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Description
Alpha particles typically exhibit collisionless dynamics throughout most of their phase space. However, their motion can be very sensitive to collisions under resonant conditions. The collision operator is required to resolve the singular behavior around resonances, allowing the formation of a collisional boundary layer, where a perturbed distribution function forms and drives radial transport. In an optimized quasisymmetric (QS) stellarator, the presence of an error field of mode numbers m,n results in a passing alpha particle resonance near the q=m/n rational surface, for those alphas whose streaming motion and tangential drift cancel. The resonance can extend over much of the radial cross section because the resonant velocity space pitch angle varies with minor radius. If the error field is sufficiently small, the collisional boundary layer around the resonance is wider than the island structure due to the drift and streaming. For larger error fields, the drift and streaming island structure prevails and the collisional boundary layer localizes around the island separatrix. In this work, we develop a drift kinetic model for alpha particles that enables an evaluation of passing alpha particle and energy collisional transport. We quantify the associated diffusivities and resulting alpha particle and energy losses, and show that these can be significant. The calculation is performed using a general QS stellarator background with an error field that causes a deviation from QS. The results are also applicable to a tokamak. In the smaller error field limit, our model predicts energy losses of up to 10% for error fields of order \delta B=10^(-3) T in a background magnetic field of 10 T.