Orbits of energetic particles near rational flux surfaces in stellarators*

We calculate the drift orbits of resonant energetic particles near rational flux surfaces in stellarators. In the vicinity of a sufficiently low-order rational surface, there is a thin boundary layer in which particle orbits exhibit several unusual features. Passing particles make large radial excursions (of order sqrt rho-star), trapped particles do not necessarily conserve the longitudinal adiabatic invariant, and islands exist [1,2] which grow with energy and which may be deleterious for energetic particle confinement. Using a new adiabatic invariant, we determine the island shape: it is sensitive to the shear and deviation from omnigeneity of the stellarator magnetic field. We also study semi-trapped particles that make many toroidal transits before bouncing. They have an enhanced probability of orbit-class transitions near the rational surface, which could facilitate losses [3].

[1]  R. White, A. Bierwage & S. Ethier, Phys. Plasmas 29(5) (2022).

[2]  R. White, Phys. Plasmas 29(9) (2022).

[3]  E. J. Paul, A. Bhattacharjee, M. Landreman, D. Alex, J. L. Velasco, & R. Nies, Nucl. Fusion 62(12), 126054 (2022).