Energy-selective confinement of fusion-born alpha particles during internal relaxations in a tokamak plasma

TL;DR

This paper demonstrates that internal relaxation events in tokamak plasmas can be exploited to selectively confine fusion-born alpha particles by energy, using a combination of magnetic geometry, crash timing, and magnetic drifts.

Contribution

It reveals a parameter window for energy-selective alpha confinement during sawtooth crashes, supported by kinetic-magnetohydrodynamic hybrid simulations.

Findings

Existence of a confinement window for alpha particles during sawtooth crashes

Magnetic drifts influence the asymmetry and reconnection in particle orbits

Optimal crash duration enhances energy-selective confinement

Abstract

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.