Measurement of depolarized Thomson scattering radiation in JET

The depolarisation of the Thomson scattering (TS) radiation, a relativistic effect noticeable only in very high Te plasmas and never before observed in a fusion machine, is measured for the first time in JET. This depolarisation effect is at the basis of polarimetric TS, an alternative method for the measurement of the plasma electron temperature, suitable for high Te plasmas such as in ITER. In our test, we employed unused fibres at the fringe of the JET HRTS system, to detect both polarisation components of the TS radiation during a high Te campaign. Polarisers perpendicular to the laser light were placed in front of a fibre pair observing a region close to the plasma core, with another adjacent fibre pair used as a control with no polariser. The intensity ratio of the unpolarised component to the full signal is found to have an approximately linear relationship with the electron temperature, as expected from the theory. The measured intensity ratio is consistent with the theory, taking into account sensitivity coefficients of the two measurement channels determined with post-experiment recalibrations and Raman scattering. As polarimetric TS is not dependent on wavelength it can provide significant advantages as an alternative to the conventional TS for measurements at high Te, and may be integrated into a standard TS system.