Fiber-optic current sensor with heterodyne detection

Abstract

The toroidal plasma current in the ZT-40M experiment (100–380 kA) has been measured using a fiber-optic sensor. This sensor consists of six turns of permanently twisted, single-mode fiber which was wound poloidally around the experiment. By modifying the laser output to a rotating linear polarization, amplitude-independent single output heterodyne detection is possible.1 The signal is proportional to the B field produced by the plasma current so integration of the data is not required to obtain the current. Comparison with the existing Rogowski coil consistently shows less than 1% difference. Present efforts are directed toward simplifying the design to minimize optical alignment and to implement real-time digital data reduction. The fiber current sensor has demonstrated accuracy comparable with existing techniques. In addition, it possesses broad frequency response, it eliminates insulation, electromagnetic interference, and ground loop problems and can easily be installed on existing plasma experiments. This work is supported by U.S. DOE.