An expanded target fabrication facility, which will be capable of producing NIF targets at the rate of one per day, is nearing completion in Bldg. 381. Replacing a 700-square-foot area in Bldg. 298, the 3,000-square-foot facility is a "class 100" cleanroom – allowing, for the first time, the entire NIF target assembly process to take place in a clean environment.
The new facility houses specialized equipment and an expanding roster of trained technicians who assemble targets by bringing together millimeter-scale components provided by NIC partner General Atomics, bonding them into subassemblies with tiny amounts of adhesive applied with whisker-thin applicators and assembling them into completed targets. The production-line-style facility includes optical coordinate measurement machines and microscopic inspections to ensure that the target capsules are clean and properly centered in the hohlraums. The facility also has the ability to bring the finished assemblies to cryogenic temperatures to test for leakage and electrical integrity.
Target Fabrication Manager Beth Dzenitis said the process is organized into four production integrated product teams: thermal-mechanical components, capsule/final assembly, tents (the plastic membranes that suspend the capsules inside the hohlraums), and special diagnostic band assemblies. "Target designs and scales are evolving as the NIC ignition tuning experiments are conducted," Dzenitis said. "The special diagnostic band element of the production line allows us to build target subassemblies with unique elements such as backlighters (for "convergent ablator" targets) and diagnostic cones (for "keyhole" targets) (targets that provide diagnostic information on shock timing, implosion velocity, and capsule mass at the end of an implosion). These targets are all very complex and require unique assembly and metrology capability to meet specifications," she said.
On August 5, members of an international study group who conducted an in-depth examination of the current NIC tuning campaign held an informal "out-brief" to summarize the results of their study for NIC researchers. The study members reported on the seven focus areas of the two weeks of intensive interaction with the NIC science team aimed at understanding the tuning campaign's experimental results and numerical simulations to date.
Facilitated by LLNL researchers Mike Key, John Lindl and Mordy Rosen, the study group consisted of 18 internationally recognized experts on inertial confinement fusion (ICF) from the United States, Europe, and Israel. The group examined key ICF issues such as the tuning of the radiation pulse and the symmetry and velocity of the capsule implosion, and made suggestions for the next phase of the tuning campaign.
The study group plans to submit a written report giving "constructive independent advice" to NIC management and the science team. NIF Director Ed Moses thanked the participants for taking the time to conduct such an "in-depth, honest scientific review" that established a useful method for obtaining future expert guidance to the NIC team.
In talks at two recent conferences, LLNL researchers discussed the requirements for crystal and glass materials for the Laboratory's Laser Inertial Fusion Energy (LIFE) power plant design.
At the American Crystal Growth Conference, held July 31 to Aug. 5 in Monterey, Kathleen Schaffers and colleagues said LIFE, with its requirement for firing diode-pumped lasers ten to 15 times a second, has prompted research into advanced materials. LIFE has selected neodymium-phosphate laser glass as its gain material, but holds open the option for adopting other materials in the longer term. Options include ytterbium ceramics or crystals, which in principle have longer lifetimes and may be able to withstand higher powers.
Other materials that will be required for the LIFE laser include large-aperture quartz, sapphire plates appropriate for high-power operations, and highly-deuterated potassium-dihydrogen-phosphate crystals for large-aperture frequency conversion crystals and Pockels cell optical switches. Joining Schaffers in the presentation were Andy Bayramian, Amber Bullington, Robert Deri, Al Erlandson, Mark Henesian, and Chris Stolz.
In an invited talk at the Aug. 3-5 University Conference on Glass Science at Rensselaer Polytechnic Institute in Troy, NY, Tayyab Suratwala described the material challenges for additional laser functionality and for next-generation fusion energy lasers, including the need to develop novel optical filters and reduce optical finishing costs while maintaining or increasing laser damage resistance.
The Advanced Radiographic Capability (ARC) beamline consists of four of the 192 NIF beamlines, with each ARC aperture operating in a split-beam architecture. An adaptive optics (AO) system on the petawatt (quadrillion-watt) ARC laser is used to improve its focusing ability. Focusing will be sensitive to shot-to-shot variations in the flashlamp-induced wavefront aberrations of the NIF beamlines.
