FIR polarimetry on the Alcator C-Mod tokamak

A three-chord FIR Polarimeter has been in operation since 2012 on Alcator C-Mod. It has been used to study changes in plasma current density profiles and magnetic and density fluctuations 1,2. Two 200 mW lasers operating near 2.55 THz, but with a frequency offset, are combined to generate a beam with a rotating polarization. Phase comparisons of signals from before and after transmission through the plasma allow the Faraday Effect, which is proportional to the line integral of the product of the magnetic field component along the beam and the electron density, to be measured. The lasers and control electronics, detectors and amplifiers, and the optical layout will be described. Sources of system noise will be discussed. Data from C-Mod plasmas showing the effects of lower hybrid current drive and ICRF heating, and both narrow-band and turbulent fluctuation spectra will be presented.