image: Left: Initial temperature and density setup, which mirrors conditions of the stable and isobaric burning process. Middle: the ion scattering trajectories for large or small angles, and the boundaries of cross-sections at various impact parameters. Right: ions in large-angle collisions exchange substantial energies during a single collision, leading to supra-thermal ions defying Maxwellian distribution, while ions in small-angle collisions continuously lose energies during multiple collisions, resulting in an equilibrium or Maxwellian ion distribution. In their simulations, integrating the screened potentials of the background with the relative motion of ions during binary collisions leads to the cutoff, which encloses an area that constitutes the cross section for large-angle collisions, while the remainder pertains to small-angle collisions.
Credit: ©Science China Press
The dream of harnessing controlled nuclear fusion for the generation of vast amounts of clean energy is increasingly becoming a reality through the advancement of inertial confinement fusion (ICF). This technology ignites deuterium-tritium (DT) fuel by achieving extreme temperatures and pressures during the implosion process. In DT fusion, the majority of the energy is carried away by neutrons for electricity production, whereas alpha particles accumulate within the fuel, thereby initiating further fusion reactions. When the deposition energies of alpha particles exceed the work accomplished by the implosion, the plasma initiates the burning process, leading to a substantial increase in energy densities. In February 2021, the National Ignition Facility (NIF) successfully achieved the state of ICF burning plasma, marking a pivotal milestone in the advancement of fusion energy and the exploration of the early universe's extreme conditions. However, within this extreme state, Hartouni and his colleagues observed novel physical phenomena in experiments conducted at the NIF: the neutron spectrum data deviated significantly from hydrodynamic predictions, indicating the emergence of supra-thermal DT ions. These observations challenge existing models that rely on Maxwell distributions and underscore the importance of previously overlooked kinetic effects and non-equilibrium mechanisms. Accurately modeling these kinetic effects, particularly large-angle collisions that involve substantial energy exchanges, poses a considerable challenge. These collisions generate supra-thermal ions during the deposition of alpha particles, causing deviations from the equilibrium state and falling outside the scope of hydrodynamic descriptions.
To address this challenge, a joint research team led by Prof. Jie Zhang from Institute of Physics of Chinese Academy of Sciences and Shanghai Jiao Tong University has innovatively proposed a large-angle collision model that integrates the screened potentials of the background ions with the relative motion of ions during binary collisions, which can comprehensively capture ion kinetics. The hybrid-particle-in-cell LAPINS code newly developed by the team, incorporating with this model, achieves high-precision simulation of ICF burning plasmas. Extensive and kinetic investigations into the implications of large-angle collisions have yielded several key findings, including an ignition moment promotion by ~10 ps, the presence of supra-thermal D ions below an energy threshold of ~34 keV, approximately twice the expected deposition of peak alpha particles densities and enhancement of alpha particles densities at the hotspot center by ~24%. The rationality of their findings is confirmed through the congruency between the neutron spectral moment analyses conducted by the NIF and their kinetic simulations, both highlighting disparities between neutron spectral moment analyses and hydrodynamics predictions, which becomes more pronounced as the yield increases.
This work not only provide novel insights for experiment interpretation but also open new research opportunities to guide the design and improvement of ignition schemes and to explore the nuclear burning plasmas, which are distinguished by their exceptionally high energy densities and hold immense potential for illuminating the intricate physics that underpins the evolution of the early universe.
Journal
Science Bulletin