Design of a dispersion interferometer combined with a polarimeter to increase the electron density measurement reliability on ITER

A dispersion interferometer is a reliable density measurement system and is being designed as a complementary density diagnostic on ITER. The dispersion interferometer is inherently insensitive to mechanical vibrations, and a combined polarimeter with the same line of sight can correct fringe jump errors. A proof of the principle of the CO2 laser dispersion interferometer combined with the PEM polarimeter was recently conducted, where the phase shift and the polarization angle were successfully measured simultaneously. Standard deviations of the line-average density and the polarization angle measurements over 1 s are 9 x 10(16) m(-2) and 0.19 degrees, respectively, with a time constant of 100 mu s. Drifts of the zero point, which determine the resolution in steady-state operation, correspond to 0.25% and 1% of the phase shift and the Faraday rotation angle expected on ITER. Published by AIP Publishing.