Low Frequency Instabilities and Alfven Eigenmodes in Mixed Species DIII-D Plasmas*

The stability of beam-ion driven instabilities in reversed-shear DIII-D plasmas is clarified. Beta-induced Alfven eigenmodes (AE) are driven by resonant co-passing high-energy deuterium ions¹ but not by hydrogen beam ions. Similarly, deuterium beams drive reversed-shear and toroidal AEs more strongly than hydrogen beams. In contrast, the instability that was previously called a ‘BAAE’ is not driven by fast ions at all.² Theory and experiment indicate that the latter modes are a low-frequency reactive instability of predominately Alfvenic polarization that occurs in plasmas with high electron temperature but modest beta; they are more unstable in hydrogen than deuterium. Simulations reproduce many features of the data but, owing to the approximation of isotropic Maxwellian fast ions, not properties that depend upon beam anisotropy.

¹Nucl. Fusion 61 (2021) 066031.

²Nucl. Fusion 61 (2021) 016029.