Collisional passing alpha particle transport in a nearly optimized quasisymmetric stellarator

Recently, nearly precise quasi-symmetric stellarator configurations have been found. These are expected to successfully reduce the non-turbulent bulk plasma transport to acceptable neoclassical levels while removing nearly all collisionless direct orbit losses of alpha particles. However, even though fusion alphas are born at high speeds, they can be very sensitive to collisions under resonant conditions. As a result, they can be lost before transferring their energy to the plasma, causing concerning energy losses and damaging plasma-facing components. We evaluate how error field departures from quasi-symmetry due to a change in the magnetic field direction give rise to a perturbed distribution function of the passing alpha particles. These collisional boundary layers, located in the barely passing region near rational surfaces, cause both sqrt(nu) and resonant plateau transport, with the latter being dominant. We evaluate the energy loss from this type of transport and check the limitations of the model.