Dual CO2 laser polarimeter for Faraday rotation measurement in tokamak plasmas

The first operational results of the dual CO2 laser polarimeter to measure the Faraday rotation in large tokamaks is described. The polarimeter consists of two same type polarimeters except those wavelengths of 10.6 and 9.27 μm. By simultaneous measurement of two wavelengths, the Faraday rotation component at vacuum windows is expected to be eliminated from the measured polarization angle. Each polarimeter utilizes photoelastic modulators for reliable polarization detection. Such a polarimeter scheme is easily compatible with the dual CO2 laser interferometer, hence, the new system is called as the dual CO2 laser polarimeter/interferometer. The Faraday rotation by the JT-60U plasma is successfully measured even at a fast disruption. The Faraday rotation angle from the 9.27 μm polarimeter shows good agreement with the calculation. It is found that the calibration of the 10.6 μm polarimeter should be improved for full two-color operation. The accuracy of the polarization angle is 0.1° for the data sampling rate of 250 Hz, which is sufficient for large devices like ITER.