S. A. Eddinger

Nondestructive quantitative dopant profiling technique by contact radiography

Abstract We have developed the only non-destructive technique to profile graded dopants in ICF shells to the precision required by the NIF specifications (Doping level must be accurate to 0.03 at. % and its radial distribution accurate to submicron precision). This quantitative contact radiography method was based on precision film digitization and a dopant simulation model. The measurements on Cu/Be and Ge/CH shells agree with those from electron microprobe and X-ray fluorescence.

Quantitative radiography : Film model calibration and dopant/impurity measurement in icf ablators

Abstract In ablator shell fabrication, trace elements and impurities are introduced in the deposition and the pyrolysis process, which must be controlled below a critical level. However, it is the opacity, not the individual elements, which matters in an Inertial Confinement Fusion (ICF) implosion. Radiography measures the opacity, allowing the accurate determination of the total impurity effect in a lump sum. Furthermore, by using the sputter target trace element information, we can determine the radial profile of oxygen to ±0.4 at. %. Oxygen is very difficult to measure by any other method, but is critically important for beryllium process development such as mandrel removal. To ensure measurement accuracy, we use a local standard to remove fluctuation in film developing and a step wedge to calibrate the film model.

Characterization of Isolated Defects for NIF Targets Using PSDI with an Analysis of Shell Flipping Capability

Abstract Capsules for the National Ignition Facility require measurement of isolated defects on the capsule surface. A phase-shifting diffraction interferometer (PSDI) is used to identify, locate, and measure defects by capturing 71 overlapping ~500-μm-diam charge coupled device height maps for software analysis. Using capsules with drilled holes for the purpose of alignment, PSDI data were confirmed with atomic force microscopy by comparing defect data from corresponding equatorial bands. We explored the limitations of the PSDI resulting from unwrapping errors caused by defect slopes greater than the Nyquist sampling theorem. White light interferometry proved to be a useful complementary tool to measure defects that could not be unwrapped by the analysis software. Implementing the PSDI in conjunction with the shell flipper, both developed at Lawrence Livermore National Laboratory, allowed for full mapping of shell surfaces by mounting corresponding hemispheres onto the PSDI within a 2-deg accuracy.

ELEMENT-SPECIFIC PROFILING FOR ICF ABLATOR CAPSULES WITH MIXED DOPANT AND IMPURITIES

Abstract National Ignition Facility (NIF) specifications require nondestructive, independent profiling of copper, argon, and oxygen in a delivered beryllium capsule. We use a combination of two methods to accomplish this goal: (a) model-enhanced energy dispersive spectroscopy (EDS) of witness shell fragments for destructive profiling of all three elements in a select sample within a batch and (b) differential radiography (DR) to profile copper and argon on multiple shells to nondestructively prove the sample-to-sample consistency within a batch. This combination fully qualifies the delivered shells. For EDS, we developed a physics model and fabricated standards to quantify low concentrations of relatively light elements in a very low-Z matrix. For model validation, we produced sputtered beryllium capsules containing a single dopant in each shell and used contact radiography (CR) to characterize the dopant profiles to 5 to 10% accuracy. The copper calibration was also checked against bulk Cu-Be standards with known composition, and the argon and oxygen calibrations were also checked against the X-ray absorption edge spectroscopy (Edge method) and the weight gain methods. Together, the EDS method gives ±0.1, ±0.05, and ±0.2 at.% accuracy for copper, argon, and oxygen, respectively, in NIF specification capsules. For DR, we conduct two CR measurements with the X-ray tube running at 9 and 30 kVp, respectively. The differential response between copper and argon enables elemental separation. The dopant profiles can be measured to ±0.1 at.% for NIF specification capsules. The oxygen profile in DR must be inferred from the EDS measurement. In the production work flow, we use EDS to obtain the oxygen profile and use it as input to the DR measurement. We then check that the copper and argon profiles obtained from DR and those from EDS are consistent. The average argon and copper contents from either method can also be checked against the results from the Edge method. These two levels of cross-checks offer critical assurances to the data integrity in production metrology.

OVERVIEW OF NATIONAL IGNITION FACILITY CAPSULE METROLOGY

Abstract A procedure has been developed to fully characterize the National Ignition Facility (NIF) capsule. A variety of techniques has been developed and deployed to characterize the critical specifications of the capsules to the precision required by NIF. Capsules are fully characterized in order to verify their critical specifications including dimensional, chemical composition, and surface roughness. It has been demonstrated that capsules can be fully characterized at a throughput that will meet NIF demand. The specific metrology procedures, techniques, and errors associated with those techniques will be described in this paper. Also, the time line that is used to fully characterize a NIF capsule will be explored.

