Magnetohydrodynamic interference with the edge pedestal motional Stark effect diagnostic on DIII-D

King, J. D.; Makowski, M. A.; Holcomb, C. T.; Allen, S. L.; Hill, D. N.; La Haye, R. J.; Turco, F.; Petty, C. C.; Van Zeeland, M. A.; Rhodes, T. L.; Meyer, W. H.; Geer, R.; Morse, E. C.
March 2011
Review of Scientific Instruments;Mar2011, Vol. 82 Issue 3, p033515
Academic Journal
Accurate measurement of internal magnetic field direction using motional Stark effect (MSE) polarimetry in the edge pedestal is desired for nearly all tokamak scenario work. A newly installed 500 kHz 32-channel digitizer on the MSE diagnostic of DIII-D allows full spectral information of the polarimeter signal to be recovered for the first time. Fourier analysis of this data has revealed magnetohydrodynamic (MHD) fluctuations in the plasma edge pedestal at ρ >= 0.92. By correlating edge localized mode fluctuations seen on lock-in amplifier outputs with MSE spectrograms, it has been shown that edge pedestal tearing mode fluctuations cause interference with MSE second harmonic instrument frequencies. This interference results in unrecoverable errors in the real-time polarization angle measurement that are more than an order of magnitude larger than typical polarimeter uncertainties. These errors can cause as much as a 38% difference in local q. By using a redundant measure of the linear polarization found at the fourth harmonic photo-elastic modulator (PEM) frequency, MHD interference can be avoided. However, because of poorer signal-to-noise the fourth harmonic signal computed polarization angle shows no improvement over the MHD polluted second harmonics. MHD interference could be avoided in future edge pedestal tokamak polarimeters by utilizing PEMs with higher fundamental frequencies and a greater separation between their frequencies.


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