HEDS Research Area

Nuclear and Atomic Physics

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At LLNL, scientists explore nuclear and atomic physics in new regimes. For example, they measure nuclear reaction rates in plasmas, and they study the behavior of atoms in hot, dense plasmas. They leverage world-class experimental and computational capabilities to explore nuclear and atomic processes that occur in plasma environments, analyzing experimental data to validate predictive multiphysics models of HED phenomena.

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Nuclear Physics

NIF capsule STR Stadermann
A target capsule at LLNL’s National Ignition Facility is doped with target materials for nuclear reaction measurements.

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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Contact

  • Dawn Shaughnessy

Atomic Physics

EBIT group
LLNL’s Electron Beam Ion Trap (EBIT) enables scientists to produce and trap highly charged ions using a high-current-density electron beam—and study atomic processes for isolated ions.

LLNL scientists explore atomic physics in new regimes, including how atoms behave in plasma environments. They combine predictive modeling with experimental validation to describe atomic processes in isolated ions and quantify plasma effects on bound electronic states. This work also helps to determine the extreme conditions obtained in high energy density experiments by connecting measured radiation to underlying temperatures, densities, and charge states.

High temperatures and plasma environments make atomic physics calculations particularly challenging for several reasons. Scientists may need to consider more than 1,000 states from the thermal ensemble for systems in thermal equilibrium, while non-thermal systems have no simple bounds on how many states to include. Interactions between the atoms and plasma are often non-negligible. Electrons can be shared with neighboring atoms and their energies and orbitals perturbed. Accurate models are needed to describe these many-body quantum mechanical systems that can be tested against well-characterized experiments.

To address these challenges, LLNL scientists use non-local thermodynamic equilibrium (NLTE) modeling, which captures atomic-physics effects without assuming local equilibrium. This approach applies to a wide range of phenomena, including time-varying, line-resolved radiation spectra emitted by hot, dilute plasmas produced at LLNL’s National Ignition Facility, and from hot gas plasmas created at x-ray free-electron laser (XFEL) and and other laser facilities. The resulting data contributes to the equations of state used in radiation-hydrodynamic simulations of target capsules (hohlraums) for laser experiments.

LLNL scientists are pursuing three complementary efforts to study nonequilibrium physics:

  • Delayed, nonequilibrium ionization—Quantifying cases where the plasma’s charge-state distribution lags behind rapidly changing temperature and density and assessing impacts on emitted spectra, opacity/emissivity, and energy transport.
  • Nonequilibrium electron energy distributions—Modeling laser- and XFEL-driven plasmas for NLTE kinetics and diagnostics and determining how they alter collisional rates, charge-state distributions, and line emission, which can shift inferred plasma conditions.
  • “Laser drive deficit” in NIF hohlraums—Applying nonequilibrium insights to discrepancies in simulated versus measured x-ray drive, including work motivated by the overpredicted 2 to 4 keV gold emission and targeted adjustments to opacities and key rates such as collisional excitation.

Experimental validation of plasma atomic processes provides essential benchmarks for complex calculations of collisional excitation rates, radiative recombination rates, and energy levels. LLNL scientists use the Electron Beam Ion Trap (EBIT) to produce and confine highly charged ions, excite selected states with an electron beam, and measure the emitted radiation with high-resolution spectroscopy. While the EBIT is important for studying atomic processes for isolated ions, many of LLNL’s experimental efforts regarding atomic processes focus on how plasmas modify bound states, especially those close to the continuum. This phenomenon can strongly influence charge states, spectra, and radiation transport in high energy density plasmas.

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Contact

  • Min Sang Cho
  • Ronnie Shepherd