Mystery of supra-thermal ions in fusion plasmas solved by advanced collision models

Mystery of supra-thermal ions in fusion plasmas solved by advanced collision models
by Simon Mansfield
Sydney, Australia (SPX) Jan 01, 2025

The pursuit of nuclear fusion as a clean and abundant energy source has made significant strides with the advent of inertial confinement fusion (ICF). This technology relies on compressing deuterium-tritium (DT) fuel to extreme temperatures and pressures to initiate fusion. While neutrons generated in this process primarily contribute to electricity production, alpha particles remain in the fuel, driving additional fusion reactions.

In ICF, when alpha particle deposition surpasses the work achieved during implosion, a self-sustaining plasma burning phase begins, resulting in a dramatic rise in energy densities. The National Ignition Facility (NIF) achieved this state in February 2021, marking a major milestone in the study of fusion energy and extreme conditions analogous to the early universe.

NIF experiments led by Hartouni revealed surprising deviations in neutron spectra from hydrodynamic predictions, indicating the presence of supra-thermal DT ions. These ions challenge the conventional Maxwellian distribution models and highlight the need to account for kinetic effects and non-equilibrium behaviors often overlooked in hydrodynamic descriptions.

Supra-thermal ions emerge from large-angle collisions during alpha particle deposition, leading to energy exchanges that disrupt equilibrium states. Modeling these collisions accurately has proven difficult, necessitating innovative approaches.

A research team led by Prof. Jie Zhang from the Institute of Physics of the Chinese Academy of Sciences and Shanghai Jiao Tong University developed a groundbreaking large-angle collision model. This model incorporates the interactions between ions' screened potentials and their relative motions during binary collisions, providing a comprehensive framework for capturing ion kinetics.

Utilizing this model, the team enhanced their hybrid-particle-in-cell LAPINS code, enabling high-precision simulations of burning plasmas. Their studies uncovered several critical findings:

- Ignition moments advanced by approximately 10 picoseconds.

- Detection of supra-thermal D ions below an energy threshold of ~34 keV.

- Alpha particle density peaks approximately doubling predicted values.

- Enhanced alpha particle density at the hotspot center by around 24%.

These findings align with neutron spectral moment analyses conducted by NIF, which further validate the team's kinetic simulations. The disparities between neutron spectra and hydrodynamic predictions grow with increasing yield, underscoring the limitations of current models.

The research offers profound insights into interpreting experimental data and opens new avenues for refining ignition strategies and exploring high-energy-density nuclear burning plasmas. These advancements have far-reaching implications for understanding the physical processes that governed the early universe's evolution.

Research Report:Mechanisms behind the surprising observation of supra-thermal ions in NIF's fusion burning plasmas