Role of Beta-Induced Alfvén Eigenmode in DIII-D high β_pscenario

Analysis of DIII-D high β_p plasmas (A. Garofalo, Nucl. Fusion, 2015) has identified beta-induced Alfvén eigenmodes (BAEs) existing in the outer regions of the plasma, between the foot of the large radius internal transport barrier (ITB) and top of the edge pedestal. While recent analysis (X.Jian, Phys. Rev. Lett, 2019) has shown that micro-tearing modes regulate residual transport in the ITB, the processes regulating transport in other regions of the plasma remain less clear. The BAEs are observed to have a frequency of ~50 kHz (~0.3 ) in the lab frame, and toroidal mode numbers of 3-4. Their amplitude is inversely correlated with the fast ion confinement level, and thermal confinement to a lesser level. Previous studies have predicted the kinetic ballooning mode (KBM) to be unstable in the same region as the BAEs (G. Staebler, Phys. Plasmas, 2019). Gyrokinetic analysis indicates that even a small fast beam ion fraction (<1%) is able to excite the BAE, which couples strongly to KBM. Calculations of fast and thermal particle transport expected to be driven by these BAEs are underway and will be reported. Implications for further optimization of the high  scenario, in terms of considerations such as first wall protection from BAE-induced fast ion transport, are discussed.