E. L. Alfonso

Using Mass Spectrometry to Characterize Permeation Half-Life of ICF Targets

Abstract A mass spectrometer-based system was developed to measure the collected gas permeating through the shell wall of a spherical ICF target. The resultant ion current was used to calculate the material’s permeation half-life. This novel technique is simple, non-destructive, and suitable for measuring targets with short or long half-lives, i.e., very permeable or impermeable target materials, typically in less than 30 min. The technique is suitable for measuring permeation at ambient and elevated temperatures. The results acquired with the mass spectrometer measurements agreed well with those from destructive techniques.

The Production and Characterization of Banded GDP Capsules for Defect Implosion Experiments on Omega

Abstract General Atomics has supported the Los Alamos National Laboratory Defect Implosion Experiment series on OMEGA with the process design and production of banded, gas-tight, glow discharge polymer (GDP) capsules. Production of a banded target is a multistep, multidisciplinary process requiring micromachining for the band, GDP coating for the capsule wall, and aluminum sputter coating to seal the capsules for subsequent gas fill. Challenges included applying a micromachining technique to create the channel that would result in the desired band after coating, and modification of the aluminum coating process to create a permeation barrier that would cover the banded region to allow for gas filling. Information describing the fabrication steps and characterization techniques employed to analyze the banded targets will be presented.

Aluminum coatings as a deuterium permeation barrier on foam shells and the dependence on foam surface finish

Abstract Low-density foam shells are currently being employed as direct drive targets on the Omega laser facility at the University of Rochester. For cryogenic shots, only a thin layer of glow discharge polymer (GDP) is required over these foam shells to hold the D2 (or DT) fill provided the capsules are re-filled after cooling. Room temperature surrogate experiments, however, require an additional permeation barrier of aluminum on GDP coated foam shells. This barrier should have a permeation time constant of at least 4 h for D2 at room temperature. To study this coating, 0.1 μm layers of Al were deposited via magnetron sputtering onto the surface of GDP shells and GDP coated foam shells. The foam shells were 180 mg/cc resorcinol formaldehyde (RF) with a GDP thickness of 3-5 μm; the GDP shells used for this study had a wall thickness of 25-30 μm. Preliminary data shows that the permeation rate of D2 for smooth GDP shells is lower than for GDP coated RF shells with a similar thickness of Al. The main factor in this difference appears to be the surface roughness of the shells.

Development of sputtered coated glass permeation barrier

Abstract We have successfully sputter deposited ≤2 μm thick layers of SiO2 on CH mandrels ∼ 2 mm in diameter to act as a permeation barrier for deuterium. Such targets can be used for experiments at Sandia’s Z facility as well as at the National Ignition Facility (NIF). This permeation barrier has been shown to have a half-life (τ1/2) of ∼2–4 weeks for a thickness of ∼ 1.5 μm. The sputter coating conditions have been successfully optimized to produce smooth uniform SiO2 coatings with enough integrity to allow routine handling as well as filling to the required pressures (20 atm). The key coating conditions investigated were the agitation mechanism and the coating pressure. We found that an agitation mechanism using gentle rolling produced coatings with a half-life of greater than three weeks, whereas a more vigorous bouncing agitation yielded half-lives of only a few days. Coating pressures of 2, 5, and 10 mTorr were studied and it was found that coatings at 5 mTorr produced coatings free of cracking. Since the sputtering is performed in a background atmosphere of argon, the sputtered SiO2 layer was found to contain trace amounts of argon as measured by x-ray fluorescence (XRF) measurements. Our work has yielded a controllable uniform alternative permeation barrier to the traditionally used poly(vinylalcohol) (PVA).

