Interaction between Faraday rotation and Cotton-Mouton effects in polarimetry modeling for NSTX

TL;DR

This paper models electromagnetic wave polarization in NSTX, revealing how Faraday rotation and Cotton-Mouton effects interact, especially at longer wavelengths, impacting polarimetry measurement interpretation.

Contribution

It demonstrates the interaction between Faraday rotation and Cotton-Mouton effects in plasma polarization modeling, highlighting their combined influence on wave elliptization.

Findings

Cotton-Mouton effect is strongly weighted to high-field regions.

Interaction between Faraday rotation and Cotton-Mouton effects increases with wavelength.

Polarimetry interpretation is complicated by these effects' interaction.

Abstract

The evolution of electromagnetic wave polarization is modeled for propagation in the major radial direction in the National Spherical Torus Experiment (NSTX) with retroreflection from the center stack of the vacuum vessel. This modeling illustrates that the Cotton-Mouton effect-elliptization due to the magnetic field perpendicular to the propagation direction-is shown to be strongly weighted to the high-field region of the plasma. An interaction between the Faraday rotation and Cotton-Mouton effects is also clearly identified. Elliptization occurs when the wave polarization direction is neither parallel nor perpendicular to the local transverse magnetic field. Since Faraday rotation modifies the polarization direction during propagation, it must also affect the resultant elliptization. The Cotton-Mouton effect also intrinsically results in rotation of the polarization direction, but this effect is less significant in the plasma conditions modeled. The interaction increases at longer wavelength, and complicates interpretation of polarimetry measurements.