P. Fitzsimmons

Polar-direct-drive experiments on the National Ignition Facilitya)

To support direct-drive inertial confinement fusion experiments at the National Ignition Facility (NIF) [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 43, 2841 (2004)] in its indirect-drive beam configuration, the polar-direct-drive (PDD) concept [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004)] has been proposed. Ignition in PDD geometry requires direct-drive–specific beam smoothing, phase plates, and repointing the NIF beams toward the equator to ensure symmetric target irradiation. First experiments to study the energetics and preheat in PDD implosions at the NIF have been performed. These experiments utilize the NIF in its current configuration, including beam geometry, phase plates, and beam smoothing. Room-temperature, 2.2-mm-diam plastic shells filled with D2 gas were imploded with total drive energies ranging from ∼500 to 750 kJ with peak powers of 120 to 180 TW and peak on-target irradiances at the initial target radius from 8 × 1014 to 1.2 × 1015 W/cm2. Results from these initial experi...

Development of a polar direct-drive platform for studying inertial confinement fusion implosion mix on the National Ignition Facilitya)

Experiments were performed to develop a platform for the simultaneous measurement of mix and its effects on fusion burn. Two polar direct drive implosions of all-plastic capsules were conducted for the first time on the National Ignition Facility (NIF). To measure implosion trajectory and symmetry, area image backlighting of these capsules was also employed for the first time on NIF, an advance over previous 1-D slit imaging experiments, providing detailed symmetry data of the capsules as they imploded. The implosion trajectory and low-mode asymmetry seen in the resultant radiographs agreed with pre-shot predictions even though the 700 kJ drive energy produced laser beam intensities exceeding laser-plasma instability thresholds. Post-shot simulations indicate that the capsule yield was reduced by a factor of two compared to pre-shot predictions owing to as-shot laser drive asymmetries. The pre-shot predictions of bang time agreed within 200 ps with the experimental results. The second shot incorporated a narrow groove encircling the equator of the capsule. A predicted yield reduction factor of three was not observed.

Surface oxygen micropatterns on glow discharge polymer targets by photo irradiation

Recent simulations predict surface oxygen may be a significant source of disruptive perturbations in the implosion process of glow-discharge polymers (GDP) ablators at the National Ignition Facility. GDP material held in ambient atmospheric conditions showed an increase in mass when stored in light transparent containers, which suggests that photo exposure is a driving force for oxygen absorption. To investigate if surface oxygen is a contributing factor of disruptive perturbations during implosion, a method to imprint a periodic micropattern of oxygen on the surface of GDP was developed and used to fabricate a flat sample for empirical testing. Photo exposure using collimated blue light was used to generate micropatterns of surface oxygen on the GDP material. The periodic oxygen micropattern was confirmed by secondary ion mass spectrometry (SIMS) and energy dispersive spectroscopy. A SIMS depth profile showed the atomic percent of oxygen ranged from 8 at. % near the surface to 1 at. % at a depth of 2 μm ...

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...

Developments in Microcoining Rippled Metal Foils

Abstract Rippled metal foils are currently sought for high-strain-rate material strength studies at laser facilities. Because these metals typically cannot be diamond turned, we employ a microcoining process to imprint the [approximately]5-μm-deep by [approximately]50-μm-long ripples into the metal surface. This work details recent process developments to fabricate these rippled metal targets, specifically for iron and tantalum. The process consists of nitriding a steel die, diamond turning the die, and then pressing the die into a polished metal foil of choice. We show: advantages of deeper-nitrided dies, improved foil thickness uniformity and characterization, variation in coining stress over different materials, pattern quality characterization, bowing reduction, and patterning of multimode ripples.

Diagnosing laser-preheated magnetized plasmas relevant to magnetized liner inertial fusion

We present a platform on the OMEGA EP Laser Facility that creates and diagnoses the conditions present during the preheat stage of the MAGnetized Liner Inertial Fusion (MagLIF) concept. Experiments were conducted using 9 kJ of 3ω (355 nm) light to heat an underdense deuterium gas (electron density: 2.5×1020 cm−3=0.025 of critical density) magnetized with a 10 T axial field. Results show that the deuterium plasma reached a peak electron temperature of 670 ± 140 eV, diagnosed using streaked spectroscopy of an argon dopant. The results demonstrate that plasmas relevant to the preheat stage of MagLIF can be produced at multiple laser facilities, thereby enabling more rapid progress in understanding magnetized preheat. Results are compared with magneto-radiation-hydrodynamics simulations, and plans for future experiments are described.

