Alfvén instabilities driven by circulating ions in optimized stellarators and their possible consequences in a Helias reactor

Abstract

This work investigates circulating-particle-induced Alfvén instabilities in optimized stellarators of the Wendelstein line [F. Wagner, Trans. Fusion Tech. 33, 67 (1998)]. A general expression for the growth rate of the instabilities is obtained and analyzed. It is shown that the absence of the axial symmetry makes it possible that various types of Alfvén eigenmodes will be destabilized; both the kind of destabilized Alfvén eigenmodes and the type of the resonances driving the instability may differ from those in tokamaks. In particular, an important role of the helicity-induced resonance is predicted. The discovered new resonances may considerably increase the instability growth rate of both the “gap” modes and the eigenmodes residing below cylindrical Alfvén continuum. The upper limits of the local energy losses of circulating α-particles caused by various Alfvén instabilities in a four-period Helias reactor [C. D. Beidler et al., in Fusion Energy 2000, 18th International Atomic Energy Agency Conference Proceedings, Sorrento, 2000 (International Atomic Energy Agency, Vienna, 2001), Report IAEA-CN-77/FT/4] are evaluated. It is found that certain destabilized Alfvén eigenmodes will affect only alphas with the energy well below 3.5 MeV, which seems to open a possibility to remove the helium ash by exciting the corresponding Alfvén eigenmodes by either energetic particles or an antenna system.