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Study of internal magnetic field via polarimetry in fusion plasmas


The internal equilibrium magnetic field and magnetic fluctuations in tokamaks were studied via polarimetry in this project. This study was performed mainly using a recently developed millimeter-wave polarimeter system, which probes the plasma along a major radial chord using a retro-reflection geometry. The system launches a rotating linearly polarized beam and detects a phase shift directly related to the polarization change caused by the plasma. The magnetic field component along major radius (BR) can be estimated from the measured phase shift. A phase resolution of <2° for f < 500 Hz and 0.3° (δBR ∼ 2 Gauss) for 500 Hz < f < 500 kHz has been achieved.

An interaction between Faraday rotation (FR) and Cotton-Mouton (CM) effects was first identified in polarimetry modeling for NSTX using a synthetic diagnostic code based on Mueller-Stokes calculus. The interaction, especially the influence of the CM effect on polarimetry measurements, was further investigated in a systematic theory-experiment comparison on DIII-D. The synthetic diagnostic code was validated by accurately reproducing the measurements over a broad range of plasma conditions, and was intensively employed in the investigation. When the CM effect is predicted to be weak, the FR effect can well approximate the measurements. As the CM effect increases, it can compete with the FR effect in rotating the polarization of the EM-wave. This results in an apparent reduced polarimeter response to the FR effect. If sufficiently large, the CM effect can even reverse the handedness of a wave launched with circular polarization. This helps to understand the surprising experimental observations that the sensitivity to the FR effect can apparently be nearly eliminated at BT = 2.0 T.

The potential of mm-wave polarimetry to measure magnetic fluctuations is explored via both modeling and experiment. Simulations suggest that microtearing modes in NSTX-U will induce >∼ 2° (f ∼ 10 kHz) variations in polarimeter phase, which is primarily due to the associated magnetic fluctuations. Therefore the polarimeter is predicted to have sufficient sensitivity to observe magnetic fluctuations associated with microtearing modes. In FR-dominant case, the magnitude of density-weighted &delta BR can be estimated from polarimeter measurements. For example, ∼ 2 Gauss δBR (δBR/B0 &sim 2 × 10-4) is estimated for a ∼ 65 kHz Toroidicity-induced Alfvén Eigenmode in a low BT (0.75 T) discharge. However, in the presence of strong CM effect, support from other diagnostics and modeling is required to determine the magnetic fluctuations.

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