Photons & Fusion is a monthly review of science and technology at the National Ignition Facility & Photon Science Directorate. For more information, submit a question.
It doesn't fetch coffee – as much as some workers in the NIF Target Bay would like it to – but the remote-controlled robot now prowling Level 2 of the Target Bay has an even more important job: checking beta and gamma radiation levels after a high-neutron-yield NIF experiment.
The robot, dubbed D2T3 in honor of the NIF fusion fuel isotopes deuterium and tritium, recently passed its first remote-control field test and AHJ (Authority Having Jurisdiction) inspection. System Manager Casey Schulz, a mechanical and robotics engineer who has helped develop a wide variety of robotic systems, said the robot can be operated remotely from anywhere in Bldg. 581 with access to the NIF wireless network.
D2T3 is equipped with six cameras and six visual laser references to provide positioning and guidance information for its operators. The Fluke Ion Chamber beta/gamma survey instrument can be lowered from its normal chest-high position so the robot can "duck" under the low-clearance target positioner housings that protrude into the Target Bay. Several safety features are built into the system, including an emergency stop function if communication is lost, either on the robot or at the control station.
"The blue boxes on the sides are a collision awareness system," Casey said. "When they sense an obstacle within three inches on any side, the remote-control screen flashes and power to the motor is cut in half so the operator can avoid a collision."
D2T3 was developed in partnership with the California Mechatronics Center within the Mechanical Engineering, Mechatronics Engineering & Manufacturing Department at California State University, Chico.
The NIF Cryogenic Target Fabrication Team celebrated the completion of its 300th "Scale 575" hohlraum with cake and award certificates on March 15. Fabrication of the Scale 575 hohlraums, which are slightly larger in diameter (5.75 millimeters) and shorter than previous hohlraums, began in June 2011; number 300 was completed in February, and the team now is producing about 20 new hohlraums a month.
"The way this team works together and takes pride in your accomplishments is just outstanding," Paul Wegner, Deputy NIF&PS Program Manager for Laser Science, Optics and Targets, told the group. NIF targets "take dedication, they take commitment, focus, incredible attention to detail to make," he said. "Our users, especially our ignition colleagues, are very highly appreciative of what you do...and that's something you should take great pride in."
Added Target Fabrication Manager Alex Hamza: "You make the best targets and it is because of your talent and your ability to produce the most complicated targets that get shot at NIF, or anywhere else."
The March 2013 issue of LLNL's scientific research publication, Science & Technology Review, takes an in-depth look at the progress NIF has made in the last 18 months toward its goal of demonstrating ignition – a fusion reaction in the laboratory that produces more energy than the amount of laser energy deposited on the target.
The article, "On the Path to Ignition," cites the accomplishments of the National Ignition Campaign in developing and validating a large body of scientific knowledge and major new experimental, diagnostic, and target manufacturing capabilities. The article also describes the remaining challenges to be met for ignition to be achieved.
In the article, NIF Chief Scientist John Lindl attributes NIF's success to the "heroic efforts" of the entire NIF staff. "In all areas of the organization – lasers, diagnostics, cryogenics, and operations – people have worked incredibly hard and with incredible skill," Lindl says. "As a result, we have made huge advances in technology, materials, and scientific understanding.
"Discovery science is the exploration of the unknown," Lindl adds. "Ignition remains a grand scientific challenge that requires unprecedented precision from the laser, diagnostics, targets, and experiments. We can't say exactly when we will achieve ignition, but we have made tremendous progress and have developed an exciting experimental plan to take us the rest of the way."
March 3 marked the 15th year without a lost work-time injury for the NIF Warehouse Group. Since warehouse operations began in 1998 at an off-site facility, the group has been involved in the receipt, storage and/or delivery of virtually every component that has gone into the construction of NIF, installation of the laser systems, and the conduct of the National Ignition Campaign. A celebration was held on March 6 to recognize the team's impressive accomplishments in both safety and productivity.
"There's a difference between paper safety and genuine safety," NIF&PS Mission Support Director Valerie Roberts told the group. "You really represent genuine safety. You have the culture, you care for each other, you watch out, you do the right thing. I'm really proud of what you've done, and I want to thank you." Added NIF Director Ed Moses, "Fifteen years without a lost-time injury, especially in a warehouse setting with the constant movement of heavy, bulky materials, is a truly remarkable achievement. Congratulations and thanks to every member of the team for your commitment to helping each other work safely."
Tammy Ma and Chris Ebbers of NIF&PS were keynote speakers at Warm Springs Elementary School's "Day of Innovation" on March 22. The presentations were part of the Fremont school's third Science Alliance Week, a celebration of scientific invention touted as a model for school science programs nationwide.
