Collisional passing alpha energy transport in quasisymmetric stellarators

Recent optimization schemes have achieved quasisymmetric stellarator designs with very small error fields to effectively reduce neoclassical transport and collisionless alpha losses. However, significant collisional alpha energy losses may still occur under resonant conditions. In the presence of deviations from quasisymmetry with poloidal and toroidal mode numbers m and n, a resonance between passing alpha particle streaming and drift motion can occur near the rational surface with safety factor q=m/n. To evaluate the resulting energy losses, we develop an analytical model to calculate the quasilinear radial energy transport of alpha particles using a drift kinetic treatment. Assuming error fields of order δB=10^-2 T in a background magnetic field B₀= 10 T, which requires a magnetic configuration precision of 0.1% after the device design and construction, passing alpha energy losses can be of the order of 10%. As expected, radial transport grows quadratically with the error field amplitude. A sensitivity analysis across different stellarator design parameters has been performed to assess their impact on alpha energy losses. In particular, configurations with very low global shear can exhibit enhanced transport over broader radial regions. Finally, the limitations of the model are evaluated.