K. C. Chen

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.

Status of the development of ignition capsules in the U.S. effort to achieve thermonuclear ignition on the national ignition facility

An important component of the U.S. effort to achieve thermonuclear ignition in 2010 on the National Ignition Facility is the development of high quality 2 mm diameter spherical capsules to function as the ablator and contain the cryogenic DT fuel. Three ignition capsule designs have been developed, and detailed fabrication specifications for each design have been established and placed under change control. A research program with activities coordinated mainly between Lawrence Livermore, General Atomics and Los Alamos is underway to demonstrate fabrication of capsules meeting specifications. The point design for ignition campaigns beginning in 2010 is a Cu-doped Be capsule that has a radial gradient in Cu dopant level in the capsule wall. This capsule is being produced by sputter deposition of Be and Cu onto either a hollow glow discharge polymer (GDP) spherical mandrel or a solid spherical mandrel, followed by removal of the mandrel and polishing of the capsule. A key goal in the U.S. is to demonstrate fabrication of this capsule by the end of 2006. Two other ignition capsule designs are also being developed as contingencies to the point design. One contingency design is a GDP capsule that has a radial Ge dopant level in its wall. This capsule is produced by co-deposition of Ge and GDP onto a PAMS mandrel followed by thermal removal of the mandrel. The second contingency design is a uniform Cu-doped Be capsule that is fabricated from high purity fine grain Be0.3at.%Cu alloy using a precision machining route followed by polishing. Ignition targets to be fielded in 2010 will be filled with DT fuel through a small fill hole. Laser drilling capability has been developed and used to drill approximately 5 μm diameter holes through capsule walls for DT filling. Characterization methods necessary for characterizing capsules are being developed.

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.

Fabrication of Graded Germanium-Doped CH Shells

Abstract One of the current capsule designs for achieving ignition on the National Ignition Facility (NIF) is a 2 mm diameter graded Ge-doped CH shell that has a 160 μm thick wall. The Ge doping is not uniform, but rather is in radial steps. This graded Ge-doped design allows rougher surface finish than the original undoped CH design, thus has a less stringent surface roughness requirement. We selected quality mandrels by coating dozens of mandrel batches to ∼70 μm thickness to amplify submicrometer defects on the mandrels and successively removed inferior batches. The Ge-doped CH layers are made by introducing (CH3)4Ge to the gas stream. The doping concentrations were determined by performing trial runs and were characterized by X-ray fluorescence and quantitative radiographic analyses, with good agreement between the methods demonstrated. The precise layer thickness and Ge concentrations were determined by a non-destructive quantitative contact radiograph. The as-deposited average layer thicknesses of the shells were 9.5 ± 1.1 μm for inner undoped CH layer, followed by a 47.1 ± 0.5 μm thick 0.83 ± 0.09 at. % Ge-doped CH, 10.0 ± 0.4 μm thick 0.38 ± 0.04 at. % Gedoped CH and then 89.2 ± 0.5 μm of undoped CH. The atomic force microscope derived power spectrum of the shell meets the new NIF standard. The shells had a root-mean-square surface roughness of ∼ 24 nm (modes 100–1000). The few surface flaws are isolated domes ∼1 μm tall and 20 μm in diameter. The PAMS mandrels were successfully removed by pyrolysis at 305°C for 10–20 h. After pyrolysis, the diameter and wall shrank 0.4% and 5.7%, respectively. Except for the outer undoped CH layer, which was 5.8 μm less than the design specification, the average thicknesses of the three other layers met the NIF design specification after pyrolysis. The averages of the Ge doping concentrations were within the tolerance limits. The shell’s inner surface has root-mean-square roughness of less than 6.5 nm.

REDUCTION OF ISOLATED DEFECTS ON Ge DOPED CH CAPSULES TO BELOW IGNITION SPECIFICATIONS

Abstract The NIF Ge-doped CH capsule should be free of isolated defects on the outer surface. The allowed number and dimensions of large isolated defects over the entire capsule surface is given by the isolated feature specification. To date NIF-thickness (146 μm) capsules are plagued by a few isolated large domes on the outer surfaces that otherwise meet the atomic force microscope (AFM) spheremap modal power spectra specification. The large domes on the capsule surfaces were mostly caused by particulate contamination from the wear of an agitation tapping solenoid inside the coater. By eliminating the solenoid and using an alternate rotation agitation, most thick-walled capsules become free of large isolated defects and meet the AFM spheremap modal power spectra standard. The number and size of the isolated defects on the outer surface were characterized with a high resolution phase-shifting diffractive spherical interferometer and checked against the NIF isolated defect specification. The results show the isolated defects on the rolled capsule are below the isolated defect specification. The growth modeling of the remaining nanometer-height domes on the capsules indicates most of these small domes come from the mandrel surface. The rolled capsules meet the layer thickness, doping levels and wall thickness specifications and have good wall uniformity of ±0.1.0.2 μm.

