S. A. MacLaren

Absolute x-ray yields from laser-irradiated germanium-doped low-density aerogels

The x-ray yields from laser-irradiated germanium-doped ultra-low-density aerogel plasmas have been measured in the energy range from sub-keV to ≈15 keV at the OMEGA laser facility at the Laboratory for Laser Energetics, University of Rochester. The targets’ x-ray yields have been studied for variation in target size, aerogel density, laser pulse length, and laser intensity. For targets that result in plasmas with electron densities in the range of ≈10% of the critical density for 3ω light, one can expect 10–11 J/sr of x rays with energies above 9 keV, and 600–800 J/sr for energies below 3.5 keV. In addition to the x-ray spectral yields, the x-ray temporal waveforms have been measured and it is observed that the emitted x rays generally follow the delivered laser power, with late-time enhancements of emitted x-ray power correlated with hydrodynamic compression of the hot plasma. Further, the laser energy reflected from the target by plasma instabilities is found to be 2%–7% of the incident energy for indiv...

Progress in hohlraum physics for the National Ignition Facilitya)

Advances in hohlraums for inertial confinement fusion at the National Ignition Facility (NIF) were made this past year in hohlraum efficiency, dynamic shape control, and hot electron and x-ray preheat control. Recent experiments are exploring hohlraum behavior over a large landscape of parameters by changing the hohlraum shape, gas-fill, and laser pulse. Radiation hydrodynamic modeling, which uses measured backscatter, shows that gas-filled hohlraums utilize between 60% and 75% of the laser power to match the measured bang-time, whereas near-vacuum hohlraums utilize 98%. Experiments seem to be pointing to deficiencies in the hohlraum (instead of capsule) modeling to explain most of the inefficiency in gas-filled targets. Experiments have begun quantifying the Cross Beam Energy Transfer (CBET) rate at several points in time for hohlraum experiments that utilize CBET for implosion symmetry. These measurements will allow better control of the dynamic implosion symmetry for these targets. New techniques are b...

Results from a scaled final focus experiment for heavy ion fusion

A one-tenth-scale version of a final focus subsystem for a heavy-ion-fusion driver has been built and used for experimental tests of concept. By properly scaling the parameters that relate ion energy and mass, current, emittance, and focusing fields, the transverse dynamics of a representative driver final focus have been replicated in a small laboratory beam. Whereas the driver beam parameters considered are 10 GeV Bi+ at 1.25 kA, the scaled experiment used a 95 μA beam of 160 keV Cs+ ions brought to a ballistic focus through a series of six quadrupole magnets. The measured focal spot size was consistent with calculations in the report of the design on which the experiment is based. In a second experimental program, a 400 μA beam was propagated through the focal system and partially neutralized after the last magnet using electrons released from a hot tungsten filament to test the predictions of the benefits of neutralization. The increase in beam current resulted in a corresponding increase in spot radi...

Symmetry tuning of a near one-dimensional 2-shock platform for code validation at the National Ignition Facility

We introduce a new quasi 1-D implosion experimental platform at the National Ignition Facility designed to validate physics models as well as to study various Inertial Confinement Fusion aspects such as implosion symmetry, convergence, hydrodynamic instabilities, and shock timing. The platform has been developed to maintain shell sphericity throughout the compression phase and produce a round hot core at stagnation. This platform utilizes a 2-shock 1 MJ pulse with 340 TW peak power in a near-vacuum Au Hohlraum and a CH ablator capsule uniformly doped with 1% Si. We have performed several inflight radiography, symmetry capsule, and shock timing experiments in order to tune the symmetry of the capsule to near round throughout several epochs of the implosion. Adjusting the relative powers of the inner and outer cones of beams has allowed us to control the drive at the poles and equator of the capsule, thus providing the mechanism to achieve a spherical capsule convergence. Details and results of the tuning e...

A platform for studying the Rayleigh–Taylor and Richtmyer–Meshkov instabilities in a planar geometry at high energy density at the National Ignition Facility

A new experimental platform has been developed at the National Ignition Facility (NIF) for studying the Rayleigh–Taylor (RT) and Richtmyer–Meshkov (RM) instabilities in a planar geometry at high-energy-densities. The platform uses 60 beams of the NIF laser to drive an initially solid shock tube containing a pre-machined interface between dense and light materials. The strong shock turns the initially solid target into a plasma and the material boundary into a fluid interface with the imprinted initial condition. The interface evolves by action of the RT and RM instabilities, and the growth is imaged with backlit x-ray radiography. We present our first data involving sinusoidal interface perturbations driven from the heavy side to the light side. Late-time radiographic images show the initial conditions reaching the deeply nonlinear regime, and an evolution of fine structure consistent with a transition to turbulence. We show preliminary comparisons with post-shot numerical simulations and discuss the impl...

