Correlation polarimeter-interferometer in the DIII-D tokamak

A correlation polarimeter-interferometer system has been developed for the DIII-D tokamak to detect small amplitude magnetic and density fluctuations. Two collinear, frequency-offset (5–15 MHz), orthogonally polarized (right- and left-handed, circularly polarized) electromagnetic waves at 650 GHz probing the plasma are used to detect the Faraday effect. A third, linearly polarized, electromagnetic wave serves as the local-oscillator to provide a measure of the line-averaged density. Correlation polarimetric measurement is accomplished by correlating the Faraday effect measured along the same line of sight using two independent detectors, whereas simultaneous correlation interferometric measurement is realized by correlating the line-averaged density measured at two different intermediate frequencies. The noise floor of the correlation polarimeter-interferometer has been demonstrated to be more than one order of magnitude lower than that of a standard polarimeter-interferometer measurement. Line-averaged correlation polarimetric-interferometric measurements in DIII-D H-mode plasmas show broadband (up to 750 kHz) magnetic and density fluctuations with amplitudes as low as $0.03Gauss/kHz$ and $4×1013m−3/kHz$⁠, respectively.