Development of Long-Term Magnetic Field Sensor Utilizing Faraday Rotation in Optical Fiber

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Accurate measurements of the plasma current and magnetic fields around the plasma are indispensable for real-time control and data analysis in magnetic fusion devices. Commonly used inductive pickup loops, however, suffer from zero-point drifts of integration in the case of long integration over several hundred seconds. To avoid this problem, a heterodyne polarimeter which measures the Faraday effect was developed as a magnetic field sensor for long-pulsed fusion devices. The wavelength of laser beam was chosen to be 850 nm since the radiation-induced loss in optical fibers is significant below 800 nm while the Faraday rotation angle decreases with increasing wavelength. By the use of a distributed Bragg reflector laser diode and a Faraday isolator, excellent stability of phase measurement was attained. We confirmed that the use of a polarization maintaining fiber between the phase shifter part and the sensor part did not degrade the phase stability when the laser beam coupling to the fiber was optimized. The phase resolution was raised to up to 0.01 deg. by the digital complex demodulation and digital band-pass filtering of beat signals. Hence a low birefringence silica fiber is sufficient for a plasma current sensor of fusion devices instead of a flint glass fiber with a high Verdet constant.Although the Verdet constant of heavily Tb^<3+>-doped paramagnetic glass is larger than that of ZnSe by about a factor of 1.4, ZnSe is superior to the former in terms of negligible temperature dependence. With a 40-mm long ZnSe sensing rod, resolutions of the magnetic field strength of about 5 and 2 gauss were found to be possible with time responses of 1 ms and 10 ms, respectively.