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

磁聚变装置中等离子体电流和磁场的精确测量是实时控制和数据分析的基础。然而，通常使用的电感拾波回路在超过几百秒的长积分的情况下遭受积分的零点漂移。为了避免这个问题，一个外差偏振计，测量的法拉第效应被开发作为磁场传感器的长脉冲聚变装置。激光束的波长选择为850 nm，因为光纤中的辐射诱导损耗在800 nm以下是显著的，而法拉第旋转角随着波长的增加而减小。采用分布布拉格反射激光二极管和法拉第隔离器，获得了良好的相位测量稳定性。我们证实，使用保偏光纤之间的移相器部分和传感器部分没有降低相位稳定性时，激光束耦合到光纤进行了优化。相位分辨率提高到0.01度。通过数字复解调和数字带通滤波差拍信号。虽然重掺Tb ^3+的顺磁玻璃的费尔德常数比ZnSe的费尔德常数大约1.4倍，但ZnSe的温度依赖性比重掺Tb ^3+的顺磁玻璃小上级，可以忽略不计。用一个40毫米长的硒化锌传感棒，约5和2高斯的磁场强度的分辨率被发现是可能的时间响应为1毫秒和10毫秒，分别。

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.