During the week of July 31 to Aug. 4, five groups of NIF users worked with LLNL researchers to carry out a successful NIF Discovery Science shot week. The teams conducted 13 experiments in five separate basic high energy density (HED) science experimental campaigns in five days. The campaigns are studying collisionless astrophysical shocks, charged particle stopping power, ionization balance at extreme density, direct-drive hydrodynamics, and amplification of planetary and astrophysical magnetic fields.
Devoting a full week of shot time to fundamental science experiments is an important aspect of NIF’s academic outreach program, said NIF Discovery Science Program Leader Bruce Remington. “We want to open up our facility at about the eight percent (of shot time) level to anyone in the world who has a great idea that would benefit from unique experiments on a very high-energy laser system. There are things you can do on NIF, a roughly two-megajoule laser facility, that you can’t do anywhere else.”
In many cases, he said, the research teams have “tried out and developed their techniques on other laser facilities, so they know what to do. All the other lasers in the world are very good at developing the techniques, but often the full energy that’s required to reach the conditions the researchers require is just not quite there. To study the properties of matter at the center of large planets like Jupiter, or the extra-solar “super-Earths,” or energetic astrophysical events such as accreting black holes or exploding stars (supernovae), they need factors of 50 to 100 more energy and power (available on NIF) to get that done.”
Discovery Science teams typically are allocated two to three laser shots in a 24-hour period for their experiments. “They have full use of the laser beams,” Remington said. “The whole facility is theirs, so they can, within the safety limits of the laser, fire any kind of laser energy and pulse shape and power they would like. They can turn on any diagnostics they would like, as long as there’s time to align them, time them, and turn them on.”
Here are summaries of the five campaigns conducted earlier this month:
Astrophysical collisionless shocks: The international Astrophysical Collisionless Shock Experiments with Lasers (ACSEL) campaign continued with three experiments studying astrophysical collisionless shock creation and magnetic field generation via high Mach number plasma flows. In collisionless shocks, the charged particles in a plasma (a medium consisting of freely moving ions and free electrons) pass by largely without colliding with each other, but trigger collective plasma instabilities that lead to shock formation; such shocks occur in many astrophysical phenomena including supernova remnants, gamma-ray bursts, and cosmic ray acceleration (see “Shedding Laser Light on Mysterious Cosmic Rays”).
“This experiment was trying to see the magnetic nature of collisionless shock formation and evolution that is relevant to cosmic ray particle acceleration,” said NIF & Photon Science physicist Hye-Sook Park, the ACSEL team’s LLNL liaison. “This was diagnosed by the protons generated by an imploding deuterium-helium-3 capsule. Initial data indicate that we achieved at least five times higher proton flux from the D3He capsule compared to our last shot day (March 29), promising high-quality proton images.” Image processing is under way at the Massachusetts Institute of Technology (MIT).
These were the first NIF shots in which Hans Rinderknecht, a Lawrence Fellow at LLNL, served as shot RI (responsible individual); Rinderknecht was a collaborator for the ACSEL project as an MIT graduate student and a participant in the NIF-MIT PhD Thesis Program. “The shot day was joined by about 20 collaborators from around the world,” Park said. “It is exciting to see that so many young researchers are so keen on the NIF Discovery Science program and their excitement over the scientific results and implications from the NIF data.”
Charged particle stopping power in dense degenerate plasmas: The “dE/dx stopping power” shots probed proton stopping power and scattering in warm dense matter (WDM), a complex state of matter that occurs during inertial confinement fusion (ICF) experiments and also is relevant to nuclear plasma-astrophysics processes. The upper NIF beams shocked a slab of glassy high-density carbon while the lower beams drove an “exploding pusher” target filled with deuterium and helium-3 as a proton source.
A new target design was successfully implemented that enables simultaneous shock characterization and proton probing by the velocity interferometer system for any reflector (VISAR) diagnostic. Previously, acquiring this data required two shots. Good data were acquired on the VISAR and secondary diagnostics; the proton data will require a few weeks for processing.
Participating on-site for the shots were Alex Zylstra of LANL, Ryan Rygg of LLNL, and Chikang Li of MIT, along with additional researchers at MIT supporting the diagnostics, and theory collaborators at LLNL, UC Irvine, and Sandia National Laboratories (SNL).
X-ray Thomson scattering in warm dense matter: An international team examined x-ray Thomson scattering (XRTS) characterization of HED plasmas to experimentally test ionization balance models (also see “NIF Experiment Challenges Standard Ionization Models”). The experiments’ goal was to measure the in-flight electron density, temperature, and ionization of imploding and stagnating plastic and beryllium capsules using XRTS. Two of the shots fielded a new hohlraum drive for a beryllium capsule implosion and characterized the capsule trajectory plus the time, temperature, and ionization at highest density of 40 grams per cubic centimeter (about 20× compression). The measurements provide the first experimental data to benchmark ionization balance models for plasmas at these extreme densities.
Participating in the shots were Benjamin Bachmann, Tilo Döppner, Annie Kritcher, Joe Nilsen, and Nino Landen of LLNL; Siegfried Glenzer of the SLAC National Accelerator Laboratory; Michael MacDonald of SLAC and the University of Michigan; Alison Saunders and Roger Falcone of UC Berkeley; Dominik Kraus of the Helmholtz-Zentrum Dresden-Rossendorf Laboratory in Dresden, Germany; and Paul Neumayer of the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany.
Planar direct-drive platform development: Long-Duration Direct-Drive Planar Hydrodynamics experiments are aimed at measuring the laser imprint and direct-drive uniformity on a planar target in order to qualify a NIF long-duration direct-drive platform. The experiments diagnosed for the first time the growth of 3D ablative Raleigh-Taylor hydrodynamic instability (RTI) seeded by intentional large imprint features, or “ripples,” on the target capsule. The growth was tracked from 12 to 24 nanoseconds over two experiments.
Raleigh-Taylor instability occurs when a dense fluid is accelerated by a less dense fluid, resulting in fingers of dense material (spikes) interpenetrating the less dense fluid which rises as “bubbles,” leading to mixing of the two fluids. Preliminary analysis indicates that up to four generations of bubbles were created as larger bubbles overtake and merge with smaller bubbles as a result of the long-duration drive, which is available only on NIF. The shots successfully concluded the direct-drive platform development effort and will help understanding of the highly nonlinear behavior of the RTI at the ablation front in direct drive.
Participating team members were Alexis Casner of the French Alternative Energies and Atomic Energy Commission (CEA), Vladimir Smalyuk, David Martinez, and Shahab Khan of LLNL, Laurent Masse of CEA and LLNL, and Corentin Mailliet, a PhD student at the CELIA Laboratory of Bordeaux University in France. Also participating in the campaign are the Nuclear Research Center Negev in Israel, the Laboratory for Laser Energetics (LLE) at the University of Rochester, Princeton Plasma Physics Laboratory, and the University of Michigan.
Turbulent dynamo generation: The Turbulent Dynamo (TDyno) Discovery Science project studies how small seed magnetic fields are amplified under turbulent plasma conditions, a process that happens throughout the universe. Cosmological magnetic fields could have played an important role in the formation of the first stars and could seed the stronger fields seen in galaxies and clusters of galaxies today. NIF is the only laboratory facility that can create the proper plasma conditions to generate fully formed astrophysical collisionless shocks and strong magnetic field amplification by a turbulent dynamo process. More information and the results of the TDyno experiments will appear in the next issue of NIF & Photon Science News.