Researchers used large-scale simulations to study how energetic alpha particles—the helium nuclei produced by fusion—affect turbulence and heat confinement in future burning plasma devices. By evolving small-scale turbulence, alpha-particle heating, and the overall plasma profiles together until they reached steady state, the team found that alpha particles can help reduce turbulent heat loss and strengthen the plasma core.

The study couples a global gyrokinetic code called GENE (a physics model that follows charged-particle motion in a magnetic field) to a transport solver named Tango to simulate reference scenarios for the planned reactors SPARC and ITER. The authors compared runs that included the fusion-born alpha population to otherwise identical runs without them. When alpha particles were included the steady-state results converged to within 1–2%, and the simulations showed an increase in alpha self-heating of up to 25% in the SPARC case and 18% in the ITER case (with reported heating uncertainties of about 3% and ±5%, respectively).

The mechanism the simulations reveal is a chain of effects rather than a single direct heating boost. Alpha particles weakly excite toroidal Alfvén eigenmodes (TAEs)—electromagnetic oscillations that travel along magnetic field lines. These TAEs then nonlinearly pump energy into large-scale, toroidally symmetric flows called zonal flows (E×B flows, where E is the electric field and B the magnetic field). The zonal flows shear apart and weaken the usual ion-scale turbulent eddies that carry heat outward. In the runs, the most active TAEs appeared around normalized radii ρtor≈0.3 in SPARC and ρtor≈0.5 in ITER, and their growth rates were smaller than the dominant drift-wave turbulence (the ratio γTAE/γDW was ≈0.3 in SPARC and ≈0.2 in ITER), consistent with a weakly unstable regime.