Effect of Off-Axis Beam Injection on Alfv\'en Eigenmodes

Off-axis injection of neutral beams provides new insights into Alfv\'en eigenmodes (AEs). Off-axis injection flattens the local fast-ion gradient $\nabla\beta_f$ in the core, completely stabilizing reversed shear AEs (RSAEs). In contrast, at larger minor radius, $\nabla\beta_f$ is similar for on- and off-axis injection. As a result, switching the angle of injection has little effect on the stability of global toroidal AEs. Two-dimensional measurements of RSAE mode structure with an electron cyclotron emission imaging diagnostic show that the phase of the eigenfunction varies with radius. The phase variation was originally attributed to symmetry breaking associated with the fast-ion gradient but, unexpectedly, changes in $\nabla\beta_f$ have little effect on radial shearing. The mode structure of lower-frequency beta-induced Alfv\'en-acoustic eigenmodes (BAAE) is similar to RSAEs. The BAAE real frequency is quite sensitive to $\nabla\beta_f$ but, surprisingly, stability is not.