First measurements of line-integrated electron density in an ITER-like configuration using the JET far infrared polarimeter diagnostic

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.