CO2 laser polarimeter for Faraday rotation measurements in the DIII-D tokamak

Van Zeeland, M. A.; Boivin, R. L.; Carlstrom, T. N.; Deterly, T. M.
October 2008
Review of Scientific Instruments;Oct2008, Vol. 79 Issue 10, p10E719
Academic Journal
A tangential viewing, 10.59 μm CO2 laser polarimeter for electron density measurements based on plasma induced Faraday rotation has been installed on DIII-D. The system uses colinear right- and left-hand circularly polarized beams with a difference frequency of 40 MHz to generate the necessary signal for heterodyne phase detection. The high-resolution phase information required to adequately resolve degree level polarization rotation is obtained using an all-digital “real-time” phase demodulation scheme based on modern digital signal processing techniques. Initial application of the system to DIII-D disruption mitigation experiments utilizing “massive gas jet” injection exhibits reliable operation and excellent agreement with CO2 interferometer measurements; interestingly, the obtained Faraday rotation angles are in the range of those expected in ITER plasmas.


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