Do $\alpha$ particles reduce or increase turbulent heat loss
In plain words
Besides heating, $\alpha$ particles change the plasma's turbulence. Simulations disagree on whether this makes confinement better or worse.
Precise statement
For alpha pressure up to about 10 percent of the total, determine the sign and size of the change in thermal ion and electron turbulent transport caused by alphas (fuel dilution, electromagnetic stabilization of ion-temperature-gradient modes, zonal-flow drive, alpha-driven modes), and the resulting change in predicted $\tau_E$ for ITER and an ARC-class reactor. An answer is a converged nonlinear gyrokinetic result reproducing JET D-T observations.
What would settle it
Global gyrokinetic simulations with kinetic alphas, validated against JET D-T and then ITER burning-plasma data.
Status in the literature
Unverified note
2026 global gyrokinetic simulations report net suppression of microturbulence by alphas (arXiv:2605.10694); this is not yet confirmed experimentally.