Exploring the limits of case-to-capsule ratio, pulse length, and picket energy for symmetric hohlraum drive on the National Ignition Facility Laser

We present a data-based model for low mode asymmetry in low gas-fill hohlraum experiments on the National Ignition Facility {NIF [Moses et al., Fusion Sci. Technol. 69, 1 (2016)]} laser. This model is based on the hypothesis that the asymmetry in these low fill hohlraums is dominated by the hydrodynamics of the expanding, low density, high-Z (gold or uranium) “bubble,” which occurs where the intense outer cone laser beams hit the high-Z hohlraum wall. We developed a simple model which states that the implosion symmetry becomes more oblate as the high-Z bubble size becomes large compared to the hohlraum radius or the capsule size becomes large compared to the hohlraum radius. This simple model captures the trends that we see in data that span much of the parameter space of interest for NIF ignition experiments. We are now using this model as a constraint on new designs for experiments on the NIF.

Radiation transport and energetics of laser-driven half-hohlraums at the National Ignition Facility

Experiments that characterize and develop a high energy-density half-hohlraum platform for use in benchmarking radiation hydrodynamics models have been conducted at the National Ignition Facility (NIF). Results from the experiments are used to quantitatively compare with simulations of the radiation transported through an evolving plasma density structure, colloquially known as an N-wave. A half-hohlraum is heated by 80 NIF beams to a temperature of 240 eV. This creates a subsonic diffusive Marshak wave, which propagates into a high atomic number Ta2O5 aerogel. The subsequent radiation transport through the aerogel and through slots cut into the aerogel layer is investigated. We describe a set of experiments that test the hohlraum performance and report on a range of x-ray measurements that absolutely quantify the energetics and radiation partition inside the target.

Streaked radiography of an irradiated foam sample on the National Ignition Facility

Streaked x-ray radiography images of annular patterns in an evolving tantalum oxide foam under the influence of a driven, subsonic radiation wave were obtained on the National Ignition Facility. This is the first successful radiography measurement of the evolution of well-defined foam features under a driven, subsonic wave in the diffusive regime. A continuous record of the evolution was recorded on an x-ray streak camera, using a slot-apertured point-projection backlighter with an 8 ns nickel source (7.9 keV). Radiography images were obtained for four different annular patterns, which were corrected using a source-dependent flat-field image. The evolution of the foam features was well-modeled using the 3D KULL radiation hydrodynamics code. This experimental and modeling platform can be modified for scaled high-energy-density laboratory astrophysics experiments.

The size and structure of the laser entrance hole in gas-filled hohlraums at the National Ignition Facility

At the National Ignition Facility, a thermal X-ray drive is created by laser energy from 192 beams heating the inside walls of a gold cylinder called a “hohlraum.” The x-ray drive heats and implodes a fuel capsule. The laser beams enter the hohlraum via laser entrance holes (LEHs) at each end. The LEH radius decreases as heated plasma from the LEH material blows radially inward but this is largely balanced by hot plasma from the high-intensity region in the center of the LEH pushing radially outward. The x-ray drive on the capsule is deduced by measuring the time evolution and spectra of the x-radiation coming out of the LEH and correcting for geometry and for the radius of the LEH. Previously, the LEH radius was measured using time-integrated images in an x-ray band of 3–5 keV (outside the thermal x-ray region). For gas-filled hohlraums, the measurements showed that the LEH radius is larger than that predicted by the standard High Flux radiation-hydrodynamic model by about 10%. A new platform using a tru...

Comparison of plastic, high density carbon, and beryllium as indirect drive NIF ablators

Detailed radiation hydrodynamic simulations calibrated to experimental data have been used to compare the relative strengths and weaknesses of three candidate indirect drive ablator materials now tested at the NIF: plastic, high density carbon or diamond, and beryllium. We apply a common simulation methodology to several currently fielded ablator platforms to benchmark the model and extrapolate designs to the full NIF envelope to compare on a more equal footing. This paper focuses on modeling of the hohlraum energetics which accurately reproduced measured changes in symmetry when changes to the hohlraum environment were made within a given platform. Calculations suggest that all three ablator materials can achieve a symmetric implosion at a capsule outer radius of ∼1100 μm, a laser energy of 1.8 MJ, and a DT ice mass of 185 μg. However, there is more uncertainty in the symmetry predictions for the plastic and beryllium designs. Scaled diamond designs had the most calculated margin for achieving symmetry a...