Validation of implosion modeling through direct-drive shock timing experiments at the National Ignition Facility.

Precise modeling of shocks in inertial confinement fusion implosions is critical for obtaining the desired compression in experiments. Shock velocities and postshock conditions are determined by laser-energy deposition, heat conduction, and equations of state. This paper describes experiments at the National Ignition Facility (NIF) [E. M. Campbell and W. J. Hogan, Plasma Phys. Control. Fusion 41, B39 (1999)10.1088/0741-3335/41/12B/303] where multiple shocks are launched into a cone-in-shell target made of polystyrene, using laser-pulse shapes with two or three pickets and varying on-target intensities. Shocks are diagnosed using the velocity interferometric system for any reflector (VISAR) diagnostic [P. M. Celliers et al., Rev. Sci. Instrum. 75, 4916 (2004)0034-674810.1063/1.1807008]. Simulated and inferred shock velocities agree well for the range of intensities studied in this work. These directly-driven shock-timing experiments on the NIF provide a good measure of early-time laser-energy coupling. The validated models add to the credibility of direct-drive-ignition designs at the megajoule scale.

Development of an x-ray radiography platform to study laser-direct-drive energy coupling at the National Ignition Facility.

A platform has been developed to study laser-direct-drive energy coupling at the National Ignition Facility (NIF) using a plastic sphere target irradiated in a polar-direct-drive geometry to launch a spherically converging shock wave. To diagnose this system evolution, eight NIF laser beams are directed onto a curved Cu foil to generate Heα line emission at a photon energy of 8.4 keV. These x rays are collected by a 100-ps gated x-ray imager in the opposing port to produce temporally gated radiographs. The platform is capable of acquiring images during and after the laser drive launches the shock wave. A backlighter profile is fit to the radiographs, and the resulting transmission images are Abel inverted to infer radial density profiles of the shock front and to track its temporal evolution. The measurements provide experimental shock trajectories and radial density profiles that are compared to 2D radiation-hydrodynamic simulations using cross-beam energy transfer and nonlocal heat-transport models.

The Scattered Light Time-history Diagnostic suite at the National Ignition Facility.

The Scattered Light Time-history Diagnostic (SLTD) is being implemented at the National Ignition Facility (NIF) to greatly expand the angular coverage of absolute scattered-light measurements for direct- and indirect-drive inertial confinement fusion (ICF) experiments. The SLTD array will ultimately consist of 15 units mounted at a variety of polar and azimuthal angles on the NIF target chamber, complementing the existing NIF backscatter suite. Each SLTD unit collects and diffuses scattered light onto a set of three optical fibers, which transport the light to filtered photodiodes to measure scattered light in different wavelength bands: stimulated Brillouin scattering (350 nm-352 nm), stimulated Raman scattering (430 nm-760 nm), and ω/2 (695 nm-745 nm). SLTD measures scattered light with a time resolution of ∼1 ns and a signal-to-noise ratio of up to 500. Currently, six units are operational and recording data. Measurements of the angular dependence of scattered light will strongly constrain models of laser energy coupling in ICF experiments and allow for a more robust inference of the total laser energy coupled to implosions.

Direct-drive laser fusion: status, plans and future.

Laser-direct drive (LDD), along with laser indirect (X-ray) drive (LID) and magnetic drive with pulsed power, is one of the three viable inertial confinement fusion approaches to achieving fusion ignition and gain in the laboratory. The LDD programme is primarily being executed at both the Omega Laser Facility at the Laboratory for Laser Energetics and at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. LDD research at Omega includes cryogenic implosions, fundamental physics including material properties, hydrodynamics and laser-plasma interaction physics. LDD research on the NIF is focused on energy coupling and laser-plasma interactions physics at ignition-scale plasmas. Limited implosions on the NIF in the 'polar-drive' configuration, where the irradiation geometry is configured for LID, are also a feature of LDD research. The ability to conduct research over a large range of energy, power and scale size using both Omega and the NIF is a major positive aspect of LDD research that reduces the risk in scaling from OMEGA to megajoule-class lasers. The paper will summarize the present status of LDD research and plans for the future with the goal of ultimately achieving a burning plasma in the laboratory. This article is part of a discussion meeting issue 'Prospects for high gain inertial fusion energy (part 2)'.

The Dynamic Compression Sector laser: A 100-J UV laser for dynamic compression research.

