Ellipticity and triangularity induced Alfven Eigenmodes: A new instability in burning plasmas

When the cross-section of the plasma column is elliptical, a new gap mode (the Ellipticity Induced Alfven Eigenmode (EAE)) can be excited with frequency near the Alfven frequency ωA. Triangularity of the plasma column can also induce a gap mode (the Triangularity Induced Alfven Eigenmode (NAE)). An energetic particle population such as alpha particles can drive the EAE and the NAE unstable by resonant interaction. Since many tokamaks have finite elongation as compared to small toroidicity, the EAE may indeed be a more robust and global mode than the TAE. The most global EAE couples some cylindrical eigenmodes. The region of strong coupling occurs at the q=2 surface and the width of the coupling region is finite. For typical q-profiles and plasma elongation values the n=1, m=(1,3) EAE involves a large portion of the plasma column. For 11 and the growth rates are of the order 10-3 ωA. It is also shown that the toroidal gap mode (TAE) is strongly stabilized via sideband resonance with the circulating ions and the EAE is stabilized by circulating electrons. Marginal stability conditions of the TAE, EAE and NAE are presented for different ignition devices. Figs