Superbanana-plateau transport of alpha particles in nearly-omnigeneous stellarators

Stellarator magnetic fields can now be optimised to provide near-perfect confinement of energetic, fusion-born alpha particles, at least in the absence of collisions. Collisional transport is expected to play an important role in determining the alpha-particle loss fraction and slowing-down distribution in such highly optimised devices. Previous work on collisional transport of alpha particles in nearly-quasisymmetric stellarators [1, 2] showed that radial transport due to trapped particles with zero poloidal drift, referred to as 'superbananas', can modify the alpha distribution and cause collisionality-independent fluxes. In this work, we generalise these calculations to arbitrary nearly-omnigeneous stellarators, allowing theoretical description of quasi-isodynamic devices. We investigate the dependence of the superbanana-plateau flux on the deviation from omnigeneity δ, and discuss some regimes that have not been studied before. For example, the superbanana flux does not grow without bound as δ increases but instead saturates at a finite value. Physically, the reason is that, on each flux surface, collisions knock particles into a 'loss cone' associated with the superbanana orbits, and this causes a bottleneck on the radial transport.

References

[1] P.J. Catto, J. Plasma Phys. 85, 905850213 (2019).

[2] P.J. Catto, E.A. Tolman & F.I. Parra, J. Plasma Phys. 89, 905890106 (2023).