CO 2 laser polarimeter for electron density profile measurement on the Large Helical Device

We developed a three-channel tangential CO2 laser polarimeter to measure electron densities on the Large Helical Device. Our system aims to obtain not only the line averaged electron density but also the electron density profile. The achieved resolution of the Faraday rotation angle was 0.01° with a response time of 3 ms by digital complex demodulation combined with digital filtering. The phase fluctuations whose amplitude is typically 0.05° with a time constant of several seconds were observed. It was found that they were caused by beam axis fluctuations from bench top experiments. In the case of pellet injected plasmas it was demonstrated that the polarimeter could measure the time evolution of the Faraday rotation with high reliability and resolution. The calibrated rotation angle of all chords were well consistent with interferometer data. Assuming the functional dependence of the density profile, we estimated the density profile after pellet injections from three-channel polarimeter data iteratively. As a result, obtained profiles agreed with those measured with the 13-channel far infrared interferometer.