Alfvén eigenmode structure during off-axis neutral beam injection

The spatial structure of Alfvén eigenmodes on the DIII-D tokamak is compared for contrasting fast ion deposition profiles resulting from on- and off-axis neutral beam injection (NBI). In both cases, poloidal mode rotation and eigenmode twist, or radial phase variation, are correlated with the direction of the normal ion diamagnetic flow and readily inverted with a reversal of toroidal magnetic field, B _T. While off-axis NBI results in weakly driven reversed shear induced Alfvén eigenmodes due to reduced fast ion pressure gradient, ∇β _fast, in the region of the mode, these marginally unstable modes exhibit a 2D phase structure that is indistinguishable from that observed during on-axis injection. This result is consistent with recent explorations using the non-perturbative codes Gyro and TAEFL that show a weak dependence of eigenmode structure on drive when fast ion density is uniformly reduced by a scalar multiplier. These codes also obtain unstable, counter-propagating modes with the inverted 2D phase structure when B _T is kept constant and the diamagnetic flow direction is reversed by making ∇β _fast sufficiently positive for an isotropic population of fast ions. While measurements of the spatial profile of fast ion D-α light from the recently upgraded charge exchange recombination diagnostic on DIII-D suggest a strong modification of fast ion pressure towards this limit, no counter-propagating modes have yet been observed in experiment.