Tammy, a physicist who grew up in Fremont and was inspired by LLNL community outreach programs, told the students that they are the future generation of scientists and engineers who will meet the grand challenge of creating life-sustaining energy from a fusion energy power plant. In a thank-you note, Clyde Mann, the event’s founder and teacher of third- through sixth-grade science, said, "You could see by the smiles on (the students') faces upon leaving the auditorium that you all did a great job."
Mann is working with members of Congress to establish a National Science Day throughout the country. "If we can execute (these programs) countrywide, the economic landscape of this country will change dramatically for the better in the next ten years," Mann said. "We will create a nation of innovators again."
Jasmine Tong-Seely, 12, the school's student body president, said in an address to the students: "I wish every child in America would have the chance to participate in a science program like the one we have. Because then the future of America will be bright and even better than before."
Ray Beach of NIF & Photon Science has been named a Distinguished Member of the Technical Staff (DMTS) in recognition of his extraordinary scientific and technical contributions to Lawrence Livermore National Laboratory and its missions.
Beach, a 26-year LLNL veteran, has pioneered many current techniques in the field of high-average-power diode-pumped lasers and demonstrated a number of firsts in the field. Two developments alone played a key role in developing the NIF laser system and have also found wide use in laser applications: two-dimensional arrays of radiance-conditioned laser diodes and their application to end-pumping laser rods, and delivery of laser power through lens ducts to deliver diode pump radiation to the ends of laser rods.
Among many other breakthroughs, Beach developed the first ground state depleted laser using Nd3+ in yttrium orthosilicate and the first end-pumped solid-state laser using composite laser rods having undoped endcaps. The latter technique has been widely adopted throughout the average power diode-pumped laser community for pushing diode end-pumped lasers to higher average powers. For his pivotal research, Beach has received three R&D 100 Awards and has been named a fellow of the Optical Society of America and SPIE.
A 2004 paper by Ray Beach and colleagues from LLNL and the U.S. Army Research Laboratory, "End-pumped continuous-wave alkali vapor lasers: experiment, model, and power scaling," was among the 15 most-cited articles published in the Journal of the Optical Society of America B over the last nine years.
The paper described the characteristics of alkali vapor lasers, which combine the best features of solid-state and gas lasers in a single laser system. Last December marked the tenth anniversary of the first demonstration of a resonance-transition alkali laser using rubidium vapor. This was the first in a series of alkali-vapor lasers developed in the NIF & Photon Science Directorate over the past decade.
A NIF Hohlraum Physics Working Group (HPWG) has been formed to provide a forum for discussing hohlraum physics in support of inertial confinement fusion and other NIF missions and to encourage future collaborations. The HPWG is a follow-on activity to the May 2012 Science of Fusion Ignition on NIF Workshop.
The first in a series of planned HPWG meetings was held as a Web conference on March 19. Of the more than 120 invited scientists from the United States and abroad, about 60 attended in person and an additional 20 participated on-line. Updates of NIF hohlraum physics experiments were provided at the meeting; subsequent meetings will provide additional updates and discuss potential future collaborative experiments, theoretical studies, and modeling the interaction of hohlraums with high-energy/high-power lasers.
LLNL researchers have performed record particle-in-cell (PIC) simulations – the largest simulations by number of cores ever performed – using all 1,572,864 cores of Sequoia, the world's largest supercomputer. PIC simulations are used extensively in plasma physics to model the motion of charged particles, and the electromagnetic interactions between them, that make up ionized matter. Supercomputers such as Sequoia enable these codes to follow the simultaneous evolution of tens of billions to trillions of individual particles in highly complex systems.
Frederico Fiuza, a physicist and Lawrence Fellow at LLNL, performed the simulations to study the interaction of ultra-powerful lasers with dense plasmas in a proposed method to produce fusion energy in a laboratory setting. The method, known as fast ignition, uses lasers capable of delivering more than a petawatt (quadrillion watts) of power in a fraction of a billionth of a second to heat compressed deuterium and tritium (DT) fuel to temperatures exceeding the 50 million degrees Celsius needed to initiate fusion reactions and release net energy.
The project is part of the U.S. Department of Energy's Office of Fusion Energy Science Program. Sequoia, based on IBM BlueGene/Q architecture, is the first machine to exceed one million computational cores. It also is No. 2 on the list of the world's fastest supercomputers, operating at 16.3 petaflops (16.3 quadrillion floating point operations per second). Fiuza has been selected to receive the Ph.D. Research Award of the European Physical Society's Plasma Physics Division. For more information, see the LLNL news release.