In an article published in Applied Optics on August 1, LLNL researchers reported on experiments that examined the repeatability of NIF beamline aberrations to determine the extent to which shot-to-shot variations may degrade the performance of a proposed adaptive optics system for the short-pulse ARC beamline. The research concluded that the effect of the aberrations will not require major changes in the current design for the ARC AO system.
Contributing to the article were LLNL researchers Doug Homoelle, Mark Bowers, Tracy Budge, Chris Haynam, John Heebner, Mark Hermann, Ken Jancaitis, Jeff Jarboe, Kai LaFortune, Thad Salmon, Tania Schindler, and Mike Shaw.
LLNL's Bruce Remington and colleagues from LLNL and General Atomics described recent studies on NIF of solid-state material properties at high pressures (more than one million Earth atmospheres) and high strain rates (106 to 108 sec-1) at the 7th International Conference on Advanced Materials (THERMEC'2011), held Aug. 1-5 in Quebec, Canada. The NIF experiments achieved quasi-isentropic, or "ramp-compressed," high-pressure conditions utilizing a unique reservoir-gap-sample configuration in which the sample materials stay well below the melting temperature while they undergo plastic deformation under high pressure.
Techniques for measuring and modeling the hydrodynamic instabilities in imploding NIF fusion target capsules were described by NIC researchers at the Third International Turbulent Mixing and Beyond Conference, held Aug. 21-28 in Trieste, Italy.
In a presentation titled, "Mix modeling for the NIF ignition capsule design," researchers Dan Clark, Steve Haan, Andy Cook, John Edwards, Bruce Hammel, Joe Koning, and Marty Marinak discussed recent efforts based on state-of-the-art hydrodynamics simulations to understand and mitigate the mixing instabilities inherent in NIF implosions. An example is the mixing that occurs when the ablator, or outer surface of the target capsule, interacts with the layer of cryogenic fuel just inside the inner surface of the capsule. The researchers said the computer simulations are used to predict mix and then modify the capsule design to mitigate its impact.
In a separate presentation titled, "Experimental techniques for measuring the Rayleigh-Taylor instability in inertial confinement fusion," LLNL researcher Vladimir Smalyuk said Rayleigh-Taylor (RT) instability, which occurs when a heavier material is decelerated by a lighter material, is a concern in inertial confinement fusion experiments because target modulations grow in both the acceleration and deceleration phases of implosions. This can lead to shell disruption and performance degradation of imploding targets. Smalyuk described experiments on the OMEGA laser at the University of Rochester which explored the linear, nonlinear, and turbulent mixing regimes of RT growth.
The findings from a series of simulations designed to improve the agreement between modeling of NIC ignition capsule performance and the results of recent experiments were reported by LLNL researchers in a Physics of Plasmas article published online on Aug. 3.
Ignition capsule designs for NIC have continued to evolve in light of improved physical data inputs, improving simulation techniques, and, most recently, experimental data from a growing number of sub-ignition experiments. The paper summarizes a number of recent changes to the cryogenic capsule design and some of the latest techniques used to simulate its performance, such as high-resolution radiation hydrodynamics simulations.
The goal of the simulations has been to model, as accurately as current computing capabilities allow, the important short-wavelength and 3-D hydrodynamic instabilities that influence ignition. As the first round of experimental data has become available from NIC, the capsule design has been re-optimized accordingly and the design choices verified against these high-resolution simulations. The researchers said that the latest experimental inputs from NIC coupled with the improving simulation capability provide a fuller and more accurate picture of NIC ignition capsule performance.
Contributing to the article were Dan Clark, Steve Haan, Andy Cook, John Edwards, Bruce Hammel, Joe Koning, and Marty Marinak.
A series of Physics of Plasmas articles describing the steps along the path to National Ignition Campaign ignition experiments in the coming months were among the journal's most-downloaded papers in June.
The papers, by lead authors Steve Haan, Nino Landen and John Edwards and their colleagues, introduced the ignition threshold factor (ITF), the criterion being used to assess progress towards achieving ignition conditions; reported on the experimental approach the researchers are using to optimize the principal implosion input variables in the presence of physics uncertainties to substantially increase the probability of ignition; and discussed the use of cryogenically layered THD targets to achieve the compressed fuel conditions required for ignition. An introductory article by Moses and NIF&PS Chief Scientist John Lindl described the key requirements for achieving ignition.