Quantitative Radiography: Submicron Dimension Calibration for ICF Ablator Shell Characterization

Abstract National Ignition Facility (NIF) ignition target specifications require submicron dimensional measurement accuracy for the spherical ablator shell, which requires the proper corrections of various distortions induced by the imaging lens, the point projection geometry, and x-ray refraction. The procedures we developed allow measurement accuracies of 0.5 μm for the capsule diameter, ±0.2 μm for the out-of-round (which is the amplitude of the radius variations), ±0.3 μm for the wall thickness (including each sub-layer), and ±0.1 μm for wall thickness profile.

Precision X-Ray Optical Depth Measurements in ICF Shells

Abstract We built a precision radiography system that measures shells for all current ablator materials to an accuracy of 1:104 in optical depth fluctuation and a spatial resolution of 120 μm. The data obtained by the precision radiography system for undoped shells was compared with the data taken using the well-known surface measurement technique Spheremapper. Since both techniques yielded the same power spectrum for the same shell, the results of the precision radiography system were verified. When this technique is compared to the Be:Cu NIF shell, there is no significant internal layer fluctuation. To account for the growing measurement demand, a new x-ray system to accommodate measurements in 1 working day was designed.

QUANTITATIVE DIMENSION MEASUREMENT OF ICF COMPONENTS USING Xradia MicroXCT MICROSCOPE

Abstract National Ignition Facility (NIF) specifications have stringent dimensional accuracy requirements on target components. For example, the laser-hole diameter on an ablator capsule must be characterized to ±0.5 μm to ensure proper fill tube insertion and to minimize the glue joint mass to <2.5 ng. A charge-coupled-device-based X-ray radiography and tomography instrument (commercially obtained from Xradia, Inc.) is used in target metrology where sample opacity precludes the use of optical techniques; however, the built-in caliper for dimensional measurement cannot provide the required accuracy. The instrument has three main error sources: (a) point projection magnification, (b) imaging lens distortion, and (c) phase contrast shift. The sample feature size dictates the calibration strategy. For large features such as the shell diameter, (a) and (b) dominate the error budget. The built-in caliper is accurate to ~2 to 3%, corresponding to a ±50-μm error for a 2000-μm NIF capsule. In this work, we developed an X-ray transmission dimension standard and developed (by measuring the standard) a software algorithm to “un-distort” the acquired images without resorting to the standard each time. The latter approach reduces the processing time by 50% and still offers a tenfold accuracy improvement and makes the Xradia instrument useful in screening components. For small features such as laser-drilled holes, (c) is dominant. It shifts the apparent wall boundary to cause a typical ~2-μm error for the 5- to 10-μm hole diameter. We developed an empirical correction technique with 0.5-μm accuracy, in which the dimensions measured by radiography were benchmarked against those by a focused ion beam and scanning electron microscope after sample cleavage. The improved accuracy allows the glue mass to be estimated to 1 ng as required by the NIF specifications.

X-RAY ABSORPTION SPECTROSCOPY FOR ICF TARGET CHARACTERIZATION

Abstract An instrumentation of X-ray absorption spectroscopy (XAS) was developed to measure the areal density of any element with an atomic number Z > 17. In contrast to X-ray fluorescence, which is affected by spatial dopant nonuniformity, an element can be accurately measured by XAS regardless of its own distribution or the presence of other elements in a sample. Furthermore, no reference standard is needed to achieve ±3% 1Ã accuracy. This method has been used to measure the average contents of specific elements in a variety of inertial confinement fusion and high energy density targets. It validates the average dopant concentration measured by contact radiography and differential radiography.

THREE-DIMENSIONAL WALLMAPPING USING Xradia WITH DISTORTION CORRECTION

Abstract The inertial confinement fusion program requires the uniformity of multilayered samples to be measured to high accuracy. We currently use a reflection spectroscopy tool to measure optically transparent shells with no more than two layers. The method cannot measure opaque samples such as beryllium shells, low-reflection samples such as foam shells, or any shells with more than two layers such as National Ignition Facility specification Ge-CH shells. We also use a white-light interferometer to measure transparent samples with multiple layers, but only at the North/South Poles for a given orientation. To complement these existing tools, we developed an X-ray technique based on a commercial X-ray microscope (Xradia MicroXCT). MicroXCT is capable of providing high-contrast, high-resolution images and allows the samples to be precision aligned and angular indexed. Dimension accuracy is achieved through the calibration of the projection magnification and the lens distortion. From each X-ray image, a wall thickness trace along the great circle is obtained by converting Cartesian coordinates into cylindrical coordinates, and edge-finding algorithms are developed for a contact radiography project. Three-dimensional reconstruction and wall thickness display allow the visualization of the sample nonuniformity. The method has a 0.3 μm measurement precision and, through phase contrast calibration, can achieve 0.3 μm accuracy.