J. S. Jaquez

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.

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).

Improvements to Fill Tube Design for Direct-Drive NIF and Fast Ignition Applications

Abstract Fill tubes are being implemented to meet direct-drive National Ignition Facility (NIF) target designs and eliminate the need for permeation filling of targets. Significant improvements have been made to the fill tube designs for the NIF-scale CD and fast ignition targets to accommodate fuel-layering experiments at the University of Rochester Laboratory for Laser Energetics. The initial fill tube design had a number of issues that contributed to the nonuniformity of the deuterium (D2) ice layer and low fabrication yield of targets. Redesign of the entire target has significantly improved the D2 ice layering by reducing thermal perturbations. These design changes also made a more robust target that can survive the handling required in fabrication and testing. This paper will detail the target design aspects that were altered, including adjusting the fill tube aspect ratio, removing the thermally conductive support stalk, and adding a thermally conductive coating on the fill tube.

Fabrication and Characterization of Hohlraums with a Co-Mixed Gold-Boron Layer

Abstract A specialized magnetron co-sputtering process to fabricate a 0.3- to 0.6-μm-thick sputtered gold-boron (AuB) liner in inertial confinement hohlraums (inertial confinement fusion) to the specified concentration of 80% Au and 20% B has been developed at General Atomics. The AuB layer concentration in atomic percent is characterized by witness pieces using depth-profiling Auger electron spectroscopy. The results of the initial research and development effort and the yield in pilot production of AuB-lined gold hohlraums are presented.

Process Developments in the Fabrication of Depleted Uranium Hohlraums

Abstract Research at General Atomics and Lawrence Livermore National Laboratory has been focused on evaluating depleted uranium (DU) hohlraum fabrication over the past 10 years to improve the yield, thereby increasing the availability of DU hohlruams required to support the increased shot rate at the National Ignition Facility. The more straightforward gold (Au) hohlraum fabrication involves four basic steps: mandrel fabrication, electroplating, back machining and milling, and leaching. For Au, the overall fabrication yield of this process approaches 98% [H. Streckert and K. Blobaum, Fusion Sci. Technol., Vol. 63, p. 213 (2013)] Depleted uranium lined hohlraum fabrication, however, requires deposition of a multilayer of thin films after the mandrel fabrication step. These thin film deposition processes have historically proven difficult to execute on a complex cylindrical geometry of a hohlraum, resulting in unacceptable stress-driven delamination, with net yields ranging 20% to 35% [H. L. Wilkens et al., Phys. Plasmas, Vol. 14, 056310 (2007)]. Recent hohlraum design and fabrication process changes, as well as material selections implemented between 2014 and 2016, have improved the fabrication yield to over 60%. These changes are discussed here as well as plans for future improvements.

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.

Fabrication, Characterization, and Modeling of Comixed Films for NXS Calibration Targets

Abstract In 2014/2015 at the Omega laser facility, several experiments took place to calibrate the National Ignition Facility (NIF) X-ray spectrometer (NXS), which is used for high-resolution time-resolved spectroscopic experiments at NIF. The spectrometer allows experimentalists to measure the X-ray energy emitted from high-energy targets, which is used to understand key data such as mixing of materials in highly compressed fuel. The purpose of the experiments at Omega was to obtain information on the instrument performance and to deliver an absolute photometric calibration of the NXS before it was deployed at NIF. The X-ray emission sources fabricated for instrument calibration were 1-mm fused silica spheres with precisely known alloy composition coatings of Si/Ag/Mo, Ti/Cr/Ag, Cr/Ni/Zn, and Zn/Zr, which have emission in the 2- to 18-keV range. Critical to the spectrometer calibration is a known atomic composition of elements with low uncertainty for each calibration sphere. This paper discusses the setup, fabrication, and precision metrology of these spheres as well as some interesting findings on the ternary magnetron-sputtered alloy structure.

Fabrication of Backlighter Laser Targets via Lithography

Abstract Lithographed wire targets have recently been fabricated for use as backlighter targets for experimental campaigns at the OMEGA laser facility. These experiments required targets with 10-μm gold wires lithographed onto plastic. A process was developed using lithography to make these targets via sputter coating and liquid solution removal of the resist. One of the challenges overcome in developing this target was depositing the gold so that it would strongly adhere to the plastic. The quality of the lithographed targets was much better than targets made by other methods, such as gluing or thermal techniques. The lithographed targets were straighter, which is important in order to minimize the emission spot size at shot time. The lithographed targets also did not have problems with plastic or glue covering the wires, which can also reduce target emission, another challenge for gluing or thermal techniques. Lithography also allows a large number of similar targets to be made at once and cut out to the individual size needed, and a wide range of complex patterns or designs are possible using this technique.

FABRICATION OF TARGETS FOR PROTON FOCUS CONE FAST IGNITION EXPERIMENTS

Abstract The fast ignition concept is a proposed method to reach fusion by two separate processes. The task of the first process is the compression of fuel and the second is the ignition of the compressed fuel by a rapid and directed energy deposition. One delivery method of this energy can be in the form of focused proton beams and this type of fast ignition target will be discussed. The target designs consisted of gold and plastic cones with a curved proton-generating surface (aluminum) within the cone and very close to the tip. The challenges of the given target specifications led to a new cone design consisting of a cone base and cone tip made in two pieces with the proton generating surface sandwiched between. The fabrication of these targets consisted of several steps and processes that included making PAMS shell mandrels, sputter coating deposition, electroplating, precision machining, chemical etching, and target assembly.