Collisional passing alpha energy transport in nearly quasisymmetric stellarators

Abstract

Recent advances in stellarator optimization have found nearly precise quasisymmetric configurations. These are expected to reduce the non-turbulent background plasma transport to acceptable neoclassical levels while removing nearly all collisionless direct orbit losses of alpha particles. Yet, alpha particles under resonant conditions can be very sensitive to collisions, causing concerning energy losses and damaging plasma facing components. For the passing alphas such resonances can happen near rational surfaces in the presence of helical error field departures from quasisymmetry that change the magnetic field direction and magnitude. The cancellation between streaming motion and tangential drift of the alphas enhances the effective collision frequency, allowing the formation of a collisional boundary layer and giving rise to a perturbed distribution function. We develop an analytical model to illustrate and evaluate the resonant plateau transport this mechanism causes by formulating a drift kinetic treatment. The results indicate the associated energy losses can become significant in the vicinity of rational surfaces at values of q=m/n when error fields with poloidal and toroidal numbers m and n are present. In addition, we investigate the validity of the quasilinear approximation to the energy flux to show that it imposes a restriction on the error field amplitude that can be considered.