Observation of beta-induced Alfvén eigenmodes in the DIII-D tokamak

Energetic ions can drive Alfv\'en gap modes unstable, causing large losses of fast ions. Toroidicity-induced Alfv\'en eigenmodes (TAE) were expected to disappear into the shear Alfv\'en continuum and become stable as the plasma beta increased. Although TAE modes may disappear, another dangerous instability with similar properties but approximately half the TAE frequency appears in a spectral gap that is created by finite beta effects. The measured frequency of the new mode agrees with the theoretical frequency of beta-induced Alfv\'en eigenmodes.