Study on creation and destruction of transport barriers via effective safety factors for energetic particles

TL;DR

This paper investigates how the topological structure of energetic particle trajectories differs from magnetic field lines in cylindrical magnetic fields, revealing mechanisms for transport barrier formation due to drift effects and resonance shifts.

Contribution

It introduces the concept of effective particle q-profile to analyze energetic particle resonances and explains how drift effects influence transport barriers.

Findings

Grad B drift shifts particle resonances.

Magnetic field line chaos affects energetic particle transport.

Two mechanisms identified for creating transport barriers.

Abstract

Charged particles with low kinetic energy move along magnetic field lines, but so do not energetic particles. We investigate the topological structure changes in the phase space of energetic particles with respect to the magnetic one. For this study cylindrical magnetic fields with non-monotonic safety factors that induce the magnetic internal transport barrier are considered. We show that the topological structure of the magnetic field line and of the particle trajectories can be quite different. We explain this difference using the concept of effective particle $q$-profile. Using this notion we can investigate the location and existence of resonances for particle orbits that are different from the magnetic ones. These are examined both numerically by integrating an equation of motion and theoretically by use of Alfv\'en's guiding center theory and by use of the effective reduced Hamiltonian for the integrable unperturbed system. It is clarified that, for the energetic particles, the grad $B$ drift effect shifts the resonances and the drift induced by curvature of the magnetic field line leads to the vanishing of the resonances. As a result, we give two different mechanisms that lead to the creation of transport barriers for energetic particles in the region where the magnetic field line is chaotic.