There is strong experimental evidence, backed up by the state-of-the art calculations, that clustering plays an important role in structure of light nuclei. Groups of levels that have large reduced alpha-width, indicating high degree of alpha clustering, have been known in some nuclei for many decades. Recent development of advanced microscopic theoretical models provided new insights into clustering problem in light nuclei. It appears that clustering is not limited to N=Z (alpha-like) nuclei and to simple alpha+core configurations. An interplay between single-nucleon and cluster degrees of freedom leads to formation of interesting structures, such as molecular-like, multi-center configurations. Manifestation of this interplay can be observed more directly in non-self-conjugate, exotic nuclei, where the thresholds for nucleon(s) and cluster(s) decays may be close and one may expect to observe nucleon(s) and cluster(s) decays from the same state. Clustering in non-self-conjugate nuclei also plays an important role in nuclear astrophysics, as near alpha-threshold cluster states often dominate the radiative alpha-capture and other reactions that involve alpha-particles and exotic nuclei, that are forming in stellar environment during certain stages of nucleosynthesis processes. A well established example of the molecular-type configuration is the alpha:2n:alpha structure in 10Be, where two valence neutrons are thought to occupy the molecular-type  pi and sigma orbitals with respect to the two alpha-particles. Recent experimental advances on identifying and characterizing these configurations in exotic nuclei will be reviewed and discussed in the context of modern theoretical understanding of clustering phenomena in atomic nuclei. 

An overarching goal of nuclear physics is to arrive at the comprehensive understanding – in terms of the laws of quantum mechanics and the underlying theory of the strong force (quantum chromodynamics) – of atomic nuclei and their interactions, and to use this understanding to accurately predict nuclear properties that play a fundamental role in explaining the inner workings of the Universe or are critical to the national security. I will present first-principles calculations of nuclear structural and reaction properties to predict thermonuclear reaction rates of interest for fusion energy technology and stellar nucleosynthesis.

The ³He+³He, T+³He, and p+D reactions directly relevant to either Stellar or Big-Bang Nucleosynthesis (BBN) have been studied at the OMEGA laser facility using inertially-confined plasmas. These high-temperature plasmas are created using shock-driven `exploding pusher' implosions. The advantage of using these plasmas is that they better mimic astrophysical systems than cold-target accelerator experiments. A new measured S-factor for the T(³He,g) ⁶Li reaction rules out an anomalously-high ⁶Li production during the Big Bang as an explanation to the high observed values in metal poor first generation stars. Our value is also inconsistent with values used in previous BBN calculations [1]. In a second experiment, proton spectra from the ³He³He and T³He reactions are used to constrain nuclear R-matrix modeling. The spectral shapes disagree with R-matrix calculations using coefficients derived from fits to T+T data at higher or lower center-of-mass energy [2]. Thirdly, recent experiments have probed the p+D reaction for the first time in a plasma; this reaction is relevant to energy production in protostars, brown dwarfs, and at higher CM energies, to BBN. The first plasma data is consistent with previous accelerator experiments at E_cm ~ 16 keV, work is ongoing to further reduce our experimental uncertainties. Currently, experiments at the National Ignition Facility are extending the ³He+³He data towards spectral and absolute cross section measurements at energies closer to the solar Gamow window. Beyond these specific results, there are numerous applications of inertial fusion capabilities to nuclear astrophysics problems, which will be discussed.

1: A.B. Zylstra et al., Phys. Rev. Lett. 117, 035002 (2016)
2: A.B. Zylstra et al., Phys. Rev. Lett. 119, 222701 (2017).