Increasing the Throughput of Phase-Shifting Diffraction Interferometer for Quantitative Characterization of ICF Ablator Capsule Surfaces

Abstract A phase-shifting diffraction interferometer provides full surface mapping of National Ignition Facility (NIF) ablator capsules for surface finish and isolated defects. To integrate this new instrument into the NIF metrology work flow, the measurement must be both quick and accurate. In this work, we developed automated processing algorithms to streamline a large number of manual steps. This enables the process time to be reduced from 1½ days to 2 h per shell, thus meeting the NIF throughput requirement of 20 capsules/week. We also developed methods to quantitatively report the isolated defects and surface roughness in formats that can be benchmarked against the NIF specifications.

MECHANICAL PROPERTIES OF THIN GDP SHELLS USED AS CRYOGENIC DIRECT DRIVE TARGETS AT OMEGA

Abstract Thin glow discharge polymer (GDP) shells are currently used as the targets for cryogenic direct drive laser fusion experiments. These shells need to be filled with nearly 1000 atm of D2 and cooled to cryogenic temperatures without failing due to buckling and bursting pressures they experience in this process. Therefore, the mechanical and permeation properties of these shells are of utmost importance in successful and rapid filling with D2. In this paper, we present an overview of buckle and burst pressures of several different types of GDP shells. These include those made using traditional GDP deposition parameters (“standard GDP”) using a high deposition pressure and using modified parameters (“strong GDP”) of low deposition pressure that leads to more robust shells.

Increased X-Ray Opacity of GDP Capsules from High Intensity X-Ray Exposure

Abstract Inertial confinement fusion capsules must be manufactured with a high degree of azimuthal symmetry to avoid degradation by Rayleigh-Taylor instabilities. Therefore, the azimuthal fluctuations of each capsule must be characterized. We have developed a precision radiography method capable of measuring X-ray optical depth fluctuations to 1 part in 104 with a spatial resolution of 120 μm. Achieving the measurement accuracy requires counting many photons. Recent measurements of glow discharge polymer (GDP) capsules show that the high X-ray intensity required to minimize measurement time modifies the GDP shell by increasing the oxygen atomic percent. An equatorial band forms that is more optically dense than the remainder of the capsule. We believe that free radicals are formed in the GDP as a result of the X-ray exposure. These free radicals preferentially absorb oxygen from the air. We will discuss how this optically dense band forms, how it is measured, and possible solutions to this issue.

UPDATE ON GERMANIUM-DOPED CH CAPSULE PRODUCTION FOR NIF: SCALE-UP ISSUES AND CURRENT YIELDS

Abstract A germanium-doped CH capsule is one of the capsule designs for the National Ignition Facility. Eight batches were made to evaluate yields and reproducibility for production. When larger batches (more than 20 capsules) were made, numerous nanometer-height domes, together with many nanometer-sized seeds and micrometer-sized beads, were observed on the capsule surface. These domes originate from abrasion-induced nanometer-sized seeds. Large batch sizes tend to slide as cohesive groups that enhance friction and abrasion. Limiting the batch size to 15 capsules prevented formation of nanometer-height domes. Roughly 80% of the capsules from 15 capsule batches meets the surface roughness specification, and 85% meets the isolated defect specification. The wall thickness and outer diameter yields, currently at 58% and 28 to 40%, respectively, are affected by variables that will be discussed. The average concentrations of the two Ge-doped layers are 0.77 and 0.50 at.%, with standard deviations of 0.15 at.%. The overall Ge-doping yield, with both layers within the most recent tolerance specification of ±0.2 at.%, is 20%. The best overall yields of 15 shell batches are currently 40 to 55%. The yield-limiting factors are wall-thickness accuracy and high mid-mode in outer surface power spectra.