Alfven continuum and high-frequency eigenmodes in optimized stellarators

An equation of shear Alfven eigenmodes (AE) in optimized stellarators of We ndelstein line (Helias configurations) is derived. The metric tensor coeffi cients, which are contained in this equation, are calculated analytically. Two numerical codes are developed: the first one, COBRA (COntinuum BRanches of Alfven waves), is intended for the investigation of the structure of Al fven continuum; the second, BOA (Branches Of Alfven modes), solves the eige nvalue problem. The family of possible gaps in Alfven continuum of a Helias configuration is obtained. It is predicted that there exist gaps which ari se due to or are strongly affected by the variation of the shape of the pla sma cross section along the large azimuth of the torus. In such gaps, discr ete eigenmodes, namely, helicity-induced eigenmodes (HAE(21)) and mirror-in duced eigenmodes (MAE) are found. It is shown that plasma inhomogeneity may suppress the AEs with a wide region of localization. (C) 2001 American Ins titute of Physics.