Proven capability to recreate cosmic forces in the laboratory has won highly competitive contract awards for physicists Will Fox and Derek Schaeffer at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and Princeton University. The joint DOE Office of Science and National Nuclear Security Administration (NNSA) three-year awards will enable Fox and Schaeffer to use powerful lasers to reproduce high-energy astrophysical plasmas under extreme conditions to probe processes such as space storms that can disrupt cell phone service and electric power grids. Funding totals $550,000 for each project.
This joint program “is an excellent example of leveraging resources within the Department of Energy to advance discovery-driven science,” said James Van Dam, Associate Director of Science for Fusion Energy Sciences in the DOE Office of Science. “These projects at the frontier of high-energy density plasma science also have the potential for advancing our nation’s industrial capabilities and homeland security.”
Astrophysical plasmas — gases composed of free electrons and atomic nuclei that make up 99 percent of the visible universe — produce a process called magnetic reconnection that occurs throughout the universe. The process, which causes the breaking apart and violent reconnection of the magnetic field lines in plasma, underlies spectacular events such as solar flares, Northern Lights, and disruptive space storms that can affect the Earth. Astrophysical plasmas also give rise to supersonic shock waves that act as cosmic accelerators that propel cosmic rays and supernova remnants to near the speed of light, making understanding how such shocks are generated a fundamental astrophysical question.
Fox will create laser-produced plasmas at the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory (LLNL) and the Omega EP laser facility at the University of Rochester. “What we want to do is make plasmas in the lab that are getting more and more like plasmas out in the cosmos,” said Fox, who will explore laser-produced plasmas that create magnetic reconnection. The more like astrophysical plasmas laboratory plasmas can become, the closer look and better understanding they can provide to researchers studying cosmic processes.
A key issue for Fox will be probing the puzzling rate of reconnection that unfolds more rapidly in astrophysical plasmas than theory predicts. Such plasmas have very low rates of magnetic dissipation that should slow down reconnection, according to theory. “What we need to understand is how reconnection remains fast even when magnetic dissipation is very low,” Fox said. “We need to do experiments in a very low dissipation regime to understand what’s happening out in the cosmos.”
Among the potential triggers for rapid reconnection to be explored are what are called “plasmoid instabilities,” bubbles that form in current-carrying plasma sheets. When these bubbles grow large enough, they create disruptions that can lead to fast reconnection. “These instabilities are quite important in laser-plasma regimes,” Fox said.
Schaeffer will investigate what are called collisionless shocks— high-energy shock waves that occur throughout the cosmos and are not caused by collisions among astrophysical particles. “We see these supersonic shocks all over the place,” Schaeffer said. “They’re very far away and hard-to-impossible for satellites to reach. The challenge has always been studying them in a controlled way and we’re developing this platform to investigate them in the lab.”
Schaeffer will conduct his experiments on the Omega laser facility in collaboration withFox and Julien Fuchs of the Laboratory for the Use of Intense Lasers (LULI) in Paris, and is upgrading a platform that he has been using there. “The main physics question in our proposal is about particle heating,” he said. “You start with a bucket of energy in which all of the ions are moving at constant speed and after they pass through the shock wave, they’re going much slower and are also much hotter. The electrons are hotter too.”
He continued: “So why does a given amount of energy go into heating the ions and a given amount go into the electrons? It’s a simple question but there’s not a simple answer that explains how the process works.” To move closer to an answer Schaeffer will compare his findings “to what theory has looked at and ideally what spacecraft have looked at as well.”
He will also serve as responsible individual (RI) for experiments on NIF that Fox will conduct. He will, for example, implement how the lasers are set up and how the diagnostics are used. “Derek and I work very closely together,” said Fox, who is also collaborating with physicist Gennady Fiskel of the University of Michigan and scientists at the University of Rochester and LLNL. Fox added that his own award “will support the experiments on Omega and NIF as well as simulations and theoretical studies to try to put together the whole story of what’s going on.”
The researchers thank the programs that provide the facilities and time to run the experiments. These include the NIF Discovery Science program at LLNL, the National Laser Users’ Facility at the University of Rochester, and the LaserNetUS program.
PPPL, on Princeton University's Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit energy.gov/science.