K. A. Moreno

Point design targets, specifications, and requirements for the 2010 ignition campaign on the National Ignition Facility

Point design targets have been specified for the initial ignition campaign on the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 443, 2841 (2004)]. The targets contain D-T fusion fuel in an ablator of either CH with Ge doping, or Be with Cu. These shells are imploded in a U or Au hohlraum with a peak radiation temperature set between 270 and 300 eV. Considerations determining the point design include laser-plasma interactions, hydrodynamic instabilities, laser operations, and target fabrication. Simulations were used to evaluate choices, and to define requirements and specifications. Simulation techniques and their experimental validation are summarized. Simulations were used to estimate the sensitivity of target performance to uncertainties and variations in experimental conditions. A formalism is described that evaluates margin for ignition, summarized in a parameter the Ignition Threshold Factor (ITF). Uncertainty and shot-to-shot variability in ITF are evaluated, and...

The experimental plan for cryogenic layered target implosions on the National Ignition Facility—The inertial confinement approach to fusion

Ignition requires precisely controlled, high convergence implosions to assemble a dense shell of deuterium-tritium (DT) fuel with ρR>∼1 g/cm2 surrounding a 10 keV hot spot with ρR ∼ 0.3 g/cm2. A working definition of ignition has been a yield of ∼1 MJ. At this yield the α-particle energy deposited in the fuel would have been ∼200 kJ, which is already ∼10 × more than the kinetic energy of a typical implosion. The National Ignition Campaign includes low yield implosions with dudded fuel layers to study and optimize the hydrodynamic assembly of the fuel in a diagnostics rich environment. The fuel is a mixture of tritium-hydrogen-deuterium (THD) with a density equivalent to DT. The fraction of D can be adjusted to control the neutron yield. Yields of ∼1014−15 14 MeV (primary) neutrons are adequate to diagnose the hot spot as well as the dense fuel properties via down scattering of the primary neutrons. X-ray imaging diagnostics can function in this low yield environment providing additional information about ...

Shock timing experiments on the National Ignition Facility: Initial results and comparison with simulation

Capsule implosions on the National Ignition Facility (NIF) [Lindl et al., Phys. Plasmas 11, 339 (2004)] are underway with the goal of compressing deuterium-tritium (DT) fuel to a sufficiently high areal density (ρR) to sustain a self-propagating burn wave required for fusion power gain greater than unity. These implosions are driven with a carefully tailored sequence of four shock waves that must be timed to very high precision in order to keep the DT fuel on a low adiabat. Initial experiments to measure the strength and relative timing of these shocks have been conducted on NIF in a specially designed surrogate target platform known as the keyhole target. This target geometry and the associated diagnostics are described in detail. The initial data are presented and compared with numerical simulations. As the primary goal of these experiments is to assess and minimize the adiabat in related DT implosions, a methodology is described for quantifying the adiabat from the shock velocity measurements. Results ...

High-density carbon ablator experiments on the National Ignition Facilitya)

High Density Carbon (HDC) is a leading candidate as an ablator material for Inertial Confinement Fusion (ICF) capsules in x-ray (indirect) drive implosions. HDC has a higher density (3.5 g/cc) than plastic (CH, 1 g/cc), which results in a thinner ablator with a larger inner radius for a given capsule scale. This leads to higher x-ray absorption and shorter laser pulses compared to equivalent CH designs. This paper will describe a series of experiments carried out to examine the feasibility of using HDC as an ablator using both gas filled hohlraums and lower density, near vacuum hohlraums. These experiments have shown that deuterium (DD) and deuterium-tritium gas filled HDC capsules driven by a hohlraum filled with 1.2 mg/cc He gas, produce neutron yields a factor of 2× higher than equivalent CH implosions, representing better than 50% Yield-over-Clean (YoC). In a near vacuum hohlraum (He = 0.03 mg/cc) with 98% laser-to-hohlraum coupling, such a DD gas-filled capsule performed near 1D expectations. A cryogenic layered implosion version was consistent with a fuel velocity = 410 ± 20 km/s with no observed ablator mixing into the hot spot.

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.

BERYLLIUM CAPSULE COATING DEVELOPMENT FOR NIF TARGETS

Abstract Various morphologies have been observed in sputter-deposited Be ablator capsules, including nodular growth, cone growth and twisted grain growth. By devising an agitation method that includes both bouncing and rolling the spherical mandrels during deposition, and by reducing the coating rate, consistent columnar grain structure has now been obtained up to 170 mm. Low mode deformation of the shells is observed on thin CH mandrels, but is suppressed if stiffer mandrels are used. Ablator density measured by weighing and x-ray radiography is 93%–95% of bulk density of Be. Transmission electron microscopy shows 100.200 nm size voids in the film and striations inside the grains. Be shells produced with rolling agitation have met most of the NIF specifications. Some of the few remaining issues will be discussed.

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.