  Long-pulse operation of a self-sustained fusion reactor using toroidal
magnetic containment requires control over the content of alpha particles
produced by D-T fusion reactions. On the one hand, MeV-class alpha particles
must stay confined to heat the plasma. On the other hand, decelerated helium
ash must be expelled before diluting the fusion fuel. Our
kinetic-magnetohydrodynamic hybrid simulations of a large tokamak plasma
confirm the existence of a parameter window where such energy-selective
confinement can be accomplished by exploiting internal relaxation events known
as `sawtooth crashes'. The physical picture -- consisting of a synergy between
magnetic geometry, optimal crash duration and rapid particle motion -- is
completed by clarifying the role played by magnetic drifts. Besides causing
asymmetry between co- and counter-going particle populations, magnetic drifts
determine the size of the confinement window by dictating where and how much
`reconnection' occurs in particle orbit topology.