SUBMICRON GOLD COATING MEASUREMENTS FOR HOHLRAUM DEVELOPMENT

Abstract A thin gold layer is deposited as a liner on the interior of a uranium hohlraum to protect from oxidation of uranium. X-ray fluorescence (XRF) spectrometry was chosen as the liner thickness measurement method for its accuracy, speed, and ease of measurement. The process is noncontact and nondestructive. The thicknesses were determined using a micro-XRF spectrometer unit with analysis software. The accuracy of the measurements was verified against qualified standards. The method was used to accurately measure gold liner thicknesses of cylindrical hohlraums, and it gave initial promising results for measuring the thickness of a boron-doped gold layer when corrected for the gold atom fraction.

Developments in capsule gas fill half-life determination

Abstract ICF experiments routinely make use of capsules filled with precise quantities of gaseous hydrogen and helium isotopes. These two gases in particular readily permeate out of capsules at rates dependent upon variables including shell wall thickness, composition and integrity. Therefore it is important that the fill half-life of these capsules be precisely known so that the exact fill pressure at shot time can be deduced, enabling valid experimental results. This presentation will describe some of our efforts to determine ICF capsule gas fill half-lives. We will compare fill half-life data obtained using weighing, interferometry and mass spectrometry techniques. In addition, we will describe our use of glass shell standards to compare the aforementioned techniques.

Evolution of the Design and Fabrication of Astrophysics Targets for Turbulent Dynamo (TDYNO) Experiments on OMEGA

Abstract Highly complex targets are constructed by General Atomics for astrophysically relevant experiments conducted by the University of Chicago on the OMEGA laser facility through the National Laser Users’ Facility (NLUF) program. Several novel target components are fabricated, precision assembled, and extensively measured in support of this campaign and have evolved over the last 3 years to improve both the science and assembly. Examples include unique laser-machined polyimide grids to enhance plasma mixing at the target center, precision-micromachined cylindrical shields that also act as component spacers, drawn glass target supports to suspend physics packages at critical distances, and tilted pinholes for collimated proton radiography. Target component fabrication and evolution details for the NLUF Turbulent Dynamo (TDYNO) campaign are presented, along with precision-assembly techniques, metrology methods, and considerations for future TDYNO experiments on OMEGA.

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.

Techniques to Measure the Refractive Index of GDP and Ge-Doped GDP with Monochromatic Light

Abstract Equation-of-state experiments using VISAR require the refractive index of undoped glow discharge polymer (GDP) and Ge-doped GDP at 532-nm-wavelength light. The index was measured with two different techniques. The first technique used measurements of pulsed laser reflections off a GDP foil suspended in refractive index fluid standards. Fluids with different indices were replaced until minimum reflection was achieved; this occurred at the matching index of the fluid and GDP film. The index of the correct matching fluid (or fluid mixture) was measured with an Abbé refractometer to produce nD (the refractive index at sodium D line, 589 nm) and was corrected for wavelength using manufacturer-supplied Cauchy equation coefficients. The second technique used interferometry to measure fringe shift over GDP and Ge-GDP bumps when submerged in various refractive index fluid standards. The fringe shift was minimized when matching the indices of the fluid and film. The refractive indices at 532 nm were 1.563 and 1.570 for undoped GDP and Ge-doped GDP, respectively.

Monolithic Fill Tube Target Fabrication

Abstract Fielding a cryogenic ICF capsule for near-term ignition shots requires a fill-tube to deliver fuel to the capsule. Polymer capsules (0.9 and 3.1 mm o.d.) with a monolithic fill-tube have been produced. The fill tube is formed on the capsule by plasma polymer coating on a mandrel with a depolymerizable stalk. We report recent advances achieved in monolithic fill-tube capsule production. The micro-assembly station for attaching PAMS stalk to the mandrels was improved to reduce fillet and fill tube diameters and increase throughput. Excimer laser etching is used to micro-machine fill-tube capsules and target components to allow proper mating for assembly. The fill-tube target is attached to the gas source by a flexible polymer fill line and was tested to confirm gas flow and retention. The effect of the mandrel stalk on the uniformity of GDP coating is investigated.