Collisional alpha particle transport in the presence of finite kinetic islands in an imperfect quasisymmetric stellarator

The trapped alpha particles in bounce or transit averaged kinetic treatments of stellarators undergo drift reversal. The velocity space pitch angle at drift reversal on each flux surface depends on the inverse aspect ratio because it determines the trapped-passing boundary location. The alphas in and adjacent to the drift reversal layer are particularly sensitive to collisions because they are close to resonance. This near resonant behavior makes them sensitive to pitch angle scattering collisions, and for a finite departure from quasisymmetry (QS) allows island formation. However, because of the presence of collisions, these are not magnetic islands in the usual sense. Instead, the behavior of the islands depends on pitch angle as well as inverse aspect ratio and they are fully kinetic in nature due to diffusive collisions. That is, the collisional transport in the presence of the island structure is due to pitch angle scatter in velocity as well as minor radius variation. When collisions are weak, as they are for alpha particles, and the departure from QS large enough, the alpha particles are only occasionally disrupted by collisions as they move in this island perturbed imperfect QS stellarator geometry. However, the pitch angle scattering due to the background ions is enhanced by the small scale introduced by the finite width of the kinetic islands. The associated transport is evaluated and found to be significant, but is shown to preferentially remove the slower speed alphas without significantly affecting the birth alphas.