Fast-ion-induced modes and fast-ion confinement in negative triangularity DIII-D plasmas

We report on fast-ion confinement and performance in DIII-D negative triangularity (NT) plasmas with β_N>2.5. NT plasmas have demonstrated H_89y2 > 1.2 and β_N ~ 3 high-performance core thermal confinement. Previous studies in wall-limited NT configurations with δ_avg = −0.4 showed that fast-ion losses are comparable to those in positive triangularity plasmas. In this work, we analyze divertor-configured NT discharges with δ_avg = −0.5, focusing on the high-current plasmas (q₉₅ < 3) and high safety factor plasmas (q_min > 1, q₉₅ > 4). By identifying fast ion-driven instabilities and estimating fast-ion confinement via the neutron deficit method, we obtain several key findings.​ In high-current shots, fishbones and resonant tearing modes are the dominant instabilities. The ratio of the measured neutron source rate to the classical neutron rate is notably lower during fishbone activity, indicating that fishbones induce more fast ion transport compared to resonant tearing modes. In shots with q_min>1, the toroidal Alfvén eigenmode (TAE) is the dominant instability. The overall neutron ratio are significantly lower than those in high-current discharges, suggesting that TAE may impede confinement improvements in NT experiments.

Keywords: negative triangularity, fast ions, Alfvén eigenmode