectroscopy of high-energy density inertial confinement fusion plasmas

Analysis is presented of K‐ and L‐shell spectra obtained from Ar and Xe dopants seeded into the fuel region of plastic capsules indirectly imploded using the Nova laser. Stark broadening measurements of the n=3‐1 lines in H‐ and He‐like Ar (Ar Ly‐β and He‐β, respectively) are used to infer fuel electron density, while spatially averaged fuel electron temperature is deduced from the ratio of the intensities of these lines. Systematic variations in Ar spectral features are observed as a function of drive conditions. A spectral postprocessing code has been developed to simulate experimental spectra by taking into account spatial gradients and line transfer effects, and shows good agreement with experimental data. It is shown that correct modeling of the x‐ray emission requires a proper treatment of the coupled radiative transfer and kinetics problem. Continuum lowering effects are shown not to affect diagnostic line ratios, within the confines of a simple model. A recently developed diagnostic based on fitti...

Soft-x-ray amplification at 50.3 Å in nickellike ytterbium

Nickellike ions of ytterbium (Yb42+) have been produced in long exploding foil plasmas. The plasmas were formed by short-pulse laser irradiation of thin foils of ytterbium with line-focused high-intensity 0.53-μm light. Soft-x-ray line emissions at 50.26 and 56.09 A are identified as 4d–4p, J = 0−1 transitions in the nickellike ionization stage of ytterbium. The energy emitted in the line at 50.26 A along the axis of the plasma is observed to increase nonlinearly with plasma length, consistent with a small-signal gain of 1.2 ± 0.4 cm−1.

K‐ and L‐shell x‐ray spectroscopy of indirectly driven implosions (invited)

Time‐resolved x‐ray spectroscopy is used to study the implosion of indirectly driven inertial confinement fusion capsules on the Nova laser. Through the use of high‐Z dopants (Ar and Xe) in the fuel, measurements of the peak temperature, from emission line ratios, and density, from line broadening, are obtained. These measurements indicate peak electron temperatures of ∼1–1.6 keV and electron (and deuteron) densities in the range of 1.0–2.0×1024 cm−3, depending on the type of laser drive used. The higher densities are achieved on targets that are driven with a shaped laser drive that allows a more isentropic compression of the fuel. Emission from high‐Z pusher dopants have also been studied. These dopants can provide information on pusher conditions and can be used to study mix at the pusher fuel interface.

Density and temperature diagnostic based on the Ar He β line and associated Li‐like satellites

We have modeled the temperature and density dependence of the Li‐like satellites of the Ar He β line by performing NLTE kinetic modeling of level populations in conjunction with Stark broadening calculations. Composite line profiles are computed including resonance and satellite line transitions that have built‐in the temperature and density dependence characteristic of the level populations and Stark broadening of these transitions. These synthetic spectra can be used to analyze experimental data, providing a simultaneous diagnostic of temperature and density.

Fusion neutrons from the gas–pusher interface in deuterated-shell inertial confinement fusion implosions

The first measurements and numerical simulations of fusion neutrons from the gas–pusher interface of indirectly-driven inertial confinement fusion implosions have been performed using hydrogen-filled capsules made with a deuterated inner layer. Nonlinear saturation of the growth of hydrodynamic perturbations in high linear growth factor (≃325) implosions was varied by adjusting the initial surface roughness of the capsule. The neutron yields are in quantitative agreement with the direct simulations of perturbation growth, and also with a linear mode superposition and saturation model including enhanced thermal loss in the mixed region. Neutron spectra from these capsules are broader than expected for the calculated ion temperatures, suggesting the presence of nonthermal broadening from mass motion during the fusion burn.