Nuclear Science using High Energy Density Plasmas - A new field of research

 

Johan Frenje

 

Plasma Science and Fusion Center, Massachusetts Institute of Technology, USA

 

Thermonuclear reaction rates and nuclear processes have been explored traditionally by means of conventional accelerator experiments, which are difficult, and at times even impossible, to execute at energies and conditions relevant to stellar nucleosynthesis.  Thus, nuclear reactions at stellar energies are often studied through extrapolations from higher energy data, or in low-background experiments such as those at the LUNA underground laboratory.   Even when measurements are possible using accelerators, thermonuclear reaction rates in burning plasmas of stars are inherently different from those in accelerator experiments.  The fusing nuclei are surrounded by bound electrons in accelerator experiments, whereas electrons occupy mainly continuum states in a stellar environment.  In accelerator experiments, extrapolations from higher energies and electron-screening corrections are also based on theoretical models that have not been checked experimentally.  Nuclear physics research will therefore benefit from an enlarged toolkit for studies of various fundamental nuclear reactions.  In this presentation, we report on the first use of laser-driven Inertial Confinement Fusion (ICF) experiments for studies of basic nuclear physics.  These experiments were carried out at the OMEGA laser facility and the National Ignition Facility (NIF), in which spherical capsules were spherically irradiated with powerful lasers to compress and heat the fuel to high enough temperatures and densities for significant nuclear reactions to occur.  Three experiments using ICF plasmas will be highlighted in this presentation. In the first experiment, the differential cross sections for the elastic neutron-triton (n-3H) and neutron-deuteron (n-2H) scattering at 14.1 MeV were measured with significantly higher accuracy than achieved in previous accelerator experiments.  In the second experiment, the 3H(3H,2n)4He reaction, which is an important mirror reaction to the 3He(3He,2p)4He reaction that plays an important role in the proton-proton chain that transforms hydrogen into ordinary 4He in stars like our Sun, was studied at center-mass energies in the range 15-40 keV, and in the third experiment, ICF plasmas were uniquely used to directly study the 3He+3He solar fusion reaction.  As the conditions in these ICF implosions better mimic the burning core in stars than the conditions in accelerator experiments, these types of studies open up new areas of research fundamental to both stellar nucleosynthesis and basic nuclear physics.  We call this field of research plasma nuclear science. The work described here was supported in part by NLUF (DOE award No. DE-NA0000877), FSC (Rochester Sub award PO No. 415023-G, UR Acct. No. 5-24431),  US DOE (Grant No. DE FG03-03SF22691), LLE (No. 412160-001G), and LLNL (No. B504974), and GA under DOE (DE-AC52-06NA27279).