Measurement of beam-driven Alfv\'{e}nic instabilities in MST and comparison to predictions

Neutral beam injection in MST drives a variety of energetic particle modes (EPMs) and Alfv\'{e}nic modes (AEs). These instabilities can lead to avalanche processes with enhanced fast ion transport, the most commonly observed avalanche involving coupling between modes with toroidal number n = 5, 4, and -1. Density fluctuations correlated with these avalanches, as well as internal magnetic fluctuations associated with the dominant, n = 5 EPM, have previously been characterized with the FIR interferometer-polarimeter. However, the n = 4 and -1 AEs were too weak to observe with polarimetry. Recent upgrades to the interferometer-polarimeter have further reduced the noise floor for fluctuation measurements. The upgraded system allows clear identification of Faraday rotation fluctuations associated with the n = -1 AE. These fluctuations are highly localized, with narrow structure peaking at the same location as the density fluctuations. Previous work has established that the n = 4 and -1 fluctuations are consistent with island-induced AEs (IAEs). Measured structures are compared to predictions for IAEs as well as helicity-induced AEs, and ongoing work to resolve the dependence of observed structures on tearing mode island phase and identify n = 4 correlated fluctuations is presented.