Polarimetry measurements of Faraday rotation on Tore Supra are systematically affected by spurious oscillations that may seriously hinder the reconstruction of plasma current profiles. The same behavior has also been regularly observed on other experiments, such as JET and as RFX. Causes of this phenomenon are interferences between the probing beam and stray radiations having the same frequency. It perturbs the intensity of the signals from the detectors installed on the two orthogonal polarization channels. Experimental tests confirm the dependence of the oscillation amplitudes on the Faraday angle as predicted by the analytical and numerical description of the phenomenon. Special care must be taken to eliminate stray beams: When the stray beam polarization is parallel to the incident one, a relative ratio of the electric field amplitudes <1% has been found to be acceptable (error <0.06°) but a much lower ratio must be achieved when polarization is perpendicular. Such oscillations can be also present on polarimeters using beams with modulating polarization. For existing and future polarimeters, special care should be taken to make sure that all the possible sources of stray beams are sufficiently minimized. © 2004 American Institute of Physics.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy (miscellaneous)
Elbéze, D., Gil, C., Giannella, R., & De Pasqual, L. (2004). Spurious oscillations affecting FIR polarimetry measurements. Review of Scientific Instruments, 75(10 II), 3405 - 3407. https://doi.org/10.1063/1.1786637