Damping of Alfvén eigenmodes on localized electrons in stellarators

Abstract

The electron collisional damping of various kinds of Alfvén eigenmodes (AE) in stellarators is studied. Simple expressions for the damping rates of various well-localized AEs are obtained and applied to evaluating the damping rates in the Helias reactor [H. Wobig et al., Fusion Energy 1998, 17th Conference Proceedings, Yokohama, Japan (IAEA, Vienna, 1999), Vol. 4, p. 1235] and quasi-poloidal stellarator [J. F. Lyon et al., Fusion Energy 2002, 19th Conference Proceedings, Lyon, France (IAEA, Vienna, 2002), Paper IC/P-05]. The estimates made indicate that the collisional damping prevents the destabilization of AEs localized at the plasma periphery of the Helias reactor. A new mechanism that can considerably affect the collisional damping of Alfvén waves in stellarators is predicted: it is found that under certain conditions the collisionless orbit transformations between the locally trapped states and locally passing states significantly change the damping rate, so that the latter becomes weakly (logarithmically) dependent on the collision frequency.