For the past six-plus years, target physics designers, target fabrication designers and target production technicians have conducted a technology development program to meet the stringent target fabrication requirements of the National Ignition Campaign (NIC).
The NIC team developed target design and assembly methods to meet requirements for capsule surface finish and positioning, performance at cryogenic temperatures and robustness during assembly and fielding. As requirements changed due to experimental feedback, new designs and processes were developed and tested. Assembly stations and tooling processes and procedures were created to meet target positioning requirements, and component design changes were implemented to improve assembly yield and throughput.
Among the many challenges in the ignition target fabrication process is the formation of the solid fuel layer inside the capsule (see Target Fabrication). Producing layers without grooves or defects requires excellent thermal control and careful monitoring. Target environment temperatures are painstakingly lowered and raised to encourage smooth, consistent layer formation. Once an acceptable layer is achieved near the triple point (the temperature and pressure of the fuel is such that solid, liquid, and gas states coexist), it is rapidly cooled another 1.5 degrees Centigrade to put the hydrogen gas pressure in equilibrium with the solid, enabling further compression during the implosion. The target then must be used within ten seconds of lowering the temperature.
Throughout the target assembly process, a critical element is cleanliness. The current focus is to eliminate any dust particles greater than five microns in diameter on the capsule. This requires assembly in a Class 100 cleanroom, special fixtures for capsule inspection and capsule cleaning, and microscopy tools to inspect capsules for particles added during assembly, including identifying the location of particles in NIF target chamber coordinates. Surface debris and imperfections can interfere with the uniformity of capsule heating and compression.
Surface conditions for the assembled fuel capsules have stringent specifications, and the capsules require polishing to meet these specifications. Conventional polishing can remove the bumps, but leaves scratches behind. The NIC team has developed a proprietary polishing technique that can remove or reduce bumps from 600 nanometers to less than 150 nanometers in height. Before polishing, larger bumps are laser-ablated to reduce their height to less than 600 nanometers. The figure above shows how polishing a carbon-hydrogen capsule dramatically improves its smoothness and consistency.
Crafting tuning targets for NIC experiments began just over a year ago. These surrogate targets are not intended to produce ignition; instead, they generate essential physics information on target conditions to help scientists refine ignition experiment parameters. The NIF Target Fabrication Group had been involved in designing tuning targets for previous experiments; the surrogates, however, were designed on the OMEGA laser at the University of Rochester's Laboratory for Laser Energetics. Designing tuning targets for NIF required customization to produce the best results with NIF's higher energy output and unique target chamber layout, array of diagnostics, and light field.
Since October 2010, four new targets for tuning platforms have been fielded on NIF, including two types of "keyhole" targets that provide information on shock timing and velocity (see October 2010 Project Status); "re-emit" targets that facilitate study of the capsule implosion shape (see November 2010 Project Status); and "convergent ablator" targets that help measure implosion velocity and capsule mass at the end of the implosion (see December 2010 Project Status). Tuning targets for studying fuel mix will soon be produced as well. This array of tuning targets will continue to be used in the coming months, enabling scientists to better assess how small changes to the laser energy, beam geometry and target conditions can affect the processes that occur in the laser-imploded target. Whether the focus is tuning or ignition targets, high-quality target design and fabrication is crucial for scientists to reach NIC's goals and attain fusion ignition.
A three-day workshop focused on the use of NIF for fundamental science research over the coming decade was held from May 10 to 12 in Washington, DC. Jointly sponsored by the National Nuclear Security Administration (NNSA) and the DOE Office of Science, the workshop was designed to inform the broad scientific research communities about NIF's research capabilities and to solicit input on indentifying, guiding, formulating, and prioritizing research that would be enabled by NIF's capabilities.
The workshop was co-chaired by John Sarrao of Los Alamos National Laboratory, Michael Wiescher of the University of Notre Dame, and Kim Budil of LLNL and featured presentations by Don Cook, NNSA Deputy Administrator for Defense Programs; Steven Koonin, DOE Under Secretary for Science; William Brinkman, Director, DOE Office of Science; and Patricia Dehmer, Deputy Director for Science Programs, DOE Office of Science. Numerous LLNL scientists attended the workshop and several provided background briefings on NIF and its capabilities. Individual panels discussed research opportunities at NIF in areas such as laboratory astrophysics, materials in extreme conditions, planetary physics, nuclear physics, and beam and plasma physics. Cross-cutting research and facility user issues also were discussed.
A report on the workshop's results will be issued in the next few months. More information is available here.
An invited presentation on the potential of using Mono-Energetic Gamma Rays (MEGa-rays) to search for hidden nuclear reactor fuel and nuclear bomb material was a highlight of the 2011 Conference on Lasers and Optics (CLEO), held May 1-6 in Baltimore, MD. NIF&PS Chief Technology Officer Chris Barty noted that gamma rays, the most energetic type of light wave, can penetrate through lead and other thick containers and can be tuned to a specific energy so they predominantly interact with only one type of material. A beam of Mega-rays, for example, might be absorbed by the nuclear fuel uranium-235 while passing through other substances including the more common (but less dangerous) isotope uranium-238. That sort of precision opens the door to "nuclear photonics," the study of nuclei with light. "It is kind of like tunable laser absorption spectroscopy but with gamma rays,"Barty said.
