Two-color polarimeter for electron density measurement on large tokamaks

A two-color laser polarimeter on large tokamaks such as ITER has been proposed for electron density measurement based on the midplane Faraday rotation. The two-color scheme in the polarimeter eliminates the Faraday rotation at vacuum windows from measured polarization angles. Two CO2 lasers which have different wavelengths of 10.6 and 9.27 μm are favorable as light sources of the two-color laser polarimeter for large tokamaks. A couple of photoelastatic modulators are used for stable polarization measurement even though the intensity and the propagation axis of an incident laser beam are changed.