Confinement degradation by Alfvén-eigenmode induced fast-ion transport in steady-state scenario discharges

Abstract

Here, analysis of neutron and fast-ion D<sub>α</sub> data from the DIII-D tokamak shows that Alfvén eigenmode activity degrades fast-ion confinement in many high β<sub>N</sub>, high q<sub>min</sub>, steady-state scenario discharges. (β<sub>N</sub> is the normalized plasma pressure and q<sub>min</sub> is the minimum value of the plasma safety factor.) Fast-ion diagnostics that are sensitive to the co-passing population exhibit the largest reduction relative to classical predictions. The increased fast-ion transport in discharges with strong AE activity accounts for the previously observed reduction in global confinement with increasing q<sub>min</sub>; however, not all high q<sub>min</sub> discharges show appreciable degradation. Two relatively simple empirical quantities provide convenient monitors of these effects: (1) an 'AE amplitude' signal based on interferometer measurements and (2) the ratio of the neutron rate to a zero-dimensional classical prediction.