In this work we provide the first explanation for observations made in 1997 on the Joint European Torus of unexpected ion heating with fusion-born alpha particles occurring over time scales much shorter than those theoretically foreseen. We demonstrate that non-thermal alpha particles above a critical concentration stabilize ion-drift-wave turbulence, therefore significantly reducing one of the main energy loss channels for thermal ions. As such ion heating occurs over times scales much shorter than those classically predicted, this mechanism opens new prospects on additional paths for the self-sustainment of thermonuclear fusion reactions in magnetically confined plasmas.