Resonant transport of fusion alpha particles in quasisymmetric stellarators

Abstract

In modern, highly optimized stellarator configurations where prompt fusion alpha particle losses from the plasma core are absent, alpha particles can still be lost due to stochastic orbits which result in delayed losses. One mechanism leading to stochastic orbits are changes in the particle trapping class during drift motion along the contours of the parallel adiabatic invariant J ∥ leading to jumps in J ∥ when crossing class boundaries. Another mechanism, which is of main interest here, is the resonance between particle drift and bounce motion (drift-orbit resonance). The first mechanism affects mainly trapped particles near the trapped-passing boundary in the phase space of quasi-symmetric and quasi-isodynamic devices, and can be minimized there by aligning local magnetic field maxima on a given flux surface. The second mechanism may affect a broader range in the trapped particle domain where contours of J ∥ still remain closed. Drift-orbit resonances modify the topology of orbits leading to island-like structures on Poincaré plots where these islands may overlap thus leading to the stochastic transport. In this report, we study this stochastization mechanism in quasi-symmetric stellarator configurations with help of the Hamiltonian drift-kinetic code NEO-RT as well as orbit classification and direct computation of fusion alpha losses within the symplectic orbit following code SIMPLE. The width and overlap of resonances in phase-space is studied using Hamiltonian perturbation theory. Based on optimized reactor configurations we assess if this approach can be used as a fast metric for fusion alpha losses in stellarator optimization.