Near-Edge Alfven Eigenmodes in DIII-D high β_pplasmas

New analysis of a DIII-D high bootstrap current fraction scenario plasma, which is characterized by high q₉₅(q₉₅>6) and a strong large radius internal transport barrier (ITB) at ρ~0.6, demonstrates that the Beta-Induced Alfven Eigenmode (BAE) plays an important role in controlling transport in the outer core region (ρ~0.75). First, multiple fluctuation measurements show evidence for a mode peaking in this region with frequency and wavenumber consistent with characteristics of the BAE. Second, a detailed stability analysis indicates that the BAE is primarily excited by the unfavorable magnetic curvature drive in this region, which lies close to the kinetic ballooning mode (KBM) boundary as is pointed by Staebler[1]. Because the BAE and KBM share the same drive from the bulk thermal plasma, only a small amount of additional free energy is needed to destabilize the BAE. Gyrokinetic simulations predict that the small but finite fast ion fraction at these radii is sufficient to provide this additional drive and robustly destabilizes the BAE. Application of such a theory is also able to reproduce the BAE behavior in shots with similar q₉₅ but the large radius ITB is not established. Finally, nonlinear CGYRO simulations predict that the BAE can efficiently drive experimentally relevant levels of the electron particle flux and electron energy flux. When additional contributions from neoclassical transport and ETG turbulence are considered, the experimental fluxes in all three channels are reproduced within uncertainties.

[1] Staebler et al. Phys. Plasmas 25, 056113 (2018)