The Dynamic Compression Sector (DCS) laser is a 100-J ultraviolet Nd:glass system designed and built by the Laboratory for Laser Energetics for experimental research at the DCS located at the Advanced Photon Source (Argonne National Laboratory). Its purpose is to serve as a shock driver to study materials under extreme dynamic pressures. It was designed to deposit energy within a uniformly illuminated 500-µm spot on target, with additional optics provided to implement spot sizes of 250 and 1000 µm. Designed after larger-scale glass lasers such as OMEGA and the National Ignition Facility, the laser consists of a fiber front end with interferometer-based pulse shaping, a Nd:glass regenerative amplifier, a four-pass rod amplifier, and a 15-cm glass disk amplifier, through which six passes are made in a bowtie geometry. The output is frequency tripled from 1053 to 351 nm by using a pair of type-II phase-matched KDP crystals, with a third to increase conversion bandwidth. The super-Gaussian spot in the far field is achieved with a distributed phase plate and a 1-m aspherical focusing lens. Beam smoothing is achieved by smoothing by spectral dispersion and polarization smoothing, resulting in a root-mean-square variation in intensity on target of ±8.7%.

First Observation of Cross-Beam Energy Transfer Mitigation for Direct-Drive Inertial Confinement Fusion Implosions Using Wavelength Detuning at the National Ignition Facility.

Cross-beam energy transfer (CBET) results from two-beam energy exchange via seeded stimulated Brillouin scattering, which detrimentally reduces ablation pressure and implosion velocity in direct-drive inertial confinement fusion. Mitigating CBET is demonstrated for the first time in inertial-confinement implosions at the National Ignition Facility by detuning the laser-source wavelengths (±2.3 Å UV) of the interacting beams. We show that, in polar direct-drive, wavelength detuning increases the equatorial region velocity experimentally by 16% and alters the in-flight shell morphology. These experimental observations are consistent with design predictions of radiation-hydrodynamic simulations that indicate a 10% increase in the average ablation pressure.

Publisher's Note: Demonstration of Fuel Hot-Spot Pressure in Excess of 50 Gbar for Direct-Drive, Layered Deuterium-Tritium Implosions on OMEGA [Phys. Rev. Lett. 117, 025001 (2016)].

This corrects the article DOI: 10.1103/PhysRevLett.117.025001.

Demonstration of Fuel Hot-Spot Pressure in Excess of 50 Gbar for Direct-Drive, Layered Deuterium-Tritium Implosions on OMEGA.

A record fuel hot-spot pressure P_{hs}=56±7 Gbar was inferred from x-ray and nuclear diagnostics for direct-drive inertial confinement fusion cryogenic, layered deuterium-tritium implosions on the 60-beam, 30-kJ, 351-nm OMEGA Laser System. When hydrodynamically scaled to the energy of the National Ignition Facility, these implosions achieved a Lawson parameter ∼60% of the value required for ignition [A. Bose et al., Phys. Rev. E 93, 011201(R) (2016)], similar to indirect-drive implosions [R. Betti et al., Phys. Rev. Lett. 114, 255003 (2015)], and nearly half of the direct-drive ignition-threshold pressure. Relative to symmetric, one-dimensional simulations, the inferred hot-spot pressure is approximately 40% lower. Three-dimensional simulations suggest that low-mode distortion of the hot spot seeded by laser-drive nonuniformity and target-positioning error reduces target performance.

Velocity and timing of multiple spherically converging shock waves in liquid deuterium.

The fuel entropy and required drive energy for an inertial confinement fusion implosion are set by a sequence of shocks that must be precisely timed to achieve ignition. This Letter reports measurements of multiple spherical shock waves in liquid deuterium that facilitate timing inertial confinement fusion shocks to the required precision. These experiments produced the highest shock velocity observed in liquid deuterium (U(s) = 135 km/s at ∼2500 GPa) and also the first observation of convergence effects on the shock velocity. Simulations model the shock-timing results well when a nonlocal transport model is used in the coronal plasma.

Studies of plastic-ablator compressibility for direct-drive inertial confinement fusion on OMEGA.

The compression of planar plastic targets was studied with x-ray radiography in the range of laser intensities of I approximately 0.5 to 1.5x10(15) W/cm2 using square (low-compression) and shaped (high-compression) pulses. Two-dimensional simulations with the radiative hydrocode DRACO show good agreement with measurements at laser intensities up to I approximately 10(15) W/cm2. These results provide the first experimental evidence for low-entropy, adiabatic compression of plastic shells in the laser intensity regime relevant to direct-drive inertial confinement fusion. A density reduction near the end of the drive at a high intensity of I approximately 1.5x10(15) W/cm2 has been correlated with the hard x-ray signal caused by hot electrons from two-plasmon-decay instability.