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Magnetohydrodynamic interference with the edge pedestal motional Stark effect diagnostic on DIII-D
J D King1, M A Makowski, C T Holcomb
1Lawrence Livermore National Laboratory, Livermore, California 94550-9234, USA.
The Review of Scientific Instruments
|April 5, 2011
Summary
Accurate tokamak magnetic field measurements using motional Stark effect (MSE) polarimetry are hindered by magnetohydrodynamic (MHD) fluctuations. New methods using higher harmonic frequencies can avoid these errors for improved plasma control.
Area of Science:
- Plasma physics
- Fusion energy research
- Diagnostic techniques
Background:
- Accurate internal magnetic field measurements are crucial for tokamak operations.
- Motional Stark Effect (MSE) polarimetry is a key diagnostic for measuring magnetic field direction.
- Previous MSE measurements were limited by noise and interference.
Purpose of the Study:
- To investigate the impact of magnetohydrodynamic (MHD) fluctuations on MSE polarimetry.
- To identify methods for mitigating interference in real-time polarization angle measurements.
- To improve the accuracy of magnetic field measurements in the tokamak edge pedestal.
Main Methods:
- Installation of a 500 kHz 32-channel digitizer for MSE diagnostic on DIII-D.
- Fourier analysis of polarimeter signals to detect MHD fluctuations.
- Correlation analysis between edge localized modes and MSE spectrograms.
- Investigation of fourth harmonic photo-elastic modulator (PEM) signals.
Main Results:
- MHD fluctuations, specifically tearing modes, were observed in the plasma edge pedestal (ρ ≥ 0.92).
- These fluctuations cause significant ( > 10x) unrecoverable errors in second harmonic MSE measurements.
- Errors can lead to substantial inaccuracies (up to 38%) in local safety factor (q) calculations.
- Fourth harmonic measurements, while avoiding MHD interference, did not improve accuracy due to lower signal-to-noise ratio.
Conclusions:
- MHD fluctuations present a major challenge for accurate real-time MSE polarimetry in tokamak edge pedestals.
- Utilizing higher harmonic PEM frequencies can mitigate MHD interference.
- Future tokamak polarimeters should employ higher fundamental PEM frequencies for improved accuracy.
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