In the last few years, early MEGa-ray prototypes have identified elements like lithium and lead hidden behind metal barriers. Barty said the next generation of MEGa-ray machines, which should come on line in a few years, will be a million times brighter, allowing them to see through thick materials to locate specific targets in less than a second. Work is under way at LLNL on a MEGa-ray technology that could be placed on a truck trailer and used to check containers for hidden bomb material. At nuclear reactors, MEGa-rays could be used to quickly identify the extent to which a spent fuel rod has been enriched in uranium-235. They could also examine nuclear waste containers to assess their contents without opening them. This technology might also be employed in medicine to track drugs that carry specific isotope markers.
A two-day workshop to plan for the installation of additional diagnostic equipment on NIF in the 2013-2015 time period was held at LLNL on May 5 and 6. About 80 members of the National Ignition Campaign and Stockpile Stewardship teams and the fundamental science user communities discussed the challenges involved in developing new diagnostics as well as those related to converting existing equipment from other high energy density facilities for use on NIF, with its higher energies and radiation levels and larger target chambers. Led by Joe Kilkenny and Dick Fortner, the workshop discussed NIF's future diagnostic requirements and the state of readiness of proposed diagnostic technology to meet those requirements.
The use of lasers to generate energy in a fusion-fission power plant was one of "7 Radical Energy Solutions" profiled in the cover article in the May issue of Scientific American.
The article, "Fusion-Triggered Fusion: Lasers coax electricity out of spent nuclear fuel," describes LLNL's hybrid fusion-fission concept, in which "neutrons from the fusion explosions generate the fission, eliminating the need to sustain a chain reaction. This arrangement broadens the menu of possible fuels to include unenriched uranium, depleted uranium (a voluminous waste product of uranium enrichment) or even spent fuel from other nuclear reactors – waste that would otherwise have to be stored for thousands of years or undergo complicated and hazardous reprocessing for reuse in a fission plant." View interactive version.
In inertial confinement fusion experiments, implosion symmetry is crucial for successful ignition. A symcap is a surrogate capsule that has the same 2-millmeter outer diameter doped shell as the ignition capsule, but replaces the deuterium-tritium fuel layer with an equivalent mass of plastic outer shell material to mimic the capsule's hydrodynamic behavior.
In a Physics of Plasmas article published online on May 5 (Phys. Plasmas 18, 056307 (2011); doi:10.1063/1.3574504), LLNL researchers and colleagues from Los Alamos National Laboratory and General Atomics discussed the use of the X-ray self-emission from imploding symcaps to measure implosion symmetry. They demonstrated the ability to transfer energy between laser beams in a gas-filled hohlraum using wavelength tuning, successfully tuning the lowest order symmetry of the symcaps in different size hohlraums at different laser energies within the specification established by calculations for successful ignition. Analysis of the imploded symcap X-ray radiation shows a symmetry that is very well correlated with the ignition capsule's shell mass shape.
A key requirement for inertial confinement fusion experiments on NIF is tripling the frequency of the infrared light generated by NIF's neodymium-glass (Nd:glass) lasers. Potassium-dihydrogen-phosphate (KDP) crystals in the final optics assemblies convert the infrared light, which has a wavelength of 1,053 nanometers, to ultraviolet light with a 351-nm wavelength. Such higher frequencies are known as "harmonics" of the initial frequency; ultraviolet is the third harmonic of infrared and is abbreviated 3-omega, or 3ω. LLNL researchers have proposed that one NIF beamline be converted to the fourth harmonic, or 4ω, to serve as a probe beam in a plasma diagnostic technique known as 4ω Thomson scattering. Efforts are under way to determine the most efficient way to accomplish this conversion.
In a paper published in the May 15th issue of Optics Letters (Opt. Lett. 36, 1824-1826 (2011); doi:10.1364/OL.36.001824), the researchers reported the first demonstration of non-critical phase-matching (NCPM) in a deuterated KDP (DKDP) crystal for converting Nd:glass laser light to 4ω (in DKDP, some hydrogen atoms have been replaced by deuterium atoms). The experiment earlier this year showed that the conversion can be efficiently achieved in a 70 percent deuterated KDP crystal by cooling the crystal to 18.5 degrees centigrade; the researchers demonstrated 2ω (green light) to 4ω peak conversion efficiency as high as 79 per cent. The paper was authored by Steven T. Yang, Mark A. Henesian, Timothy L. Weiland, James L. Vickers, Ronald L. Luthi, John P. Bielecki, and Paul J. Wegner, all members of the NIF optics and target group.