On August 3, NIF Director Ed Moses gave a talk on NIF and the Laser Inertial Fusion Energy (LIFE) concept at a Washington, DC, briefing on "Fusion Energy: An Opportunity for American Leadership and Security." Sponsored by the American Security Project in conjunction with the Congressional Research & Development (R&D) Caucus, the briefing was intended to enhance policymakers' awareness of the benefits of fusion energy as a potential solution to the energy use and production challenges of the next few decades. Also presenting were Rep. Rush Holt (D-NJ), co-chair of the R&D caucus; Lt. Gen. Daniel Christman of the U.S. Chamber of Commerce; and Dr. Stewart Prager of the Princeton Plasma Physics Laboratory.
A new Website providing detailed information on the design, operation, and commercialization potential of a Laser Inertial Fusion Energy (LIFE) power plant in the 2020-2030 time frame is now available to the public. To visit the site, go to https://life.llnl.gov/.
HYDRA, a three-dimensional radiation hydrodynamics code that contains the physics necessary for modeling imploding NIF ignition capsules, can simulate the entire ignition target in 3D, including the hohlraum, capsule and all relevant features. It also simulates the hydrodynamic instabilities that occur when the capsule implodes.
To validate the HYDRA code during the later stages of an implosion, LLNL researchers recently conducted a high-energy-density (HED), X-ray-driven, high-velocity single-jet experiment on the OMEGA laser at the University of Rochester (a jet is a rapid stream of liquid or gas forced out of a small opening). The experiment imaged the jets into the late time regime when hydrodynamic instabilities have had a chance to develop. The experiment significantly extended the data set of HED jets into the late time regime, with improved image quality over previous experiments.
The experiment showed that HYDRA numerical simulations match the experimental results reasonably well, but "exterior" details of the laser target must be included to obtain a match at late times in the implosion. Reporting the results in a Physics of Plasmas paper published online on Aug. 5 were former NIF&PS employee Freddy Hansen and LLNL researchers Thomas Dittrich, James Elliott, Gail Glendinning, and David Cotrell.
Optical components made of fused silica play a key role in shaping NIF's laser beams and delivering them to the target. High fluence (energy per unit area) laser light, however, can produce significant damage in fused silica optics. The lateral size of this damage grows exponentially with the number of laser pulses, shortening the optics' lifetime.
In an article published online on Aug. 17 in the journal Physical Review B, LLNL researchers described recent experiments and hydrodynamics modeling of the early response of bulk fused silica materials to the energy deposited by nanosecond-long laser pulses. The authors said that images from a time-resolved microscope system, as well as the simulations, revealed a clear boundary between two distinct phases – a hot plasma and solid silica – during laser-induced breakdown of the material. Hydrodynamic instabilities that arise where the two phases meet, known as Rayleigh-Taylor instabilities, may be responsible for the formation of cracks in the stressed material that leads to irreversible damage, according to the researchers.
Contributing to the paper were researchers Paul DeMange, Raluca Negres, Rajesh Raman, Jeff Colvin, and Stavros Demos.
A third presentation at the Turbulent Mixing conference, "Radiation hydrodynamics experiments at the National Ignition Facility," discussed current laboratory astrophysics experiments being conducted at NIF by University of Michigan, LLNL, and Florida State University researchers. The experiments are relevant to the complex radiation hydrodynamics that occur during the core-collapse explosions of red supergiant stars.
The presentation summarized two test shots that were performed on NIF and described the integrated physics experiments currently scheduled at the facility. Contributing to the presentation were Carolyn Kuranz, Paul Drake, and Channing Huntington of the University of Michigan, along with LLNL researchers Hye-Sook Park, Bruce Remington, and Aaron Miles, and Tomek Plewa of Florida State University.
Several Laboratory researchers gave presentations on the instruments designed for use in high-yield experiments on NIF from Aug. 21 to 25 at the annual SPIE Optics & Photonics Conference in San Diego.
The talks covered radiation-induced noise in x-ray imagers, by Christian Hagmann and colleagues; the effects of radiation on laser beam alignment, by Abdul Awwal and colleagues; and the use of phosphors with a high optical output and fast decay time to improve the signal-to-background ratio in x-ray framing cameras used in harsh radiation environments, by Nobuhiko Izumi and colleagues. Other talks discussed recent results obtained from NIF experiments using gated x-ray imagers, by Steven Glenn and colleagues; and NIF's laser alignment tolerancing tools and alignment methods, by Scott Burkhart.