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

在磁聚变装置中，对等离子体周围等离子体电流和磁场的准确测量对于实时控制和数据分析是必不可少的。然而，在长积分超过数百秒的情况下，通常使用的电感拾取环路会受到积分零点漂移的影响。为了避免这一问题，研制了一种测量法拉第效应的外差偏振仪，作为长脉冲聚变装置的磁场传感器。由于光纤的辐射损耗在800 nm以下显着，而法拉第旋转角随波长的增加而减小，因此选择850 nm作为激光波长。通过使用分布式布拉格反射式激光二极管和法拉第光隔离器，获得了良好的相位测量稳定性。我们证实，当激光与光纤的耦合优化时，在移相器部分和传感器部分之间使用保偏光纤不会降低相位稳定性。相分辨率提高到0.01度。通过对差拍信号进行数字复解调和数字带通滤波。因此，对于聚变装置的等离子体电流传感器来说，低双折射石英光纤是足够的，而不是具有高Verdet常数的打火石玻璃纤维。尽管重掺Tb^&lt；3+&Gt；的顺磁玻璃的Verdet常数比ZnSe大约1.4倍，但在可以忽略的温度依赖性方面，ZnSe优于前者。对于40 mm长的ZnSe传感棒，在时间响应分别为1ms和10ms的情况下，磁场强度约为5和2高斯的分辨率是可能的。

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.