Faraday rotation densitometry for Large Helical Device

Abstract

A CO2 laser polarimeter to measure the Faraday rotation in the large helical device (LHD) was developed and installed on a LHD. A CO2 laser whose wavelength is 10.6 μm was selected to avoid effects of the refraction. The Faraday rotation angle along toroidally tangential chords is of the order of one degree. The polarimeter utilizes the frequency-shift heterodyne technique with the use of acousto-optic modulators for high resolution. This polarimeter can be operated with a maximum of three channels. The accuracy of the Faraday rotation angle with a time resolution of 16 ms is about 0.01 deg by digital complex demodulation. We succeeded in performing preliminary measurements of the Faraday rotation angle from LHD plasmas and the values were 10%–30% larger than those estimated with the electron density profiles from interferometry.