Polarimetry exploits the optical activity and birefringent properties of thermonuclear plasmas to calculate some important quantities for their control like the line-integrated electron density and magnetic field distribution. The Joint European Torus (JET) far infrared polarimeter shares the same probing laser beams of the interferometer, with eight channels, four vertical and four lateral. While the vertical channels were already optimised to provide accurate measurements of the Faraday rotation angle, Cotton–Mouton (CM) phase shift and ellipticity, the lateral channels had been only optimised for Faraday rotation angle alone. By setting the initial polarisation angle of the lateral channel at zero degrees the CM effect is minimised, the ellipticity is almost zero, and the CM phase shift angle is impossible to measure. During the recent JET experimental campaign (C38 in 2019–2020), the input polarisation for the lateral channels was altered for a class of pulses to analyse the possibility to measure the CM phase shift angle and the ellipticity, and, more important, to assess if it is possible to provide information of line-integrated electron density using the lateral channels of JET polarimetry in an ITER-like configuration. As a note, ITER will have only tangential channels with reflectors buried deep inside the first wall, so this setup was truly an ITER-like configuration. The results clearly show huge improvements, which can be achieved by just changing the input polarisation. Moreover, the analysis of the measurements shows that the polarimetric measurements have a systematic error, which is probably due to the effect of refraction and to the in-vessel mirrors, which was only partially taken into account during the calibration phase before the plasma. Thus, a new calibration method was developed and the results presented on a statistical basis. It has been demonstrated that, varying the input polarisation of the polarimeter and using the new calibration method, it is possible to measure the line-integrated electron density, using the CM phase shift (or the ellipticity) of the lateral channels, with good accuracy with respect to the electron density measured by the interferometer that was considered the reference.

旋光法利用热核等离子体的光学活性和双折射特性来计算一些重要量，例如线积分电子密度和磁场分布，以进行控制。欧洲联合圆环（JET）远红外旋光仪与干涉仪共享相同的探测激光束，具有八个通道，四个垂直通道和四个横向通道。虽然已经对垂直通道进行了优化，以提供对法拉第旋转角度，棉花木桐（CM）相移和椭圆度的准确测量，但横向通道仅针对法拉第旋转角度进行了优化。通过将横向通道的初始偏振角设置为零度，可以将CM效应降至最低，椭圆率几乎为零，并且无法测量CM相移角。在最近的JET实验活动（2019-2020年为C38）期间，针对一类脉冲改变了横向通道的输入极化，以分析测量CM相移角和椭圆率的可能性，更重要的是，评估如果有可能使用类似ITER的JET偏光测定法的横向通道提供线积分电子密度的信息。需要注意的是，ITER将只有切线通道，而反射器则埋在第一壁的深处，因此这种设置确实是一种类似于ITER的配置。结果清楚地表明了巨大的改进，只需更改输入极化即可实现。此外，对测量结果的分析表明，偏振测量结果存在系统误差，这可能是由于折射和血管内镜的影响，在等离子之前的校准阶段仅部分考虑了这一点。因此，开发了一种新的校准方法，并在统计基础上显示了结果。已经证明，改变偏振计的输入偏振并使用新的校准方法，可以使用横向通道的CM相移（或椭圆率）来测量线积分电子密度，并且精度很高。相对于通过干涉仪测得的电子密度（被视为参考）。