Coupling of Alfvén and sound waves in stellarator plasmas

Recently, a low frequency mode possibly driven unstable by hot electrons [J. M. Canik et al., 21st IAEA Fusion Energy Conference, Chengdu (China) (IAEA, Vienna, 2006), CN-149EX/5-2] has been observed in the Helically Symmetric Experiment [F. S. B. Anderson et al., Fusion Technol. 27, 273 (1995)]. The emergence of such a mode has been investigated within the framework of computational three-dimensional magnetohydrodynamics (MHD). A three-dimensional MHD continuum code CONTI has been developed for a fast computation of the continuous Alfvén and sound spectrum. The global modes have been calculated using the CAS3D code. A possibly new class of modes has been identified which is generated by the interaction between sound and Alfvén waves. The respective gap still contains a continuum branch, so that because of the continuum interaction, a conclusive answer if these modes really exist cannot be achieved within the computational ideal MHD picture. Additionally, continuum gaps of similar type have been investigated also for other stellarators.