The Joint National Security Applications Council Peer Review Panel (JNSAC-PRP) held the first peer review of NIF national security applications experiments via Web conference on March 28. The 11-member panel reviewed and rated 10 proposed NIF experiments for fiscal years 2014 and 2015 in areas such as nuclear forensics, Department of Defense-related x-ray effects, and other topics. The review is one aspect of the development NIF as a user facility.
For studies of the evolution of astrophysical objects such as the Carina and Eagle nebulae, scaled, laser-driven high-energy-density experiments can bridge the gap between observation and simulation.
In the interstellar medium, where the star-forming regions of dense clouds of gas and dust exist, some regions of interstellar gas can vary greatly in density. As a star begins to form, it bombards the non-uniform clouds with radiation, producing radiation-hydrodynamics-driven effects, such as shocks and radiative heat waves.
To better understand these effects, lead author Amy Cooper was joined by fellow LLNL colleagues and collaborators from AWE and General Atomics to conduct a campaign of experiments to develop a platform for radiation hydrodynamics studies on NIF. As reported in the March issue of Physics of Plasmas, a subsonic radiative heat wave was driven through a tantalum oxide foam. Because the wave was subsonic, it affected the evolution of the features as it passed through the foam. Streaked x-ray radiography images of the patterns in the evolving foam were the first successful radiography measurement of the evolution of well-defined foam features under a driven, subsonic wave in the diffusive regime.
The researchers said the experiments mark the successful development of an experimental platform for executing, modeling, and imaging an evolving radiation hydrodynamics experiment. "With careful scaling," they said, "this platform could be used for a radiation hydrodynamics laboratory astrophysics experiment targeted towards a specific event or object, such as the Carina or Eagle nebulae."
LLNL researchers reported on the results of numerical modeling of the effects of capsule shape asymmetries on NIF implosion experiments in a Physical Review Letters paper published online on Feb. 14. The major asymmetry factor currently under exploration at NIF is referred to as "P4" or "low-mode" asymmetry and leads to a diamond-shaped, as opposed to spherical, imploded deuterium-tritium (DT) region.
The study showed that large-amplitude P4 implosion asymmetries imposed by hohlraum-generated radiation flux asymmetries cause spatial variations in capsule and fuel momentum that prevent the DT ice layer in the NIF target capsule from being decelerated uniformly by the hot spot pressure. This asymmetry inhibits the transfer of implosion kinetic energy to internal energy of the central hot spot by as much as 50 percent, significantly reducing capsule performance and neutron yield. The study also found that these long-wavelength asymmetries may be playing a significant role in the observed yield reduction of NIF DT implosions compared to detailed post-shot two-dimensional simulations.
The researchers said low-mode capsule shape distortions "may explain some of this apparent discordancy, as these simulations indicate they can reduce the conversion of implosion kinetic energy to hot spot internal energy, thereby bringing hot spot mass, energy, temperature, and neutron yield more in line with experiments."
Lead author Robbie Scott of LLNL and the Central Laser Facility at the Rutherford Appleton Laboratory (RAL) in the United Kingdom was joined by Peter Norreys of RAL and by LLNL researchers Dan Clark, Dave Bradley, Debbie Callahan, John Edwards, Steve Haan, Oggie Jones, Brian Spears, Marty Marinak, Richard Pj Town, and Larry Suter.
Tests and simulations show that inexpensive alternate hohlraum materials for inertial fusion energy (IFE) perform as well as the gold hohlraums currently used in ICF experiments on NIF.
In a Scientific Reports paper published March 14 by Nature.com, LLNL researchers and a collaborator from General Atomics reported on the results of experiments on the OMEGA Laser Facility at the University of Rochester comparing the performance of alternate hohlraums with the gold hohlraums. They said deuterium-filled plastic target capsules inside the hohlraums produced similar total neutron yields, “a direct indication that both (materials) produce comparable radiation drive in the integrated geometry required for ICF.”
They said both types of hohlraums “have very similar bang times for (two different laser) pulse shapes, providing strong evidence that the capsule implosion velocities are similar and that the hohlraums produce a similar capsule drive environment.” (Bang time is the time interval between the start of the laser pulse and peak x-ray emission from the fuel core.)
Family members and friends of Lewis H. Strauss, son of former U.S. Atomic Energy Commission Chairman Lewis L. Strauss, visited NIF for briefings and a tour on March 22. Strauss, AEC chairman from 1953 to 1958, was a major supporter of Project Sherwood, an early U.S. government program to develop controlled fusion.