The Active MHD diagnostic system excites Alfv\'{e}n eigenmodes in Alcator C-Mod and measures their damping rates, or margin to instability. Resonant modes are stimulated with two moderate-n antennas positioned above and below the outboard midplane. The antennas are fed by power amplifiers equipped with automatically tuned capacitive networks capable of matching in the range of 30kHz to 1MHz. Pick-up coils located in a toroidal and poloidal array around the vessel and just outside the last closed flux surface measure poloidal magnetic field fluctuations. The excitation frequency follows in real-time the Toroidal Alfv\'{e}n Eigenmode (TAE) frequency, \omega_{TAE}=\frac{v_A}{2qR}, making small excursions away from the gap to observe the modes' damping characteristics.

Studies are conducted on the scaling of damping rate with plasma shape and the dimensionless parameters collisionality, beta and normalized ion gyroradius. The variables are scanned over a sequence of shots and TAEs are excited and observed to measure their damping rate. The effect of fast ions is also studied by operating the diagnostic in conjunction with various levels of ICRF heating and measuring the fast particle population with the Compact Neutral Particle Analyzer.

Work is being performed to improve the diagnostic in several areas. More pickup coils are being installed to more accurately measure the toroidal mode number. Signal processing of the recorded signals is being refined to more precisely determine the damping rate of the observed modes and to observe weaker modes.

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