New era of plasma nuclear science opens on the Omega Laser Facility

CAMBRIDGE, Mass. -- The University of Rochester’s Omega Laser Facility has given rise to many innovative ways to probe matter under conditions of extreme energy density. These conditions include pressures of 100 billion atmospheres, temperatures of 200,000,000 kelvins and densities 20 times that of gold; they occur in nature only at the center of the Giant Planets Jupiter and Saturn and inside stars. In a unique experiment, published Sept. 15 in Physical Review Letters, MIT’s Johan Frenje and fellow researchers used the Omega Laser Facility to make precise measurements of a fundamental nuclear process: the elastic scattering of neutrons off heavy forms of hydrogen, deuterium (D) and tritium (T). This is the first time a fundamental nuclear physics experiment has been achieved using a high-energy-density laser facility.

Researchers typically probe nuclear reactions using conventional accelerators. In the new work, the research team -- Frenje of MIT’s Plasma Science and Fusion Center and his colleagues from Lawrence Livermore National Laboratory (LLNL) and the University of Rochester -- 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. This plasma state comprises stars, fluorescent lights, lighting bolts and in fact 99 percent of the visible universe -- it’s often referred to as the fourth state of matter, after solids, liquids and gases. To achieve this plasma state on the Omega laser, all 60 of its powerful laser beams strike the outer surface of a glass capsule one millimeter in diameter filled with D and T. The laser beams generate a rapidly expanding high-temperature plasma gas on the surface of the capsule, causing the capsule to implode on itself. This implosion, in turn, creates inside the capsule an extremely hot (100,000,000 kelvins) plasma of D and’T ions and electrons. A small fraction of these D and’T ions fuse together, a process that generates a neutron 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 race out of the imploding capsule, a small fraction collide and scatter like billiard balls off the surrounding D and’T ions. From these rare collisions, and from the corresponding transfer of energy from the neutrons to these D and’T ions, a highly accurate measurement of the nuclear-collision process is obtained. Accelerator-based measurements are used to normalize the absolute cross section. Once normalized, the shape of the low-energy cross section is obtained with more accuracy than possible with accelerators. Importantly, the experimental results match theoretical calculations, providing not only a boost to nuclear theory but also data on reactions of crucial importance to nuclear astrophysics and to fusion energy research.

The researchers believe that important variations of the technique in the promising plasma environment will soon emerge, leading to innovative experiments into other fundamental nuclear processes. One such experiment is the fusion of 3He and 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. With this class of experiments imminent, a new and exciting field of research is ushered in: plasma nuclear science, blending the disciplines of plasma and nuclear physics.
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.