Investigation of fast particle driven modes on Alcator C-Mod

High-frequency coherent magnetic fluctuations have been observed with hydrogen minority ion cyclotron resonant frequency (ICRF) heating in Alcator C-Mod in low- or reversed-shear current rise conditions. The mode frequencies (150-450 kHz) are in the range expected for toroidal Alfvén eigenmodes (TAEs) with central qvalues greater than unity. These modes have low n 3 and appear to rotate in the ion diamagnetic drift direction. Under these relatively low-density current rise conditions with low or reversed magnetic shear, TAEs can be destabilized by a predicted fast hydrogen ion tail of just over 100 keV. Calculations with the CASTOR code indicate that a broad eigenmode can exist with a calculated frequency at the measured mode frequency, which falls in a clear TAE gap across the profile. The frequency of the current rise modes can increase or decrease rapidly from 150 to 400 kHz in less than 15 ms, which cannot be explained by linear theory. In some cases, only a single mode is excited with a linearly increasing frequency, in contrast to the multiple modes with decreasing frequency usually observed on other devices.