Initial active MHD spectroscopy experiments exciting stable Alfvén eigenmodes in Alcator C-Mod

A pair of toroidally localized in-vessel antennas in Alcator C-Mod were used to excite and detect stable toroidal Alfvén eigenmode (TAB) resonances in the frequency range of 400-500 kHz. By ramping the toroidal field, the plasma TAB frequency was swept through the constant antenna excitation frequency to find a resonance and the width of the resonance was measured to determine the effective damping rate of the mode. The toroidal field and density at the resonances were observed to scale as B_T α n _e^0.5, as expected for Alfvén eigenmodes. TAB damping rates were measured in both inner wall limited and diverted discharges. The inner wall limited damping rates for these discharges were in the range of 1% < |γ/ω| < 4%. Diverted plasmas, on the other hand, required a very small outer gap between the last closed flux surface and the outboard limiter of less than 1 cm to observe the resonances and the damping rates were then very low with |γ/ω| < 1%.