Supra thermal ions in burning plasmas physics explained

Los Angeles CA (SPX) Jan 07, 2025
The pursuit of controlled nuclear fusion as a source of abundant clean energy is advancing through inertial confinement fusion (ICF). This method ignites deuterium-tritium (DT) fuel by subjecting it to extreme temperatures and pressures during implosion. In DT fusion, most energy is carried away by neutrons for electricity generation, while alpha particles remain in the fuel, promoting further fusion reactions. When the energy deposited by alpha particles surpasses that produced by the implosion, the plasma enters a burning state, significantly increasing energy density.

In February 2021, the National Ignition Facility (NIF) achieved ICF burning plasma, marking a significant milestone in fusion energy development and the study of extreme conditions akin to the early universe. However, in this state, Hartouni and colleagues observed new physical phenomena: neutron spectrum data deviated notably from hydrodynamic predictions, indicating the presence of supra-thermal DT ions. These findings challenge existing models based on Maxwellian distributions, highlighting the importance of previously overlooked kinetic effects and non-equilibrium mechanisms.

Modeling these kinetic effects, especially large-angle collisions involving substantial energy exchanges, is challenging. Such collisions produce supra-thermal ions during alpha particle deposition, leading to deviations from equilibrium beyond hydrodynamic descriptions.

To address this, a research team led by Prof. Jie Zhang from the Institute of Physics of the Chinese Academy of Sciences and Shanghai Jiao Tong University proposed a large-angle collision model. This model integrates the screened potentials of background ions with the relative motion of ions during binary collisions, effectively capturing ion kinetics. Their newly developed hybrid-particle-in-cell LAPINS code, incorporating this model, enables high-precision simulation of ICF burning plasmas.

Their kinetic investigations into large-angle collisions have led to several key findings:

- Advancement of ignition timing by approximately 10 picoseconds.

- Detection of supra-thermal deuterium ions below an energy threshold of about 34 keV.

- Nearly double the anticipated peak alpha particle densities.

- Enhancement of alpha particle densities at the hotspot center by around 24%.

The validity of these findings is supported by the alignment between neutron spectral moment analyses from NIF and their kinetic simulations. Both reveal discrepancies between neutron spectral analyses and hydrodynamic predictions, which become more significant with increasing yield.

This research offers new insights for interpreting experiments and opens avenues for improving ignition designs. It also facilitates exploration of nuclear burning plasmas, characterized by extremely high energy densities, shedding light on the complex physics underlying the early universe's evolution.