The effect of local magnetic wells produced by the external helical windings of a stellarator reactor on alpha -particle confinement is investigated by using the Fokker-Planck equation. It is found that alpha -particles can deposit most of their energy into the background plasma before they scatter into the trapped region in velocity space and are lost. An estimate is made on the steady-state, low energy (thermal) alpha -particle concentration. The result shows that this concentration will be less than a few per cent of the background plasma density.