Collisionless alpha -particle confinement in stellarators

In many stellarators-envisaged as fusion devices-any alpha -particle which ever gets reflected ( nu /sub ///=0) is collisionlessly lost in a time which is orders of magnitude smaller than the typical slowing-down time of approximately=10 -1 s. Two classes of stellarators to which this general picture does not apply are described: quasi-helically symmetric stellarators and a class of stellarators with vanishing bootstrap current in which the collisionless alpha -particle confinement sufficiently improves at finite beta . The influence of the modular ripple in optimized coil systems realizing these configurations, the angular distribution of the fast alpha -particle losses, and the application of the results to alpha -particle confinement simulation experiments in next-generation stellarators are also discussed.