A landmark has been reached at the University of Rochester's Laboratory for Laser Energetics (LLE). The Omega Laser Facility has been used to measure a nuclear scattering cross-section with more precision than previously achieved with particle accelerators.
But in many ways, the data are the least important part of the experiment. "This is the first time a high-energy-density laser facility has been used to advance the field of nuclear physics," said Deputy Director David Meyerhofer.
Researchers from the Massachusetts Institute of Technology, the University of Rochester, and Lawrence Livermore National Laboratory (LLNL), collaborated on the project and published the results in a recent Physical Review Letters.
The research team created a hot, dense plasma in which electrons are stripped off their parent atoms to create an interpenetrating gas, or "soup," of positive and negative charges. To achieve this plasma state, all 60 of the facility's powerful laser beams were used to strike the outer surface of a glass capsule, which was one millimeter in diameter and filled with deuterium (D) and tritium (T), heavy forms of hydrogen. The laser beams generated a rapidly expanding high-temperature plasma gas on the surface of the capsule, causing the capsule to implode on itself. This implosion, in turn, created an extremely hot (100,000,000 kelvin) plasma of D and T ions and electrons inside the capsule. A small fraction of these D and T ions fused together, a process that generated neutrons travelling at one-sixth the speed of light with about 14.1 million electron volts of energy. (In contrast, an ordinary chemical reaction — such as burning wood or coal — generates about one electron volt of energy.) As these energetic neutrons raced out of the imploding capsule, some collided and scattered like billiard balls off the surrounding D and T ions. The researchers then measured the energy transferred from the neutrons to the ions.
Meyerhofer said it is difficult for accelerators to measure nuclear scattering cross-sections at lower temperatures (e.g. the temperature of the sun) than what is typically reached in particle accelerators. The Omega laser, by contrast, was able to provide more data points and more precise measurements.
"This is the first step in using the Omega Laser Facility to measure cross-sections that are relevant to the way the sun works," said Meyerhofer.
The researchers believe that variations of the technique will soon emerge, leading to innovative experiments into other fundamental nuclear processes. One such experiment is the fusion of 3He ions, important because it is the dominant energy-producing step by which the sun generates its vast energy (thus illustrating that solar power is in fact fusion power). By successfully measuring the cross-sections in a dense "soup" that resembles the plasma found in the Sun and other stars, scientists should have a better understanding of the nuclear reactions that power the Sun and stars.
"The University of Rochester has had a long and productive collaboration with the excellent MIT team, led by Dr. Richard Petrasso" said LLE Director Robert McCrory, "This new ground-breaking work in nuclear science resulted from the joint effort of the MIT, Rochester, and Lawrence Livermore teams on the Omega laser. High energy density laser facilities will open new frontiers in science."
This research was supported by National Nuclear Security Administration, an agency within the U.S. Department of Energy (DOE). The administration funds the operation of the Omega Laser Facility and the National Laser Users' Facility (NLUF) through which this research was carried out, as well as operations at LLNL. NLUF allows university researchers access to the Omega Laser Facility to perform basic high-energy-density physics experiments. This research was also partly funded by the DOE's Office of Fusion Energy Science through the University of Rochester's Fusion Science Center, and the Laboratory Directed Research and Development Program at LLNL. The DOE Offices of Nuclear Physics and Advanced Scientific Computing Research supported the development of the nuclear-theory techniques used to perform the calculations presented in this work.