S. Felker

Hydrodynamic instability growth of three-dimensional modulations in radiation-driven implosions with “low-foot” and “high-foot” drives at the National Ignition Facility

Hydrodynamic instability growth has been studied using three-dimensional (3-D) broadband modulations by comparing “high-foot” and “low-foot” spherical plastic (CH) capsule implosions at the National Ignition Facility (NIF) [E. M. Campbell et al., AIP Conf. Proc. 429, 3 (1998)]. The initial perturbations included capsule outer-surface roughness and capsule-mounting membranes (“tents”) that were similar to those used in a majority of implosions on NIF. The tents with thicknesses of 31-nm, 46-nm, and 109-nm were used in the experiments. The outer-surface roughness in the “low-foot” experiment was similar to the standard specification, while it was increased by ∼4 times in the “high-foot” experiment to compensate for the reduced growth. The ablation-front instability growth was measured using a Hydrodynamic Growth Radiography platform at a convergence ratio of ∼3. The dominant capsule perturbations, generated by the tent mountings, had measured perturbation amplitudes comparable to the capsule thickness with ...

Mitigation of X-ray shadow seeding of hydrodynamic instabilities on inertial confinement fusion capsules using a reduced diameter fuel fill-tube

We report a reduced X-ray shadow imprint of hydrodynamic instabilities on the high-density carbon ablator surface of inertial confinement fusion (ICF) capsules using a reduced diameter fuel fill tube on the National Ignition Facility (NIF). The perturbation seed mass from hydrodynamic instabilities was reduced by approximately an order of magnitude by reducing both the diameter and wall thickness of the fill tube by ∼2×, consistent with analytical estimates. This work demonstrates a successful mitigation strategy for engineered features for ICF implosions on the NIF.

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).

A “polar contact” tent for reduced perturbation and improved performance of NIF ignition capsules

In indirectly driven Inertial Confinement Fusion implosions conducted on the National Ignition Facility (NIF), the imploding capsule is supported in a laser-heated radiation enclosure (called a “hohlraum”) by a pair of very thin (∼15–45u2009nm) plastic films (referred to as a “tent”). Even though the thickness of these tents is a small fraction of that of the spherical capsule ablator (∼165u2009μm), both numerical simulations as well as experiments indicate that this capsule support mechanism results in a large areal density (ρR) perturbation on the capsule surface at the contact point where the tent departs from the capsule. As a result, during deceleration of the deuterium-tritium (DT) fuel layer, a jet of the dense ablator material penetrates and cools the fuel hot spot, significantly degrading the neutron yield (resulting in only ∼10%–20% of the unperturbed 1-D yield). In this article, we present a hypothesis and supporting design simulations of a new “polar contact” tent support system, which reduces the contact area between the tent and the capsule and results in a significant improvement in the capsule performance. Simulations predict a ∼70% increase in neutron yield over that for an implosion with a traditional tent support. An initial demonstration experiment was conducted on the NIF and produced highest ever recorded primary DT neutron yield among all layered DT implosions with plastic ablators on the NIF, though more experiments are needed to comprehensively study the effect of the polar tent on implosion performance.

Hydrodynamic instabilities seeded by the X-ray shadow of ICF capsule fill-tubes

During the first few hundred picoseconds of indirect drive for inertial confinement fusion on the National Ignition Facility, x-ray spots formed on the hohlraum wall when the drive beams cast shadows of the fuel fill-tube on the capsule surface. Differential ablation at the shadow boundaries seeds perturbations which are hydrodynamically unstable under subsequent acceleration and can grow to impact capsule performance. We have characterized this shadow imprint mechanism and demonstrated two techniques to mitigate against it using (i) a reduced diameter fuel fill-tube, and (ii) a pre-pulse to blow down the fill-tube before the shadow forming x-ray spots from the main outer drive beams develop.

Evaluation of Polyimide/Carbon Composite Films for Capsule Support

Abstract Capsules in National Ignition Facility targets are conventionally supported by thin polymer films. Recent experiments have shown that these films add significant perturbations to the implosion. Here, we evaluate stiffer polyimide composite films for use in a new target design that has been predicted to reduce these perturbations. The films are evaluated by their contact radius to the capsule for different deflections and the force they generate at those deflections to center the capsule. We find that a composite film with a single-sided coating of carbon produces the best results and show the performance of these films in target assemblies, highlighting the importance of the indentation depth.

Novel Capsule Fill Tube Assemblies for the Hydrodynamic Growth Radiography Targets

Abstract Capsule fill tube assemblies (CFTAs) consist of an ablator capsule and fill tube via a laser-drilled funnel hole. This hole tapers from 17-μm diameter at the outer surface of the ablator capsule to less than 5-μm diameter on the inside of the capsule over approximately 200 μm of wall thickness. Demand for better understanding of the fill tube perturbation during the capsule implosion has driven advancements in the fill tube design. Engineering efforts have been made on hydrodynamic growth radiography assemblies (HGRs) using multiple tube-design variations, including alternative angles, depths, sizes, and location with engineered defects to showcase fill tube effects during an implosion. Testing has shown that these CFTAs and HGRs have survived all fabrication and transport to and from General Atomics (GA) to Lawrence Livermore National Laboratory. These assemblies have also passed cryogenic testing at GA. An overview of alternative CFTA designs, fabrication methods, and developments is presented.