Nuclear Physics
Interactions between nuclei are responsible for nuclear power generation, nuclear fusion that powers the Sun, and the formation of the elements on Earth and in the Solar System. Measuring the reaction rates of these nuclear interactions is important for understanding the evolution of matter.
Particle accelerators are typically used to bombard one nucleus into another so that the reaction kinetics can be measured. A target of one material is irradiated with a high-energy ion beam of another, causing the nuclei to fuse and form a different nucleus altogether. However, in a stellar environment, nuclei exist in a plasma environment, where the nuclei are surrounded by a sea of electrons that screen their charge—making it easier for them to fuse. Recreating and measuring this very high temperature and high density environment is extremely challenging, but it is important for understanding fusion.
At LLNL, scientists use the National Ignition Facility (NIF) to measure stellar nuclear reactions rates on Earth. During a NIF experiment, the interior of the NIF capsule reaches temperatures and pressures that more closely mimic the conditions found inside stellar interiors, making it one of the only ways to measure stellar nuclear reaction rates in a plasma and compare them to equivalent reactions measured at accelerator facilities.
Small quantities of dopant atoms (~1015 or fewer) are added to the inner surface of a NIF capsule, which is then subsequently filled with deuterium and tritium. The NIF lasers cause the capsule to compress, which fuses the deuterium and tritium, creating a large quantity of high-energy neutrons. The target dopant atoms capture these neutrons, forming new nuclei, which are then collected and analyzed to determine the number of nuclear reactions that occurred.
Since the conditions in the capsule create a plasma environment, it is one of the only ways to measure the effects of the plasma on nuclear reaction rates. As neutron yields at NIF continue to increase with advances in capsule and laser design, it may be possible in the future to measure the most elusive reactions responsible for stellar nucleosynthesis and to understand the role of the plasma on the underlying reaction rates.
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- Plasma studies yield new insights into NIF implosions and ‘failed stars’
- Providing data for nuclear detectives
- Radiochemical analysis of gaseous samples used to probe reactions
- Nuclear Counting Facility supports nuclear science research at LLNL
Contact
- Dawn Shaughnessy
- shaughnessy2 [at] llnl.gov (shaughnessy2[at]llnl[dot]gov )
Atomic Physics
LLNL scientists explore atomic physics in new regimes, including the behavior of atoms in plasma environments. They combine predictive modeling and experimental validation to explore this complex research space, including atomic processes for isolated ions and plasma effects on bound states. They also use their atomic physics knowledge to determine the extreme conditions obtained in their experiments.
Atoms are systems of electrons bound to charged nuclei, and the electrons occupy orbitals with discrete energies. No two electrons can occupy the exact same state (per the Pauli exclusion principle), and so in the ground state, electrons fill up all the lowest energy orbitals. Excited atomic states have one or more electrons in higher energy orbitals. Transitions between these states can occur via photon emission and absorption or from collisions with other electrons in the plasma.
The high temperatures and plasma environment make atomic physics calculations particularly challenging. Scientists may need to consider 1020 or more states from the thermal ensemble for systems in thermal equilibrium, while non-thermal systems have no simple bounds on how many states to include. To make matters worse, interactions between the atoms and the plasma are often non-negligible. Electrons can be shared with neighboring atoms and their energies and orbitals perturbed. Accurate models and experiments are needed to describe these many-body quantum mechanical systems.
LLNL scientists use Non-Local-Thermodynamic Equilibrium (NLTE) modeling to capture the effects of atomic physics without resorting to the assumption of local equilibrium. This applies to a wide range of phenomena—from the time-varying detailed radiation spectrum emitted from a hot, dilute plasma produced on LLNL’s National Ignition Facility, or from a cold-dense plasma driven with an X-ray Free Electron Laser (XFEL), to the equation of state underlying radiation-hydrodynamic simulations of hohlraums.
Experimental validation of plasma atomic processes provides a benchmark for complex calculations of collisional excitation rates, radiative recombination rates, and energy levels. Scientists use a powerful platform to measure fundamental quantities—the Electron Beam Ion Trap (EBIT). With EBIT, a near mono-energetic beam of electrons strips atoms to a high level of ionization, then traps them in an “electro-magnetic bottle.” Excited states in the ions are produced by collisions with the electron beam tuned around specific bound states. The subsequent relaxation of the excited states produce radiation that is spectroscopically resolved and used to determine collisional excitation rates, radiative lifetimes, and energy level positions.
While the EBIT is important for studying atomic processes for isolated ions, effects due to the plasma environment require a different platform. Because of the broad and fundamental impact, much of LLNL’s experimental efforts regarding atomic processes focuses on the plasma effects on bound states. The impact is greatest on bound states close to the continuum, commonly called continuum lowering or ionization potential depression (IPD).
Currently, LLNL scientists are engaged in two experimental efforts to study IPD, and both techniques use isochoric heating to produce a solid density or greater plasma for short durations (<20 picoseconds). One technique (used by a research group at Oxford University) both heats and excites bound electrons in solid-density matter using an XFEL. The XFEL is tuned to specific excitation energies, and the subsequent florescence is used to determine where bound states change from bound to free. The second technique, used by researchers at the Atomic Weapons Establishment and at LLNL, uses a high-energy, long-pulse laser to compress a solid density target. After compression, an ultrashort pulse laser heats the compressed target. The emission spectra are used to determine when the upper bound electrons transition from bound to free.
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Contact
- Paul Grabowski
- grabowski5 [at] llnl.gov (grabowski5[at]llnl[dot]gov )
- Ronnie Shepherd
- shepherd1 [at] llnl.gov (shepherd1[at]llnl[dot]gov)
- Howard Scott
- scott6 [at] llnl.gov (gscott6[at]llnl[dot]gov )
