Bench testing of magneto-optic polarimeters for long-pulsed fusion devices

We have developed magneto-optic polarimeters as magnetic field sensors for long-pulsed fusion devices. In this paper, results of bench testing of a polarimeter using a pair of photoelastic modulators (PEMs) and a superluminescent diode (SLD) with a center wavelength of 822 nm are described. Angle resolutions of 0.004° and 0.002° with response times of 1 ms and 10 ms, respectively were achieved at an incident polarization angle of about 0° while those at 21° were 0.07° independently on the response time. When a 40-mm-long ZnSe sensing rod was used, the resolution of 0.07° corresponds to a magnetic field resolution of 7 gauss. The long-time continuous measurement demonstrated good stability and little dependence on the room temperature in comparison with a polarimeter by the polarization rotation method.