How Fusion-Born Alpha Particles Suppress Microturbulence in Burning Plasmas

TL;DR

Self-consistent simulations show that fusion-born alpha particles can enhance plasma confinement by suppressing microturbulence through nonlinear interactions with toroidal Alfven eigenmodes, leading to increased core heating.

Contribution

First demonstration that alpha particles can improve confinement via TAE-induced zonal flows in burning plasmas, revealing an intrinsic self-reinforcing mechanism.

Findings

Alpha particles weakly destabilize TAEs.

TAEs enhance zonal flows that suppress turbulence.

Core heating increases by up to 25% due to reduced turbulence.

Abstract

A central unresolved question in fusion energy research is whether energetic alpha particles, the primary products of deuterium-tritium fusion reactions, enhance or degrade plasma confinement. In burning plasmas, the operating regime of future devices such as ITER and SPARC, alpha particles become the dominant heating source, yet their impact on confinement has remained uncertain. Here, we present self-consistent simulations of burning plasmas that simultaneously evolve microturbulence, alpha-particle heating, and macroscopic plasma profiles to steady state, and find that alpha particles can substantially improve confinement. Fusion-born alpha particles weakly destabilize toroidal Alfven eigenmodes (TAEs), which nonlinearly enhance zonal flows that shear apart and suppress ion-scale turbulence. The resulting reduction in turbulent heat transport drives stronger core profile peaking, increasing alpha heating by up to 25% and establishing a self-reinforcing feedback loop. This mechanism has no direct analogue in present-day experiments, where external heating dominates, and reveals an intrinsic pathway toward improved confinement in burning plasmas.