Merging of the superbanana plateau and √ν transport regimes in nearly quasisymmetric stellarators.

Alpha particle confinement is one of the most demanding issues for stellarators. Here the collisional confinement of barely trapped alphas in an optimized stellarator is considered by accounting for the resonance due to tangential drift reversal and investigating the sensitive role of magnetic shear in keeping this resonance close to the passing boundary in some nearly quasisymmetric stellarator configurations. The treatment relies on a narrow collisional boundary layer formulation that combines the responses of both these resonant pitch angle alphas and the remaining barely trapped alphas. A novel merged regime treatment leads to explicit expressions for the energy diffusivity for both superbanana plateau (or resonant plateau) and √ν transport in the large aspect ratio limit for a slowing down tail alpha distribution function, where ν is the pitch angle scattering collision frequency of the alphas off the background ions.