Far-infrared tangential interferometer/polarimeter design and installation for NSTX-U

The Far-infrared Tangential Interferometer/Polarimeter (FIReTIP) system has been refurbished andis being reinstalled on the National Spherical Torus Experiment-Upgrade (NSTX-U) to supplyreal-time line-integrated core electron density measurements for use in the NSTX-U plasma controlsystem (PCS) to facilitate real-time density feedback control of the NSTX-U plasma. Inclusionof a visible light heterodyne interferometer in the FIReTIP system allows for real-time vibrationcompensation due to movement of an internally mounted retroreflector and the FIReTIP front-endoptics. Real-time signal correction is achieved through use of a National Instruments CompactRIOfield-programmable gate array.