NIF Liquid-Hydrogen Target Gets Its First Test

On April 21, a team of researchers from LLNL, Los Alamos National Laboratory (LANL), and General Atomics conducted the first liquid deuterium-tritium (DT) fuel layer implosion at NIF using a "wetted-foam" target design.

The LANL-led experiment used a target capsule lined with a DT-saturated polymer foam developed over the last decade by members of LLNL’s Material Sciences Division and target specialists at General Atomics (see "Solving the Challenges of Making Liquid-Hydrogen Targets"). The foam-layered fuel capsules open a new NIF experimental platform, providing an opportunity to explore the relationship between hot-spot convergence ratio (the initial fuel radius divided by the imploded radius) and the robustness of hot-spot formation during inertial confinement fusion (ICF) implosions.

The new platform also enables designs which should allow dopants (small, controlled amounts of impurities) to be in direct contact with the fuel. Researchers said doping the foam could be used to diagnose the plasma conditions, such as electron temperature, at the edge of the target’s central hot spot; the density of the cold fuel surrounding the hot spot; and the mix of cold fuel into the hot spot.

The primary goal of the experiment was "to demonstrate our ability to field DT layers at low convergence ratios," said LANL scientist Rick Olson, one of the wetted-foam campaign’s responsible individuals. "The baseline DT-ice-layer ICF ignition design requires a very high- convergence implosion (a convergence ratio of about 35) with a hot spot that is dynamically formed from a thin, inner portion of the DT ice layer," he said. "Several years ago, (LANL colleague) Ray Leeper and I began thinking about a more robust hot-spot formation process that could have a reduced convergence implosion, with a hot spot formed mostly or entirely from mass originating within a spherical volume of DT vapor.

"It soon became clear to us that the path to such a design would involve replacing the DT ice layer with a DT liquid layer," Olson said. "After several years of effort, we had an opportunity to test the liquid DT layer concept in this collaborative LANL/LLNL experiment at the NIF."

The wetted-foam target design allows for flexibility in the hot-spot convergence ratio through adjustment of the initial cryogenic capsule’s temperature and, thus, DT vapor density. Olson said higher initial vapor density both reduces the capsule convergence, leading to more symmetric implosions, and changes both the process of hot-spot formation and the burn history of the hot spot. Both changes are expected to ameliorate challenges to high-convergence, DT-ice-layer implosions, such as the growth of hydrodynamic instabilities, while preserving the physics of layered implosions. Olson said the lower-convergence implosion also could be less sensitive to the x-ray flux asymmetries that occur in the NIF hohlraum and could help increase the NIF shot rate for layered ICF capsules.

The experiment used a liquid DT fuel layer at a temperature of 26 kelvin (-412 degrees Fahrenheit). NIF delivered an 876-kilojoule pulse of ultraviolet light with a peak power of 297 terawatts. Preliminary results show the implosion had a convergence ratio of about 14 and was quite round, with a slightly "sausaged" shape in the equatorial view and with little time-dependent change in shape, as expected for a low-convergence implosion. Neutron yield was about 4.5 × 1014 (450 trillion), close to the yield predicted by one-dimensional simulations.

Development of the wetted-foam capsule began as a Laboratory Directed Research and Development strategic initiative in 2007 with the goal of enabling researchers to "dial in" a desired concentration of dopants in the foam so they would come into direct contact with the fuel and provide diagnostic information about the implosion. The project evolved into a design for mass-produced targets for inertial fusion energy, then was taken up by the LANL researchers to investigate the relationship between convergence ratio and hot-spot robustness.

NIF completed two DT layered implosions in one week for the first time with the liquid layer experiment. It took less than six hours to form the liquid layer once the target was cold, demonstrating the potential for fast turnaround times for wetted foam capsules.

Olson said the next wetted-foam experiment will change the target fielding temperature to increase the convergence by lowering the liquid DT vapor pressure, thereby demonstrating control of the convergence. The platform’s longer-term goals are to provide insights into hot-spot formation through control of the experiment’s initial conditions.

Participating in the first experiment along with Olson and Leeper were John Kline, Alex Zylstra, Austin Yi, Bob Peterson, Paul Bradley, Brian Haines, and Lin Yin of LANL and LLNL’s Bernie Kozioziemski, Tom Braun, Juergen Biener, Jim Sater, Alex Hamza, Abbas Nikroo, Laura Berzak Hopkins, Sebastien LePape, and Nathan Meezan.