Alexis Diaz Torres

The process of carbon burning is vital to understanding late stage stellar evolution of massive stars and the conditions of certain supernovae. Carbon burning is a complex problem, involving quantum tunnelling and nuclear molecular states. Quantum dynamical calculations of carbon burning are presented, combining the time-dependent wave-packet method and the density-constrained time-dependent Hartree-Fock (DC-TDHF) approach. We demonstrate that the state-of-the-art DC-TDHF interaction potential successfully explains the appearance of some resonant structures in the sub-barrier fusion cross-section. We study the dynamic response of the compound nucleus to further explain resonant structure seen in the Gamow energy region. The results show the critical role of nucleon-nucleon interactions and compound nucleus excitations in the 12C + 12C fusion resonances observed at astrophysical energies.

Low-energy fusion of heavy ions is a fascinating coupling-assisted quantum tunnelling problem, whose understanding is crucial for advancing the synthesis of new elements and isotopes. Quantum dynamical coupled-channels calculations of laser-assisted 16O + 238U fusion are presented for both a central collision and the total fusion cross-sections, suggesting that laser-nucleus interaction can enhance the average 16O + 238U fusion probability by 6 − 60% at subbarrier energies using quasi-static laser fields of intensity 10^27 − 10^29 Wcm^−2 and photon's energy of 1 eV. Femtosecond laser pulses are shown to reduce this enhancement by many orders of magnitude.

Stars are slowly developing objects; the lifetimes of the different burning phases are determined by the strength of nuclear reactions, which in turn are defined by the quantum structure of the associated nuclei at the threshold and the respective reaction mechanisms. Stars, from the nuclear physics perspective, are cold environments where only a very few of the key nuclear reactions have been measured at the actual stellar plasma temperatures. This is also the case for more dynamic astrophysical phenomena from Big Bang to stellar explosions. Most of the nuclear reaction rates are therefore based on theoretical extrapolations. A number of discrepancies between these predictions and the associated stellar signatures have been observed and many may be due to low-energy or near-threshold quantum effects. These effects need to be understood in order to reliably model nuclear reaction processes, not only for stars, but also for low-temperature plasma environments such as controlled magnetic or inertial confinement fusion systems, which operate in similar temperature regimes. This article will summarize the various theoretical techniques presently used for deriving reaction rates and will discuss possible quantum effects that may impact the reaction cross-section near the reaction threshold. These resemble enhanced single-particle and cluster structures in the vicinity of threshold and associated interference effects. New experimental techniques such as deep underground accelerators or the study of transfer reactions to mimic the quantum mechanical transition strength, the so-called Trojan horse method, provide ways to directly or indirectly probe the reaction features that determine the reaction rates at stellar energies. This will be demonstrated on a number of key nuclear reactions for different nucleosynthesis environments. Finally, current inconsistencies between experimental prediction and observation will be discussed.

Deuterium-Tritium (D-T) fusion is a key to generating safe, clean and limitless energy on Earth in future fusion power plants. Its understanding at low collision energies is incomplete, as D-T fusion is a quantum tunneling process affected by resonances whose origin is linked to properties of not fully understood nuclear forces. Simplified quantum dynamical calculations of laser-assisted D-T fusion are presented, suggesting that laser-nucleus interaction can enhance the average D-T fusion probability by 7 − 70% at deep subbarrier energies using laser fields of intensity 10^27 − 10^29 Wcm −2 and photon's energy of 1 eV.

Nuclear friction causes energy dissipation in heavy-ion collisions. Its understanding and inclusion in quantum mechanical reaction models are crucial for advancing the physics of heavy-ion reactions forming heavy elements. The effects of nuclear friction on heavy-ion fusion reactions are investigated using the coupled-channels density-matrix method. In this open-quantum-system description, a phenomenological nuclear friction form factor is introduced along with coherent coupled-channels effects. The key nucleus was the 92 Zr target, due to its high density of low-lying non-collective excited states, which was recently theorised to be cause of nuclear friction. The calculations using the 16O + 92Zr collision showed that the inclusion of nuclear friction effects increased the fusion probability significantly, and that the agreement between the theoretical and experimental fusion barrier distributions was improved when nuclear friction effects were included.