Using a Laser-Generated Plasma to Stimulate the Field of Nuclear Physics

February 2012

CPS Installation

Installation of a charged-particle spectrometer (CPS) used for n–D and n–T differential cross-section measurements on OMEGA

The University of Rochester's Laboratory for Laser Energetics (LLE) ushered in a new frontier of plasma nuclear science at the Omega Laser Facility by measuring a nuclear scattering cross section more precisely than ever determined before with particle accelerators. "This is the first time a high-energy-density laser facility has been used to advance the field of nuclear physics," said Dr. David Meyerhofer, LLE Deputy Director and Professor of Mechanical Engineering and Physics & Astronomy. A research team from the Massachusetts Institute of Technology, Lawrence Livermore National Laboratory (LLNL), and the University of Rochester worked on the project and published their findings in the 16 September 2011 Physical Review Letters.

Dr. Steven Koonin

Dr. Steven Koonin (former Under Secretary for Science, U. S. Department of Energy) is shown here discussing the current status of the U.S. Inertial Confinement Fusion program and its path toward Inertial Fusion Energy at the 2011 Inertial Fusion Sciences and Applications conference

Typically, researchers use conventional accelerators to investigate nuclear reactions. With the new work, the research team developed a hot, dense plasma in which electrons are stripped off their parent atoms to create an interpenetrating gas of positive and negative charges. To arrive at this plasma state of matter on the OMEGA laser, all 60 of its powerful laser beams were employed to strike the outer surface of a 1-mm glass capsule filled with deuterium (D) and tritium (T), heavy forms of hydrogen. The laser beams produced a rapidly expanding high-temperature plasma gas on the surface of the capsule, prompting the capsule to implode. This implosion, in turn, created an extremely hot (100 million kelvin) plasma of D and T ions and electrons inside the capsule. A small fraction of D and T ions fused together—a process that generates neutrons traveling at one-sixth the speed of light with about 14.1 million electron volts of energy. (In contrast, an ordinary chemical reaction—such as the burning of wood or coal—generates about one electron volt of energy). As these energetic neutrons sped out of the imploding capsule, a small fraction of them collided and scattered 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, the researchers obtained an accurate measurement of the nuclear collision process.

CPS

(a) The charged-particle spectrometer (CPS) on the OMEGA chamber simultaneously measures deuterons and tritons elastically scattered by 14.1-MeV neutrons and protons from D–D reactions in a deuterium–tritium-gas–filled, thin-glass capsule implosion. A bending magnet (gray) was used for momentum analysis.

(b) Schematic drawing of the CPS, which uses a 2-mm-wide aperture in front of a 7.6-kG permanent magnet (Nd-Fe-B) for dispersion and high-resolution measurements of charged particles. Two pieces of CR-39 were used to detect the DD protons and the elastically scattered deuterons and tritons. The energies are given in MeV for protons.

Meyerhofer said it is difficult for accelerators to measure nuclear scattering cross sections at lower temperatures (e.g., the temperature of the sun). The OMEGA laser, on the other hand, provided more data points and more accurate 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. This promises to lead to innovative experiments into other fundamental nuclear processes. For example, an exciting experiment, one illustrating that solar power is fusion power, is the fusion of 3He ions, which is significant since it is the dominant energy-producing step by which the sun generates its enormous energy.

The National Nuclear Security Administration (NNSA), an agency within the Department of Energy, supported this research. The NNSA funds the operation of the Omega Laser Facility, the National Ignition Facility Laser at LLNL, as well as the National Laser Users' Facility (NLUF) through which this research was carried out. The NLUF allows university researchers access to the Omega Laser Facility to perform basic high-energy-density-physics experiments. This research also was 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 groundbreaking nuclear theory techniques used to perform the calculations presented in this work.

The American Physical Society also highlighted this work on their web site.