Faraday rotation measurements on compact helical system by using a phase sensitive heterodyne polarimeter

A phase sensitive heterodyne polarimeter was installed on compact helical system (CHS). An HCN laser (337 μm) and Schottky barrier diodes are used for the radiation source and for detection, respectively. By using a phase sensitive heterodyne scheme, the Faraday rotation angle can be measured from the phase variations caused by the phase difference between the right- and left-handed waves in the magnetized plasma. The purpose of the polarimeter on CHS is for a stable density monitor which is free from fringe jump and for the measurements of the internal magnetic field induced by the plasma current. The present resolution of the Faraday rotation angle is 0.1° with 10 ms time resolution and 1° with 1 ms. These are mainly determined by the laser instability.