Laser irradiance scaling in polar direct drive implosions on the National Ignition Facility

Polar-direct-drive experiments conducted at the National Ignition Facility [E. I. Moses, Fusion Sci. Technol. 54, 361 (2008)] performed at laser irradiance between 1 and 2×1015 W/cm2 exhibit increased hard x-ray emission, decreased neutron yield, and reduced areal density as the irradiance is increased. Experimental x-ray images at the higher irradiances show x-ray emission at the equator, as well as degraded symmetry, that is not predicted in hydrodynamic simulations using flux-limited energy transport, but that appear when non-local electron transport together with a model to account for cross beam energy transfer (CBET) is utilized. The reduction in laser power for equatorial beams required in the simulations to reproduce the effects of CBET on the observed symmetry also reproduces the yield degradation consistent with experimental data.

Capsule Shimming Developments for National Ignition Facility (NIF) Hohlraum Asymmetry Experiments

Abstract Capsule drive in National Ignition Facility indirect-drive implosions is generated by X-ray illumination from cylindrical hohlraums. The cylindrical hohlraum geometry is axially symmetric but not spherically symmetric, causing capsule–fuel drive asymmetries. It is hypothesized that fabricating capsules asymmetric in wall thickness (shimmed) may compensate for drive asymmetries and improve implosion symmetry. Simulations suggest that for high-compression implosions, Legendre mode P4 hohlraum flux asymmetries are the most detrimental to implosion performance. General Atomics has developed a diamond-turning method to form a glow discharge polymer capsule outer surface to a Legendre mode P4 profile. The P4 shape requires full capsule surface coverage. As a result, in order to avoid tool-lathe interference, flipping the capsule part way through the machining process is required. This flipping process risks misalignment of the capsule, causing a vertical step feature on the capsule surface. Recent trials have proven this step feature height can be minimized to ~0.25 µm.

Zinc Oxide–Coated Poly(HIPE) Annular Liners to Advance Laser Indirect Drive Inertial Confinement Fusion

Abstract Laser indirect drive is hindered, in part, by two problems: “wall motion” resulting from ablation of the hohlraum inner wall and “preheat” of the fuel capsule. To mitigate wall motion and preheat, a mid-Z–coated high internal phase emulsion, poly(HIPE) foam liner (5.7-mm diameter, 150 μm thick, 2.8 mm long, 33 mg/cm3) was developed and integrated into the hohlraum interior. A zinc oxide coating was applied throughout the poly(HIPE) foam using atomic layer deposition to achieve 149 ± 14 mg/cm3 bulk density. Preliminary data collected from actual shots at the National Ignition Facility suggest the inclusion of the poly(HIPE) liner reduced preheat threefold and stimulated Brillouin scattering (SBS) fivefold relative to an existing reference shot on a gold hohlraum (wavelength shift also contributed to SBS reduction).

Development of a polar direct drive platform for mix and burn experiments on the National Ignition Facility

Capsules driven with polar drive [1, 2] on the National Ignition Facility [3] are being used [4] to study mix in convergent geometry. In preparation for experiments that will utilize deuterated plastic shells with a pure tritium fill, hydrogen-filled capsules with copper- doped deuterated layers have been imploded on NIF to provide spectroscopic and nuclear measurements of capsule performance. Experiments have shown that the mix region, when composed of shell material doped with about 1% copper (by atom), reaches temperatures of about 2 keV, while undoped mixed regions reach about 3 keV. Based on the yield from these implosions, we estimate the thickness of CD that mixed into the gas as between about 0.25 and 0.43 μm of the inner capsule surface, corresponding to about 5 to 9 μg of material. Using 5 atm of tritium as the fill gas should result in over 1013 DT neutrons being produced, which is sufficient for neutron imaging [5].