A Fast MSE Measurement of MHD Magnetic Fluctuations on DIII-D

Resolved local magnetic fluctuation measurements, along with other fluctuation diagnostics, could provide a means of accurately measuring magnetic island size and evolution. The ability to make such measurements has significant implications for the detailed analysis of the structure of NTMs and other MHD. A new 16 bit, 500~kS/s data acquisition system has been installed on all channels of the existing MSE diagnostic on DIII-D. In addition to the fundamental 2f$_1$ and 2f$_2$ signals used for existing MSE polarimetry measurements, the spectrum contains peaks that strongly correlate to MHD fluctuations. Of particular interest are sideband spectral peaks at $2\omega_{1,2}\pm\omega_{MHD}$, which provide information needed for recovering the amplitude of local magnetic field fluctuations. In this work, a brief quantitative validation of the underlying theory of magnetic fluctuation measurements is presented, relationships between spectral harmonics are shown, and a first attempt at a temporally and spatially resolved MHD measurement is discussed.